The rotationally resolved electronic spectra of several conformers of 1-hexoxy and 1-heptoxy

Article abstract of DOI:10.1139/v04-056

Laser-induced fluorescence excitation spectra of five vibronic bands of 1-hexoxy and three bands of 1-heptoxy have been recorded in a jet-cooled environment. Experimental values of rotational constants for both the X and B states and components of the spin-rotational tensor for the X state were obtained by an analysis of the partially resolved rotational structure of the vibronic bands. Comparing these experimental results with quantum chemistry calculations, and using corresponding assignments of smaller alkoxy radicals as a guide, permitted unambiguous conformational assignments for the bands. The extension of similar assignments to larger alkoxy radicals is also discussed.

Full text of DOI:10.1139/v04-056

854
The rotationally resolved electronic spectra of
several conformers of 1-hexoxy and 1-heptoxy1
Lily Zu, Jinjun Liu, Sandhya Gopalakrishnan, and Terry A. Miller
Abstract: Laser-induced fluorescence excitation spectra of five vibronic bands of 1-hexoxy and three bands of 1-heptoxy
ꢀ
ꢀ
have been recorded in a jet-cooled environment. Experimental values of rotational constants for both the X and B states
ꢀ
and components of the spin-rotational tensor for the X state were obtained by an analysis of the partially resolved
rotational structure of the vibronic bands. Comparing these experimental results with quantum chemistry calculations,
and using corresponding assignments of smaller alkoxy radicals as a guide, permitted unambiguous conformational
assignments for the bands. The extension of similar assignments to larger alkoxy radicals is also discussed.
Key words: electronic spectroscopy, organic radicals, combustion, atmospheric chemistry.
Résumé : Les spectres de fluorescence induite par laser de cinq bandes vibroniques du 1-hexoxy et de trois bandes du
1-heptoxy ont été enregistrés en jet froid. Une analyse de la structure rotationnelle partiellement résolue des bandes
ꢀ
ꢀ
vibroniques a permis d’obtenir les valeurs expérimentales des constantes rotationnelles des deux états X et B, ainsi que
ꢀ
les composantes tensorielles de spin-rotation de l’état X. La comparaison des résultats expérimentaux avec des calculs de
chimie quantique, ainsi qu’une utilisation des attributions correspondantes de plus petits radicaux alkoxy comme guide, a
permis une attribution des conformères sans ambiguité possible pour chaque bande. L’extension d’attribution similaire à de
plus gros radicaux de type alkoxy est discutée.
Mots clés: spectroscopie électronique, radicaux organiques, combustion, chimie de l’atmosphère.
[Traduit par la Rédaction]
ꢀ
ꢀ
1. Introduction
generate E ground state decomposes into A and X states, of
Aꢀ and Aꢀꢀ symmetry assuming a plane of symmetry exists.
ꢀ
ꢀꢀ
ꢀ
It is a privilege to contribute to this Gerhard Herzberg memo-
rial issue. One of his abiding interests was the spectroscopy of
free radicals (1). Arguably his electronic spectra of the simplest
hydrocarbonfreeradicals(2–6)CH, CH2, andCH3, rankamong
the most significant achievements in his illustrious career.
In this spirit, this paper reports the electronic spectroscopy
of some simple hydrocarbon-derived, oxygen-containing, free
radicals, the alkoxies (CnH2n+1O). The importance of alkoxy
radicals in atmospheric chemistry is well known (7). They are
key intermediates in the oxidation of hydrocarbons. More than
100 studies have been published alone over the last 50 years
on the spectroscopic studies of the smallest alkoxy radical,
methoxy. Other small alkoxies like ethoxy (8, 9), C2H5O, and
2-propoxy (9, 10), (CH3)2HCO, have also been studied inten-
sively.
Whether the X state is A or A depends upon the radical. Tan
ꢀ
ꢀ
et al. (14) determined the symmetry of the X and A electronic
states of ethoxy, vinoxy, and isopropoxy, using the technique
of laser-induced fluorescence (LIF) combined with supersonic
jet cooling. The complete rotational analysis of the LIF spec-
ꢀ
ꢀ
trum of the ethoxy (14) B – X transition further revealed a
significant ground-state, spin-rotation splitting due to the close
ꢀ
ꢀ
spacing of the low-lying A and X states. Within experimental
resolution (≈200 MHz) no excited-state, spin-rotation splitting
was observed.
While the smaller alkoxy radicals have drawn significant
attention, the spectroscopic study of larger alkoxy radicals is
much sparser. It was generally felt that the fluorescence quan-
tum yield for the B – X transition would rapidly decrease as
non-radiative decay processes became dominant in the larger
alkoxy radicals. That perception began to change when LIF
spectra of t-pentoxy, 3-pentoxy, t-butoxy, and 2-butoxy were
observed at near ambient temperature in 1999 (15, 16). How-
ever, the failed attempts to observe several other larger alkoxy
radicals in the same LIF studies left the overall situation unclear.
Clarification occurred when the LIF spectra of nearly 20 large
alkoxy radicals containing 5–12 carbon atoms were reported
by Carter et al. (17) in 2001. The studies showed that under
ꢀ
ꢀ
The early laser magnetic resonance investigation of Radford
and Russell (11) in the far-IR and the microwave studies of
Endo et al. (12) yielded considerable information about the
2
ground E state of methoxy. Its electronic transition has long
2
ꢀ
been known to be an excitation from the ground X E state to
2
ꢀ
the A A state (13). When a hydrogen is substituted in methoxy,
the C3v symmetry of CH3O is at least lowered to Cs; the excited
ꢀ
ꢀ
ꢀ
A A1 state of CH3O correlates to a B A state while the de-
Received 13 April 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca/ on 31 August 2004.
L. Zu, J. Liu, S. Gopalakrishnan, and T.A. Miller.2 Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University,
120 W. 18th Avenue, Columbus, OH 43210, USA.
1This article is part of a Special Issue dedicated to the memory of Professor Gerhard Herzberg.
2 Corresponding author (e-mail: tamiller@chemistry.ohio-state.edu).
Can. J. Chem. 82: 854–866 (2004)
doi: 10.1139/V04-056
© 2004 NRC Canada

