Comparative matrix isolation infrared spectroscopy study of 1,3- and 1,4-diene monoterpenes (α-phellandrene and γ-terpinene)

Article abstract of DOI:10.1021/jp2013122

In the present work, γ-terpinene (a 1,4-diene derivative) and α-phellandrene (1,3-diene derivative) were isolated in cryogenic argon matrices and their structures, vibrational spectra, and photochemistries were characterized with the aid of FTIR spectroscopy and quantum chemical calculations performed at the DFT/B3LYP/6-311++G(d,p) level of approximation. The molecules bear one conformationally relevant internal rotation axis, corresponding to the rotation of the isopropyl group. The calculations provide evidence of three minima on the potential energy surfaces of the studied molecules, where the isopropyl group assumes the trans, gauche+, and gauche conformations (T, G+, G- ). The signatures of all these conformers were identified in the experimental matrix infrared spectra, with the T forms dominating, in agreement with the theoretical predicted abundances in gas phase at room temperature. In situ UV (λ > 200 nm) irradiation of matrix-isolated α-phellandrene led to its isomerization into an open-ring species. The photoproduct was found to exhibit the ZE configuration of its backbone, which to be formed from the reactant molecule does not require extensive structural rearrangements of both the reagent and matrix. γ-Terpinene was photostable when subjected to irradiation under the same experimental conditions. In addition, the liquid compounds at room temperature were also investigated by FTIR-ATR and FT-Raman spectroscopies.

Full text of DOI:10.1021/jp2013122

ARTICLE
pubs.acs.org/JPCA
Comparative Matrix Isolation Infrared Spectroscopy Study of 1,3- and
1,4-Diene Monoterpenes (r-Phellandrene and γ-Terpinene)
K. M. Marzec,† I. Reva,‡ R. Fausto,*,‡ and L. M. Proniewicz*,§
†Jagiellonian Centre for Experimental Therapeutics, Jagiellonian University, Bobrzynskiego 14, 30-348 Krakow, Poland
‡Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal
§Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland
S Supporting Information
b
ABSTRACT: In the present work, γ-terpinene (a 1,4-diene
derivative) and R-phellandrene (1,3-diene derivative) were
isolated in cryogenic argon matrices and their structures,
vibrational spectra, and photochemistries were characterized
with the aid of FTIR spectroscopy and quantum chemical
calculations performed at the DFT/B3LYP/6-311þþG(d,p)
level of approximation. The molecules bear one conformation-
ally relevant internal rotation axis, corresponding to the rotation
of the isopropyl group. The calculations provide evidence of
three minima on the potential energy surfaces of the studied molecules, where the isopropyl group assumes the trans, gaucheþ, and
gaucheꢀ conformations (T, Gþ, Gꢀ). The signatures of all these conformers were identified in the experimental matrix infrared
spectra, with the T forms dominating, in agreement with the theoretical predicted abundances in gas phase at room temperature. In
situ UV (λ > 200 nm) irradiation of matrix-isolated R-phellandrene led to its isomerization into an open-ring species. The
photoproduct was found to exhibit the ZE configuration of its backbone, which to be formed from the reactant molecule does not
require extensive structural rearrangements of both the reagent and matrix. γ-Terpinene was photostable when subjected to
irradiation under the same experimental conditions. In addition, the liquid compounds at room temperature were also investigated
by FTIR-ATR and FT-Raman spectroscopies.
’ INTRODUCTION
The crystallographic structure of γ-terpinene was studied at 150 K
by the X-ray method.11 The obtained crystals were orthorhombic,
space group Pnma, Z =4, witha = 18.1968 (13) Å, b =7.2601(5) Å,
c = 6.7498 (3) Å, and θ = 1.0 ( 27.5°. Within the crystal, the
molecules of γ-terpinene assume a conformation where the
C(H2)ꢀCꢀC(CH3)2ꢀH dihedral angle is 180° (trans; T), and
the molecules are stacked along the b axis and packed in a herring-
bonetype arrangement. On the other hand, the structure of crystal-
line R-phellandrene has not yet been reported.
The photochemical behavior of terpenes has been investigated
to shed light on decomposition processes that might be relevant
for their above-mentioned applications.12ꢀ14 It was found that
the UV irradiation of essential plant oils such as those of the tea or
lemon tree, under conditions mimicking the natural atmospheric
conditions, increases the concentration of p-cymene and simul-
taneously lowers of the amount of other monoterpenoids, such
as R-terpinene, γ-terpinene, terpinolene, and limonene.14ꢀ17
These results suggest that the deterioration process of food
products containing γ-terpinene occurs through the sterically
favored photooxidation mechanism that leads to conversion of
γ-Terpinene (1-isopropyl-4-methyl-1,4-cyclohexadiene; 1) and
R-phellandrene (5-isopropyl-2-methyl-1,3-cyclohexadiene; 2) be-
long to the group of monocyclic terpinenes (Scheme 1) having the
molecular formula C10H16 (class of monoterpenes). They are
natural products that are isolated from a variety of plant sources.
γ-Terpinene has been mainly isolated from citrus fruit oils.1
R-Phellandrene is isolated from the plant species Eucalyptus
radiata. Its name is derived from the plants’ former scientific name,
Eucalyptus phellandra.2 The growing interest for studying these
compounds is mainly related to their bioactivity and odor, which
justify their pharmacological, cosmetic, and food uses.3,4 While
γ-terpinene and R-phellandrene have been determined as compo-
nents in plants using many quantitative analysis methods,5ꢀ9 they
were not studied extensively as main topic compounds.10
In R-phellandrene, the cis-1,3-diene fragment inserted in a
6-membered ring is a conjugated π system. On the other hand,
γ-terpinene contains two double bonds which are not conjugated
(1,4-diene group). As it will be shown in this paper, the presence
or absence of the conjugated π system is a fundamental factor in
determining the properties of the two compounds and, in
particular, their different photochemical behavior.
Received: February 9, 2011
To the best of our knowledge, the structures of monomeric
γ-terpinene and R-phellandrene have never been reported hitherto.
Revised:
Published: April 04, 2011
March 20, 2011
r
2011 American Chemical Society
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Scheme 1. Structures of γ-Terpinene (1), R-Phellandrene
(2), and R-Terpinene (3)
digital temperature controller (Scientific Instruments, Model
9650-1), which provides an accuracy of 0.1 K. The matrix
isolation IR spectra were collected on a Nicolet 6700 FTIR
spectrometer, equipped with a deuterated triglycine sulfate
(DTGS) detector and a Ge/KBr beam splitter, with 0.5 cmꢀ1
spectral resolution.
The matrices were irradiated (180 min for γ-terpinene and
330 min for R-phellandrene) using a 200 W output power of
a 300 W Hg/Xe lamp (Oriel, Newport) and a series of long-
pass optical filters, through the quartz (λ > 200 nm) or KBr
(λ > 235 nm) external windows of the cryostat.
γ-terpinene mainly into p-cymene.18 To the best of our knowl-
edge, no data were reported previously on the photochemistry of
isolated γ-terpinene. On the other hand, the photochemistry of
R-phellandrene was investigated before using ab initio multistate
second-order perturbation theory and gas-phase femtosecond
time-resolved spectroscopy,19 where it was shown that R-phellan-
drene may undergo the ring-opening conversion leading to the
cyclohexatriene-type product.
The two title molecules are isomers of R-terpinene (3, in
Scheme 1), which was the subject of our previous investigation.12
Like R-phellandrene, R-terpinene contains also a cis-1,3-diene
conjugated fragment inserted in a 6-membered ring. UV
(λ > 235 nm) irradiation of matrix-isolated R-terpinene led to
its isomerization into an open-ring species, 2,6-dimethyl-5-
methylenehepta-1,3-diene, in a concerted σ2 þ π4 electrocyclic
ring-opening reaction, the photoproduct being generated in
the Z configuration around the central C3dC4 double bond
and in conformations requiring the smallest structural rearrange-
ments of both the reagent and matrix. The photochemistry of
R-phellandrene and R-terpinene parent molecule, 1,3-cyclohex-
adiene, was also previously investigated by several theoretical and
experimental methods.19ꢀ24 It has been found that 1,3-cyclohex-
adiene undergoes exclusively the ring-opening photoreaction.25
It was also found experimentally that R-phellandrene in solutions
may undergo similar ring-opening reactions26 as well as other
types of isomerizations.27
In the present work, a detailed investigation of the structure,
spectroscopy, and photochemistry of γ-terpinene and R-phellan-
drene is reported. The monomers of these compounds were
isolated in cryogenic argon matrices, their structures characterized
and the corresponding infrared spectra interpreted and assigned. In
addition, in situ UV irradiation of the matrix-isolated compounds
was performed, and their photochemistries probed by FTIR
spectroscopy. The neat liquid substances, at room temperature,
were also investigated by FTIR-ATR and FT-Raman spectro-
scopies. The experimental studies were complemented by quantum
chemical calculations, which allowed for a detailed characterization
of the conformational space of the two molecules and identification
of the spectroscopically observed photoproducts.
