Conformation-specific spectroscopy and populations of diastereomers of a model monolignol derivative: Chiral effects in a triol chain

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pubs.acs.org/JPCA

Conformation-Specific Spectroscopy and Populations of

Diastereomers of a Model Monolignol Derivative: Chiral Effects

in a Triol Chain

Jacob C. Dean, Evan G. Buchanan, William H. James III,^†Anna Gutberlet,^‡Bidyut Biswas,

P. Veeraraghavan Ramachandran, and Timothy S. Zwier*

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States

S Supporting Information

b

ABSTRACT: Single-conformation spectroscopy of two dia-

stereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,

3-triol (HMPPT) has been carried out under isolated, jet-cooled

conditions. HMPPT is a close analog of coniferyl alcohol, one of

the three monomers that make up lignin, the aromatic biopo-

lymer that gives structural integrity to plants. In HMPPT, the

double bond of coniferyl alcohol has been oxidized to produce

an alkyl triol chain with chiral centers at C(R) and C(β),

thereby incorporating key aspects of the β-O-4 linkage between

monomer subunits that occurs commonly in lignin. Both (R,S)-

and (R,R)-HMPPT diastereomers have been synthesized in

pure form for study. Resonant two-photon ionization (R2PI),

UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB),

and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S₀ꢀS₁

region (35200ꢀ35800 cm^ꢀ1) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-

HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for Rꢀβꢀγ,

γꢀβꢀR, and Rꢀγꢀβꢀπ chains and two cyclic forms [(Rβγ) and (Rγβ)] of the glycerol side chain are determined. Infrared ion-

gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations

present in (R,R). The two diastereomers have very diﬀerent conformational preferences. More than 95% of the population of (R,R)

conﬁgures the glycerol side chain in a γꢀβꢀR triol chain, while in (R,S)-HMPPT, 51% of the population is in Rꢀβꢀγ chains that

point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared

with calculations to provide a consistent explanation of the diastereomer-speciﬁc eﬀects observed.

they are oxidized and polymerized via radical coupling reactions.^3,7

I. INTRODUCTION

Regulation of the relative abundances of the monolignols during

Lignin is an aromatic heteropolymer found in all vascular

polymerization leads to the polymer containing a variety of chemical

plants, ranking second only to cellulose as the most abundant

linkages that determine the level of branching and cross-linking in

biopolymer found in nature.^1ꢀ3As a major component of secondary

the lignin produced. In this way, the degree of structural rigidity is

thickened cell walls, lignin gives the plant its structural rigidity,

controlled in plants ranging from hardwoods to grasses.^3,8,9Among

thereby enabling the transport of water and nutrients through the

the chemical linkages between monomer units, the most abundant is

vascular system.^1,2,4Lignin also protects the cell wall from degrada-

the β-O-4 linkage, derived from the coupling of two coniferyl alcohol-

tion by microorganisms, increasing its resistance to decay and

derived (guaiacyl) units, as shown in Figure 1b.⁸

prolonging the plant’s life.³The same chemical composition that

Many analytical methods, including NMR, HPLC, and GC/

MS, have been employed to examine the relative abundances of

increases resistance to decay also makes the lignin polymer

diﬃcult to break down during the conversion of lignocellulosic

biomass to harvestable biofuels. Understanding the chemical

the monomers in samples ranging from simple dimers to large

oligomers extracted directly from plant samples.^2,10,11The NMR-

composition, variability, and energetics of the chemical linkages

based methods are also capable of quantifying the types and

within the lignin network is therefore needed to eﬃciently

amounts of each chemical linkage, and can detect the presence of

convert plant mass to harvestable sugars.^5,6

Lignin is made up of just three chemically distinct monomers,

the monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl

alcohol, whose structures are shown in Figure 1a. In the ligniﬁcation

process, the monolignols are transported to the cell wall where

Received: May 10, 2011

Revised: June 15, 2011

Published: July 14, 2011

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2011 American Chemical Society

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Despite the racemic nature of the polymer, studying diastereo-

meric eﬀects within a single linkage can provide insight to the

folding and energetics at a local scale.

Recently, double-resonance jet spectroscopy has also been

used to directly measure the relative abundances of observed

conformational isomers of ﬂexible molecules via IR population

transfer (IRPT) spectroscopy¹⁶or its recently implemented analog,

IR ion-gain (IRIG) spectroscopy.¹⁷Such measurements provide

incisive tests of quantum chemical calculations, and deeper

insight to the relationship between the thermal conformational

populations present before supersonic expansion, and those

observed downstream in the expansion after the cooling process.¹⁸

Given the importance of the β-O-4 linkages in most lignin, we

report here a study of the single-conformation ultraviolet and

infrared spectroscopy of the coniferyl alcohol/β-O-4 derivative

1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT,

Figure 1b). HMPPT is a model lignin monomer derivative that

diﬀers from coniferyl alcohol in having its double bond oxidized to

produce a triol chain closely analogous to that found in the β-O-4

linkage in larger lignin oligomers. As with lignin, formation of the

β-O-4 linkage produces two chiral centers on R- and β-carbons that

produce two diastereomeric forms, (R,R)- and (R,S)-HMPPT.

Single-conformation UV spectra in the S₀ꢀS₁region and IR

spectra in the OH stretch region are reported for (R,R)- and

(R,S)-HMPPT, providing a basis for comparison with density

functional theory (DFT) calculations that lead to ﬁrm assign-

ments for all nine observed conformers of the two diastereomers.

As we shall see, one of the intriguing aspects of the structure of

HMPPT is its glycerol-like triol side chain, which places three

OH groups on adjacent carbon atoms where they can H-bond

with one another. The intramolecular H-bonds so produced

serve as a source of diagnostic OH stretches that report on the

H-bonded network formed. Interestingly, the triol side chain

shows remarkable structural diversity, forming H-bonded chains

that are directed either toward or away from the aromatic ring,

clockwise and counter-clockwise H-bonded cycles, and extended

chains involving the aromatic π cloud as terminal H-bond

acceptor. Several of these structural types are present already

in glycerol itself, whose conformers have been probed in a recent

microwave study under jet-cooled conditions, aided by ab initio

calculations.^19,20The present work bears some similarity with

work from Simons and co-workers on carbohydrates, which also

possess an array of OH groups whose H-bonding patterns can be

used to distinguish them.^21,22The triol side chain also bears a

resemblance to trimeric water and methanol clusters, where

cycles and chains compete with one another for the lowest

energy structures formed.^22,23

Figure 1. (a) Chemical structures of the three monolignols. (b) Left:

HMPPT structure with asterisks indicating the two chiral centers. Right:

β-O-4 dimer unit.

other minor monolignols that occur in specialized circumstances.²

Recently, mass spectrometry has also served as a signiﬁcant tool

for the sequencing of small oligomers via MSⁿ-based fragmenta-

tion, with the goal of analyzing longer oligomeric samples by

building a library of characteristic fragmentation pathways of

speciﬁc lignin structural units.^4,12

The fact that lignin is composed entirely of aromatic mono-

mers raises the possibility that ultraviolet spectroscopy could play

a role in lignin structural characterization. With this in mind, our

group has recently begun a systematic study of lignin structural

units under isolated, jet-cooled conditions. The initial study

focused attention on the three monolignols p-coumaryl alcohol,

coniferyl alcohol, and sinapyl alcohol.⁹The three monolignols

show characteristic UV spectral signatures that can be readily

used to distinguish between them. Furthermore, double-reso-

nance spectroscopy was used to determine UV and IR spectra of

single conformations of the three molecules, thereby character-

izing their preferred conformations. In each case, there were only

two conformational isomers present, associated with a syn/anti

pair between the vinyl group and OH group in the para position

on the aromatic ring.

The utility of these methods also extends to exploring chiral

eﬀects within and between molecules. When more than one

chiral center is present in the same molecule, diﬀerences in the

conformational preferences of the diastereomers so produced

can be explored.¹³When the chiral centers are on diﬀerent

molecules, chiral recognition can be probed by forming com-

plexes between them and exploring their spectroscopic properties.¹⁴

In lignin, chiral centers are abundant in the growing polymer

since at every β-coupling, at least one chiral center is created.

However, naturally occurring lignin shows no net chirality, indicat-

ing a lack of direct stereocontrol in the biosynthetic process.¹⁵

The two diastereomers of HMPPT show distinctly diﬀerent

conformational preferences that reﬂect a shift in the delicate

balances between the various types of cycles and chains when the

β-carbon is changed from R to S chirality. We explore the reason

for these shifts by comparing the observations with the predictions

of dispersion-corrected DFT calculations. Finally, the present results

are to be seen in the context of future studies of larger oligomers,

where the structural components present here are incorporated into

a sequence of linkages. It is hoped that characterization of each

linkage by itself will provide a road map for such studies.