Zu et al.
855
Fig. 1. Structures of representative examples of the (a) 41 1-hexoxy conformers and (b) 122 1-heptoxy conformers. The corresponding
Newman projections are also shown.
cold jet conditions, all of the alkoxy radicals exhibit sharp LIF
excitation spectra with well-characterized vibrational structure.
The S/N ratio of the spectra did become somewhat poorer as the
number of carbon atoms (n) increased but not dramatically so,
and the authors attributed the gradual reduction to the decreased
vapor pressure of the higher molecular weight precursors.
Assignment of the observed spectral bands in the large alkoxy
radicals has turned out to be very challenging. Unlike methoxy
and ethoxy, larger alkoxy radicals have structural isomers, and
the same structural isomer can exist in different conformations.
Alkoxy conformers are typically labelled according to whether
the non-H substituents are gauche (G) or trans (T) with respect
to one another for each of the C—C bonds in the molecules,
(with the C—C bond labelled 1 being the one that is closest to
the O atom) (Fig. 1).
The spectral positions of structural isomers are usually well-
separated but those of conformers are not. Indeed, closely-
spaced vibronic bands can be expected from the various
conformers, e.g., an origin band from one conformer can lie
close to a C-O stretch band belonging to an entirely differ-
ent conformer of the radical. Gopalakrishnan et al. (18) made
states as well as components of the electron spin-rotation tensor
in the X state were determined for each conformer. Similar ro-
ꢀ
tational analyses (19) were recently published for 1-butoxy and
1-pentoxy, again correlating the strongest spectral lines with
specific conformers.
The method used in the 1-propoxy, 1-butoxy, and 1-pentoxy
analyses is computationally intensive (18, 19). It involves
identifying all likely stable conformations using the systematic
pseudo-Monte Carlo (SPMC) methods (20) available in Macro-
Model(21).Thisisfollowedbyquantumchemistrycalculations
ꢀ
ꢀ
forboththeX andB states, optimizingeachofthesegeometries
to predict rotational constants. Components of the ground-state
electronic spin – molecular rotation tensor were also predicted
by a semiempirical method using the experimentally observed
spin-rotation tensor of ethoxy as a reference. Finally, compar-
isons between molecular constants obtained experimentally and
those from the calculations are used to identify the carrier of a
given band.
When one examines larger open-chain, primary alkoxies, the
number (19) of conformers expected (41 for 1-hexoxy, 122 for
1-heptoxy) becomes so large that the computations described
above become infeasible. In this paper, we suggest a new tech-
nique, which lets us select a candidate conformer for a given
band based only upon a comparison of the given experimental
spectra with those of smaller alkoxy radicals. Using this ap-
ꢀ
ꢀ
significant progress analyzing the rotationally resolved B – X
LIF spectra of all the strong bands of 1-propoxy. Five vibronic
bands were unambiguously assigned to two conformers (G
ꢀ
ꢀ
and T). Experimental rotational constants for both X and B
© 2004 NRC Canada

856
Can. J. Chem. Vol. 82, 2004
Fig. 2. LIF excitation spectra of the primary alkoxy radicals (CnH2n+1O) from n = 3 − 10. Persistent lines in the spectrum are marked
A, B, and C. Conformationally assigned bands are labeled (G for gauche, T for trans) (18, 19) according to the various dihedral angle
minima. Excited vibrational levels are indicated as subscripts by their approximate motion, e.g., CO (stretch), CCO (bend), etc.
proach, four conformers were assigned for five vibronic bands
of 1-hexoxy and two conformers for three vibronic bands of
1-heptoxy. Extending the analysis to even larger alkoxy radi-
cals is also discussed.
(22). A few torr (1 torr = 133.322 Pa) of the alkyl nitrite vapor
was entrained into the jet flow by passing helium at 6 atm (1 std
atm = 101.325 kPa) over the liquid contained in a steel bomb
maintained at a suitable temperature depending upon the vapor
pressure.
The rotationally resolved LIF spectra of the 1-alkoxy rad-
icals were obtained on our high-resolution apparatus, which
has been described elsewhere (23). Briefly, a cw ring dye laser
(Coherent 899-29 Autoscan) pumped by an Ar+ laser (Coher-
ent Innova Sabre) was pulse amplified and frequency doubled
using a KDP crystal to generate the UV radiation required to
2. Experimental
The alkoxy radicals were generated in a supersonic free jet
expansion by UV laser photolysis (XeF) of the corresponding
alkyl nitrites at the base of a 0.5 mm circular nozzle. The alkyl
nitrites were synthesized according to well-known procedures
© 2004 NRC Canada