The room temperature FTIR-ATR spectra (650ꢀ4000 cmꢀ1
)
of the liquid compounds (ca. 5ꢀ10 μL of sample placed on the
surface of a diamondꢀZnSe ATR crystal with 0.5 mm diameter)
were recorded using a portable diamond TravelIR spectrometer,
fitted with a deuterated L-alanine-doped triglycine sulfate
(DLATGS) detector, in a single reflection configuration. Sixteen
scans, with a spectral resolution of 4 cmꢀ1, were accumulated.
The Raman spectra of the liquid compounds were recorded at
room temperature on a Nicolet NXR 9650 FT-Raman spectro-
meter, equipped with a Nd3þ:YAG laser, emitting at 1064 nm
(9398.5 cmꢀ1), and a germanium detector cooled with liquid
nitrogen. The laser power at the sample position was 100 mW. 512
scans with a resolution of 4 cmꢀ1 were collected and averaged.
Density functional theory (DFT) calculations were carried out
using the Gaussian 0928 program package. The geometries of the
conformers of R-phellandrene and γ-terpinene as well as possible
photoproducts were fully optimized at the B3LYP/6-311þþG-
(d,p) level of theory.29 Additionally, the influence of a polar
environment on the stability of the minima was estimated using
the conductor-like polarized continuum C-PCM approach
(PCM) at the same level of theory, with toluene as a solvent.30,31
Theoretical Raman intensities (Ii) were obtained from the
Gaussian calculated Raman scattering activities (Si) according to
the expression Ii = 10ꢀ12(ν0 ꢀ νi)4νiꢀ1Biꢀ1Si, where Bi is a
temperature factor which accounts for the intensity contribution
of excited vibrational states and is represented by the Boltzmann
distribution [Bi = 1 ꢀ exp{ꢀ(hνic)/(kT)}]; h, k, c, and T are
Planck and Boltzmann constants, speed of light, and temperature
in kelvin, respectively; ν0 is the frequency of the laser excitation
line (ν0 = 1/λ0, where λ0 is the laser wavelength); νi is the
frequency of normal mode.32
Prior to the comparison of the calculated wavenumbers with
their experimental counterparts (IR and Raman spectra), the first
were scaled down by appropriate scale factors (0.978 and 0.960
for the 0ꢀ2000 and 2000ꢀ4000 cmꢀ1 regions, respectively33,34),
mainly to account for anharmonicity effects, basis set truncation,
and the neglected part of electron correlation. To provide an
unequivocal assignment of the calculated IR and Raman spectra,
normal-coordinate analysis calculations were performed using a
set of internal coordinates defined as suggested by Pulay,
Fogarasi and co-authors.35,36 Program GAR2PED was used for
calculation of potential energy distribution (PED) of normal
modes in terms of natural internal coordinates.37
’ EXPERIMENTAL AND COMPUTATIONAL METHODS
γ-Terpinene and R-phellandrene were provided by Fluka
(purum, g98.5% (GC) and g95.0% (GC), respectively). The
matrix gas, argon N60, was supplied by Air Liquide.
The low-temperature equipment was based on an APD
Cryogenics closed-cycle helium refrigerator with a DE-202A
expander. For preparation of the matrices, the studied compound
(R-phellandrene or γ-terpinene) and argon were premixed in a
molar ratio of ca. 1:500, and deposited through a needle valve.
The valve nozzle was kept at room temperature, which defined
the conformational distribution in the gas phase prior to deposi-
tion. The temperature of the cold CsI window of the cryostat was
kept at about 13 K and was measured directly at the sample
holder by a silicon diode temperature sensor, connected to a
’ RESULTS AND DISCUSSION
Conformational Space of γ-Terpinene and r-Phellandrene.
The molecule of R-phellandrene has one chiral center, carbon atom
C2 (see Figure 1 for atom numbering). The geometric parameters
of R-phellandrene reported in this work refer to the S-enantiomer.
R-Phellandrene has two conformationally relevant internal rotation
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Figure 1. Trans conformers of R-phellandrene (top) and γ-terpinene (bottom) with atom numbering. The conformers are named according to the
value of the C10ꢀC2ꢀC4ꢀH26 (γ-terpinene) or C6ꢀC2ꢀC4ꢀH25 (R-phellandrene) dihedral angle (see arrows in the figure), defining the orientation
of the isopropyl group in relation to the ring. The dihedral angle C6ꢀC2ꢀC4ꢀH25 (R-phellandrene) for T, Gþ, and Gꢀ is equal to 179.6°, 69.6°, and
ꢀ48.1°, respectively. The dihedral angle C10ꢀC2ꢀC4ꢀH26 (γ-terpinene) for T, Gþ, and Gꢀ is equal to 180.0°, 47.5°, and ꢀ47.5°, respectively.
Table 1. DFT(B3LYP)/6-311þþG(d,p) Calculated Relative
Energies without (ΔE°) and with (ΔE°ZPVE) Zero-Point
Corrections, Relative Gibbs Energies at 298 K (ΔG°), and
Dipole Moments (|μ|) of γ-Terpinene and r-Phellandrene
Conformersa
found, this parameter was very rigid, falling in a narrow range of
values from ꢀ44° to ꢀ46°. Therefore, the unique set of conformers
of R-phellandrene can be defined by a single parameter, that is, by
the internal rotation of the isopropyl group.
The molecule of γ-terpinene does not have chiral centers.
Furthermore, the two sp3 carbon atoms of its ring are separated
by two sp2 carbon atoms from each side, thus allowing for the
planar arrangement of the ring. Indeed, in all the found structures
of γ-terpinene minima the ring subunit is essentially planar.
Then, γ-terpinene has only one conformationally relevant inter-
nal rotation axis, which corresponds to the rotation of the
isopropyl group in relation to the ring.
The DFT calculations led to the identification of three
minima on the potential energy surface of each molecule,
corresponding to the trans, gaucheþ, and gaucheꢀ conformers
(Figure 1). The conformers of R-phellandrene are named here
according to the orientation of the C6ꢀC2ꢀC4ꢀH25 dihedral
angle, while for γ-terpinene according to the orientation of the
C10ꢀC2ꢀC4ꢀH26 dihedral angle. In R-phellandrene, the three
unique forms, trans (179.6°), gaucheþ (69.6°), and gaucheꢀ
(ꢀ48.1°), belong to the C1 symmetry point group. In
γ-terpinene, the trans (180.0°) conformer has Cs symmetry,
while the gaucheþ (47.5°) and gaucheꢀ (ꢀ47.5°) conformers
(C1 symmetry) are mirror images. The optimized geometries of
these conformers are given in Table S1 (Supporting In-
formation), whereas the values of relative energies and dipole
moments are presented in Table 1.
compound
conformer
ΔE°
ΔE°ZPVE
ΔG°
|μ|
γ-terpinene
trans (T)
0.00
1.73
0.00
0.80
0.42
0.00
1.78
0.00
1.00
0.37
0.00
1.77
0.00
1.06
0.27
0.07
0.05
0.23
0.21
0.29
gauche (G()
trans (T)
R-phellandrene
gauche (Gþ)
gauche (Gꢀ)
a Energies in kJ molꢀ1. The absolute energies (all in hartrees; 1 hartree ≈
2625.5 kJ molꢀ1) of the most stable conformer for γ-terpinene are
ΔE° = ꢀ390.788 785, ΔE°ZPVE = ꢀ390.555 543, and ΔG° (at 298.15 K) =
ꢀ390.592 109; for R-phellandrene they are ꢀ390.783 951, ꢀ390.550
626, and ꢀ390.586 473, respectively. Dipole moments in debyes.
axes: (i) rotation of the isopropyl group in relation to the ring and
(ii) twist of the ring, which can be described by the orientation of the
dihedral angle around the two neighboring sp3 carbon atoms of the
ring (C10ꢀC2ꢀC6ꢀC7). The second parameter can assume two
values, gaucheþ and gaucheꢀ, producing two equivalent-by-sym-
metry geometries of the ring subunit. In the present study, only
gaucheꢀ orientation of the ring was considered. In all minima
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Figure 3. (Top) Experimental FTIR spectra of R-phellandrene em-
bedded in argon (diluted matrix, 13 K). (Bottom) Simulated spectrum
obtained as a sum of theoretical infrared spectra of R-phellandrene
conformers with their intensities scaled proportionally to their estimated
room-temperature gas-phase populations (Gþ/Gꢀ/T is 25.6:35.2:39.2).