II. METHODS

A. Experimental Methods. The (R,R)- and (R,S)-diastereo-

mers of HMPPT were prepared starting with commercial

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Figure 2. (a) Schematic diagram for IR ion-gain spectroscopy. Blue arrows denote resonant excitation in R2PI in the absence of IR excitation, while

orange arrows identify R2PI ionization after IR excitation. (b) R2PI spectrum for RR-HMPPT with a dashed red line indicating the UV wavelength used

in IRIGS. (c) Top: IRIG spectrum for RR-HMPPT under unsaturated conditions. Lower: IRIG spectrum at higher powers showing an increase in

intensity for weak transitions not seen without saturation.

(Aldrich) ferulic acid via AD-mix R- and β-mediated Sharpless

lasers were used for the S₀ꢀS₁resonant step (279ꢀ285 nm, 0.lꢀ0.3

mJ/pulse) and S₁-ionization steps (285 nm, ∼0.3ꢀ0.5 mJ/pulse).

Conformation-speciﬁc electronic spectra were recorded using

either UVꢀUV holeburning (UVHB) or IR-UV holeburning

(IRUV HB) spectroscopy, while resonant ion-dip infrared spec-

troscopy was used to record infrared spectra in the OH stretch

region (3480ꢀ3700 cm^ꢀ1). These methods have been described

elsewhere.³⁰Details speciﬁc to the present work are included in

the Supporting Information.

The present work also utilizes infrared ion-gain (IRIG) spectro-

scopy, a close analog of methods implemented previously by

the Quack³¹and Fujii groups.^32ꢀ34In the present context, IRIG

spectroscopy served as a sensitive method for recording the total

IR spectrum containing contributions from all conforma-

tional isomers present in the jet-cooled expansion. A schematic

diagram for the method is shown in Figure 2a. The experimental

setup is much like that in RIDIR spectroscopy, except that the

wavelength chosen for the ﬁxed probe laser is deliberately chosen

so as not to be resonant with any of the conformational isomers’

jet-cooled transitions, well to the red of the S₀-S₁origins of all

conformers present. As a result, no ion signal is observed without

external excitation, as shown in Figure 2b for RR-HMPPT where the

nonresonant UV wavelength was 284 nm (35211 cm^ꢀ1).

To record IRIG spectra, the IR laser is tuned through the

region of interest, and when resonant with a vibrational transition

of any conformer, a gain in ion signal is produced by the UV

probe laser. As shown in Figure 2a, the gain in ion yield is due to

the redistribution of energy in the vibrationally excited molecule

from the initial vibration (OH stretch) to other vibrational

degrees of freedom (dominated by low-frequency modes) via

intramolecular vibrational redistribution (IVR). Many of the

modes that participate in IVR are those that lead to isomeriza-

tion. If the IR energy is above the barrier to isomerization,

conformationally mixed states can be produced, many of which

absorb to the red of the S₀ꢀS₁origin. The result is that excitation

asymmetric dihydroxylation,^24,25as described by Previtera and

1

co-workers.²⁶The H and ¹³C NMR spectra and the optical

rotations of the triols matched with those reported in the literature.²⁷

A diﬀerentially pumped time-of-ﬂight (TOF) mass spectro-

meter that has been described previously was used in this work.²⁸

Introduction of the sample into the gas phase could not be

achieved by heating methods due to rapid polymerization, so

laser desorption was implemented instead. The method used for

desorption of the neutral molecules was an adaptation of the

desorption/ionization on porous silicon (DIOS) method used

to introduce nonvolatile samples as ions in mass spectrometry

applications.²⁹As a method for laser desorption of neutrals, we

refer to it as laser desorption on (etched) silicon (LDOS).

General information on the preparation of the silicon chips used

in this experiment is included in the Supporting Information,

while details on the procedure for HF-etching and preparation of

the silicon wafers for LDOS/DIOS is described by Woo et al.²⁹

The doped silicon chips were fastened into the back of a tube

(7 mm long, 2 mm channel width) that was ﬁxed onto the front

face of a Series 9 Parker General Valve, serving as a stationary

desorption source. The chips were ﬁxed so that their edges

bordered the nozzle oriﬁce allowing for desorption to occur

along the molecular beam axis. A schematic diagram of the LDOS

setup is included in the Supporting Information.

The third harmonic of a Nd:YAG laser (Continuum Minilite

II) operating at 20 Hz was used as the desorption laser at 1.3 mJ/

pulse power. The Series 9 General Valve was operated at 20 Hz

and was used to form the supersonic expansion. The desorbed

sample was entrained in argon at a backing pressure of 3ꢀ4 bar,

expanded into the vacuum and skimmed, forming the molecular

beam upon entering the ionization region. Two-color resonant

two-photon ionization (2C-R2PI) was used to record mass-

selected UV spectra of the two diastereomers in the S₀ꢀS₁region.

The collimated, doubled outputs of two Nd:YAG-pumped dye

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out of the conformationally mixed “hot bath” subsequent to IR

absorption, leads to an increase in ion signal against zero back-

ground. In most circumstances, the IR power used for IRIG scans

was identical to that used in recording the corresponding RIDIR

spectra (0.5ꢀ2.0 mJ/pulse). At times, the contributions from

weak transitions or minor population conformers were enhanced

by raising the IR power to ∼2.5ꢀ3.0 mJ/pulse, as demonstrated

in Figure 2c. This proved very helpful in identifying the presence

of minor conformers with unique IR absorptions.

Besides acquiring the IR-induced gain spectrum due to all

conformers, I_IRIG(ν), IRIG scans were also used as a method for

determining fractional abundances of the conformers present in

the jet.¹⁷By deﬁnition, the gain spectrum is the weighted sum of

the RIDIR spectra for all conformers:

I_IRIGðνÞ ¼ c F_iγ I_Rⁱ_IDIRðνÞ

ð1Þ

Figure 3. Conformational families and chain types for HMPPT,

including, (a) H-bonded chains and (b) H-bonded cycles. (c) Syn/anti

pairing occurs for every conformational family as a result of the R-OH

orienting syn or anti to the methoxy group on the aromatic ring.

∑

i

Here, “c” is a scaling factor required to scale the RIDIR-based ﬁt

to the IRIG spectrum, F_iis the fractional abundance of conformer

i, Iⁱ_RIDIRis the RIDIR spectrum of conformer i measured as a

fractional depletion, and γ_iis the detection eﬃciency of con-

former i. To avoid systematic errors from saturation eﬀects, the

IRIG and RIDIR spectra are recorded under identical IR power

and beam conditions. We have shown elsewhere¹⁷that popula-

tions extracted from ﬁts to eq 1 assuming that the detection

eﬃciency of all conformers is equal at the probe wavelength (i.e.,

γ_i= γ_jfor all j ¼ i) give the same fractional populations as when

IRPT methods are used. Equal detection eﬃciencies of the IR-

excited molecules are likely a natural consequence of giving the

ground state molecules suﬃcient energy (from IR excitation) to

isomerize, thus, producing a conformationally mixed set of

molecules with identical R2PI eﬃciencies, independent of their

starting conformation. In the present case, we have checked this

by recording IRIG scans at a series of UV wavelengths. These

scans are identical to one another, apart from a slow drop-oﬀ in

intensity as the probe wavelength is shifted further to the red. As a

result, we assume equal detection eﬃciencies in using eq 1 to

extract fractional abundances of the conformers of HMPPT. The

ﬁtting procedure used in determining the fractional abundances

utilized the LevenbergꢀMarquardt algorithm, as implemented in

Mathcad 14.0.³⁵

B. Computational Methods. To identify the possible con-

formational minima associated with each of the two diastereo-

mers of HMPPT, a conformational search was carried out using

the Amber* force field³⁶in the MACROMODEL suite of

programs.³⁷Conformational space was sampled up to a maximum

energy of 50 kJ/mol, identifying approximately 230 structures for

each of the diastereomers. Approximately 50 of the lowest energy

structures obtained from the force field search (for each dia-

stereomer) were used as starting structures for full optimization

and harmonic frequency calculations by density functional

theory (DFT) using the B3LYP³⁸and M05-2X³⁹functionals

with the 6-31+G(d) basis set. All of the DFT calculations were

carried out using the Gaussian 09 suite of programs.⁴⁰For the

geometry optimizations, a tight optimization convergence criter-

ion was specified, and all of the calculations were run with an

ultrafine grid.

energies (after zero-point correction) and for comparison with

experimental infrared spectra.

Vertical excitation energies were also calculated for the lower

energy, optimized structures (<20 kJ/mol) using time-depen-

dent DFT (TDDFT/M05-2X/6-31+G(d)) to compare with the

experimental S₀ꢀS₁origin frequencies. Because the experimen-

tal spectra show substantial FranckꢀCondon activity, the use of

vertical excitation energies are used here only as a consistency

check on the assignments based on the IR spectroscopy.

III. RESULTS

A. Computational Prediction of Conformational Families.

In the structural optimizations for both RS- and RR-HMPPT, a

remarkable variety of structures for the H-bonded triol were

predicted. All low energy conformers retain an OH OCH₃

3 3 3

H-bond between the ortho-substituted OH and OCH₃substit-

uents. In what follows, the presence of this H-bond is assumed

and is indeed present in all observed structures.

