Infrared and Raman spectra, conformations, ab initio calculations and spectral assignments of 1-fluoro-1-silacyclohexane

Article abstract of DOI:10.1016/j.molstruc.2012.02.016

Raman spectra of 1-fluoro-1-silacyclohexane as a liquid were recorded at 293 K and polarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143 K, and intensity changes of certain bands with temperature were investigated. An apparently plastic phase was observed around 170 K, but no definite crystallization was ever obtained on cooling. The infrared spectra have been studied of the vapor, of an amorphous solid at 78 K and of the liquid in the range 600-100 cm^-1. No infrared bands present in the vapor or liquid vanished upon cooling. The compound exists a priori in two conformers, equatorial (e) and axial (a), and the experimental results suggest an equilibrium in which the a-conformer has 1.2 kJ mol^-1 lower enthalpy than the e-conformer in the liquid, leading to 60% a-conformer at ambient temperature. B3LYP calculations with various basis sets and the G3 model chemistry gave conformational enthalpy difference ΔH(e - a) in the range 0.6 and 1.8 kJ mol^-1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The wavenumbers of the vibrational modes were derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations. An average relative deviation of ca. 1% between the observed and calculated wavenumbers for the 48 modes of the e and a conformers was found.

Full text of DOI:10.1016/j.molstruc.2012.02.016

Journal of Molecular Structure 1015 (2012) 120–128
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Infrared and Raman spectra, conformations, ab initio calculations and spectral
assignments of 1-fluoro-1-silacyclohexane
Peter Klaeboe a, , Valdemaras Aleksa a,b, Claus J. Nielsen a, Anne Horn a, Gamil A. Guirgis c,
⇑
Michael D. Johnston c
a CTCC, Department of Chemistry, University of Oslo, P.O. Box 1033, NO-0315 Oslo, Norway
b Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania
c Department of Chemistry & Biochemistry, College of Charleston, Charleston, SC 29403, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Raman spectra of 1-fluoro-1-silacyclohexane as a liquid were recorded at 293 K and polarization data
obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143 K,
and intensity changes of certain bands with temperature were investigated. An apparently plastic phase
was observed around 170 K, but no definite crystallization was ever obtained on cooling. The infrared
spectra have been studied of the vapor, of an amorphous solid at 78 K and of the liquid in the range
600–100 cmꢀ1. No infrared bands present in the vapor or liquid vanished upon cooling.
The compound exists a priori in two conformers, equatorial (e) and axial (a), and the experimental
results suggest an equilibrium in which the a-conformer has 1.2 kJ molꢀ1 lower enthalpy than the
e-conformer in the liquid, leading to 60% a-conformer at ambient temperature.
Received 2 December 2011
Received in revised form 7 February 2012
Accepted 7 February 2012
Available online 17 February 2012
Keywords:
Infrared and Raman spectra
Conformations
DFT calculations
1-Fluoro-1-silacyclohexane
B3LYP calculations with various basis sets and the G3 model chemistry gave conformational enthalpy
difference
D
H(e ꢀ a) in the range 0.6 and 1.8 kJ molꢀ1. Infrared and Raman intensities, polarization ratios
and vibrational frequencies for the e and a conformers were calculated. The wavenumbers of the vibra-
tional modes were derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations. An average
relative deviation of ca. 1% between the observed and calculated wavenumbers for the 48 modes of the e
and a conformers was found.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
increasing the energy of this conformer, although additional effects
might be involved.
The stereochemistry of cyclohexane has been studied for more
than 70 years, and substituted cyclohexanes were among the first
examples of molecules exhibiting conformational equilibria. Thus,
it has been established from numerous studies that the low energy
form for the cyclohexane ring is the chair, while the craddle and
twisted forms have much higher energies and are not observed
under ordinary conditions. Moreover, in monosubstituted
cyclohexanes the e-conformer has generally lower energy than
the a-conformer, leading to a larger abundance of the e-conformer
in the vapor and in the liquid states. In the crystalline state, how-
ever, obtained by cooling or by high pressure, the monosubstituted
cyclohexanes generally exist only in the e-conformation since the
a-conformer is not accommodated in the crystal lattice. Tradition-
ally, the preference for the e-conformer has been explained as a
1,3-diaxial repulsion between the parallel C–X and C–H bonds
Much less information is available on the steric effect when a
carbon atom in cyclohexane is substituted with a neighboring
atom in the periodic system – silicon. Silicon differs from carbon
in various respects, it has lower electronegativity and larger cova-
lent radius than carbon, leading to increased conformational flexi-
bility. It was suggested [1] from force field calculations that
silacyclohexane and various substituted silacyclohexanes exist in
a modified chair form in which the ring is more flattened than
cyclohexane in the region of silicon and is more puckered in the
opposite part of the ring. The parent molecule, silacyclohexane
and a few molecules with a substituted atom on the silicon have
been investigated. The preparation of silacyclohexanes and their
derivatives is difficult, and these compounds are much more unsta-
ble than the corresponding cyclohexanes, and therefore few exper-
imental studies have been made.
Many quantum chemical calculations of silacyclohexane [1–4]
revealed the chair form to be the dominant conformer. 1-Methyl-
1-silacyclohexane was studied by gaseous electron diffraction
and by NMR spectroscopy, giving the e-conformer to be the more
stable form [4–6]. Quantum chemical calculations indicated [7–9]
⇑
Corresponding author. Tel.: +47 22855678; fax: +47 22855441.