Zu et al.
857
Fig. 3. Rotationally resolved spectra of primary alkoxy radicals from 1-propoxy to 1-heptoxy: (a) band A, (b) band B, and (c) band C.
ꢀ
ꢀ
pump the B – X transition of the alkoxy radicals. The pulse
amplifier (Lambda Physik, FL 2003 model) was pumped by an
excimer laser (XeCl, Lambda Physik, MSC 103 model) with a
resulting line width of ≈100 MHz in the fundamental (200 MHz
when doubled into the UV). This radiation was directed into a
vacuum chamber ≈15 mm downstream of the nozzle orifice
in the jet expansion. This signal was then preamplified and sent
to a boxcar averager (Stanford Research, SR250 boxcar) for
integration. The amplified signal was then sent to the Coherent
Autoscan unit for signal averaging and recording.
3. Theory
ꢀ
ꢀ
to excite the B – X transition. The resulting fluorescence was
collected perpendicular to the probe laser beam with a 2.5 cm
f/1 lens. The light was then focused with a f/3 lens through an
adjustable slit onto a photomultiplier tube (PMT). The slit can
be adjusted to allow only the central portion of the fluorescence
from the jet to be detected by the PMT, thereby reducing the
Doppler width of the signal from off-axis velocity components
The theory upon which the analysis of the rotational struc-
ture of the bands is based is described in detail in ref. 18, hence
only a brief description will be provided here. An asymmet-
ric top Hamiltonian is used to model the rotational structure
of the high-resolution electronic spectra of the primary alkoxy
radicals. Because of the very low temperature in the jet, only
© 2004 NRC Canada

858
Can. J. Chem. Vol. 82, 2004
Fig. 3. (continued).
the lowest few rotational states are populated. Therefore, cen-
trifugal terms in the rotational Hamiltonian (HRot ) may be ne-
glected. However, the spin-rotation coupling certainly needs to
be considered. Hence, the total Hamiltonian can be written:
where N and S are the molecular rotational and electron spin
angular momentum, respectively. A, B, and C are the rotational
constants, and ꢀαβ are the components of the spin-rotation ten-
sor in the inertial axis system (a, b, c).
The matrix elements of the Hamiltonian of a nonrigid, non-
linear, polyatomic molecule in a multiplet electronic state have
been derived by Raynes (24), and these were programmed into
a graphical user interface program SpecView, which was devel-
oped in our laboratory (25) to model the spectra of the alkoxy
radicals.
[1]
H = HRot + HSR
In which
[2]
[3]
HRot = ANa2 + BNb2 + CNc2
The six rotational constants, namely, Aꢀꢀ, Bꢀꢀ, and Cꢀꢀ in
ꢁ
1
HSR
=
ꢀαβ (NαSβ + SβNα)
ꢀ
ꢀ
ꢀ
ꢀ
the ground state and A , B , and C in the excited B state,
2
αβ
need to be fit to the spectra. For the spin-rotation interaction,
© 2004 NRC Canada

Zu et al.
859
Fig. 3. (concluded).
only ground-state constants need to be fitted because the spin-
rotation interaction in the excited state causes splittings unre-
solvable with the experimental resolution, as confirmed by the
study of ethoxy (14). According to Brown et al. (26), six unique
spin-rotation constants can be determined for molecules with
C1 symmetry, while only four can be determined for molecules
with Cs symmetry. In the general case, the spin-rotation con-
stants that can be determined for a single isotopic species are
given below in terms of their relationship to the components of
the reduced spin-rotation tensorꢀꢀ introduced by Brown et al.:
√
T01(ε˜) = (−1/ 2)i(ε˜cb − ε˜bc) = 0
T 1 (ε˜) = (1/2)[(ε˜ba − ε˜ab
)
(ε˜ca − ε˜ac)] = 0
1
√
√
T02(ε˜) = (1/ 6)(2ε˜aa − ε˜bb − ε˜cc) = 6a
T 21(ε˜) = (1/2)[(ε˜ba + ε˜ab
)
(ε˜ca + ε˜ac)] = (d ie)
√
√
T00(ε˜) = (−1/ 3)(ε˜aa + ε˜bb + ε˜cc) = 3a0
T 2 (ε˜) = (1/2)[(ε˜bb − ε˜cc) (ε˜bc + ε˜cb)] = (b ∓ ic)
2
© 2004 NRC Canada