Figure 3 shows representative examples of the various con-

formational family types, using RR-HMPPT as an example. In the

ﬁgure, R/β/γ refers to the OH group attached to that carbon on

the glycerol substituent, with the order giving the direction of

H-bond donation (e.g., Rꢀβꢀγ = R(OH) f β(OH) f

γ(OH)). Of the six H-bonded chain types (Figure 3a), the linear

chains γꢀβꢀR and Rꢀβꢀγ were the most commonly observed

types and also constituted the lowest energy structures among

the H-bonded chains. These linear chains H-bond to one another

sequentially along the propyl chain, with H-bond direction either

toward (“out f in”) or away from (“in f out”) the aromatic

ring. In RR-HMPPT, the low energy structures are those that

orient the carbon backbone of the triol group perpendicular to

the ring. Another subset of linear chains are those that do not

hydrogen-bond sequentially but incorporate a H-bond interac-

tion between the two terminal OH groups with the β-OH

H-bonding with one or the other. These chains are denoted as

βꢀγꢀR or βꢀRꢀγ, depending on the directionality of the

Rꢀγ bond with respect to the aromatic ring. The remaining two

H-bonded chain structures are either branched chains (Rꢀβ/

γꢀβ), which involve H-bond donation of both terminal OH

groups to the central β-OH, or extended chains (Rꢀγꢀβꢀπ),

While quantitatively diﬀerent, the two DFT methods yielded a

similar energy ordering of the conformers of HMPPT, which are

dictated largely by H-bonding. The dispersion-corrected func-

tional M05-2X calculations will be used in reporting the relative

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Figure 4. (a) R2PI spectrum of RS-HMPPT (top trace) and UVHB and IR-UV HB spectra of the observed conformers AꢀE. (b) R2PI spectrum of

RR-HMPPT (top trace). The inset displays an expanded view of the origin transitions for conformers A and B. Lower: UVHB and IR-UV HB spectra of

the observed conformers AꢀD. Asterisks indicate the UV transition used as the hole-burn transition in the UVHB spectra; spectra without asterisks are

IR-UV HB spectra. The IR holeburn laser was ﬁxed at 3508, 3520, and 3633 cm^ꢀ1for RS(C), RR(C), and RR(D), respectively.

which have an additional hydrogen-bonding interaction between

the β-OH and the π-cloud of the aromatic ring.

the single-conformation UV spectroscopy are shown in Figure 4.

Figure 4a shows the RS-HMPPT electronic spectra for a total of five

conformers with S₀ꢀS₁origin transition frequencies spread over

256 cm^ꢀ1. The S₀ꢀS₁origin transitions for conformers AꢀE

appear at 35215, 35308, 35333, 35464, and 35472 cm^ꢀ1, respec-

tively. In all the holeburn spectra there is extensive low-frequency

The second general class of conformations predicted by

calculations was the H-bonded triol cycle (Figure 3b). Two

types of cycles were predicted by the calculations, depending on

the relative directionality of the H-bonds in the cycle; either

clockwise or counter-clockwise relative to a ﬁxed position of the

aromatic ring. The cycles are labeled by parentheses with (Rβγ)

denoting a counterclockwise R f β f γ f R arrangement and

the clockwise direction as (Rγβ).

The ﬁnal structural feature distinguishing the conformational

isomers is due to the asymmetry of the guaiacyl ring inherited

from coniferyl alcohol. The relative positions of the methoxy

group on the ring compared to the R-OH on the glycerol side

chain are syn or anti, with most triol structures appearing in dual

form as a syn/anti pair as shown in Figure 3c.

B. R2PI Spectroscopy. The R2PI spectra in the S₀ꢀS₁region

of RS-HMPPT and RR-HMPPT are shown in Figure 4a and b,

respectively (top trace). Both R2PI spectra are quite congested,

with low frequency vibronic structure built off the S₀ꢀS₁origins

of the conformers spanning some 300ꢀ400 cm^ꢀ1. The largest

peaks in the two spectra and the corresponding vibronic structure

blue of those transitions are both quite similar between the two

diastereomers. An immediately obvious difference between the

spectra is the intensities of transitions to the red of the dominant

peaks, in the 35200ꢀ35350 cm^ꢀ1region. RS-HMPPT has signifi-

cant intensity and congestion in this region while the correspond-

ing bands in the RR-HMPPT spectrum are a small fraction of the

larger bands to the blue. These differences are already indicative

of a change in conformational preferences, with a switch in

chirality at the β-position. In addition, the electronic frequency

shift between the largest transitions in RS- and RR-HMPPT is

approximately 60 cm^ꢀ1, suggesting that the dominant conforma-

tion may be different between the two.

vibronic activity, with the lowest frequency fundamental (∼25 cm^ꢀ1

)

associated with torsion of the glycerol side chain about the C_ϕꢀC_R

bond. In all but one of the spectra the largest peak is the origin

transition, indicating that the same H-bonded architecture is likely

retained in the S₁state. This is also likely the case in conformer A,

where the largest band is a combination band between the two

lowest frequency modes at +61.5 cm^ꢀ1(25.4 + 34.8 cm^ꢀ1).

b. RIDIR Spectra in the OH Stretch Region. To better

characterize the structures of the conformational isomers found via

UVHB and IR-UV HB spectroscopy, conformation-specific infrared

spectroscopy in the OH stretch region (3480ꢀ3700 cm^ꢀ1

)

was carried out using RIDIR spectroscopy. In the RIDIR spectra,

four OH stretch fundamentals are expected for each of the four

OH groups present in the molecule. The OH stretch funda-

mentals directly report on the molecule’s intramolecular

H-bonding arrangement, with frequency and intensity patterns

that are sensitive to the H-bonded architectures present. Three of

the transitions are due to the glycerol substituent and are the

primary structural diagnostic for this group. The fourth OH

stretch comes from the para-OH on the ring. As mentioned

already, the OH OCH₃H-bond is present in all observed

3 3 3

conformers, appearing near 3599 cm^ꢀ1, much as it does in

guaiacol, which has an identical OH OCH₃arrangement.⁴¹

3 3 3

The RIDIR spectra for conformers A, D, and E for RS-

HMPPT are shown in Figure 5, with the IRIG spectrum below

it, recorded with the UV laser ﬁxed at 35149 cm^ꢀ1. A comparison

between the two conﬁrms the fact that the IRIG spectrum is

the weighted sum of the RIDIR spectra above it. The OH stretch

transitions for these three conformers span a range from 3580

to 3690 cm^ꢀ1. As expected, a strong transition appears at

C. Conformation-Specific Spectroscopy.

1. RS-HMPPT. a. UV-UV and IR-UV HB Spectroscopy. The results of

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Figure 6. RS-HMPPT RIDIR spectra for conformers B and C with

calculated (scaled) vibrational frequencies and IR intensities for the

assigned structure shown as stick spectra below them, using the DFT

Figure 5. RS-HMPPT RIDIR spectra for conformers A, D, and E with

calculated (scaled) vibrational frequencies and IR intensities for the

assigned structure shown as stick spectra below them, using the DFT

M05-2X/6-31+G(d) level of theory. Red sticks are calculated OH

3 3 3

M05-2X/6-31+G(d) level of theory. Red sticks are calculated OH

3 3 3

OCH₃bands. Bottom: IRIG spectrum for RS-HMPPT.

∼3600 cm^ꢀ1in all three, consistent with the presence of an

centered about the OH OCH₃frequency with triol frequen-

3 3 3

OH OCH₃H-bond in each conformer. Further evidence for

cies of 3574, 3585, and 3621 cm^ꢀ1all signiﬁcantly red-shifted

from the observed triol frequencies of the chains. On this basis,

we tentatively assign this spectrum to a hydrogen-bonded triol

cycle. The small gain signals at 3610 and 3686 cm^ꢀ1in the

spectrum (Figure 6B) are due to interference from a background

signal created by the large population conformer D at the UV

probe wavelength used to monitor conformer B when the IR

absorptions of conformer D are encountered. This is the basis of

the IRIG signal, but here contributes a small gain on top of the

depletions we seek to measure.

3 3 3

this fact is seen in the IRIG spectrum, where the summed

contributions produce the large intensity and broadening at this

central peak.