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2012.02.016

P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
121
that most of the substituted silacyclohexanes have the a-form as
the preferred conformer (Fig. 1).
resolution. Depolarization measurements were carried out in the
90° illumination mode, employing a polarizer and a scrambler
between the sample and the monochromator. The polarizer unit
was calibrated with carbon tetrachloride filled into a cylindrical
tube identical to the one used for FSC. Except for very weak and/
or overlapping bands, fairly comprehensive depolarization data
were obtained.
Raman spectra of the liquid, including depolarization measure-
ments were recorded at room temperature. Additional spectra
were obtained for every 10° between 293 and 143 K in a capillary
tube of 2 mm inner diameter. The tube was surrounded by a De-
war, cooled by gaseous nitrogen evaporated from a liquid reservoir
[12]. From these spectra, the enthalpy difference DconfH between
the conformers were calculated. Various (a)/(e) band pairs were at-
tempted for determining the enthalpy difference. The vapor of FSC
was condensed on a copper finger at 78 K. An amorphous phase
was formed, and the bands were quite similar to those of the li-
quid. Many annealing experiments were tried, but the sample
never crystallized in any of the temperature ranges employed.
In the present study, we present an infrared and Raman spec-
troscopic investigation of 1-fluoro-1-silacyclohexane (later to be
abbreviated FSC). Special emphasis is put on the conformational
equilibrium between the e and a conformers, and a comparison
with the results of fluorocyclohexane. The stereochemistry of FSC
has previously been investigated by microwave spectroscopy
[10], and by gaseous electron diffraction, low temperature NMR
and Raman spectroscopy, and by quantum chemical calculations
[11].
2. Experimental
2.1. Preparation
First, a sample of 1-chloro-1-silacyclohexane was prepared by
the formation of a double Grignard of 1,4-dibromopentane in dry
ethyl ether. The Grignard was then coupled with freshly distilled
trichlorosilane in dry ethyl ether overnight with refluxing. The
magnesium salt was separated from the ethereal solution by filtra-
tion under dry nitrogen. The ether was distilled off at reduced pres-
sure and the final purification was obtained by a trap to trap
distillation. The sample of 1-fluoro-1-silacyclohexane was pre-
pared from the chloro derivative using freshly sublimed antimony
trifluoride. The sample purity was checked by infrared spectros-
copy giving distinct bands for the Si–H and Si–F stretches. 13C
NMR spectra in CDCl3 solution gave three signals for the carbons
with the positions at 13.333 ppm, 23.469 ppm and 29.395 ppm.
2.3. Infrared measurements
The middle infrared spectra (MIR) were recorded on two Fourier
transform spectrometers: a Bruker spectrometer, Vertex 80 and a
Perkin–Elmer model 2000 (4000–400 cmꢀ1). Additional far infra-
red (FIR) spectra were obtained using the Vertex 80 spectrometer
(600–100 cmꢀ1) in the FIR set-up. Both spectrometers had DTGS
detectors. Beamsplitters of Ge coating on KBr substrate were em-
ployed in the MIR region, and a multilayer coating on Mylar was
used in the FIR region.
The vapor was studied in cells with CsI windows (10 cm path) in
the MIR region and polyethylene widows (18 cm path) in FIR. The
spectrometer was flushed with dry nitrogen gas to diminish the
absorption of water vapor in the FIR region. FIR spectra were re-
corded with the sample confined in a liquid cell of polyethylene
(throw away cells) of ca. 0.2 mm path. The vapor was deposited
on a CsI window in a glass cryostat, cooled with liquid nitrogen,
and an amorphous solid was observed on the window. The sample
was annealed to various temperatures between 120 and 150 K and
recooled to 78 K before recording of the spectra. When heated
above 180 K the sample melted.
2.2. Raman measurements
The Raman spectra were recorded with a multichannel spec-
trometer from Horiba (Jobin Yvon) model T 64000 employing both
single and triple monochromator detection. The spectrometer was
fitted with a CCD detector, cooled to ca. 130 K. The spectra were
excited by a Milennia Pro diode-pumped (Nd:YVO4 crystal) laser
from Spectra-Physics (Model J 40), using 90° scattering geometry,
adjusted to give approximately 50 mW of the 532 nm line on the
sample. The spectrometer was applied with: (1) a single mono-
chromator using a notch filter, or a triple monochromator with
(2) additive or (3) subtractive collection. A higher signal to noise
ratio was achieved with the single monochromator, the triple addi-
tive collection gave slightly higher resolution, while the triple sub-
tractive set-up allowed the spectra to be recorded at wavenumbers
closer to the exciting line. In the liquid sample the low wavenum-
ber modes were recorded to 60 cmꢀ1 with the triple subtractive
mode, compared to 120 cmꢀ1 with the single monochromator
and the notch filter.
3. Results
3.1. Raman spectral results
Raman spectra of FSC as a liquid at ambient temperature are
presented in Figs. 2 and 3, respectively, whereas depolarization
spectra are given in Fig. 4. A Raman spectrum of the amorphous
sample deposited on a Cu finger at 78 K is presented in Fig. 5, quite
similar to that recorded of the liquid at ambient temperature. The
The high sensitivity of the CCD detector gave spectra with large
signal/noise ratio, and the spectra were recorded at ca. 2 cmꢀ1
Fig. 1. The equatorial (e) and axial (a) conformers of 1-fluoro-1-silacyclohexane (FSC), including numbering of atoms for the definition of symmetry coordinates.