The theoretical spectra in the CH stretching (3100ꢀ2750 cmꢀ1) and
fingerprint (1800ꢀ400 cmꢀ1) regions were simulated by Lorentzian
functions centered at the DFT(B3LYP)/6-311þþG(d,p) calculated
wavenumbers scaled by 0.960 and 0.978, respectively, and with fwhm
equal to 10 and 4 cmꢀ1, respectively.
Figure 2. DFT(B3LYP)/6-311þþG(d,p) calculated potential energy
(radial coordinate) profiles for interconversion between the confor-
mers of (top) R-phellandrene and (bottom) γ-terpinene. Minima are
labeled by the conformer symbol (see Figure 1). Each profile was
obtained by performing a relaxed scan along the conformationally
relevant torsional coordinate of the molecule (C6ꢀC2ꢀC4ꢀH25 for
R-phellandrene and C10ꢀC2ꢀC4ꢀH26 for γ-terpinene; dihedral
angles defining the internal rotation of the isopropyl group), by varying
this coordinate in steps of 10° and optimizing all the remaining internal
coordinates.
trapped in the argon matrices should then be 49.5%:50.5%
(i.e., ∼1:1), as estimated from the calculated relative Gibbs free
energies at 298.15 K of the conformers using the Boltzmann
statistics, while in R-phellandrene the Gþ:Gꢀ:T population
ratio should be 25.6%:35.2%:39.2% (i.e., ∼0.65:0.9:1).
Infrared Spectra of the As-Deposited Matrices of γ-Terpi-
nene and r-Phellandrene. The infrared spectra of the as-deposited
argon matrices of the studied compounds are shown in Figures 3
and 4. These spectra are compared with the simulated spectra of the
compounds obtained by adding the DFT calculated infrared spectra
of the individual conformers of each molecule with intensities scaled
by factors proportional to their estimated populations.
For both compounds, the most stable conformer is the trans (T)
form. In the case of γ-terpinene, the two gauche conformers (G()
are degenerate by symmetry and were calculated to be 1.78 kJ
molꢀ1 higher in energy than the trans form (zero-point-corrected
energies). In R-phellandrene, Gþ and Gꢀ are calculated to be 1.00
and 0.37 kJ molꢀ1 higher in energy than T, respectively.
The good agreement between the experimentally observed
spectra and their simulated counterparts confirms the presence in
the matrices of the two conformers of γ-terpinene (T, G() and
the three conformers of R-phellandrene (T, Gþ, Gꢀ) predicted
theoretically. The detailed assignment of the spectra is shown in
Tables S2ꢀS5 (Supporting Information).
The calculated potential energy profiles for interconversion
between the three conformers of the studied compounds
(Figure 2) show that the barriers for T f G( and Gþ f Gꢀ
isomerizations in γ-terpinene amount to ca. 15 and 3 kJ molꢀ1
,
In the more detailed analysis of the spectra presented below,
we focus on only three spectral regions. These regions contribute
to the identification of the individual conformers in the matrices
(1200ꢀ1000 cmꢀ1) or are related to the molecular fragments
which are expected to be directly involved in the photochemical
processes described in detail later (in particular, CdC stretching
and CH out-of-plane modes).
respectively. In R-phellandrene, the T f Gþ, Gþ f Gꢀ, and
T f Gꢀ barriers are predicted as 17, 14, and 32 kJ molꢀ1
,
respectively. These results are particularly relevant for the
interpretation of the matrix isolation experiments presented later
in this paper. Indeed, all relevant barriers are high and the
conversion between the conformers during their landing and
cooling on the surface of the optical substrate during deposition
of the matrices can be neglected. [The Gþ T Gꢀ barrier in
γ-terpinene is low, but interconversion between the two equiva-
lent-by-symmetry gauche forms is irrelevant for the performed
spectroscopic experiments, since their spectra are equal.] Be-
cause, the matrices presented here were obtained from the vapor
of the compounds at room temperature, one can assume that the
relative populations of the conformers trapped into the matrices
should reflect the conformational equilibrium in the gas phase at
that temperature. The G(:T population ratio for γ-terpinene
According to the previously observed photoreactions of the
analogous molecules, R-terpinene12 and 1,3-cyclohexadiene,25 the
photochemical excitation of the studied compounds can be
expected to strongly affect the π system of their rings. Then, the
CdC stretching vibrations appear as good probes of the photo-
chemical events. In addition to the CdC stretching vibrations, the
out-of-plane deformation vibrations of the CꢀH groups, specifi-
cally those in which the carbon atom is involved in a double bond
(sp2 carbon atoms), are very sensitive to changes in the π system
and rearrangements of the rings. These vibrations were also shown
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group), while for γ-terpinene the highest percent contribution is
connected with the C2dC6 bond (adjacent to the isopropyl
group). The situation is inverse regarding the compositions of
the CdC symmetric stretching in the two molecules. The
contributions of the individual double bonds to the antisym-
metric stretching CdC vibration amount to 45ꢀ47% (ν(C7C8)) þ
22ꢀ23% (ν(C9C10)) for R-phellandrene, and 23ꢀ25%
(ν(C8C9)) þ 48ꢀ50% (ν(C2C6)) for γ-terpinene. The corre-
sponding symmetric CdC vibrations are composed as 23ꢀ24%
(ν(C7C8)) þ 48ꢀ49% (ν(C9C10)) for R-phellandrene, and
46ꢀ47% (ν(C8C9)) þ 22ꢀ23% (ν(C2C6)) for γ-terpinene
(see Tables S3 and S5).
Regarding the CꢀH deformations, three of such vibrations are
expected for R-phellandrene, according to the number of the CH
groups linked to the ring. One of the CH groups is attached to the
double CdC bond bearing the methyl group on the other side.
The deformation vibration of this CH group (ω(C7H22)) is
uncoupled from the other two CH groups and is related with an
Figure 4. (Top) Experimental FTIR spectra of γ-terpinene embedded
in argon (diluted matrix, 13 K). (Bottom) Simulated spectrum obtained
as a sum of theoretical infrared spectra of γ-terpinene conformers with
their intensities scaled proportionally to their estimated room-tempera-
ture gas-phase populations (G/T is 49.5:50.5). The theoretical spectra
intenseIRbandobservedat795 cmꢀ1 (calculated795ꢀ798 cmꢀ1
,
77ꢀ80% PED). The out-of-plane vibrations of the other
two CH groups (ω(C9H23), ω(C10H24)), making part of the
CdCH—CHdC fragment of the molecule, are coupled in the
symmetric and antisymmetric ways. The symmetric combination
is related with the most intense IR band in the entire fingerprint
region (calculated intensity 21ꢀ23 km molꢀ1) appearing at
730 cmꢀ1 (calculated 726ꢀ731 cmꢀ1, 62ꢀ63% PED). The
antisymmetric combination gives origin to a weak IR absorption
(calculated intensity ∼1 km molꢀ1) in the 962ꢀ968 cmꢀ1 range
(calculated 959ꢀ964 cmꢀ1, 63ꢀ84% PED).
in the CH stretching (3100ꢀ2750 cmꢀ1
) and fingerprint
(1800ꢀ400 cmꢀ1) regions were simulated by Lorentzian functions
centered at the DFT(B3LYP)/6-311þþG(d,p) calculated wavenum-
bers scaled by 0.960 and 0.978, respectively, and with fwhm equal to 10
and 4 cmꢀ1, respectively. An asterisk designates the band due to the
atmospheric CO2.
previously to be useful to identify the type of photoreactions
observed due to UV irradiation of this type of compounds.12
The vibrations of the CdC bonds are expected to give rise to
bands in a spectral region free from other bands originated in the
compounds. The antisymmetric stretching vibration of the
conjugated CdC bonds of the R-phellandrene ring are calcu-
lated at 1675, 1676, and 1673 cmꢀ1 for conformers T, Gþ, and
Gꢀ, respectively, and observed in the experimental IR spectra at
1663 cmꢀ1 as a medium-intensity band (Figure 3). In turn, the
CdC symmetric stretching vibration is predicted at 1603 cmꢀ1
in both T and Gþ conformers and at 1601 cmꢀ1 in Gꢀ, being
observed experimentally at 1601 cmꢀ1. This relatively weak IR
band is strong in the Raman spectrum of this compound in the
liquid state, where it is observed at 1590 cmꢀ1 (see Table S3 and
Figure S1 in the Supporting Information).