In conformer A, the band near 3600 cm^ꢀ1has an asymmetric

shape, with a sharp transition peaked at 3598 cm^ꢀ1, indicating

the presence of multiple transitions in this region. The stretch

at 3686 cm^ꢀ1is consistent with the presence of a free OH,

immediately suggesting that this conformation possesses a

H-bonded triol chain. Despite the 250 cm^ꢀ1separation between

the electronic origins of conformer A and conformer D, in the

infrared, conformer D is quite similar to A with a closely spaced

set of transitions around 3600 cm^ꢀ1that are in this case resolved

and a free OH stretch at about the same frequency as in A. On

this basis, we tentatively assign conformer D also to a H-bonded

triol chain conformer. Directly evident in the RIDIR spectrum of

conformer E is the presence of a total of six resolved transitions

instead of the expected four. It seems plausible that spectrum E is

actually the RIDIR spectrum of two very similar conformers

whose electronic spectra are partially overlapped. The primary

feature of this spectrum is its free OH stretch frequency, which is

red-shifted from RS(A) and RS(D) by nearly 30 cm^ꢀ1. The

observed red shift is not large enough to indicate formation of a

H-bonded cycle. Firm assignment, thus, will require comparison

with the predictions of theory.

Conformer C also has a unique spectrum with an unusually

large shift for its lowest frequency OH stretch fundamental,

which appears at 3508 cm^ꢀ1. This shift to lower frequency and

the inherent broadening of the band both point to the OH group

involved in this hydrogen bond being particularly strong. There

are a total of three resolved bands, with the band around

3600 cm^ꢀ1possessing asymmetry, suggesting that the fourth

unresolved transition contributes to this asymmetry. On this

basis, we assign both conformers B and C as H-bonded triol

cycles, with signiﬁcant diﬀerences between the strengths of the

H-bonds involved in the two cases.

2. RR-HMPPT.

a. UV-UV and IR-UV HB Spectroscopy. The conformation-

specific UV spectra for RR-HMPPT are shown in Figure 4b.

Transitions due to the conformers labeled A and B dominate the

R2PI spectrum and display an obvious resemblance. The origin

region of the R2PI spectra of these two conformers is shown in

expanded form as an inset. The two origin transitions are

separated by only 7.5 cm^ꢀ1. UVHB clearly separates the two

spectra, and there is significant FranckꢀCondon activity in the

two lowest frequency modes of both conformers at approximately

The RIDIR spectra for the remaining two conformers in RS-

HMPPT are presented in Figure 6. The infrared spectra of

conformers B and C have a pattern quite diﬀerent from those of

A, D, and E, particularly in having all four infrared transitions

below 3640 cm^ꢀ1, indicating that all OH groups are involved in

H-bonds. In particular, conformer B has four well resolved transitions

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difference between these spectra and RS(A,D) is the 30 cm^ꢀ1red

shift associated with the free OH frequency, which also appears in

RS(E). This is a point to which we will return shortly.

The RIDIR spectrum of conformer C looks very similar to that

of RS(C), with a set of three H-bonded OH stretch transitions

characteristic of a H-bonded cycle. The most notable features

of this spectrum are the intense and broadened transition at

3520 cm^ꢀ1and the lack of a free OH stretch fundamental. As in

RS(C), the transition at 3520 cm^ꢀ1is reﬂecting the presence of

one particularly strong OH H-bond. When this transition is

followed down to the corresponding frequency in the IRIG

spectrum (nonsaturated) at the bottom of Figure 7, it can barely

be seen. This is evidence of its small fractional population relative

to the other conformers. An IR-saturated IRIG spectrum was

required to identify it in the IRIG spectrum. In fact, the small

sizes of its transitions in the UV led to diﬃculties in taking a dip

spectrum that did not suﬀer from interferences from other con-

formers, due to the large size of the gain signal when transitions

due to the dominant conformers were encountered. The spec-

trum in Figure 7 is a diﬀerence of two RIDIR scans taken with UV

laser on and oﬀ resonance with the RR(C) origin transition. Even

so, some of the gain was left in the spectrum as an artifact of this

subtraction.

The weak intensity and extensive FranckꢀCondon activity

prevented us from recording a RIDIR spectrum of RR(D)

without any interference from other conformers. As a result,

the spectrum of RR(D) has some contribution from conformer

A, which has a similar IR spectrum. Nevertheless, there are

diﬀerences of a few wavenumbers in the peak positions of the

bands, conﬁrming the unique presence of conformer D. Further

evidence is provided by the unique IR absorption at 3633 cm^ꢀ1

in the RIDIR and IRIG spectra. The close similarity with the

spectra of A and B indicate that conformer D is another member

of the same conformational family as these more dominant

conformers, tentatively ascribed to a hydrogen-bonded triol chain.

D. Conformational Assignments and the Comparison with

Calculation. To make firm assignments and to gain additional

insight to the source of the observed IR patterns, the experimental

spectra were compared with calculated spectra for optimized

conformational minima. Figures 5ꢀ7 include stick spectra showing

the scaled vibrational frequencies and IR intensities predicted by

DFT M05-2X/6-31+G(d) calculations for conformers that best

match the experimental spectra. As anticipated, four OH stretch

fundamentals are present in each calculated spectrum due to the

Figure 7. RR-HMPPT RIDIR spectra for conformers AꢀD with

calculated (scaled) vibrational frequencies and IR intensities for the

assigned structure shown as stick spectra below them, using the DFT

M05-2X/6-31+G(d) level of theory. Red sticks are calculated OH

OCH₃bands. Bottom: IRIG spectrum for RR-HMPPT.

3 3 3

28 and 40 cm^ꢀ1. A third conformershows weak transitions starting

at a red-shifted origin frequency of 35237 cm^ꢀ1, shifted from the

conformer A origin by ꢀ160 cm^ꢀ1. In Figure 4b, the hole-burning

spectra of conformer C is magnified by a factor of 6 to better

present its UV spectrum. In the region blue of the origins of A

and B, the holeburning spectrum suffers from incomplete sub-

traction of transitions from these conformers. Nevertheless, in

the S₀ꢀS₁origin region, a long progression in its lowest fre-

quency mode at 34.6 cm^ꢀ1is clearly present. The holeburning

spectrum for RR(C) was taken using IR-UV holeburning as it had

a unique, large transition in its IR spectrum. The IR-UV HB

spectrum of conformer D is also weak, and its spectrum proved

difficult to cleanly record due to extensive overlap with other

conformers both in the IR and UV. An IR-UV HB scan is shown

in Figure 4 with the IR holeburning transition at 3633 cm^ꢀ1, a

unique IR absorption indicated by the IRIG spectrum.

b. RIDIRS in the OH Stretch Region. The RIDIR spectra for

the four conformers of RR-HMPPT are shown in Figure 7 along

with the IR-unsaturated and -saturated IRIG spectra recorded

with the UV laser at 35211 cm^ꢀ1. Once again, the dominant feature

in the IRIG spectrum is at 3596 cm^ꢀ1, ascribable to the guaiacyl

OH stretch, as it did in the RS-diastereomer. As suggested by the

IRIG spectrum, all of the RIDIR spectra have transitions at

approximately this frequency. Consistent with the close similarity

and small shift in the UV spectra of conformers A and B, these

two conformers also have very similar IR spectra, indicating that

these two dominant conformers belong to the same structural

type. Their pattern is similar to those observed for RS(A) and

RS(D) with two triol stretches appearing close to one another,

and a free OH stretch well to the blue. On this basis, we assign

RR(A) and RR(B) as H-bonded triol chains. One interesting

single OH OCH₃on the aromatic ring and the three alkyl OH

3 3 3

groups on the glycerol substituent on the opposite side of the ring.

The harmonic frequencies of the three triol-OH stretches in the

calculated spectra were scaled by a scale factor of 0.9535, a value

determined by scaling the free OH of the calculated spectrum in

RR(A) to the experimental value, a case where the assignment was

especially clear. The frequency of the OH OCH₃stretch was

3 3 3

scaled by a factor of 0.9495 chosen from the fit to the experimental

spectrum of guaiacol.⁴¹The sticks colored red in all of the calculated

spectra are the transitions corresponding to the OH OCH₃

3 3 3

stretch. The tentative assignments to particular conformations made

based on the spectral patterns in the experimental scans are

strengthened and further clarified by the comparison with calculated

IR spectra.