860
Can. J. Chem. Vol. 82, 2004
Table 1. Experimentally determined and calculated (in brackets) values of molecular constants for several conformers of
1-hexoxy.
d
e
Constants (GHz)
Band Aa
Band Bb
Band Cc
Band Ba
Band Cb
Aꢀꢀ
Bꢀꢀ
Cꢀꢀ
8.884(3)[9.062] 12.817(13)[13.549] 13.633(8)[13.549] 7.741(19)[7.704] 8.322(23)[8.561]
0.833(1)[0.818] 0.767(1)[0.728]
0.770(1)[0.788] 0.697(1)[0.709]
–0.34(1)[–0.41] –3.41(6)[–3.82]
–0.19(1)[–0.19] –0.17(1)[–0.04]
–0.10(1)[–0.05] 0.02(0)[–0.00]
0.748(1)[0.728]
0.722(1)[0.709]
–4.08(4)[–3.82]
–0.01(1)[–0.04]
· · · [–0.00]
0.996(3)[0.853]
0.909(3)[0.802]
–0.33(6)[–1.44]
0.844(6)[0.825]
0.830(6)[0.789]
–3.14(30)[–2.70]
ꢀꢀ
ꢀꢀ
aa
ꢀꢀ
ꢀꢀ
bb
–0.22(19)[–0.13] –0.00(21)[0.01]
ꢀꢀ
ꢀꢀ
cc
· · · [–0.01]
· · · [0.72]
· · · [0.19]
· · · [0.03]
–0.07(21)[–0.01]
· · · [0.35]
ꢀꢀ
ꢀꢀ
|ꢀꢀ | = |ꢀꢀ |
· · · [0.50]
· · · [0.27]
· · · [0.10]
· · · [1.00]
· · · [0.04]
· · · [0.00]
· · · [1.00]
ab
ba
ꢀꢀ
ꢀꢀ
|ꢀꢀ | = |ꢀꢀ |
· · · [0.04]
· · · [0.34]
ac
ca
ꢀꢀ
ꢀꢀ
|ꢀꢀ | = |ꢀꢀ |
· · · [0.00]
· · · [0.01]
bc
cb
Aꢀ
8.142(2)[8.501] 12.974(8)[12.807]
0.836(1)[0.837] 0.745(1)[0.738]
0.792(1)[0.802] 0.715(1)[0.716]
12.474(3)[12.807] 7.149(9)[7.287]
8.193(11)[8.624]
0.847(4)[0.827]
0.815(5)[0.793]
29 658.59(1)
· · · [377]
Bꢀ
0.746(1)[0.738]
0.729(1)[0.716]
29 662.05(1)
· · · [57]
0.926(2)[0.869]
0.966(2)[0.814]
28 983.21(1)
· · · [369]
Cꢀ
T0 (cm−1
)
28 636.60(1)
· · · [0.0]
28 989.34(1)
· · · [57]
Relative energies (cm−1
)
a
(G T T T ) 116 lines assigned, σ = 43 MHz.
1
2 3 4
b
c
(T T T T ) 129 lines assigned, σ = 121 MHz.
1
2 3 4
2 3 4
(T T T T ) 111 lines assigned, σ = 35 MHz.
1
d
e
(T T G T ) 116 lines assigned, σ = 194 MHz.
1
2 3 4
(T T T G ) 60 lines assigned, σ = 91 MHz.
1
2 3 4
Table 2. Experimentally determined and calculated (in brackets) values of molecular
constants for several conformers of 1-heptoxy.
Constants (GHz)
Band Aa
Band Bb
Band Cc
Aꢀꢀ
Bꢀꢀ
Cꢀꢀ
7.496(6)[7.492] 11.674(32)[11.716] 11.738(26)[11.716]
0.530(4)[0.542] 0.512(2)[0.489]
0.518(4)[0.525] 0.492(3)[0.479]
–0.08(4)[–0.35] –3.79(4)[–3.47]
–0.01(2)[–0.13] 0.00(9)[–0.03]
0.484(1)[0.489]
0.489(1)[0.479]
–3.49(5)[–3.47]
· · · [–0.03]
ꢀꢀ
ꢀꢀ
aa
ꢀꢀ
ꢀꢀ
bb
ꢀꢀ
ꢀꢀ
cc
0.00(2)[–0.06]
· · · [0.41]
–0.05(9)[0.00]
· · · [0.85]
· · · [0.00]
ꢀꢀ
ꢀꢀ
ꢀꢀ
|ꢀꢀ | = |ꢀꢀ
· · · [0.85]
ab
ba|
ca
ꢀꢀ
|ꢀꢀ | = |ꢀꢀ |
· · · [0.22]
· · · [0.03]
· · · [0.03]
ac
ꢀꢀ
ꢀꢀ
|ꢀꢀ | = |ꢀꢀ |
· · · [0.07]
· · · [0.00]
· · · [0.00]
bc
cb
Aꢀ
6.872(4)[7.120] 11.085(9)[11.354]
0.548(3)[0.554] 0.503(3)[0.495]
0.526(3)[0.534] 0.502(3)[0.484]
11.124(9)[11.354]
0.493(1)[0.495]
0.491(1)[0.484]
29 647.11(1)
· · · [56]
Bꢀ
Cꢀ
T0 (cm−1
)
28 635.10(1)
28 975.04(1)
Relative energies (cm−1
)
· · · [0.0]
· · · [56]
a
(G T T T T ) 109 lines assigned, σ = 116 MHz.
1
2 3 4
5
b
c
(T T T T T ) 71 lines assigned, σ = 66 MHz.
1
2 3 4
5
2 3 4
5
(T T T T T ) 69 lines assigned, σ = 75 MHz.
1
The parameters a0, a, b, c, d, and e were defined by Raynes (24)
and because of their notational convenience are used in our
spectral analysis.
The prediction of rotational constants was performed via cal-
culations using the Gaussian98 program (27) at the B3LYP/6-
4. Results
The number (N) of conformers grows rather dramatically ac-
3n−2−1
cording to the formula (19), N =
+1, as the number (n)
2
of carbon atoms in the alkoxy radical increases. However, the
LIF excitation spectra (Fig. 2) show, if anything, fewer features
with an increase of n instead of more congestion. Our approach
to understanding these spectra builds upon corresponding anal-
yses (18,19) for 1-propoxy, 1-butoxy, and 1-pentoxy for which
multiple conformers were “frozen out” in the jet. For these
ꢀ
ꢀ
31+G* and the CIS/6-31+G* levels for the X and B states,
respectively. As already mentioned in the Introduction, a semi-
empirical approach based upon the knowledge of the experi-
mentally determined spin-rotation constants of the ethoxy
radical was used to estimate the spin-rotation constants (28).
© 2004 NRC Canada