122
P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
1400
1200
1000
800
600
400
200
3000 2700 2400 2100 1800 1500 1200 900
Wavenumber / cm-1
600
Wavenumber / cm-1
Fig. 5. Raman spectra of FSC in the range 1550–80 cmꢀ1, recorded of a sample
deposited on a cold finger at 78 K.
Fig. 2. Raman spectra of FSC as a liquid at 293 K in the range 3200–600 cmꢀ1 in a
capillary with 2 mm inner diameter.
cooling to 78 K, but negligible spectral changes were observed.
The experimental data for the Raman and infrared spectra are col-
lected in Table 1.
Raman spectra of the liquid were recorded at 15 temperatures
(every 10°) in two series of experiments between 293 and 143 K.
Minor intensity variations with temperature of some Raman bands
were observed relative to neighboring bands, interpreted as a dis-
placement of the conformational equilibrium. The theoretical cal-
culations give an enthalpy difference between the conformers
amounting to 0.6–1.8 kJ molꢀ1 (see below), with a being the more
stable conformer. The Raman bands which are enhanced at lower
temperatures belong to the a, and those which diminish upon cool-
ing should belong to the e-conformer. However, most of the Raman
(and infrared) bands observed do not belong to a single conformer,
but to both the e and a-conformers Thus, only a few Raman bands
are suitable for calculations of the enthalpy differences.
600
500
400
300
200
100
Wavenumber / cm-1
The intensities of each band pair were fitted to the van’t Hoff
equation: ln{Ia(T)/Ie(T)} = ꢀDconf /RT + constant; where Ia/Ie is the
ratio in peak heights or integrated areas, and it is assumed that
Fig. 3. Raman spectra of FSC as a liquid at 293 K in the range 700–100 cmꢀ1 in a
capillary with 2 mm inner diameter.
ꢀDconfH is constant with temperature. Both peak heights and inte-
grated band areas were employed for determining band intensities.
The following two pairs of bands were employed in the van’t Hoff
plots: 630/659 and 395/311 cmꢀ1 (liquid) employing peak heights
(Fig. 6), giving the values 0.94 and 1.62 kJ molꢀ1, respectively,
(293–143 K). The band pair 630/659 cmꢀ1 was also treated in inte-
grated areas, giving the value 0.98 kJ molꢀ1. An average value of
1.2 kJ molꢀ1 was obtained for the DconfH, in reasonable agreement
with our quantum chemical results 0.7–1.8 kJ molꢀ1. Earlier deter-
minations of DconfH from variable temperature Raman spectros-
copy [11], all based upon the 630/659 cmꢀ1 band pair, gave the
values: 1.04, 0.92 and 1.17 kJ molꢀ1 in neat liquid, pentane and
dichloromethane, respectively.
3.2. Infrared spectral results
A MIR spectrum of FSC is presented in Fig. 7. Some rotational
vapor contours with resolved P, Q and R peaks were observed in
the spectra both for the assumed e and a conformers, but in other
cases apparent fine structure is attributed to close-lying conformer
peaks. A FIR spectrum of the sample is given in Fig. 8 while MIR
spectra of FSC deposited on a CsI window at 78 K are shown in
Fig. 9. A complete list of the infrared and Raman bands for FSC
including assignments are listed in Table 1, based upon the results
of the calculations and on experimental Raman depolarization data
and infrared vapor contours.
3000
2500
2000
1500
1000
500
Wavenumber / cm-1
Fig. 4. Raman spectra of FSC in the range 3200–200 cmꢀ1 recorded at 293 K, giving
the parallel and perpendicular polarization directions.
sample was subsequently annealed to 90, 110, 130, 150 and 180 K
and recooled to 78 K before recording. Visual observation sug-
gested that the sample formed a solid at 170 K, maintained after

P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
123
Table 1
Infrared and Raman spectral dataa for 1-fluoro-1-silacyclohexane, C5H11SiHF.
Infrared
Raman
Assignments
Equatorial
Vapor
Liquid
298 Kb
Solid
78 Kb
Liquid
298 Kb
Solid
78 Kb
Axial
298 Kb
2944 s, Qc
2940 s, P
2938 vs
2934 vs
2931 m
2927 vs
2924 s
2921 s
2939 vvs
m
1A0
2937 vvs, P
m
m
m
2A0
00
2931 s, sh