In the case of γ-terpinene, the order of frequencies of the two
CdC stretching modes is reversed compared to R-phellandrene.
The CdC antisymmetric stretching is calculated at 1675 and
1678 cmꢀ1 for T and G( conformers, respectively, and observed
at 1670 cmꢀ1 (Figure 4). The CdC symmetric stretching is
predicted at ca. 1715 cmꢀ1 with a very low intensity in the
infrared (<0.1 km molꢀ1) and could not be observed in the
experimental spectrum of the matrix isolated compound. How-
ever, as in the case of R-phellandrene, this mode gives rise to a
very strong band in the Raman spectrum of the liquid compound,
where it appears at 1701 cmꢀ1 (see Table S5 and Figure S1 in the
Supporting Information).
In the case of γ-terpinene, only two CH groups are linked to
the ring. Like in R-phellandrene, it is expected that their out-of-
plane vibrations are coupled and their symmetric and antisym-
metric combinations should give a strong and a weak absorption
in infrared. Indeed, a strong (symmetric) infrared band is
observed at 783 cmꢀ1 (calculated 781ꢀ784 cmꢀ1, ω(C9H23)
35ꢀ37% PED, and ω(C6H20) 10ꢀ11% PED) and a weak
(antisymmetric) band appears at 832 cmꢀ1 (836ꢀ837 cmꢀ1
,
ω(C6H20) 52ꢀ58% PED, and ω(C9H23) 25ꢀ27% PED). These
two coordinates are partially mixed with the rocking motion of
the two methylene groups linked to the ring. These mixtures give
rise to two more infrared bands (medium and strong intensity)
both in the 950ꢀ965 cmꢀ1 spectral range (see Table S5), i.e.,
in the same range as for the out-of-plane CH vibrations in
R-phellandrene (see Table S3).
The conformer-sensitive range in the IR spectra of γ-terpinene
and R-phellandrene extends from 1200 to 1000 cmꢀ1 (Figures 5
and 6; see also Tables S3 and S5 in the Supporting Informat-
ion for results of normal-coordinate analysis). In the case of
γ-terpinene (see Figure 5), the assignments are doubtless and
could be made easily. In the considered region, the bands
observed at 1012 cmꢀ1 (delocalized vibration with a consider-
able degree of the stretching of the C2ꢀC(CH3)2 bond; cal-
culated 1001 cmꢀ1), 1076 cmꢀ1 (stretching of the C9ꢀC10
bond; calculated 1067 cmꢀ1), 1101 cmꢀ1 (CꢀC antisymmetric
stretching of the isopropyl moiety; calculated 1089 cmꢀ1),
1148 cmꢀ1 (extensively mixed vibration delocalized over the
whole molecule; calculated 1141 cmꢀ1) and 1162 cmꢀ1
(predominantly C8ꢀCH3 bond; calculated 1150 cmꢀ1) are
marks of the trans (T) conformer. On the other hand, the bands
appearing at 1021 cmꢀ1 (mostly the symmetric stretching of the
C6ꢀC7 and C9ꢀC10 bonds; calculated 1008 cmꢀ1), 1092 cmꢀ1
(antisymmetric stretching of the C6ꢀC7 and C9ꢀC10 bonds;
The reason for the different order of frequencies of the CdC
stretching vibrations in γ-terpinene and R-phellandrene results
from the different composition of these modes in the two
molecules, as shown in the potential energy distribution results
presented in Tables S3 and S5 in the Supporting Information. In
R-phellandrene, the highest contribution to the antisymmetric
stretching comes from the C7dC8 bond (adjacent to the methyl
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Figure 6. Comparison of the FTIR spectrum of R-phellandrene
isolated in an argon matrix with the calculated IR spectra of T, Gþ,
and Gꢀ conformers. The theoretical spectra were calculated at the
DFT(B3LYP)/6-311þþG(d,p) level of theory, and wavenumbers were
scaled by a factor of 0.978. Absorption bands were simulated with
Gaussian profiles, centered at the calculated (scaled) wavenumbers and
Figure 5. Comparison of the FTIR spectrum of γ-terpinene isolated in
an argon matrix, its FTIR-ATR spectrum in liquid phase at 298 K, and
the calculated IR spectra of T and G( conformers. The theoretical
spectra were calculated at the DFT(B3LYP)/6-311þþG(d,p) level of
theory, and wavenumbers were scaled by a factor of 0.978. Absorption
bands were simulated with Gaussian profiles, centered at the calculated
having fwhm of 4 cmꢀ1
.
(scaled) wavenumbers and having fwhm of 4 cmꢀ1
.
pointing to a similar equilibrium conformational composition in
both the gas and liquid phase of the compound at room
temperature. In the case of R-phellandrene the liquid-phase
FTIR-ATR spectrum is also composed by broad bands and is
shown in Figure S1 (Supporting Information). The comparison
between the experimental FT-Raman spectra of the two com-
pounds at room temperature and the theoretical Raman spectra
of their conformers in the conformer-sensitive region (1200ꢀ
1000 cmꢀ1, Figure S1) shows also that the population of the
conformers in the liquid at room temperature is similar to the
population in the gas phase at the same temperature. This result
could be anticipated, considering the similar values of the dipole
moments for the different conformers of each molecule (see
Table 1).
Photochemical Processes and Analysis of the Photopro-
ducts. Scheme 2 summarizes the experimentally observed
photochemistry of compounds analogous to R-phellandrene
which have a cis-1,3-diene fragment inserted in a six-membered
ring: R-terpinene,12 1,3-cyclohexadiene,25 and R-pyrone.38 In
the last case, besides the ring-opening reaction, isomerization to
the R-pyrone Dewar form was also observed. Bearing the same
conjugated π-system, R-phellandrene can be expected to show a
similar photochemistry, as represented in Scheme 2. On the
other hand, γ-terpinene has a different arrangement of the
double bonds in the ring (the 1,4-diene moiety), and despite
its UV spectrum not differing very much from those of com-
pounds with cis-1,3-diene fragment (Figure S2 in the Supporting
Information), it can be expected to behave differently when
exposed to UV light.
calculated 1082 cmꢀ1), 1108, 1120, and 1167 cmꢀ1 (all exten-
sively mixed vibrations with predominant contribution from the
C2ꢀC(CH3)2 stretching coordinate; calculated at 1098, 1112,
and 1152 cmꢀ1, respectively) belong to the gauche forms (G(),
and the strong band at 1033 cmꢀ1 contains contributions from both
forms (predominantly C6ꢀC7 stretching; calculated 1019 cmꢀ1 T,
1022 cmꢀ1 G().