Zu et al.
861
Fig. 4. Rotationally resolved band C of 1-hexoxy: (a) experimental spectrum, (b) simulation using only calculated rotational constants
of conformer T1T2T3T4, (c) simulation including spin-rotation component a0 + a/2, and (d) spectrum simulated from fit constants of
Table 1. The ratio of the electric dipole moments a:b:c for the simulation is 0:0:1. The rotational temperature is 1.2 K.
species, detailed spectroscopic analyses determined rotational
and spin-rotation constants for two of two 1-propoxy conform-
ers, three of a possible five 1-butoxy conformers, and five of 14
possible conformers of 1-pentoxy.
compares the rotational structure for bands A, B, and C for the
alkoxy radicals (CnH2n+1O) with 3 ≤ n ≤ 7. Figure 3 shows,
as expected, that the rotational structure becomes increasingly
compressed as the molecule increases in size and its rotational
constants concomitantly decrease. However, allowing for this
compression, we see clearly a great deal of similarity in struc-
ture in each labelled band for all the alkoxy radicals. Previous
detailed assignments for the propoxy, butoxy, and pentoxy rad-
icals helped guide our present assignments. Based upon the ab-
solute and relative positions of the lines relative to the smaller
alkoxies, it seems very reasonable to assign bands B and C to
the origin and first C-O stretch of the conformer with Cs sym-
metry (in 1-hexoxy and 1-heptoxy T1T2T3T4 and T1T2T3T4T5,
respectively), while band A is assigned to the origin of the con-
former with the oxygen out of the Cs plane (G1T2T3T4 and
G1T2T3T4T5 for 1-hexoxy and 1-heptoxy, respectively).
Ab initio calculations were performed for these two con-
formers for both 1-hexoxy and 1-heptoxy, and the calculated
molecular constants are given in Tables 1 and 2 and were used
Figure 2 also shows that there are three bands that appear
persistently for all the alkoxy radicals, labelled bands A, B,
and C. Interestingly, for 1-propoxy, 1-butoxy, and 1-pentoxy,
band B is always correlated with the all-trans conformer, which
uniquely has Cs symmetry. Band B has been assigned as the
origin band of the T1T2...Tn−2 conformer with all the bands
C being assigned as the C-O stretch vibrational fundamental
of the same conformer. Similarly, all A bands are assigned to
the G1T2...Tn−2 conformer, which retains the Cs symmetry of
the carbon chain but has the oxygen rotated 60◦ out of the
carbon-chain plane. The structures of these conformations are
illustrated in Fig. 1.
Figure 2 shows that nearly all the strong bands in the spec-
tra of pentoxy and smaller alkoxy radicals have been assigned
to specific conformers and their vibronic structure. Figure 3
© 2004 NRC Canada