2929 s
2927 vs
2923 s, sh
2921 vs
2890 w, br
2871 vw
2857 vs
A
28
3A0
2932 sh
2925 vvs, D
2922 vvs
m
m
m
m
m
m
1A0
2A0
m
m
m
4A0
5A0
6A0m
00
00
2919 m, sh
2889 vs P
m
30
A
A
3A0m28A0
29
00
00
00
2890 w
2871 m, sh
2865 m
2890 vs
2872 vvw
2853 vs
4A0m
5A0m
6A0m
A
29
A
30
A
31
00
2859 vvs, P
2162 vsP
m
31
A
2175 s, R
2170 vvs, Q
2165 s, P
2162 vs R
2158 vs Q
2150 m, P
1498 vw
1468 w
1461 w
1456 w
1454 w, R
1450 s, Q
1446 w, P
1418 w
1409 s
1354 w
1348 w
1341 vw
1299 w
1288 w
1277 w
1269 w
2166 s
2163 s, br
m
7A0
m
7A0
00
1494 vw
1464 vw
1462 vw
1455 vw
1496 vw
m A
8A0m
32
m
m
m
8A0
00
1462 w, P
1458 w
1447 s
m
9A0
A
32
9A0
00
1446 m
1451 m, D
m
m
A
m
10A0
33
10A0
00
1406 m, D
1351 vw
1344 m
1334 w, D
1305 vw,
1293 m, P
1271 w, D
1267 w, br
1254 m
m
m
m
m
A
33
11A0
m
m
m
m
11A0
00
00
00
00
1347 w, D
1341 w
1306 vw
1289 m
A
A
A
A
34
34
35
12A0
35
1303 vw
1291 m
1275 m
1269 w
1251 m
m
m
12A0
00
A
36
00
00
m
m
A
A
36
00
1249 m
1195 m
m
m
A
37
37
1194 m, R
1192 s, Q
1189 vs P
1186 m, R
1184 vs Q
1179 s, P
1105 vw
1098 vw
1197 s
1198 m, P
1184 m, D
m
13A0
13A0
1182 vs
1180 m
m
m
m
m
m
m
14A0
m
m
m
m
m
m
m
m
14A0
00
00
1106 w
1099 m
1074 w
1106 w, D
1097 vw
1077 w, D
1103 m
1094 w, sh
1074 m
A
A
38
38
00
00
A
A
39
39
1048 w, R
1042, w, Q
1039 w, P
1009 s
1001 vs, R
996 vs Q
991 s, P
920 s
00
00
1032 vw
1008 m
996 s
1038 w, br
1006 m, P
A
A
40
40
1004 m
15A0
16A0
15A0
16A0
995 w, sh, br
997 m, sh
914 w
909 m, D
m
m
17A0
17A0
00
916 vs
910 s
912 vs
891 m, sh
887 m
905 m
A
41
00
A
41
00
885 s, sh
881 vs R
878 vvs, Q
873 vs P
869 m
887 w, D
875 m
885 w, sh
m
42
A
877 s
m
18A0
00
866 m
863 w
873 w, D
863 w
m
42
A
845 s
855 vs
m
18A0
840 vs R
835 vs Q
808 vw
796 vw
780 m R
776 s, Q
771 m, P
742 w
721 w
670 vw
659 w
628 vw
615 m, Q
00
00
833 vvs
814 s
794 m
835 vw, sh,
818 m, P
796 s, P
830 w, br
811 m
793 s
m
m
m
A
m
m
A
43
43
19A0
20A0
19A0
771 s
768 vw
m
21A0
m
m
m
20A0
743 m
719 m
741 m, P
720 w, D
671 vw, sh
659 vs P
630 vs P
611 vw, sh
741 m
717 w
671 w, sh
660 s
630 vvs
21A0
00
m
44
A
00
A
44
660 m
631 w
614 m
m
m
m
22A0
22A0
45A0
m
45A0
(continued on next page)

124
P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
Table 1 (continued)
Infrared
Raman
Assignments
Equatorial
Vapor
Liquid
298 Kb
Solid
78 Kb
Liquid
298 Kb
Solid
78 Kb
Axial
298 Kb
610 m, P
511 w, R
506 s, Q
499 w,P
480 vw
396 m
508 s
506 s
506 w, br, P
m
23A0
482 m
395 s
482 w
397 s
385 m
365 m
482 m, P
395 m, P
385 vw, sh
366 w, D
346 m, P
311 m, P
279 w, D
262 w, P
233 w, br,D
219 w, P
166 vw
477 w
394 m
m
23A0
m
24A0
389 w
391 m
363 s, br
347 w
313 w
280 w
263 w
235 m
220 vw
Comb
00
00
A
46
364 w
344 w
311 w
282 w
260 vw
233 vw
217 w
166 vw
158 vw
141 vw
106 w ?
m
m
m
A
m
m
46
353 w
312 vvw
281 w
260 w
235 w
24A0
25A0
25A0
314 vw
283 w
262 vw
237 vw
217 vw
170 w
00
A
47
m
m
m
26A0
00
A
47
m
26A0
171 vw
Comb
00
155 m
155 w
137 w
102 w
98 w
m A
48
00
A
48
140 vw
105 vw
101 w
m
104 w, sh
102 w
m
27A0
m
27A0
a
Wavenumbers.
Recording temperature.
b
c
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; bd, broad; P, polarized; D, depolarized in the Raman spectra; P, Q and R signify rotational band
contours in the infrared vapor spectra.
0.8
1.0
0.5
0.0
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
3000
2500
1200
900
600
Waavenumber / cm-1
0.003
0.0035
0.004
0.0045
0.005
0.0055
1/T
Fig. 7. Mid infrared spectra (MIR) of FSC as a vapor at room temperature in the
range 3200–2000 and 1350–480 cmꢀ1 in a 10 cm cell with KBr windows, pressures
Fig. 6. Van’t Hoff plots of the axial/equatorial band pairs 630/659 (open circles) and
395/311 (black squares) cmꢀ1 of FSC in the temperature range 293–143 K, employ-
ing peak heights as measures of the scattering intensities.
20 (solid curve) and 4 Torr (broken curve), resolution 0.5 cmꢀ1
.
(6-31G(d)), 0.9 (6-31++G(d)), 1.0 (6-311++G(d)) and ꢀ0.4 kJ molꢀ1
(6-311G(2df,2pd)) [9], suggesting an average [7,9] value of Dconf
3.3. Quantum chemical calculations
H(e ꢀ a)=0.85 kJ molꢀ1
.