1. RS-HMPPT. In RS-HMPPT, conformers A and D have the

highest observed OH stretch fundamental frequency at 3687 cm^ꢀ1

(Figure 5A,D), which is characteristic of a free “terminal” γ-OH

in an Rꢀβꢀγ H-bonded chain. The frequency of this transition

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Table 1. Experimental and TDDFT Calculated S₀ꢀS₁Origin Frequencies and Zero-Point Corrected Energies for the Assigned

Structures in RS- and RR-HMPPT

conformer

assignment

expt S₀ꢀS₁origin frequency (cm^ꢀ1

)

TDDFT^acalcd S₀ꢀ S₁origin (cm^ꢀ1

)

relative energy^b(kJ/mol)

RS-HMPPT

A

Rꢀβꢀγ (anti)

CW cycle (syn)

CCW cycle (syn)

Rꢀβꢀγ (syn)

γꢀβꢀR (syn)

35215 (0)

42045 (0)

2.27 [9.13]

B

35308 (+93)

35333 (+118)

35464 (+249)

35472 (+257)

42137 (+92)

0 [6.86]

C

42180 (+135)

1.77 [8.63]

D

42328 (+283)

0.17 [7.03]

E1/E2

42256/42143 (+211/+98)

2.68/3.08 [9.54/9.94]

RR-HMPPT

A

B

γꢀβꢀR (anti)

γꢀβꢀR (syn)

Rꢀγꢀβꢀπ (anti)

γꢀβꢀR (syn)

35397 (0) [+182]

35404 (+7) [+189]

35238 (ꢀ159) [+23]

35395 (ꢀ2) [+180]

42150 (0) [+105]

42157 (+7) [+112]

42080 (ꢀ80) [+35]

42151 (+1) [+106]

0

1.46

7.69

5.02

C

D

^aTDDFT/M05-2X/6-31+G(d) S₀ꢀS₁vertical excitation energies (cm^ꢀ1) with relative wavenumber in parentheses. The bracketed values compare

relative frequencies between diastereomers, with RS(A) as zero. ^bZero-point corrected relative energies at the same level of theory. Bracketed values

compare energies of the two diastereomers, with the global minimum of RR-HMPPT (RR(A) γꢀβꢀR (anti)) as zero.

is very close to the calculated free OH stretch in glycerol itself,

confirming this assignment. In these chains, the γ-OH accepts a

hydrogen bond from the β-OH, but is otherwise free because the

direction of the H-bonded chain places this terminal OH group

far from the aromatic ring, absent of any interactions with it. The

remaining two transitions in the glycerol chain are shifted down

(Rγβ) cycle with calculated relative energy only 1.8 kJ/mol

higher than that for the clockwise cycle assigned to B. Here, the

large frequency shift of the band at 3508 cm^ꢀ1can only be ﬁt by

the optimized counter-clockwise cycle, with an R f γ H-bond

that is unusually short (1.96 Å) and oriented so that it forms an

unusually strong H-bond, somewhat at the expense of the γ f β

H-bond.

to ∼3600 cm^ꢀ1, in close proximity with the OH OCH₃

3 3 3

fundamental. This places three OH bands in the same region.

In RS(A), the experimental spectrum does not resolve these

three transitions, instead possessing an asymmetrically broa-

dened single band. In RS(D), on the other hand, the three bands

are just resolved, with the Rꢀβꢀγ(syn) conformer providing a

best match with experiment. Thus, we assign conformers RS(A)

and RS(D) as a syn/anti pair that shares the same Rꢀβꢀγ linear

H-bonded chain, differing only in the orientation of the aromatic

ring relative to the chain. According to the calculations, the

conformer D structure is the second-lowest in energy for RS-

HMPPT at 0.17 kJ/mol above the RS global minimum, and this

syn/anti pair differs from one another by ∼2 kJ/mol.

The assignment for RS(E) is somewhat more tentative, given

the “extra” transitions present. However, the location of the

highest frequency OH stretch (3660 cm^ꢀ1) is clearly not that of a

free γ-OH but, instead, is characteristic of a free R-OH group.

The calculations predict such a shift, as shown in the stick spectrum

in Figure 5E. This shift is also conﬁrmed by IR-UV double

resonance studies of Mons et al. on benzyl alcohol (C₆H₅CH₂-

OH) that locate its R-OH stretch fundamental at 3649 cm^{ꢀ1 42}

.

As a result, conformer E is assigned with some conﬁdence as a

member of the γꢀβꢀR family of H-bonded chains. If the

postulated contributions from two conformers of the same family

to the RIDIR spectrum of RS(E) is correct, the two calculated

structures that best account for observation are the lowest-energy

γꢀβꢀR structures, both of which are syn conformers diﬀering in

their glycerol chain dihedrals (OCCO and OHOH), with

calculated energies 2.68 and 3.09 kJ/mol above the global

minimum for RS-HMPPT. The stick spectrum in Figure 5E is

a combination of the two calculated spectra, with transitions to

the ﬁrst conformer shown with dashed lines.

Based on the RIDIR spectrum alone, it is not possible to ﬁrmly

assign A to anti and D to syn. However, the UV spectrum provides

further evidence in support of this assignment. A surprising aspect

of the assignment of RS(A) and RS(D) as a syn/anti pair is that

one might have anticipated the two to have electronic origins

near to one another, much as was the case for the IR transitions.

However, the S₀ꢀS₁origin transition of RS(A) is 238 cm^ꢀ1to

the red of that of RS(D). As Table 1 summarizes, TDDFT calcula-

tions of the vertical excitation energies of the S₀ꢀS₁transitions

predict that the anti conformer will be 283 cm^ꢀ1to the red of syn,

providing a means of distinction between the two, and leading to

the assignments for RS(A) and RS(D) shown in Figure 8a.

We have already noted that RS(B) and RS(C) have all three

OH stretch fundamentals involved in H-bonds. Only four

H-bonded cycle minima were found by the calculations, corre-

sponding to the syn/anti pairs of the clockwise and counter-

clockwise cycle conformations. Among those possibilities, the

unique pattern of OH stretch transitions in the spectrum of

conformer B can only be ﬁt as a clockwise (Rβγ) H-bonded

cycle. This structure, shown in Figure 8a, is calculated to be the

global minimum for RS-HMPPT. The best ﬁt to the spectrum of

RS(C) is also to a H-bonded cycle, here a counter-clockwise

The proposed assignments for RS(AꢀD) are conﬁrmed by

TDDFT single-point vertical excitation energy calculations, which

match well with experimental S₀ꢀS₁frequencies. Only RS(E)

lacks this close correspondence, possibly due to the overlapping

transitions in the UV.

2. RR-HMPPT. RR-HMPPT has two dominant conformers, RR-

(A) and RR(B), which have been tentatively assigned as H-bonded

triol chains. The calculated stick spectra in Figure 7A,B confirm

those assignments, and lead to the more specific attribution to linear

γꢀβꢀR H-bonded chains. These chains have a free R-OH stretch

fundamental at 3657 cm^ꢀ1, very close to that in RS(E). The

calculated spectra capture the frequency shifts and splitting of the

H-bonded OH stretches as well, lending confidence to the assign-

ments. It is important to note that the direction of the H-bonds

(γ f β f R or out f in) is opposite to that in the dominant

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Figure 8. Optimized structures of (a) conformers AꢀE for RS-HMPPT and (b) conformers AꢀD for RR-HMPPT at the DFT M05-2X/6-31+G(d)

level of theory. The dashed blue lines represent hydrogen bonds between H-bonding pairs.

conformers of the RS diastereomer, where R f β f γ chains

dominate. Thus, the RR-HMPPT diastereomer prefers γ f β f R

chains that curl back in toward the aromatic ring, while RS-HMPPT

favors the reverse directionality (R f β f γ).

OH π H-bond with the aromatic ring. The large, intense

3 3 3

OH stretch at 3520 cm^ꢀ1is accounted for by this structure,

ascribable to the R-OH, which is conﬁgured to form a strong,

linear R f γ H-bond (R_Rꢀγ= 1.92 Å). The β f π, OH

3 3 3

RR(A) and RR(B) form a syn/anti pair in which the only

diﬀerence between the two is the orientation of the triol chain

relative to the aromatic ring (Figure 8b). The anti structure is

calculated to be the global minimum, lower in energy than RR(B)

by 1.5 kJ/mol and consistent with the assignment of RR(A) as

anti, with its transitions among the most intense in the R2PI

spectrum of Figure 4b. This assignment is also strengthened by

the predicted close proximity of the two S₀ꢀS₁origins, with

TDDFT calculations predicting a vertical splitting in which

RR(B) is blue-shifted from RR(A) by 7 cm^ꢀ1, nearly identical

to that observed (Table 1).

The spectrum of RR(D) is very similar to that of RR(A,B), with

a free OH stretch also characteristic of a γꢀβꢀR chain. Here we

tentatively assign RR(D) as γꢀβꢀR(syn), diﬀering from RR(A)

and RR(B) by its glycerol chain type/dihedrals (Z versus C1

type, Table 3) pushing the structure up 5.0 kJ/mol above the

global minimum, in keeping with its small intensity in the R2PI

spectrum. The less stable glycerol chain type in RR(D) weakens

the β f R H-bond, producing the larger splitting between the

H-bonded OH stretch fundamentals observed in the RIDIR scan

(Figure 7D). The TDDFT prediction also places its S₀ꢀS₁origin

within a few cm^ꢀ1of those of RR(A) and RR(B). The tentative

assignment of the S₀ꢀS₁origin of RR(D) is indeed close to those

of RR(A,B), although the UVHB spectrum is admittedly noisy

due to its weak intensity in the R2PI spectrum (Figure 4b).