The analysis of the data in the case of R-phellandrene
(Figure 6) is more complicated, because of the larger number
of conformers, leading to a substantial increase in the complexity
of the experimental spectra. According to the predicted relative
conformational populations presented in the previous section,
the IR spectrum of matrix-isolated R-phellandrene is dominated
by the bands due to the T and Gꢀ conformers, which are
expected to be present in the argon matrix with larger popula-
tions. The bands assigned only to T conformer are observed at
1078, 1100, and 1135 cmꢀ1, and are ascribed to vibrations with
dominant contributions of the C6ꢀC7, isopropyl CꢀC antisym-
metric, and C2ꢀC6 stretching coordinates, respectively, which
were calculated at 1067, 1088, and 1166 cmꢀ1. Those due to Gꢀ
form only are the bands observed at 1024, 1102, 1145, and
1171 cmꢀ1, which correspond to the calculated bands at 1012,
1091, 1136, and 1168 cmꢀ1, with substantial contributions from
the C2ꢀC6, isopropyl CꢀC antisymmetric, isopropyl group
rocking coordinates, and in-plane H23—C9dC10—H24 defor-
mation, respectively. Finally, the bands observed at 1120
(shoulder) and 1123 cmꢀ1 are due to Gþ (C2ꢀC(CH3)2 and
isopropyl CꢀC antisymmetric stretching modes; calculated
wavenumbers: 1110 and 1127 cmꢀ1). The remaining bands
appearing in this spectral range contain contributions from more
than one conformer, e.g., bands at 1013 and 1090 cmꢀ1 from Gþ
and Gꢀ, the band at 1030 cmꢀ1 from T and Gþ, and those at
1039 and 1159 cmꢀ1 from all three forms.
The matrix-isolated R-phellandrene and γ-terpinene were sub-
jected to a series of UV irradiations using different cutoff filters. For
irradiations with wavelengths longer than 288 nm, no changes were
observed in the infrared spectrum of R-phellandrene. This is
consistent with the UV absorption spectrum of the compound
(Figure S2) as well as with that of the parent molecule 1,3-
cyclohexadiene,19,39 both compounds not absorbing above
280 nm. On the other hand, irradiation at λ > 200 nm
The FTIR-ATR spectrum of liquid γ-terpinene (see Figure 5)
compares well with the matrix isolation spectrum of the com-
pound, the main difference being the expected band broadening,
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Scheme 2. Experimentally Observed Photochemistries for
(Top to Bottom) R-Pyrone,38 1,3-Cyclohexadiene,25 and
R-Terpinene,12 and the Possible Analogous Photoreactions of
R-Phellandrene
fragment, decreases upon irradiation, whereas new bands at ca.
900 cmꢀ1 and in the 967ꢀ990 cmꢀ1 range increase. The new
bands are ascribable to the CH out-of-plane wagging mode of the
rearranged CdC8—C7HdC fragment and the wagging vibra-
tion of the =CH2 terminal group (at C6 position) of the open-
ring isomeric photoproduct, 3,7-dimethylocta-1,3,5-triene
(abbreviated hereafter as DOT, see Figure 8 and Figure S3 in
the Supporting Information). These new bands also appear at
similar wavenumbers to those previously observed for the related
compound 2,5-dimethyl-1,3,5-hexatriene isolated in an argon
matrix as well as 1,3,5-hexatriene in the gas phase.42
In an attempt to identify more precisely the resulting photo-
products of matrix-isolated R-phellandrene subjected to UV-C
irradiation, we performed an extensive series of theoretical
calculations on the various possible open-ring isomers of DOT.
The basis of the nomenclature here used to name the possible
DOT species have been described in detail elsewhere.12 Quickly,
the capital letters Z or E define the configuration around the two
central CdC bonds of the molecule. For each possible Z/E
combination around these bonds, nine reference structures can
be devised, with trans (t) or gauche (gþ; gꢀ) arrangements of
the two CdC—CdC moieties (see Figure 8 and Figure S3 in
the Supporting Information, which contains the full set of
reference structures for DOT). Each reference structure was
allowed to have three different arrangements of the isopropyl
group (according to the conformation defined by the
HꢀC4ꢀC2ꢀC6 dihedral: cis (0°; C), skewþ (þ120°; SKþ), and
skewꢀ (ꢀ120°; SKꢀ). Hence, the molecular conformations of
DOT can be uniquely described by combinations of five letters,
which define the arrangements around the conformationally
relevant bonds of the H2CdC—CdC—CdC—CH(CH3)2
backbone, starting from the left to the right.
Thirty-eight different minima could be found on the PES of
DOT, corresponding to 14 ZE-type structures, 5 ZZ-type
structures, 14 EE-type structures, and 5 EZ-type structures. For
simplicity, the minimum energy structures are named here using
the notation of the closest resembling reference structure repre-
sented in Figure S3 (Supporting Information). The minimum-
energy structures and their defining dihedral angles, dipole
moments, and relative energies are given in Table 2. The
extended tEtE/C and tZtE/C forms were predicted by the
calculations to be the most stable species.
(where R-phellandrene, 1,3-cyclohexadiene, and R-terpinene exhi-
bit their main absorptions) led to the emergence of new bands in the
infrared spectrum of the irradiated matrix of R-phellandrene. These
results show that R-phellandrene is photolabile in the range of
germicidal UV-C light only. No changes were observed in the
infrared spectrum of the irradiated γ-terpinene matrix in the whole
range of wavelengths used in our experiments. This result demon-
strates the relevance of the different arrangement of the double
bonds in the rings of the two molecules in terms of photostability in
the matrix. It is found that 1,4-cyclohexadiene, which is γ-terpinene’s
parent molecule, shows a complex photochemistry in solution and
in the gas phase;40 however, there are no reported studies of its
photochemistry in cryomatrices. To the best of our knowledge,
there are no examples of molecules with the 1,4-hexadiene moiety
isolated in matrix that are reactive upon irradiation at λ > 200 nm.
For example, attempts to introduce photochemical transformations
in matrix-isolated γ-pyrone41 were unsuccessful.
Figure 9 shows the theoretical spectra of R-phellandrene and
of its open-ring isomerscalculated atthe B3LYP/6-311þþG(d,p)
level of theory. The 1800ꢀ400 cmꢀ1 spectral region can be used
as a spectral marker for discriminating different isomers taking
into account their characteristic strong infrared absorbances,
particularly those related with the out-of-plane vibrations of the
two central CH groups in the 1000ꢀ800 cmꢀ1 range. Besides
open-ring isomers, Figure 9 also shows the calculated spectra and
possible structures for the Dewar isomer of R-phellandrene (top
frame). The Dewar isomer, which might exist in three different
conformational states differing by the orientation of the isopropyl
substituent, has been considered in this analysis by the analogy
with the photochemistry of R-pyrone.38
The Dewar isomer of R-phellandrene is predicted to give rise
to an intense band (due to the out-of-plane bending vibration of
the CH groups attached to the central CdC bond) at ca.
830 cmꢀ1. In the experimental spectra of the photolyzed matrix,
such band does not exist (see Figure 7), which indicates that the
Dewar form is not formed upon irradiation of matrix-isolated
Inorder to identify the photoproduct(s) of the UVꢀC irradiated
matrix-isolated R-phellandrene, the spectral region of the CdC
stretching modes (1550ꢀ1700 cmꢀ1) was first examined
(Figure 7). The observed spectroscopic changes in this spectral
region indicate a rearrangement of the π-system of the molecule
after reaction. The strong ν(CdC) band observed in the spectrum
of R-phellandrene at 1663 cmꢀ1 as well as the less intense band at
1601 cmꢀ1, assigned respectively to the antisymmetric and sym-
metric stretching vibrations of the conjugated CdC bonds, con-
siderably decrease in intensity, being replaced by bands at 1642,
1624, and 1583 cmꢀ1. The number of new bands in this spectral
region points to a photoproduct structure that contains an addi-
tional double bond compared to R-phellandrene, thus being in
agreement with the photoconversion of the compound into an
open ring species bearing the 1,3,5-hexatriene skeleton.
The analysis of the 1010ꢀ760 cmꢀ1 spectral range (Figure 7)
presents further support for this conclusion. For example, the
strong IR band of R-phellandrene at ca. 795 cmꢀ1 due to the
ω(C7H22) out-of-plane wagging mode of the C8dC7—H ring
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Figure 7. (Bottom) Experimental FTIR spectra of R-phellandrene in an argon matrix in the ranges 1680ꢀ1620 cmꢀ1 (concentrated sample) and
1010ꢀ760 cmꢀ1 (diluted sample), before (0 min) and after UV (λ > 200 nm) irradiation (280 min). (Middle and Top) Theoretical infrared spectrum
of the equilibrium mixture of the conformers at room temperature (100% RT) simulates the substrate absorption. In the simulation of the spectrum
upon irradiation, the RT spectrum was reduced to 70%, and the resulting scaled spectrum was coadded with the spectrum of the putative open-ring
product (3ꢀ, 3þ, 3C or 7ꢀ, 7þ, 7C), which was reduced to 30%. The abbreviated names 3þ, 3ꢀ, and 3C stand for the tZgþE/SKþ, tZgþE/SKꢀ, and
tZgþE/C, and names 7þ, 7ꢀ, and 7C stand for the gꢀZgꢀE/SKþ, gꢀZgꢀE/SKꢀ, and gꢀZgꢀE/C conformers of the photoproduct (see also
Table 2 and Figure 9). The theoretical spectra were simulated by Lorentzian functions centered at the DFT(B3LYP)/6-311þþG(d,p) calculated
wavenumbers scaled by 0.978 and with fwhm equal to 4 cmꢀ1 (see also Figure 8 and Figure S3 in the Supporting Information).