MP2 and DFT calculations were performed using Gaussian 09
[13]. The minima on the potential surface were found by relaxing
the geometry. The Cartesian coordinates and the structural param-
eters for the e- and a-conformers from B3LYP/cc-pVTZ calculations
are given in Tables S1 and S2, respectively (Supporting informa-
tion). The conformational energy difference, DconfH(e ꢀ a), obtained
from the MP2/6-31G calculations was 0.7, while B3LYP calcula-
tions gave 1.8, 0.7 and 0.6 kJ molꢀ1 employing the cc-pVDZ, cc-
pVTZ and cc-pVQZ basis sets, respectively. A value of 1.0 kJ molꢀ1
was obtained in our G3 calculations [14], giving an average value
of DconfH(e ꢀ a)=0.96 kJ molꢀ1. Earlier density functional calcula-
tions DconfH equal to: 0.5 (B3LYP), 1.1 (B3LYP-D), 0.6 (B2PLYP),
1.0 (B2PLYP-D) and 0.7 kJ molꢀ1 (MO6-2X) were reported with
widely different basis sets [7]. MP2 calculations gave: 0.6
The vibrational wavenumbers of the e- and a-conformers of FSC,
resulting from three B3LYP/cc-pVXZ (X = D, T, Q) calculations are
compared in Tables S3 and S4, respectively. Only minor differences
were obtained between the results with triple and quadruple zeta
basis sets, suggesting that the B3LYP electron density is described
with sufficient accuracy employing the triple zeta basis set. Pro-
vided an adequate basis set is used in calculations of the normal
modes of vibration, the calculated wavenumbers are invariably
too high. A common procedure to remedy this discrepancy is to
scale the calculated wavenumbers to compensate for, among oth-
ers, mechanical anharmonicity. In the present study the cubic
and quartic force constants were obtained in the B3LYP/cc-pVTZ
calculations allowing a prediction of the anharmonic vibrational
wavenumbers directly.

P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
125
2 and 3 and the largest term in PED is also described in terms of
valence coordinates in these tables.
1.0
0.5
3.5. Conformations
It can be seen from Tables 1–3 that all the e and a-bands have
been tentatively identified. The experimental enthalpy difference
DconfH(e ꢀ a)=1.2 kJ molꢀ1 (see above) favors a, and the calculated
entropy difference was calculated to be DconfS(e ꢀ a) = 0.59
J molꢀ1 Kꢀ1
DconfG(e ꢀ a):
. The conformational equilibrium is derived from
Dconf Gðe ꢀ aÞ ¼ Dconf Hðe ꢀ aÞ ꢀ TDconf Sðe ꢀ aÞ
¼ 1:2 ꢀ 293 ꢁ 0:59 ꢁ 10ꢀ3
600
500
400
300
200
100
¼ 1:03 kJ molꢀ1; giving 60% a:
Wavenumber / cm-1
In the gaseous electron diffraction study [10,11], an equilibrium
of 63 % axial and 37% equatorial conformer was observed at 298 K.
Since the e and a conformers of FSC are expected to have nearly the
same dipole moments, the conformational equilibrium should be
nearly the same in the vapor phase and in the neat liquid and also
in solvents of different polarity previously recorded [11]. In fluoro-
cyclohexane the enthalpy difference DconfH(a ꢀ e) is reported to be
5.4 [17] and 5.7 kJ molꢀ1 [18] measured by infrared spectroscopy
in liquid krypton solution. Thus, a comparison between the mono-
substituted 1-silacyclohexanes with the corresponding cyclohex-
anes reveals the former group of molecules to have a much more
stable a-conformer than the latter.
Fig. 8. FIR spectra of FSC as a liquid in a polyethylene cell with ca. 0.2 mm path at
room temperature in the range 600–100 cmꢀ1
.
4
2
0
This trend has also been found by quantum chemical calcula-
tions [7,9]. From B3LYP/6-311+G(d,p) calculations the enthalpy dif-
ference between the twists and boat transition state to chair–chair
conversion in silacyclohexane [1–3] is given as 16.3–21.6 kJ molꢀ1
.
In cyclohexane the twist, boat and half chair/sofa conformers are
compared with the stable chair form in various MP2 and B3LYP cal-
culations [19]. In B3LYP/6-311+G(d,p) calculations the much higher
values 26.5, 30.0 and 46.1 kJ molꢀ1 in twist, boat and half chair com-
pared to the chair conformation were derived [19]. From kinetic
3000
2500
1000
500
Wavenumber / cm-1
measurements in krypton, the
DH of activation between the a and
Fig. 9. MIR spectrum of FSC as a solid on a CSI window at 78 K in a cryostat in the
range 3400–2000 and 1500–480 cmꢀ1
e-conformers in fluorocyclohexane were measured as 37.8 kJ molꢀ1
[17]. With lower barrier to e ꢀ a conversion in silacyclohexane than
in cyclohexane, the spectral shifts from equatorial to axial con-
former should be smaller in FSC, leading to more frequent coinci-
.
3.4. Normal coordinate calculations
dences of the
e and a vibrational modes in FSC than in
Harmonic force constants were obtained for each of the two
conformers of FSC in B3LYP/cc-pVTZ calculations. The harmonic,
unscaled force constants were transformed from Cartesian to sym-
metry coordinates, listed in Table S5, derived from a set of valence
coordinates to obtain an approximate description of the normal
modes. These calculations were carried out both for the e- and
the a-conformers employing the VIBROT program [15] and the re-
sults are listed in Tables 2 and 3. The calculated infrared intensities
and Raman polarization ratios, in the harmonic approximation, are
included in these tables combined with the experimental wave-
numbers, The theoretical Raman intensity, R, which mimics the
measured Raman spectrum is related to the Raman scattering
fluorocyclohexane [18,20]. As is apparent both from Tables 2 and
3, the vibrational modes derived from the anharmonic calculations
are with a few exceptions at lower wavenumbers than those from
the harmonic approximation both for the e and a-conformations.