It was noted previously that the spectrum of RR(C) shows

three H-bonded OH stretch transitions ascribable to the triol,

suggesting initially that this spectrum was due to a H-bonded

triol cycle analogous to RS(B) and RS(C). However, in the RR

diastereomer, no low energy H-bonded cycles were predicted by

the calculations. Instead, the stick spectrum that matches best

with experiment is due to an Rꢀγꢀβꢀπ H-bonded extended

chain, in which the terminal β-OH in the chain forms an

OCH₃, and γ-OH stretch fundamentals appear as a triplet of

transitions near 3600 cm^ꢀ1, just resolved in the experimental

spectrum. The conﬁguration of the Rꢀγꢀβ chain is nominally

like that in the counter-clockwise cycle RS(C) (Figure 8a), except

that in the RR-diastereomer the β-OH group is positioned to

form a π hydrogen-bond rather than closing the H-bonding cycle

by forming a β f R H-bond. The calculated relative energy of

this structure (7.7 kJ/mol) is consistent with its small intensity in

the R2PI spectrum. Of the syn/anti pair of Rꢀγꢀβꢀπ struc-

tures, the observed structure was assigned to the anti structure,

despite being higher in energy by about 3.5 kJ/mol, using the

TDDFT vertical excitation energies as a discriminating factor.

The vertical excitation calculations for the syn conformer yields a

blue shift of approximately 60 cm^ꢀ1from RR(A), which clearly is

not the case experimentally, while the calculated excitation

energy for the anti conformer red shifts from RR(A) by about

ꢀ70 cm^ꢀ1closer to the experimental red shift of ꢀ159 cm^ꢀ1

.

H-bonded triol cycles of RR-HMPPT are all signiﬁcantly higher

in energy (starting near 10 kJ/mol), a point to which we will

return in the Discussion section.

E. IRIG Spectra and Fractional Abundances. 1. RS-HMPPT.

The IRIG spectrum shown in the bottom trace of Figure 5 was

used to determine fractional abundances for RS-HMPPT, using

the RIDIR spectra of RS(AꢀE) as basis functions in fitting

eq 1 to the IRIG spectrum, as described in section IIA. Figure 9

shows the IRIG spectrum of RS-HMPPT (black trace) and the

weighted sum (red trace) that minimizes the difference between

the two. This difference is shown above the IRIG scan. Fractional

abundances and errors associated with the best fit are collected in

Table 2. As anticipated, the fractional abundances are dominated

by the three triol chain structures, conformers RS(D), RS(E), and

RS(A), with percent abundances of 31, 28, and 20%, respectively.

Two of these (A and D) are Rꢀβꢀγ chains forming a syn/anti

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complexity to the molecule’s potential energy landscape.^19,20

No less than six H-bonded triol chain types are possible (Figure 3a),

with OH groups forming (i) linear chains pointing toward

(γꢀβꢀR) or away from (Rꢀβꢀγ) the aromatic ring or folding

back on itself (βꢀγꢀR or βꢀRꢀγ); (ii) branched chains

(Rꢀγ/βꢀγ) with central OH group acting as double acceptor;

or (iii) extended chains that use the aromatic ring as a terminal

acceptor (Rꢀγꢀβꢀπ). H-bonded triol cycles (Figure 3b) are

also among the possible structures, differing in the direction of

the H-bonds in the cycle, nominally clockwise or counter-clock-

wise relative to a given aromatic ring orientation. The other two

aromatic substituents, OH and OCH₃, are in adjacent positions

on the ring, as they are in coniferyl alcohol, locking in their

relative orientation in forming an OH OCH₃intramolecular

3 3 3

H-bond far more stable than any other configuration. This

locked-in asymmetry in the ring substituents constitutes a structural

marker that distinguishes a given triol configuration relative to the

Figure 9. Bottom: IRIG spectrum for RS-HMPPT (black trace) with

best-ﬁt, weighted sum of RIDIR spectra (red trace). Top (black):

diﬀerence of IRIG spectrum and weighted sum of RIDIR spectra.

OH OCH₃H-bond, either syn or anti (Figure 3c).

3 3 3

This structural diversity is in stark contrast to its parent

molecule, coniferyl alcohol (Figure 1a). There, the vinyl group

in the allyl alcohol side chain prefers an in-plane structure that

retains conjugation with the aromatic ring, and locks the terminal

OH group into a single conformation. When combined with the

Table 2. Fractional Abundances and 1σ Error Bars from IRIG

Fitting Analysis for RS- and RR-HMPPT

RS conformer

F_i^RS

RR conformer

F_i^RR

energetic asymmetry of the OH OCH₃substituents across

3 3 3

the ring, the population of coniferyl alcohol is funneled almost

exclusively into a single conformation under jet-cooled conditions.⁹

A total of nine conformations were observed for the two

diastereomers of HMPPT, ﬁve for RS and four for RR. Seven of

the nine contain H-bonded triol chains, which dominate the

populations of both, with triol cycles accounting for 21% of the

RS population, but no measurable fraction of the RR diastereo-

mer. Three of the six possible chain types are observed experi-

mentally, with in f out dominating (Rꢀβꢀγ) in RS and out f

in (γꢀβꢀR) in RR. More detailed structural data for the

assigned structures (dihedrals, chain types) are given in the

Supporting Information.

A

B

C

D

E

0.20 ( 0.02

0.14 ( 0.02

0.07 ( 0.02

0.31 ( 0.03

0.28 ( 0.02

A + D

0.54 ( 0.03

0.43 ( 0.03

0.03 ( 0.01

B

C

pair, while RS(E) is tentatively assigned as a γꢀβꢀR(syn) chain.

Together these three chain conformers account for 79% of the

conformer population in the jet. The remaining population

resides in the two cyclic conformers, RS(B) and RS(C), in a

2:1 ratio (14:7%) between clockwise (Rβγ) and counter-clock-

wise (Rγβ) cycles.

B. Infrared Spectral Signatures. The OH stretch region of

2. RR-HMPPT. In RR-HMPPT, overlapping OH stretch transi-

tions in the three γꢀβꢀR chains RR(A,B,D) prevent us from

obtaining quantitative fractional abundances for all three inde-

pendently. Nevertheless, even a qualitative look at the IRIG

spectrum in Figure 7 makes it clear that these three triol chain

conformers dominate the population, with the extended chain

Rꢀγꢀβꢀπ RR(C) accounting for no more than 3% of the

population. Unique IR transitions due to B and C are evident in

the IRIG spectrum, but at best, the sum of the fractional

abundances for A + D can be determined, recognizing that D

is a minor conformer in the R2PI spectrum (Figure 4b). Table 2

presents fractional abundances and error bars for (A + D)/B/C

of 0.54 ( 0.03:0.43 ( 0.03:0.03 ( 0.01. Thus, once again, the

triol chains dominate, here exclusively of the γꢀβꢀR type, with

only a small fraction in the unusual Rꢀγꢀβꢀπ extended chain.

the infrared spectrum is an extraordinarily powerful probe of

intramolecular H-bonding. The OH OCH₃fundamental is

3 _ꢀ3 3₁

positioned characteristically at 3599 cm , signaling the presence

of the OH OCH₃H-bond, but offering little further by way of

3 3 3

diagnostics of what else is present. The three OH stretch

transitions of the glycerol side chain enable a characterization

of the diverse set of H-bonded architectures present when three

OH groups are present on adjacent carbons along an alkyl

backbone. The H-bonded chains show two H-bonded OH

stretch transitions near 3600 cm^ꢀ1and a single free OH stretch

some 60ꢀ90 cm^ꢀ1higher in frequency. The exact position of this

free OH depends on the direction of the triol chain, with

Rꢀβꢀγ chains appearing at ∼3690 cm^ꢀ1, while γꢀβꢀR chains

are nearer 3660 cm^ꢀ1, providing a diagnostic of chain direction.

The obvious diagnostic signature of the triol cycles is the lack of a

free OH group. Beyond this, the two examples in hand (RS(B)

and RS(C)) present quite different spectral patterns (Figure 6B,

C) arising from the two directions of the cycle (Rβγ) for

clockwise and (Rγβ) for counter-clockwise. In the latter case,

an unusually short R f γ H-bond pushes this frequency all the

way down to 3508 cm^ꢀ1, some 60 cm^ꢀ1below any other

absorption of the triol OH groups in the other conformations.

C. Ultraviolet Spectral Signatures. Figure 10 compares the

R2PI spectra of RS- and RR-HMPPT, with S₀ꢀS₁origins,

structural assignments, and fractional populations labeled to

IV. DISCUSSION

A. Structural Complexity in RS- and RR-HMPPT. Interest in

the conformational preferences of HMPPT is fueled by its close

structural similarity with the β-O-4 lignin dimer of coniferyl

alcohol, sharing the same aromatic ring substituents, with the

dimer’s aromatic ether linkage replaced by a β-OH group

(Figure 1b). The glycerol side chain so created, with its three

OH groups on adjacent carbons, lends substantial conformational

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member is in this set. For instance, the RS(A) and RS(D)

conformers form a syn/anti structural pair of Rꢀβꢀγ triol

chains, with a frequency shift of almost 250 cm^ꢀ1between them.

However, in RR-HMPPT, RR(A) and RR(B) also constitute a

syn/anti pair, but here of γꢀβꢀR type, and their origins are split

by only 7 cm^ꢀ1

.