862
Can. J. Chem. Vol. 82, 2004
Fig. 5. Trace of band A of 1-hexoxy: (a) experimental spectrum, (b) simulation using calculated rotational constants of conformer
G1T2T3T4, (c) simulation including spin-rotation constants components, (d) pre-fit spectrum, and (e) spectrum simulated from fit
constants of Table 1. The ratio of the electric dipole moment a:b:c is 3:1:0.5. The rotational temperature is 1.3 K.
as a starting point for a full rotational analysis. Details of the
analysis are given below for bands A and C of 1-hexoxy. The
assignments of other bands were performed in a similar manner.
Figure 4 shows the experimental trace of band C and the
Simulations including the calculated spin-rotational compo-
nent a0 +a/2 (Fig. 4c) enabled us to assign most of the remain-
ing experimental lines. At this point, least-square fits of 111
ꢀ
ꢀ
lineswereusedtodeterminetheoriginfrequency, X andB state
ꢀ
ꢀ
ꢀ
simulation of the B – X transition using the rotational con-
stants from the ab initio calculations of conformer T1T2T3T4 of
1-hexoxy.As the T1T2T3T4 conformer has Cs symmetry, only a
c-type transition dipole is allowed, making the structure of band
C (or B) rather simple. The simulation in Fig. 4b clearly resem-
bles the experimental spectrum (trace a). The P and R branches
of the Kꢀꢀ = 0 ↔ Kꢀ = 1 transition can be easily identified and
assigned although there are clearly small discrepancies between
the experimental and simulꢀated structure. The positions of the
Q branch for Kꢀꢀ = 0 ↔ K = 1 as well as Kꢀꢀ = 1 ↔ Kꢀ = 0
and Kꢀꢀ = 1 ↔ Kꢀ = 2 can easily be located, but the latter two
appear as doublets experimentally. At this stage, combinations
of rotational constants, (B + C)/2 and A – (B + C)/2 of the
rotational constants, and the two most significant X state spin-
rotation tensor components (ꢀꢀaa,ꢀꢀbb). The simulation based
upon this fit is given in Fig. 4d and the corresponding molec-
ular parameters are listed in Table 1. Good agreement is seen
between the experimental results and values from the ab initio
calculations. For the rotational constants, the values are within
5% for both ground and excited states. For the spin-rotation
constants, the values were within 7% for the larger compo-
ꢀꢀ
nent, ꢀꢀ . Several of the spin-rotation constants could not be
aa
determined experimentally because the effect they had on the
spectrum was below the experimental resolution (≈200 MHz).
In this case, those spin-rotation constants were fixed at zero in
the fitting.
ꢀ
ground state and (B +C)/2 of the excited B state, can be fitted.
Band B was assigned according to the same procedure as
described for band C. Its analysis was made after the assignment
of band C because there was another band partially overlapping
it (Fig. 3b). The overlap will be discussed later in this paper.
In spite of the overlap, unambiguous assignments can still be
made and the resulting parameters are listed in Table 1.
As noted in the beginning of this section, the likely assign-
ment for band A is to conformer G1T2T3T4. The structure of
bandA appears more complicated than that of bands B and C be-
cause conformer G1T2T3T4 only has C1 symmetry rather than
The splitting between the doublets of the Q branches is
about 2.2 GHz. It is close to the calculated splitting of
2.6 GHz [3/2(a0 + a/2)] for transitions involving |Kꢀꢀ| = 1
and Nꢀꢀ = 1, due to the spin-rotation interaction in the ground
ꢀꢀ
ꢀ
Xꢀ state. Smaller splittings are ꢀobserved in the P(K = 0 ↔
K = 1) and R(Kꢀꢀ = 0 ↔ K = 1) branches (but not com-
pletely resolved); these observations are consistent with the cal-
culated value of 0.06 GHz for transitions involving Kꢀꢀ = 0,
which are split by the term (Nꢀꢀ + 1/2)(a0 − a).
© 2004 NRC Canada

Zu et al.
863
Fig. 6. Survey LIF spectra of 1-hexoxy and 1-heptoxy. Expanded views of the regions of bands B and C show additional lines in the
1-hexoxy spectrum, but not in the 1-heptoxy spectrum.
Cs symmetry as for the T1T2T3T4 conformer. There are now
three nonzero components of the transition dipole rather than
one. The increase in allowed transitions leads to increased spec-
tral congestion, making the spectral analyses somewhat more
difficult.
sulting simulation is shown in Fig. 5e, and the corresponding
molecular constants are listed in Table 1.
For 1-heptoxy, bandA is assigned to conformer G1T2T3T4T5
despite the increasing congestion in the rotationally resolved
spectrum. For bands B and C, overlap with an additional band
makes the structure even more complicated. However, a reason-
able fit can still be made for the T1T2T3T4T5 conformer portion
of the spectrum using the same approach as for the 1-hexoxy
analysis. The results are given in Table 2.
However, the analysis of bands B and C and other alkoxy
bands in our previous work (18,19) indicated that ab initio pre-
dictions of the molecular constants should be quite reliable. In-
deed, the simulation using the calculated molecular constants of
conformer G1T2T3T4 shows rather good agreement with the ex-
perimental spectrum of bandA (Fig. 5b). Nonetheless, the spec-
tral congestion makes it difficult to distinguish the rotational
and spin-rotation splittings in the center of the band. The spin-
rotation splitting is more easily discerned at the ends of the spec-
trum where higher Nꢀꢀ, |Kꢀꢀ| > 0 transitions lie. By including
Bandsfromadditionalconformerswereidentifiedin1-butoxy
and 1-pentoxy. The bands identified included the origin of the
T1G2 conformer of 1-butoxy, the origin of the T1T2G3 and
T1G2T3 conformers of 1-pentoxy, and the C-O stretch bands
of the T1T2G3 and T1T2T3 conformers of the 1-pentoxy spec-
trum. In addition, low-frequency fundamentals and combina-
tion bands of these conformers appeared in the spectra. The
additonal conformers observed in 1-butoxy and 1-pentoxy all
have a nearly straight chain structure obtained by rotating one
C-C-C dihedral angle of the Cs (all-trans) conformer. One ex-
pects that bands of this nature will shift closer to each other in
1-hexoxy as the difference between these conformers becomes
less significant with the increasing length of the carbon chain.
The expanded region in Fig. 6 of the 1-hexoxy LIF spectrum
shows one extra band (Ba) close to band B and two extra bands
(Ca and Cb) close to band C. From 1-heptoxy to 1-decoxy,
there is no clear separation between these bands, implying they
might completely overlap. An expanded view of the 1-heptoxy
the calculated value of the spin-rotation constantsꢀꢀ andꢀꢀbꢀꢀb in
ꢀꢀ
aa
the simulation, all the doublets at the ends of the experimental
spectrum are accounted for in the simulation (Fig. 5c)ꢀ. Then
assignments can be made for branches Q(Kꢀꢀ = 1 ↔ K = 2),
R(Kꢀꢀ = 2 ↔ Kꢀ = 3), and Q(Kꢀꢀ = 2 ↔ Kꢀ = 1). Using
these assignments, an initial fit can be made to obtain experi-
mental rotational constants and spin-rotation components, and
the resulting simulation is shown in Fig. 5d. At this stage, more
lines in Q(Kꢀꢀ = 2 ↔ Kꢀ = 3), R(Kꢀꢀ = 1 ↔ Kꢀ = 2),
P(Kꢀꢀ = 1 ↔ Kꢀ = 0), and R(Kꢀꢀ = 0 ↔ Kꢀ = 1) branches
can be assigned. The final fit was then performed with all the
off-diagonal spin-rotation components fixed at zero. The re-
© 2004 NRC Canada