A number of monosubstituted cyclohexanes crystallize readily
in the e-conformer at low temperature and at high pressures
[21], giving experimental support for the spectral interpretation.
An exception was fluorocyclohexane which formed a plastic phase,
containing both conformers after cooling to 78 K [20]. Only after
prolonged annealing did the compound crystallize. Bands of the
amorphous solid were observed at 90 K, bands of the plastic phase
at 190 K and an anisotropic crystal was observed at 160 K when the
a-conformer vanished. It has been observed that when a plastic
phase is formed by cooling, the conversion to an anisotropic phase
is frequently difficult to achieve [21–23]. Because of the close sim-
ilarity between FSC and fluorocyclohexane, it is possible that also
FSC forms a plastic phase.
activity: Ri = C(mL
ꢀ
m
i)4 ꢁ miꢀ1 ꢁ Biꢀ1 ꢁ Si, where mi is the frequency
and Si the corresponding Raman scattering activity of mode i, mL
is the laser excitation frequency, Bi = 1 ꢀ exp(ꢀh ꢁ mi ꢁ c/kT), and C
a constant [16]. Finally, the wavenumbers derived from the anhar-
monic calculations are included in Tables 2 and 3 to be directly
compared with the experimental values as observed in the infrared
and Raman spectra.
The PED (potential energy distribution) is expressed in terms of
the symmetry coordinates. They are constructed from a set of va-
lence coordinates, and the numbering of the atoms appears in
Fig. 1. Only PED terms larger than 10% have been included in Tables
4. Spectral interpretation
With 18 atoms, each conformer of FSC has 48 modes of vibra-
tion. Both the e-and a-conformers have a plane of symmetry (Cs

126
P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
Table 2
Observed and calculated vibrational modes of the e- conformer of 1-fluoro-1-silacyclohexane (FSC).
No. Sym. spec. Harma wave. Anharm. wave. Obs.b wave. IRc int. Rad int. Dep. ratio PEDe
Description
1
28
2
3
4
A0
A00
A0
A0
A0
A0
A00
A00
A0
A00
A0
A0
A00
A0
A0
A00
A0
A00
A00
A0
A00
A00
A0
A0
A00
A00
A00
A0
A0
A0
A00
A00
A0
A00
A0
A0
A0
A00
A0
A00
A0
A00
A0
A0
A0
A00
A00
A0
3007
3065
3066
2997
3049
3045
3040
3015
3017
3001
2204
1493
1498
1507
1457
1453
1380
1381
1369
1324
1301
1282
1227
1209
1125
1084
1061
1017
1014
930
919
900
870
843
821
796
784
709
650
622
481
374
339
309
258
227
2945
2916
2915
2914
2910
2904
2899
2891
2882
2853
2142
1544
1500
1464
1418
1414
1351
1342
1341
1298
1269
1255
1202
1177
1105
1062
1036
999
995
911
899
886
859
828
812
780
778
696
641
619
488
367
353
309
2944
2934
2938
2931
2924
2921
2921
2890
2890
2865
2158
1498
1498
1461
1418
1450
1354
1348
1341
1299
1269
1254f
1192
1184
1105
1077f
1042
1009
996
34
20
27
9
57
55
47
0
26
14
146
8
3
4
4
8
3
0
0
5
3
0
8
26
1
2
6
27
0
64
50
25
148
69
4
130
141
254
476
225
458
132
91
238
22
447
35
53
20
36
57
2
0.16
0.75
0.36
0.15
0.35
0.13
0.75
0.75
0.15
0.75
0.18
0.73
0.75
0.58
0.74
0.75
0.75
0.75
0.75
0.69
0.75
0.75
0.10
0.75
0.75
0.75
0.75
0.45
0.69
0.75
0.75
0.75
0.70
0.75
0.75
0.07
0.72
0.75
0.04
0.75
0.12
0.75
0.20
0.41
0.61
0.75
0.75
0.74
34S8 + 62S10
11S35 + 86S36
88S13
45S8 + 19S9 + 20S10 + 15S11
13S8 + 79S9
76S11
11S33 + 79S34
77S35
80S12
77S33 + 16S34
100S14
90S16
90S39
94S18
93S22
92S43
CH2 symmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
Si–H stretch
CH2 scissor
CH2 scissor
CH2 scissor
CH2 scissor
CH2 scissor
CH2 rock
CH2 twist
CH2 twist
CH2 twist
CH2 twist
CH2 twist
CH2 rock
CH2 twist
CH2 rock
C–C stretch
CH2 twist
CH2 wag
C–C stretch
CH2 wag
CH2 twist
CH2 wag
Si–F stretch
C–C stretch
FSiH bend
5
29
30
6
31
7
8
32
9
10
33
11
34
35
12
36
37
13
14
38
39
40
15
16
17
41
42
18
43
19
20
21
44