R-phellandrene. This result parallels that found previously for
R-terpinene.12
Because in Figure 9 the ordinate scales are equal in all frames,
one can easily notice that the infrared intensities of the open-ring
photoproducts are larger than in the starting compound. The
main observations and conclusions extracted from this figure,
aside from the nonproduction of the Dewar isomer, are the
following:
(i) In the theoretical spectra of EE-type DOT forms (8, 10,
11, 13, 14) intense bands are predicted at about 900 and
1700 cmꢀ1
;
(ii) In the theoretical spectra of EZ-type DOT forms (9, 12)
intense bands are predicted at about 800 and 900 cmꢀ1
(iii) In the theoretical spectra of ZZ-type DOT forms (2, 5)
intense bands are predicted at about 800 and 1000 cmꢀ1
;
;
(iv) In the theoretical spectra of ZE-type DOT forms (1, 3, 4,
6, 7) intense bands are predicted at about 900 and
Figure 8. Selected reference structures and notation adopted to name
the possible isomeric structures and conformers of DOT. The names of
symmetry-equivalent mirror images reference structures are given in
parentheses. Figure S3 (Supporting Information) shows the full set of
considered reference structures of DOT.
1000 cmꢀ1
.
Since each type of configuration about the two central double
bonds in DOT has a characteristic signature, comparison of these
signatures with the experimental data (IR spectrum of the photolyzed
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Table 2. Structural and Energetic Parameters of r-Phellandrene Conformers and of the Corresponding Open-Ring Isomers
Calculated at the DFT(B3LYP)/6-311þþG(d,p) Level of Theorya
dihedral angles
relative energy
name
short name
A
B
C
D
E
|μ|
ΔE°
ΔE°ZPVE
ΔG°
trans
T
179.6
ꢀ48.1
69.6
0.23
0.29
0.21
0.57
0.73
0.60
0.75
0.50
0.56
0.60
0.08
0.16
0.22
0.44
0.38
0.36
0.18
0.24
0.14
0.14
0.30
0.26
0.95
1.05
0.53
0.77
0.91
1.03
1.04
0.65
0.70
0.70
0.15
0.35
0.36
0.73
0.86
0.88
0.61
0.76
0.74
0.00
0.42
0.80
35.1
38.4
41.9
59.2
51.8
55.2
55.4
49.1
52.4
52.4
55.4
72.2
72.0
77.6
81.5
81.7
75.0
78.6
78.1
31.7
35.0
38.4
55.4
49.5
53.4
53.2
46.0
49.3
49.4
52.5
69.4
69.5
61.6
65.5
65.2
65.1
68.8
69.0
0.00
0.37
1.00
27.7
31.7
35.5
54.1
44.7
48.7
48.9
40.7
44.7
45.0
47.8
66.2
65.7
68.9
73.3
73.6
65.3
70.2
69.1
24.3
27.9
31.6
49.9
42.5
46.9
46.8
37.5
41.4
41.5
44.7
63.0
63.2
53.9
58.0
58.0
56.8
61.2
61.1
0.00
0.27
1.06
20.6
25.3
27.2
46.7
37.9
42.9
43.1
33.1
38.0
38.7
39.7
59.1
58.1
60.6
65.1
65.4
56.2
63.0
61.3
17.1
21.3
22.2
42.1
36.0
40.9
40.8
29.0
33.6
33.8
34.9
54.1
54.5
46.9
51.3
51.6
47.9
54.1
53.4
gaucheꢀ
Gꢀ
Gþ
1C
gaucheþ
tZtE/C
180.0
180.0
0.0
0.2
0.0
0.2
5.6
5.7
5.7
180.0
ꢀ179.8
180.0
179.9
39.9
180.0
179.7
0.0
0.0
tZtE/SKþ
tZtZ/C
1þ
2C
120.3
0.0
180.0
tZtZ/SKþ
tZgþE/C
tZgþE/SKþ
tZgþE/SKꢀ
gꢀZtE/C
gꢀZtE/SKþ
gꢀZtE/SKꢀ
gꢀZtZ/C
gꢀZtZ/SKþ
gꢀZtZ/SKꢀ
gꢀZgþE/C
gꢀZgþE/SKþ
gꢀZgþE/SKꢀ
gꢀZgꢀE/C
gꢀZgꢀE/SKþ
gꢀZgꢀE/SKꢀ
tEtE/C
2þ
3C
179.9
ꢀ0.1
129.8
ꢀ0.4
121.6
ꢀ120.2
0.2
ꢀ172.0
ꢀ171.9
ꢀ172.1
ꢀ45.4
ꢀ45.6
ꢀ45.6
ꢀ44.2
ꢀ43.6
ꢀ43.7
ꢀ65.1
ꢀ65.5
ꢀ66.2
ꢀ55.2
ꢀ56.1
ꢀ54.6
180.0
178.8
178.5
179.1
178.3
177.8
178.6
ꢀ3.2
3þ
41.0
3ꢀ
41.2
4C
ꢀ3.8
ꢀ3.8
ꢀ3.8
ꢀ4.5
ꢀ4.7
ꢀ4.9
ꢀ2.2
ꢀ2.2
ꢀ2.0
ꢀ5.4
ꢀ5.3
ꢀ5.2
180.0
179.9
180.0
ꢀ179.8
178.3
178.4
178.6
178.2
178.3
178.2
177.9
177.9
177.5
177.7
177.8
177.8
179.5
179.4
179.5
176.1
176.7
176.8
173.2
171.1
171.8
41.6
4þ
120.5
ꢀ120.5
1.3
4ꢀ
5C
5þ
ꢀ4.5
ꢀ3.8
131.1
ꢀ127.7
0.5
5ꢀ
6C
ꢀ179.8
ꢀ179.9
ꢀ179.5
ꢀ177.5
ꢀ178.0
ꢀ178.2
180.0
179.6
0.0
6þ
42.6
120.9
ꢀ120.9
0.5
6ꢀ
43.1
7C
ꢀ41.1
ꢀ41.7
ꢀ42.6
180.0
179.7
180.0
179.6
ꢀ35.7
ꢀ36.5
ꢀ36.6
178.7
178.9
178.8
178.3
177.6
177.6
ꢀ34.0
ꢀ35.2
ꢀ35.3
33.2
7þ
121.1
ꢀ119.5
0.0
7ꢀ
8C
tEtE/SKþ
tEtZ/C
8þ
9C
180.0
120.2
0.0
180.0
tEtZ/SKþ
tEgꢀE/C
tEgꢀE/SKþ
tEgꢀE/SKꢀ
gꢀEtE/C
gꢀEtE/SKþ
gꢀEtE/SKꢀ
gꢀEtZ/C
gꢀEtZ/SKþ
gꢀEtZ/SKꢀ
gꢀEgꢀE/C
gꢀEgꢀE/SKþ
gꢀEgꢀE/SKꢀ
gꢀEgþE/C
gꢀEgþE/SKþ
gꢀEgþE/SKꢀ
9þ
180.0
ꢀ0.3
130.2
ꢀ1.2
119.5
ꢀ122.1
0.2
10C
10þ
10ꢀ
11C
11þ
11ꢀ
12C
12þ
12ꢀ
13C
13þ
13ꢀ
14C
14þ
14ꢀ
ꢀ177.1
ꢀ177.1
ꢀ177.3
ꢀ34.0
ꢀ34.0
ꢀ34.1
ꢀ34.4
ꢀ34.4
ꢀ34.2
ꢀ36.9
ꢀ37.4
ꢀ37.2
ꢀ34.8
ꢀ35.0
ꢀ35.3
ꢀ178.6
ꢀ179.0
ꢀ178.5
179.7
179.4
180.0
ꢀ0.3
121.2
ꢀ120.4
1.5
ꢀ0.7
ꢀ0.3
129.2
ꢀ128.4
ꢀ0.7
120.2
ꢀ122.0
0.2
ꢀ179.1
ꢀ179.6
ꢀ178.9
178.6
178.4
179.0
37.2
121.9
ꢀ120.1
36.5
a DFT(B3LYP)/6-311þþG(d,p) calculated energies of the closed-ring trans conformer of R-phellandrene was chosen as the zero level. The relative
calculated energies of the remaining forms are given in kJ molꢀ1. The absolute energies of T conformer are given in the caption of Table 1. Dipole
moments (|μ|/debye). Dihedral angles A, B, C, D, E (degrees) stand for C6dC7—C8dC9, C7ꢀC8dC9ꢀC10, C8dC9—C10dC2, C9ꢀC10dC2—C4
(isopropyl side), and C10dC2—C4—H25 (isopropyl). For the closed-ring molecule, the values of dihedral angle E (defining orientation of the isopropyl
group) refer to the H2CꢀCꢀCꢀH angle. In the abbreviated names, the backbone structure is designated by an arabic numeral followed by þ (skewþ),
ꢀ (skewꢀ) or C (cis), defining the orientation of isopropyl group.