864
Can. J. Chem. Vol. 82, 2004
Fig. 7. Simulations using the calculated constants of the T1T2T3G4, T1T2G3T4, and T1G2T3T4 conformers of 1-hexoxy, as well as
rotationally resolved experimental spectra of additional bands of 1-hexoxy close to bands B and C.
Fig. 8. Comparison of experimental rotationally resolved spectra of band Cb (top) and Ba (bottom) with their respective simulations
using the constants from Table 1.
© 2004 NRC Canada

Zu et al.
865
LIF spectrum in the band B and C region only shows a single
band, but the rotationally resolved spectra of each band displays
overlap with an additional transition.
Our earlier work on smaller alkoxies reached the conclu-
sion (18,19) that multiple conformers exist in the cold-jet envi-
ronment, i.e., the sample is not at thermal equilibrium with the
rotational temperature — indeed the conformers do not appear
to form an equilibrium distribution for any temperature. How-
ever, the conformers are not all populated. The bands assigned
arise from all-trans conformers or conformers with at most one
gauche linkage. Three bands, the T1T2...Tn−2 origin and CO
stretch and the G1T2...Tn−2 origin, form the main “structure”
of the LIF spectra of large primary alkoxy radicals. The ori-
gin, and occasionally the CO stretch fundamental bands from
other nearly straight chain conformers, “decorate” the spectra
and coalesce into the three dominant bands as the number of
carbons in the radicals increases.
High-resolution spectra of bands Ba, Ca, and Cb were ob-
tained and are shown in Fig. 7. Of the 41 conformers of
1-hexoxy, there are three conformers derived from the all-trans
T conformer by changing only one C-C-C dihedral angle, i.e.,
T1T2T3G4, T1T2G3T4, and T1G2T3T4. Simulations using the
computed rotational constants (Fig. 7) indicated that the
T1T2T3G4 conformer yields the closest match for band Cb,
and the T1T2G3T4 conformer for Ba. Results of fits to these
are given in Fig. 8 and Table 1. There is no obvious conformer
assignment for band Ca.
Band Ba is assigned as the origin band of conformer
T1T2G3T4, as no band with comparable intensity was assigned
below this frequency. A definite vibrational assignment cannot
be made for band Cb, although it appears in the C-O stretch
frequency region. A band partially overlapped with band B
appearing in the rotationally resolved spectrum might be the
buried origin of the T1T2T3G4 conformer, but an unambiguous
assignment cannot be made due to the low signal-to-noise ratio
and the congestion.
With these results in hand, we can return to the spectra of
1-octoxy, 1-nonoxy, and 1-decoxy in Fig. 2. As with 1-hexoxy,
1-heptoxy, and the smaller alkoxies, it is easy to identify bands
A, B, and C. We have not attempted to obtain rotationally re-
solved spectra of these bands to obtain a definitive conforma-
tional assignment. Nonetheless, we feel that the close analogy
between the 1-octoxy to 1-decoxy spectra in Fig. 2 and the
spectra of the smaller alkoxies leaves little doubt as to the con-
formational assignments. It appears highly probable that band
A is the origin of the G1T2...Tn−2 conformer, while bands B
and C are, respectively, the origin and CO stretch fundamental
of conformer T1T2...Tn−2. One would also expect this pattern
to continue in even larger alkoxy radicals.
Acknowledgements
The authors wish to acknowledge György Tarczay for help-
ful discussions and the Chemical Sciences, Geosciences and
Biosciences Division, Office of Basic Energy Sciences, Office
of Science, US Department of Energy, via Grant DE-FG02-
01ER15172.
References
1. G. Herzberg. The spectra and structures of simple free radicals:
An introduction to molecular spectroscopy. Cornell University
Press, Ithaca, N.Y. 1971.
2. G. Herzberg and J.W.C. Johns. Astrophys. J. 158, 399 (1969).
3. G. Herzberg and J. Shoosmith. Can. J. Phys. 34, 523 (1956).
4. G. Herzberg. Proc. R. Soc. A, 262, 291 (1961).
5. G. Herzberg. Can. J. Phys. 39, 1511 (1961).
6. G. Herzberg and J.W.C. Johns. Proc. R. Soc. A, 295, 107 (1966).
7. W.P.L. Carter and R. Atkinson. J. Atmos. Chem. 3, 377 (1985).
8. T.M. Ramond, G.E. Davico, R.L. Schwartz, andW.C. Lineberger.
J. Chem. Phys. 112, 1158 (2000).
5. Conclusions
9. J. Bai, H. Okabe, and J.B. Halpern. Chem. Phys. Lett. 149, 37
(1988).
10. K. Ohbayashi, H.Akimoto, and I. Tanaka. J. Phys. Chem. 81, 798
(1977).
11. H.E. Radford and D.K. Russell. J. Chem. Phys. 66, 2222 (1977).
12. Y. Endo, S. Saito, and E. Hirota. J. Chem. Phys. 81, 122 (1984).
13. S.C. Foster, Y.-C. Hsu, C.P. Damo, X. Liu, C.-Y. Kung, and T.A.
Miller. J. Phys. Chem. 90, 6766 (1986).
14. X.Q. Tan, J.M. Williamson, S.C. Foster, and T.A. Miller. J. Phys.
Chem. 97, 9311 (1993).
15. C. Wang, L.G. Shemesh, W. Deng, M.D. Lilien, and T.S. Dibble.
J. Phys. Chem. A, 103, 8207 (1999).
16. C. Wang, W. Deng, L. Shemesh, M.D. Lilien, D.R. Katz, and T.S.
Dibble. J. Phys. Chem. A, 104 , 10 368 (2000).
17. C.C. Carter, S. Gopalakrishnan, J.Atwell, andT.A. Miller. J. Phys.
Chem. A, 105, 2925 (2001).
18. S. Gopalakrishnan, C.C. Carter, L. Zu, V. Stakhursky, G. Tarczay,
and T.A. Miller. J. Chem. Phys. 118, 4954 (2003).
19. S. Gopalakrishnan, L. Zu, and T.A. Miller. J. Phys. Chem.A, 107,
5189 (2003).
Rotationally resolved spectra of six bands of 1-hexoxy and
three bands of 1-heptoxy were obtained. A new strategy was
used for the analysis of the 1-hexoxy and 1-heptoxy spectra
and is applicable for larger primary alkoxy radicals. Using this
approach, five bands of 1-hexoxy were assigned to four differ-
ent conformers, i.e., band A to G1T2T3T4, bands B and C to
T1T2T3T4, band Ba to T1T2G3T4, and band Cb to T1T2T3G4.
For 1-heptoxy, bandA was assigned to conformer G1T2T3T4T5,
while bands B and C were assigned to conformer T1T2T3T4T5.
Experimental molecular parameters were obtained from the ro-
tational analyses, which are in rather good agreement with the
values predicted for the given conformers by quantum chem-
istry calculations.
In earlier work (29, 30), probing a jet-cooled sample via
microwave spectroscopy, it was suggested that little confor-
mation relaxation takes place from the room-temperature con-
former distribution. This work includes studies on 1-pentene
and 1-hexene, which of course bear similarities to the presently
studied alkoxies. However, with the alkoxy radicals there is an
important difference in that they are created by photolysis in
the jet.
20. J. Goodman and W.C. Still. J. Comput. Chem. 12, 1110 (1991).
© 2004 NRC Canada