22
45
23
46
24
25
26
47
48
27
77S20
6
13S28 + 63S37 + 12S42
12S29 + 16S38 + 56S41
69S21 + 20S24
12
41
15
19
14
29
8
15
3
22
58
8
24
34
20
22
48
85
17
65
314
21
25
60
36
116
38
20
17
99
15S31 + 42S42 + 26S45
19S38 + 19S44 + 18S46 + 19S48
21S19 + 13S21 + 33S24
12S2 + 21S24 + 35S25
11S41 + 14S42 + 31S45
28S28 + 35S29 + 10S42
11S29 + 26S46 + 36S48
11S17 + 22S19 + 33S25
19S1 + 37S2 + 13S24
12S2 + 13S5 + 18S17 + 12S19
27S29 + 32S31 + 29S44
15S29 + 24S40 + 45S44
55S15 + 22S26
18S29 + 18S32 + 15S40 + 20S44
29S15 + 15S23 + 41S26
54S1 + 21S2
16S17 + 42S23 + 20S26
53S30 + 28S32
73S3
57S44 + 37S48
24S4 + 28S5 + 10S17 + 14S19
56S31 + 14S32 + 24S48
26S4 + 30S5 + 18S7 + 10S23
16S4 + 18S6 + 23S7 + 26S27
920
916
885
845
835
808
796
776
0
CC stretch
CH2 wag
C–Si antisymmetric stretch
C–Si symmetric stretch
CH2 wag
C–C–C bend
C–C–C bend
CCC bend
C–Si–F bend
47
14
13
15
2
0
2
1
5
721
659
615
480
366f
353
312
260
264
217
152
99
14S6 + 33S7 + 10S23 + 10S25 + 17S27 C–Si–C bend
18S31 + 11S47 + 69S48
50S32 + 33S48
235
7
1
1
Ring deformation
C–C–Si bend
C–C–C bend
159
94
155f
105
38S6 + 16S7 + 35S27
a
b
c
d
e
f
Calculated with B3LYP/cc-pVTZ basis set.
The observed wavenumbers are derived from IR vapor spectra. except when noted.
Calculated infrared intensities in km/mole.
Calculated Raman scattering intensities in arbitrary units. see text and Ref. [16].
For definition of symmetry coordinates. see Table S5.
From the Raman spectra of the liquid.
symmetry). The fundamentals will divide into 27 modes of species
A0 and 21 A00, and give rise to polarized and depolarized Raman
be 0.75 (contrary to the theory), justifying the observed value D
for these A0 modes. The smallest and intermediate axes of inertia
lie in the symmetry plane and the largest axis is perpendicular to
this plane, leading to AB hybrid contour for the A0 modes and C
type contour for the A00 modes for both conformers. As apparent
from Table 1, only the vapor bands at 2170, 2158, 878 and
506 cmꢀ1 with PQR contours are attributed to a single conformer,
while those at 1450, 1192, 1184, 1042, 996 and 776 cmꢀ1 are coin-
ciding e and a conformer bands with very uncertain overlapping
contours. A careful investigation of the vapor spectra suggests no
definite cases in which the AB hybride contour or the C-type con-
tours give a definite clue to the attributions.
bands, respectively. They are numbered m1
–
m27 for species A0 and
m28
–
m48 for A00. Since the order of the fundamentals sometimes
change between the harmonic and the anharmonic calculations
(Tables 2 and 3), the latter are made the basis for the numbering.
The assignments agree well with the results of the B3LYP/cc-pVTZ
anharmonic calculations, and are supported by the Raman depolar-
ization measurements and the infrared vapor contours. As is
apparent from Table 1 practically all the A0 modes appearing at a
separate wavenumber are described as P or the polarization ratio
is uncertain and left out. Similarly, the A00 modes appear as D or
are left out. An exception is
m
14 of both e and a conformers coincid-
From the experimental results it was concluded that a majority
of the e-conformer bands overlapped those of the a-bands in agree-
ment with the results of the calculations. Sometimes the A0 and A00
ing at 1184 cmꢀ1 and observed as D. However, from Tables 2 and 3
the depolarization ratios for both these modes were calculated to

P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
127
Table 3
Observed and calculated vibrational modes of the a- conformer of 1-fluoro-1-silacyclohexane (FSC).
No
Sym
Harma wave.
Anharm. wave.
Obs.b wave.
IRc Int.
Rad Int.