R-phellandrene matrix; see Figure 7) was carried out in an attempt to
identify the isomers resulting from the observed photochemical
reaction. In the spectrum of the irradiated matrix, prominent bands
thus allowing to conclude that the observed photochemistry leads to
the predominant formation of the ZE-isomer of DOT.
Comparison of the spectra of the different conformers of a given
type of isomeric structures sharing the same configuration about
due to the photoproduct are observed at about 900 and 1000 cmꢀ1
,
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Figure 9. Theoretical spectra of R-phellandrene (bottom frame) and its possible isomers calculated at the DFT(B3LYP)/6-311þþG(d,p) level of approximation
(see Figure S3 in the Supporting Information). The calculated frequencies and intensities are not scaled. Note that ordinate scales are equal in all frames.
the two central CdC bonds shows that they are very similar. There-
fore, the identification of the particular conformer(s) generated in
the photoreaction is difficult. However, simulations of infrared
spectral changes occurring upon photolysis were made assuming
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the different sets of conformers sharing the common ZE-config-
uration as possible photoproducts. Based on the close match
between the experimentally observed spectroscopic changes upon
irradiation and the simulation resulting when open-ring isomers
with the tZgE and gZgE backbone were used (see Figure 7), the
most likely conformersof DOT resultingfromphotolysis of matrix
isolated R-phellandrene seemto be those exhibitingthis backbone,
i.e., 3C, 3þ, 3ꢀ and 7C, 7þ, 7ꢀ. These conformers require the
smallest structural rearrangement of both the reactant and sur-
rounding matrix, thus being in concordance with the principle of
least motion,43 which has been formulated in the context of the
ring-opening reaction in related compounds, and proved to be
obeyed by the analogous molecule, R-terpinene.12 The expected
mechanism of the photoprocess is a concerted σ2 þ π4 electro-
cyclic ring-opening conrotatory reaction taking place in S1
(according to the WoodwardꢀHoffmann orbital symmetry
rules44), closely resembling that proposed for R-terpinene.12
The sole production of ZE-type conformers of the photo-
product of R-phellandrene is in line with the photochemistry of
matrix-isolated 1,3-cyclohexadiene, which was found to undergo
rapid photolysis exclusively to a specific type of open-chain
structure (Z forms in that case),45 but with different isopropyl
group orientations. This is in agreement with NEER principle
(nonequilibration of excited rotamers), which notes that equili-
bration of the excited rotamers during the singlet-excited-state
lifetime does not occur, implying unique photochemistry and
photophysics for each rotamer42). For R-phellandrene, if the
photoisomerization of each conformer is highly stereoselective
giving the product anticipated in a model emphasizing ground-
state geometry as a stereochemical determinant and accounting
for the principle of least motion,43 then two different conformers
with a given backbone type and different isopropyl group
orientation should be produced from each conformer of the
reactant (for CCW and CW rotation of the isopropyl fragment).
On the whole, all six tZgE-type or gZgE-type conformers of DOT
would then be produced. This is in agreement with the spectro-
scopic data shown in Figure 7, showing that structures 3C, 3þ,
3ꢀ as well as 7C, 7þ, 7ꢀ exhibit infrared spectra that are
congruous with the experimental spectrum of the photoproduct.
1200ꢀ1000 cmꢀ1 region shows that conformational population in
the liquid phase at RT is comparable with the population in the gas
phase at RT. Moreover, Raman spectroscopy aided in the conforma-
tional characterization of the studied compounds providing the
experimental frequencies for the vibrations that are weakly absorbing
in infrared but have high Raman activities, in agreement with the DFT
calculations.
Vibrational analysis of the matrix spectra was focused on the
conformer-specific regions corresponding to the CdC stretching
and CꢀH bending vibrations. These spectral regions, 1700ꢀ1600
and 1200ꢀ760 cmꢀ1, were especially sensitive to the photoche-
mical changes in the studied molecules. These changes are related
with reorganization of the π-system in the course of the UV-
induced photoreaction. Upon irradiation at λ > 200 nm, no change
was observed in the spectrum of γ-terpinene, while new bands
emerged in the spectrum of R-phellandrene. These results show
that R-phellandrene is photolabile under germicidal UV-C light
and demonstrate the relevance of the different double bonds
arrangement in the two rings on their photostability. The observed
photoprocess for R-phellandrene is a typical concerted electro-
cyclic ring-opening reaction. The series of theoretical calculations
(which were carried out on the whole set of possible open-ring
isomers of R-phellandrene) and analysis of the spectra of the
irradiated matrices suggest that the most likely photoproducts are
the open-ring isomers with the tZgE backbone, i.e., those that
require the smallest changes of the reacting species as well as the
smallest changes of the local environment.
’ ASSOCIATED CONTENT
S
Supporting Information. Figure S1, showing the com-
b
parison of the FT-Raman spectra of liquid R-phellandrene and
γ-terpinene at 298 K with the simulated Raman spectra for their
conformers; Figure S2, with the UVꢀvis spectra of 10ꢀ4
M
ethanol solutions of R-terpinene, R-phellandrene, and γ-terpi-
nene; Figure S3, showing the reference structures and notation
adopted to name the open-ring isomers of R-phellandrene; Table
S1, presenting the structural parameters of the conformers of
R-phellandrene and γ-terpinene calculated at the DFT(B3LYP)/6-
311þþG(d,p) level of theory; Tables S2, S4, and S6, with the
definition of internal coordinates used in the normal coordinate
analysis calculations for R-phellandrene, γ-terpinene, and DOT
(tZgE form), respectively; Tables S3, S5, and S7, presenting the
experimental wavenumbers and intensities (qualitative) of the
infrared spectra R-phellandrene, γ-terpinene, and DOT (tZgE
form), respectively, as well as the calculated infrared spectra for
the relevant conformers of each molecule and potential energy
distributions resulting from the normal coordinate analysis
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ CONCLUSIONS
This paper compares molecular structures and photochemical
properties of γ-terpinene and R-phellandrene which are two struc-
tural monoterpene isomers with 1,4- and 1,3-diene six-membered
rings, respectively. The conformational space of the two compounds
was fully characterized using DFT calculations. Each molecule can
adopt three conformations related with the internal rotation of the
exocyclic isopropyl group. The potential energy profiles for inter-
conversion between conformers were also characterized. For both
compounds, the gas-phase conformational population characteristic
of room temperature (RT) could be efficiently trapped in cryogenic
Ar matrices. High energy barriers (at least 15 kJ molꢀ1) separating
the unique conformers prevent conformational relaxation during the
matrix deposition. The signatures of all conformers were identified in
the experimental IR spectra with the lowest energy trans forms
dominating for both matrix-isolated compounds. The FTIR-ATR
spectrum of liquid R-phellandrene and γ-terpinene and the IR
spectra obtained for the compounds isolated in argon matrices
show only minor differences. The comparison between the experi-
mental FT-Raman spectra of R-phellandrene and γ-terpinene at
RT and the theoretical Raman spectra of their conformers in the
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: proniewi@chemia.uj.edu.pl (L.M.P.); rfausto@ci.uc.pt
(R.F.). Phone: þ48 12 663 2288 (L.M.P.); þ351 91 9236971
(R.F.).