866
Can. J. Chem. Vol. 82, 2004
21. F. Mohamadi, N.G.J. Richards, W.C. Guida, R. Liskamp, M.
Lipton, C. Caufield, G. Chang, T. Hendrickson, and W.C. Still.
MacroModel — an Integrated Software System for Modeling Or-
ganic and Biorganic Molecules Using Molecular Mechanics. J.
Comput. Chem. 11, 440 (1990).
22. A.H. Blatt. Organic synthesis, collective volume. Wiley, New
York. 1943.
23. X. Liu, S.C. Foster, J.M. Williamson, L.Yu, and T.A. Miller. Mol.
Phys. 69, 357 (1990).
24. W.T. Raynes. J. Chem. Phys. 41, 3020 (1964).
25. V.L. Stakhursky and T.A. Miller. 56th OSU International
Symposium on Molecular Spectroscopy (Ohio State Uni-
versity, Columbus, Ohio, 2001). Program available from
http://molspect.mps.ohio-state.edu/goes/specview.html.
26. J.M. Brown, T.J. Sears, and J.K.G. Watson. Mol. Phys. 41, 173
(1980).
Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q.
Cui, K. Morokuma, D.K. Malick,A.D. Rabuck, K. Raghavachari,
J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Ste-
fanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gom-
perts, R.L. Martin, D.J. Fox, T. Keith, M.A.Al-Laham, C.Y. Peng,
A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.
Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M.
Head-Gordon, E.S. Replogle, and J.A. Pople. Gaussian 98. Re-
vision A.7 [computer program]. Gaussian, Inc., Pittsburgh, Pa.
1998.
28. G. Tarczay, S. Gopalakrishnan, andT.A. Miller. J. Mol. Spectrosc.
220, 276 (2003).
29. G. Fraser, R. Suenram, and C. Lugez. J. Phys. Chem. A, 104,
1141 (2000).
27. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria,
M.A. Robb, J.R. Cheeseman,V.G. Zakrzewski, J.A. Montgomery,
R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D.
30. G. Fraser, L. Xu, and R. Suenram. J. Chem. Phys. 112, 6209
(2000).
© 2004 NRC Canada