Finally, the two cyclic conformers (RS(B, C)) have S₀ꢀS₁

origins that appear about midway between these two extremes,

with frequency shifts +93 and +118 cm^ꢀ1from RS(A). In these

structures, the R-OH group acts as both donor and acceptor, with

the γ-OH playing some role in the observed shift, both through

reorientation of the R-OH and through its direct interaction with

the aromatic π cloud. The eﬀect of the γ-OH group is evident by

comparison of RS(D) with RS(B), which share the same

orientation and position for the R-OH relative to OH OCH₃

3 3 3

(Figure 8a), but diﬀer in the position and orientation of the

γ-OH, which is free and displaced away from the phenyl ring in

RS(D) (Δν = +249 cm^ꢀ1), but reaching back down toward the

ring to close the H-bonded cycle in RS(B) (Δν = +93 cm^ꢀ1).

D. Conformational Preferences: Diastereomeric Effects on

Populations. The polymerization of the three lignin monomers

(coniferyl alcohol, p-coumaryl alcohol, and sinapyl alcohol,

Figure 1a), involves radical attack on the CdC double bonds

of these molecules, producing chiral centers along the various

chemical linkages that join monomer units. The β-O-4 linkage

will have two such chiral centers at the R- and β-carbons,

producing (R,R)- and (R,S)-diastereomers just like those present

in RR- and RS-HMPPT.

Naturally occurring lignin oligomers show no net chirality,

consistent with a polymerization mechanism without stereo-

chemical control, with each site of random chirality.¹⁵This is

thought to be true even for the nearest-neighbor sites on the

same chemical linkage, although no detailed characterization has

been made.

One of the goals in studying the two diastereomers of RR-

HMPPT and RS-HMPPT is to see the extent to which these

diastereomers diﬀer in their conformational preferences. Inter-

estingly, the two diastereomers have strikingly diﬀerent single-

conformation IR (Figures 5ꢀ7) and UV (Figure 10) spectra that

reﬂect wholesale changes in the conformational preferences. In

seeking to understand these diﬀerences, we have measured the

fractional abundances of the conformers using the recently

developed method of IRIG spectroscopy. In HMPPT, the

strikingly diﬀerent conformational preferences of the two dia-

stereomers, and the observed fractional abundances obtained by

IRIG spectroscopy can be accounted for largely by changes in the

relative energies of the various conformational types.

Figure 10. (a) R2PI spectrum for RS-HMPPT in the S₀ꢀS₁origin region

with S₀ꢀS₁origins labeled for conformers AꢀE with their respective

fractional abundances. (b) R2PI spectrum for RR-HMPPT in the S₀ꢀS₁

origin region with S₀ꢀS₁origins labeled for conformers AꢀD.

facilitate comparison between the two. Table 1 summarizes both

experimental and TDDFT vertical excitation energies for the

transitions. In Table 1, the electronic frequency shifts (in cm^ꢀ1

)

are given with respect to the most red-shifted S₀ꢀS₁origin, due

to RS(A), serving as zero on the relative frequency scale. The

TDDFT calculations generally capture the directions and approx-

imate magnitudes of the shifts with conformation and diastereomer.

In both diastereomers, the largest population conformers appear

nearer the blue end of the range, with red-shifted transitions very

weak in the RR diastereomer (Figure 10b) and much stronger in

RS (Figure 10a). In seeking to understand the structural cause of

the electronic frequency shifts, one can consider how the shifts

depend on triol structural type (chain vs cycle, type of chain),

position of the triol chain relative to the OH OCH₃groups

3 3 3

(syn vs anti), and diastereomer-specific effects.

The correlation of electronic frequency shift with structural

type is not a simple function of any one structural feature.

Because the R-OH group is closest to the aromatic ring and

orients in opposite directions in the two types of chains, both its

syn/anti position (relative to OH OCH₃) and orientation are

3 3 3

important in modulating the magnitude and direction of their

electronic eﬀect on the S₀ꢀS₁origin transition of the adjacent

aromatic ring. All the γꢀβꢀR chains (RR(A, B, D), RS(E)),

whether syn or anti, are on the blue end of the spectrum, as is the

Rꢀβꢀγ(syn) conformer (RS(D) = +180 cm^ꢀ1). The two anti

Rꢀβꢀγ chains (RR(C) and RS(A)) are the two furthest red-

shifted conformers (0, +23 cm^ꢀ1). We surmise on this basis that

when the R-OH acts solely as H-bond donor in an Rꢀβꢀγ

chain, it produces a red-shift when in the anti conformation. The

syn position for the R-OH is, by comparison relatively insensitive

to R-OH orientation or H-bonding conﬁguration when in a

H-bonded chain (Rꢀβꢀγ or γꢀβꢀR).

H-bonded γꢀβꢀR triol chains (out f in) dominate the

population in RR, accounting for over 95% of the population

(Table 2), with a single Rꢀγꢀβꢀπ extended chain accounting

for the remainder. In the RS diastereomer, 80% of the population

is also in triol chains, but in this case, the favored orientation is in

the opposite direction (Rꢀβꢀγ, in f out) by a 2:1 margin. In

addition, a sizable fraction of the population (∼20%) in RS-

HMPPT is in H-bonded cycles, again split about 2:1 in favor of

(Rβγ) over (Rγβ) cycles. It remains to understand why.

Figure 11 compares energy level diagrams for the (R,R)

(Figure 11a) and (R,S) (Figure 11b) diastereomers. The energy

levels have been labeled by triol H-bonding type, as indicated in

the ﬁgure. As diastereomers, the two energy level diagrams can be

placed on a single energy scale, as they are in the ﬁgure. Several

deductions can be immediately drawn from the ﬁgure. First, in

One consequence of the unique red-shift of the Rꢀβꢀγ anti

conformers is that the splitting between two conformers forming

a syn/anti pair diﬀers dramatically depending on whether one

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Figure 11. Conformational energy level diagram for all (a) RR-HMPPT and (b) RS-HMPPT structures with energies in the ﬁrst 25 kJ/mol. Black lines

represent the zero-point corrected energies with respect to the global minimum energy for all structures (RR(A)). Designations next to energy levels

specify glycerol chain type and syn/anti orientation. Levels shown in bold are assigned structures. See text for details.

both diastereomers, the observed conformers have been assigned

to structures among the lowest in energy, providing some

conﬁdence that the calculations are capturing the important

interactions that produce these preferences. Second, there are far

fewer low energy conformers in RR than in RS, with 4(10)

conformations in the ﬁrst 5(10) kJ/mol for RR-HMPPT, but

13(21) in the ﬁrst 5(10) kJ/mol in RS. Third, the calculations

correctly predict a clear preference in RR-HMPPT for γꢀβꢀR

triol chains, as observed, with Rꢀβꢀγ chains and (Rβγ) and

(Rγβ) cycles higher in energy by at least 10 kJ/mol, consistent

with their absence in the expansion. By contrast, in RS-HMPPT,

H-bonded cycles and H-bonded chains of the Rꢀβꢀγ type are

lowest energy, with several examples of each in the ﬁrst few kJ/

mol, consistent with the large populations observed in conforma-

tions of these structural types.

The relative energies of the conformers are dictated by three

sometimes competing factors: the structure of the glycerol side

chain, the position of the RꢀOH group, and the orientation of

the RꢀOH group relative to the aromatic ring. Not surprisingly,

the glycerol side chain in HMPPT takes on the same H-bonding

conﬁgurationsasglycerolitself(CH₂OHꢀCHOHꢀCH₂OH).^19,20

Figure 12. (a) Optimized glycerol chain types observed in HMPPT in

order of increasing energy. (b) Relaxed potential energy scan along the

C_ϕC_ROH torsional coordinate of 1-(4-hydroxy-3-methoxyphenyl)-R-

propanol, with syn in blue and anti in black. Relative energies and

dihedral angles of assigned HMPPT structures labeled by squares (syn)

or diamonds (anti) on the diagram.

Figure 12a shows the calculated structures of ﬁve conformers

of glycerol, consisting of a H-bonded cycle, and four types of

H-bonded triol chains. Table 3 lists key structural features of the

glycerol conformations, for ready comparison with the analogous

side-chain conformations in HMPPT. The complete table of all

optimized glycerol conformations and their OCCO and OHOH

dihedrals can be found in the Supporting Information. As the

labels in the ﬁgure indicate, the oxygen atoms in the triol chains

take up two diﬀerent spatial arrangements, forming either a

smooth curve (C1, C2, C3⁰) or zigzag (Z), with the two OCCO

dihedral angles having the same (C1ꢀC3⁰) or opposite signs (Z).