’ ACKNOWLEDGMENT
This work was supported by The Polish State Committee for
Scientific Research, 2009/2011 (Grant No. N N204 341437),
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the Portuguese “Fundac-~ao para a Ci^encia e a Tecnologia”, FCT
(Projects PTDC/QUI/71203/2006 and PTDC/QUI-QUI/
111879/2009, also supported by QREN-FEDER/COMPETE),
and the European Union from the resources of the European
Regional Development Fund under the Innovative Economy
Programme (grant coordinated by JCET-UJ, No POIG.01.01.
02-00-069/09).
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich,
S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.;
Fox, D. J. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT,
2009.
(29) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. 1988, B37, 785–789.
(30) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys.
2002, 117, 43–54.
(31) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem.
2003, 24, 669–681.
(32) Michalska, D.; Wysokinski, R. Chem. Phys. Lett. 2005,
403, 211–217.
(33) Reva, I.; Lopes Jesus, A. J.; Rosado, M. T. S.; Fausto, R.;
Eusꢀebio, M. E.; Redinha, J. S. Phys. Chem. Chem. Phys. 2006,
8, 5339–5349.
(34) Andrzejewska, A.; Lapinski, L.; Reva, I.; Fausto, R. Phys. Chem.
Chem. Phys. 2002, 4, 3289–3296.
(35) Fogarasi, G.; Zhou, X.; Taylor, P. W.; Pulay, P. J. Am. Chem. Soc.
1992, 114, 8191–8201.
(36) Pulay, P.; Fogarasi, G.; Pang, F.; Boggs, J. E. J. Am. Chem. Soc.
1979, 101, 2550–2560.
(37) Martin, J. M. L.; Van Alsenoy, C. Gar2ped; University of
Antwerp: Antwerp, Belgium, 1995.
(38) Breda, S.; Reva, I. D.; Lapinski, L.; Fausto, R. Phys. Chem. Chem.
Phys. 2004, 6, 929–937.
’ REFERENCES
(1) Shaw, P. E.; Moshonas, M. G.; Bowman, K. D. Phytochemistry
2000, 53, 1083–1086.
€
(2) Jacobs, S. W. L.; Pickard, J. Plants of New South Wales;
Government Printing: Sydney, NSW, Australia, 1981; ISBN 0-7240-1978-2.
(3) Pongprayoon, U.; Soontornsaratune, P.; Jarikasem, S.; Sematong,
T.; Wasuwat, S.; Claeson, P. Phytomedicine 1996/97, 3, 319–22.
(4) Williams, A. C.; Barry, B. W. Pharm. Res. 1991, 8, 17–24.
(5) Al-Burtamani, S. K. S.; Fatope, M. O.; Marwah, R. G.; Onifade,
A. K.; Al-Saidi, A. S. H. J. Ethnopharmacol. 2005, 96, 107–112.
(6) Cheng, S.; Liu, J.; Hsui, Y.; Chang, S. Bioresour. Technol. 2006,
97, 306–312.
(7) Nhu-Trang, T.; Casabianca, H.; Grenier-Loustalot, M. J. Chro-
matogr. A 2006, 1132, 219–227.
(8) Schulz, H.; Baraꢀnska, M. Vibrat. Spectrosc. 2007, 43, 13–25.
€
€
(9) Schulz, H.; Ozkan, G.; Baranꢀska, M.; Kruger, H.; Ozcan, M.
Vibrat. Spectrosc. 2005, 39, 249–259.
(10) Breitmaier, E. Terpenes: Flavors, Fragrances, Pharmaca, Phero-
mones; Wiley-VCH: Weinheim, Germany, 2006.
(11) Bond, A. D.; Davies, J. E. Acta Crystallogr. 2001, 57, 1032–1033.
(12) Marzec, K. M.; Reva, I.; Fausto, R.; Malek, K.; Proniewicz, L. M.
J. Phys. Chem. A 2010, 114, 5526–5536.
(13) Harkenthal, M.; Reichling, J.; Geiss, H. K.; Saller, R. Pharm. Z.
1998, 143, 26–30.
(14) Gallivan, J. B.; Brinen, J. S. Chem. Phys. Lett. 1971, 10, 455–459.
(15) Iwanami, Y.; Tateba, H.; Kodama, N.; Kishino, K. J. Agric. Food
Chem. 1997, 45, 463–466.
(16) Land, E. J.; Truscott, T. G. Photochem. Photobiol. 1979, 29,
861–886.
(17) Hausen, B. M.; Reichling, J.; Harkenthal, M. Am. J. Contact
Dermat. 1999, 10, 68–77.
(18) Avciba-si, N.; Ic-li, S.; Gilbert, A. Turk. J. Chem. 2003, 27, 1–7.
(19) Garavelli, M.; Page, C. S.; Celani, P.; Olivucci, M.; Schmid,
W. E.; Trushin, S. A.; Fuss, W. J. Phys. Chem. A 2001, 105, 4458–4469.
(20) Dou, Y.; Yuan, S.; Lo, G. V. Appl. Surf. Sci. 2007, 253,
6404–6408.
(39) Harris, D. A.; Orozco, M. B.; Sension, R. J. J. Phys. Chem. A
2006, 110, 9325–9333.
(40) White, L. S.; Rossi, A. R.; Epling, G. A. J. Am. Chem. Soc. 1981,
103, 7299–7304.
(41) Seixas de Melo, J.; Quinteiro, G.; Pina, J.; Breda, S.; Fausto, R. J.
Mol. Struct. 2001, 59, 565–566.
(42) (a) Brouwer, A. M.; Jacobs, H. J. C. Recl. Trav. Chim. Pays-Bas.
1995, 114, 449–458. (b) Craig, N. C.; Leyden, M. C.; Moore, M. C.;
Patchen, A. K.; Van den Heuvel, T.; Blake, T. A.; Masiello, T.; Sams, R. L.
J. Mol. Spectrosc. 2010, 262, 49–60.
(43) Baldwin, J. E.; Krueger, S. M. J. Am. Chem. Soc. 1969,
91, 6444–6447.
(44) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital
Symmetry; Verlag Chemie: Weinheim, Germany, 1970.
(45) Danen, W. C.; Tipton, T. J.; Saunders, D. G. J. Am. Chem. Soc.
1971, 93, 5186–5189.
(21) Fuss, W.; Schikarski, T.; Schmid, W. E.; Trushin, S.; Kompa,
K. L. Chem. Phys. Lett. 1996, 262, 675–682.
(22) Kauffmann, E.; Frei, H.; Mathies, R. A. Chem. Phys. Lett. 1997,
266, 554–559.
(23) Dudek, R. C.; Weber, P. M. J. Phys. Chem. A 2001, 105,
4167–4171.
(24) Reid, P. J.; Doig, S. J.; Wickham, S. D.; Mathies, R. A. J. Am.
Chem. Soc. 1993, 115, 4754–4763.
(25) (a) Geppert, D.; Seyfarth, L.; De Vivie-Riedle, R. Appl. Phys. B:
Lasers Opt. 2004, 79, 987–992. (b) Ruan, C.-Y.; Lobastov, V. A.;
Srinivasan, R.; Goodson, B. M.; Ihee, H.; Zewail, A. H. Proc. Natl. Acad.
Sci. U.S.A. 2001, 98, 7117–7122. (c) Kurtz, L.; Hofmann, A.; de Vivie-
Riedle, R. J. Chem. Phys. 2001, 114, 6151–6159.
(26) (a) de Kock, R. J.; Minnaard, N. G.; Havinga, E. Recl. Trav.
Chim. Pays- Bas. 1960, 79, 922–934. (b) Crowley, K. J.; Erickson, K. L.;
Eckell, A; Meinwald, J. J. Chem. Soc., Perkin Trans. 1 1973, 2671–2679.
(c) Reid, P. J.; Doig, S. J.; Mathies, R. A. J. Phys. Chem. 1990,
94, 8396–8399.
(27) Climent, M. J.; Miranda, M. A.; Roth, H. D. Eur. J. Org. Chem.
2000, 1563–1567.
(28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
4353
dx.doi.org/10.1021/jp2013122 |J. Phys. Chem. A 2011, 115, 4342–4353