The distinction between C1, C2, and C3⁰structures is in the

C3⁰denotes diﬀerences in the signs of the dihedral angles, as

described further in the Supporting Information. For reference,

Table 3 also includes an alternative labeling scheme associated

with the glycerol conformations identiﬁed in the previous

microwave study of Ilyushin et al.^19,20The reader is referred to

that work for details of their labeling scheme. For our purposes

here, we simply note that the microwave study identiﬁed ﬁve

OH OH dihedrals, which reﬂect diﬀerent angles of attack for

3 3 3

the donor OH relative to the acceptor OH. The prime labeling

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Table 3. Optimized Glycerol Chain Types and Dihedrals for Chain Types Observed in HMPPT Structures

chain type

labeling scheme 2^b

OCCO 1 (°)^a

OCCO 2 (°)

OHOH 1 (°)^a

OHOH 2 (°)

rel. E (kJ/mol)^c

cycle

C1

Z

G⁰Ggg⁰g

G⁰G⁰tgg

GG⁰t⁰g⁰g

G⁰G⁰g⁰gg

G⁰G⁰gg⁰g⁰

ꢀ60.8

ꢀ56.8

ꢀ57.9

ꢀ55.2

ꢀ52.8

47.4

ꢀ54.8

58.0

ꢀ99.8

ꢀ110.9

ꢀ120.6

95.6

ꢀ75.3

ꢀ122.1

117.5

0

1.82

4.79

6.49

9.52

C2

C3⁰

ꢀ55.0

ꢀ49.4

ꢀ122.2

96.0

95.3

^aDihedral angles are listed in order of the direction of H-bonding on the chain (donor f acceptor). ^bLabeling scheme 2 orders the dihedrals from the

terminal acceptor f donor. ^cEnergies relative to the glycerol cycle (global minimum), calculated at the DFT//M05-2X/6-31+G(d) level of theory.

glycerol conformers, which in our parlance constitute the cycle,

C1, two slightly diﬀerent versions of C2, and Z chain structures.²⁰

The most stable conformation of glycerol is the H-bonded cycle,

with C1, Z, C2, and C3⁰chains 1.8, 4.8, 6.5, and 9.5 kJ/mol higher

in energy, respectively.

chains that incorporate the most stable type of triol chain

(C1) and R-OH orientation.

(2) In the RS diastereomer, no low-energy γꢀβꢀR C1 struc-

tures are possible, because the change in chirality at C(β)

forces the C1 chain out over the aromatic ring, where it

interacts with the ring to destabilize it. The corresponding

C1⁰structure is more than 10 kJ/mol above the RS global

minimum.

In the presence of the aromatic ring attached at the R-carbon,

the glycerol side chain must reconﬁgure itself to maximize its

stabilization relative to the aromatic ring. Aromatic ring substitu-

tion renders the R-carbon chiral (R), with the R-OH and β-

carbons competing for positions relative to the ring. In benzyl

alcohol, where the CH₂OH group lacks the β-carbon, the OH

group prefers a position perpendicular to the aromatic ring, with

the lowest energy conformer having the OH group pointing back

over the phenyl ring.⁴²Alkyl benzenes also place C(β) prefer-

entially in a perpendicular conﬁguration. In HMPPT, all low-

energy conformations put C(β) in the perpendicular conﬁgura-

tion, with the R-OH at an angle of (30°, in relative close

proximity to the aromatic ring.

Given this close proximity, it seemed likely that the orientation

of the R-OH relative to the aromatic ring plays a signiﬁcant role

in the conformational stability. To test this hypothesis, relaxed

potential energy scans of the R-OH torsional angle were carried

out on 1-(4-hydroxy-3-methoxyphenyl)-R-propanol, a close

analog of HMPPT in which the β- and γ-OH groups were

replaced by hydrogen. As Figure 12b) shows, two minima appear

at C_ϕC_ROH dihedrals of ꢀ50° and +170°, with the former

preferred over the latter by ∼7 kJ/mol. The preferred angle and

overall shape of the relaxed potential energy curve is maintained

in both syn and anti positions for the R-OH.

Figure 12b identiﬁes the C_ϕC_ROH dihedrals and relative

energies of the nine conformers of HMPPT on the same plot

as the relaxed potential energy curves. It is immediately obvious

from the plot that the R-OH group in the γꢀβꢀR and Rꢀβꢀγ

chains is oriented at the same dihedral angles (ꢀ50° and +170°,

respectively) as the two minima in the potential energy scan. In

forming the cycles (RS(B,C)), on the other hand, the R-OH

group reorients into a less favorable angle to close the ring with

the β-OH (Rβγ) or γ-OH (Rγβ) at a cost of ∼7 kJ/mol.

Finally, the relative energies of syn/anti pairs that have the

same glycerol side chain conformation are typically within 2ꢀ3

kJ/mol of one another, indicating that such electronic eﬀects play

a minor role in the energies of the conformers in the ground

electronic state.

(3) The other observed γꢀβꢀR chains (RR(D) and RS(E1,

E2)) are destabilized from RR(A, B) by amounts that

reﬂect the less stable chain types (Z, C2) taken up by

these structures. The small population of RR(D) reﬂects

this destabilization. The large population in RS(E) (28%)

is in part the result of its postulated contributions from

two γꢀβꢀR conformers. More importantly, RS(E1,E2)

are near to the global minimum in energy for the RS

diastereomer (Figure 11).

(4) The RS(B,C) cycles incorporate the lowest energy gly-

cerol structure, but are destabilized by the poorer R-OH

orientation required to do so. Because no RS γꢀβꢀR C1

structures are possible, the RS cycles are among the lowest

energy structures formed.

(5) C1-type Rꢀβꢀγ chains are possible in both diastereo-

mers (RR(C) and RS(A,D)) but are destabilized by their

poor R-OH orientation. This energetic disadvantage is

realized in RR-HMPPT, making RR(C) a small popula-

tion conformer relative to the global minimum RR(A).

The RS(A,D) pair do not have C1 γꢀβꢀR structures in

competition with them, giving them energies near to the

RS cycles.

V. CONCLUSION

The present spectroscopic study has focused attention on a

close analog of one of the three lignin monomers (coniferyl

alcohol) in which the double bond is oxidized to incorporate two

OH groups, thereby producing a glycerol side chain. By stereo-

controlled synthesis, two diastereomers are produced, the same

ones known to be present in the β-O-4 linkage in lignin oligomers.

This study has thus aﬀorded a detailed look at diastereomer

speciﬁc eﬀects on the conformational preferences in HMPPT, an

issue of some relevance to lignin itself, where chiral eﬀects are

largely unexplored. A combination of hole-burning spectroscopy,

resonant ion-dip infrared spectroscopy, and population analysis

via infrared ion-gain spectroscopy has been used to obtain

conformation-speciﬁc IR and UV spectra and fractional abun-

dances of a total of nine conformers of the two diastereomers of

HMPPT. Because the C(β) oxygen is uncapped in HMPPT, the

triol so produced provides its own fascinating spectroscopic

puzzle. The OH stretch infrared spectra are exquisitely sensitive

We are now in a position to account for the diﬀering

conformational preferences and relative populations of RR- and

RS-HMPPT.

(1) The strong preference in RR-HMPPT for γꢀβꢀR chains

over all other structural types is a simple consequence of

the particular stability of its two lowest-energy structures

(RR(A,B)), which are a syn/anti pair of C1 γꢀβꢀR

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to these various conformational types, leading to ﬁrm conforma-

tional assignments for all nine conformers. The RR diastereomer

prefers “out f in” H-bonded chains, while chains of reverse

direction (“in f out”) and H-bonded cycles dominate the

population in the RS counterpart. These changes can be under-

stood as a combined consequence of the innate conformational

preferences of glycerol, suitably modiﬁed by the restrictions

imposed by its attachment to the aromatic ring. The most stable

structure for the molecule is one that can only be taken up by the

RR diastereomer, in which the R-OH group prefers to conﬁgure

itself gauche to the ring plane with its OH group pointed near-

perpendicular to the plane of the aromatic ring, oriented so as to

accept a H-bond from another OH group in the most stable

glycerol chain conformation (C1). Extension of studies such as

these to lignin dimers and larger oligomers has the potential to

shed further light on the preferred conformations and spectro-

scopic signatures of the various chemical linkages that make up

this important biopolymer.

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’ ASSOCIATED CONTENT

S

Supporting Information. Description of the experimen-

b

tal methods used to prepare the etched silicon chips and the laser

desorption source that uses them, further details of the double

resonance schemes used, and further details regarding the assigned

HMPPT structures and a wider range of glycerol structural types.

This material is available free of charge via the Internet at http://

pubs.acs.org.

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^†SCHOTT North America, Inc., 400 York Avenue, Duryea,

PA 18642.

^‡Federal Institute for Occupational Safety and Health, BAuA,

Friedrich-Henkel-Weg 1-25, D-44149 Dortmund, Germany.

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Article Doi

Conformation-specific spectroscopy and populations of diastereomers of a model monolignol derivative: Chiral effects in a triol chain

DOI: 10.1021/jp204367k

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Article abstract of DOI:10.1021/jp204367k

Full text of DOI:10.1021/jp204367k

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