Dep. ratio
PEDe
Description
1
28
2
3
29
4
30
5
31
6
7
32
8
A0
A00
A0
A0
A00
A0
A00
A0
A00
A0
A0
A00
A0
A0
A0
A00
A0
A00
A00
A0
A00
A00
A0
A0
A00
A00
A00
A0
A0
A0
A00
A00
A0
A00
A0
A0
A0
A00
A00
A0
A0
A0
A00
A0
A00
A0
A00
A0
3061
3061
3049
3046
3041
3011
3005
3008
3011
2995
2213
1498
1508
1493
1448
1445
1381
1381
1370
1325
1303
1284
1229
1214
1132
1086
1057
1024
1015
928
913
887
875
842
817
795
752
673
634
620
508
397
369
328
270
210
2912
2912
2908
2906
2897
2894
2878
2874
2860
2845
2148
1498
1468
1457
1413
1403
1351
1343
1340
1295
1276
1251
1199
1180
1112
1062
1032
1002
996
909
893
869
864
827
805
779
747
670
621
609
512
397
364
351
266
2927
2921
2924
2921
2890
2890
2871
2871
2865
2865
2170
1461
1468
1456
1450
1409
1354
1348
1341
1288
1277
1254f
1192
1184
1105
1077f
1042
1009
996
920
910
869
878
835
808
776
742
670
615
628
506
38
18
54
55
39
37
6
12
19
17
185
3
3
7
6
9
5
0
0
2
4
0
3
25
0
0
2
24
3
41
9
10
164
157
13
2
6
1
1
2
242
156
153
502
107
136
21
442
115
299
610
54
19
38
31
53
1
0.23
0.75
0.64
0.18
0.75
0.06
0.75
0.13
0.75
0.18
0.21
0.75
0.59
0.73
0.75
0.75
0.68
0.75
0.75
0.68
0.75
0.75
0.16
0.75
0.75
0.75
0.75
0.13
0.71
0.71
0.75
0.75
0.75
0.75
0.56
0.17
0.36
0.75
0.75
0.06
0.24
0.48
0.75
0.30
0.75
0.69
0.75
0.72
11S12 + 79S13
14S35 + 81S36
64S9 + 16S11
15S8 + 67S11
91S34
84S10
69S35
74S12 + 17S13
89S33
68S8 + 22S9
100S14
89S39
94S18
92S16
94S22
91S43
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 antisymmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
CH2 symmetric stretch
Si–H stretch
CH2 scissor
CH2 scissor
CH2 scissor
CH2 scissor
CH2 scissor
CH2 rock
CH2 rock
CH2 rock
CH2 twist
CH2 twist
CH2 twist
CH2 rock
CH2 twist
CH2 rock
C–C stretch
CH2 twist
CH2 wag
C–C stretch
CH2 wag
C–C stretch
CH2 rock
F–Si–H bend
C–Si–F bend
Si–F stretch
C–C stretch
9
10
33
11
34
35
12
36
37
13
14
38
39
40
15
16
17
41
42
18
43
19
20
21
44
45
22
23
24
46
25
47
26
48
27
77S20
6
12S28 + 63S37 + 12S42
12S29 + 16S38 + 54S41
69S21 + 19S24
15S31 + 19S42 + 27S45
11S31 + 20S38 + 18S44 + 21S46 + 17S48
19S19 + 13S21 + 33S24 + 10S25
13S2 + 22S24 + 35S25
13S29 + 15S42 + 33S45
30S28 + 36S29
13
46
10
19
19
28
17
15
3
33S46 + 29S48
11
61
4
11S2 + 12S17 + 19S19 + 30S25
20S1 + 34S2 + 15S24
13S2 + 14S5 + 18S17 + 17S19
43S29 + 37S31
13S32 + 28S40 + 10S44 + 10S46 + 15S48
33S15 + 32S26
43S44 + 28S48
50S15 + 36S26
52S1 + 19S2
13S17 + 44S23 + 15S26
78S44
28
30
31
27
55
88
58
46
76
389
15
60
16
19
35
42
31
38
CH2 wag
CH2 wag
CH2 wag
C–Si sym stretch
C–C–C bend
C–Si–C bend
C–C–C bend
C–C–C bend
Ring deformation
C–Si–F bend
13S30 + 42S44 + 33S48
76S3
16
13
6
0
5
0
1
2
13S4 + 19S5 + 17S6 + 10S17 + 17S19
25S4 + 18S5 + 11S6 + 14S7 + 11S27
57S31 + 19S32 + 21S48
25S4 + 18S5 + 11S6 + 14S7 + 11S27
73S48
36S7 + 39S27
14S31 + 37S32 + 10S47 + 29S48
52S6 + 26S27
396
366f
353
281
213
139
98
219f
140
143
81
C–C–Si bend
C–C–Si bend
101
a
b
c
d
e
f
Calculated with B3LYP/cc-pVTZ basis set.
The observed wavenumbers are derived from IR vapor spectra. except when noted.
Calculated infrared intensities in km/mole.
Calculated Raman scattering intensities in arbitrary units. see text and Ref. [16].
For definition of symmetry coordinates. see Table S5.
From the Raman spectra of the liquid.
modes of the same conformer coincided as a result of accidental
degeneracy. Ca. 15 separate e and a pairs were tentatively assigned
(Tables 1–3). As expected, the observed wavenumbers in FSC are in
many cases very similar to those of fluorocyclohexane [18,20].
The 6 A0 and 4 A00 modes of the e and a conformers, attributed to
CH2 antisymmetric and symmetric stretch are observed as overlap-
ping IR and Raman bands between 3000 and 2850 cmꢀ1 in which
the e-modes were calculated at slightly higher wavenumbers than
the a-modes. They were quite similar to the CH2 stretching vibra-
tions in fluorocyclohexane [18,20] and in 1-chloro-1-silacyclohex-
ane [24]. The Si–H stretch m7 was attributed to the very intense IR
vapor bands with Q-branches at 2170 (a) and 2158 cmꢀ1 (e), sepa-
rated 12 cmꢀ1 while the calculations suggested 6 cmꢀ1 distance.
They have one strong, overlapping Raman peak at 2162 cmꢀ1 in
the liquid, common for both conformers. In 1-chloro-1-silacyclo-
hexane the Si–H stretches were observed at 2174 (a) and
2163 cmꢀ1 (e) as Q-branches in the IR vapor spectra [24] very close
to the present results in FSC.
With 10 hydrogens in FSC attached to carbon, we expect 20 fun-
damentals involving CH2 scissor, wag, twist and rock. These vibra-
tional modes are expected in the range below 1500 and as far as
700 cmꢀ1, although a few modes mainly involving C–C stretch
m39
,
m16
,
m43), Si–F stretch (
m18), F–Si–H bend (m19) and C–Si–C
stretch (m44 22) are also anticipated in this range (Tables 2 and
,
m
3). Prominent vapor bands with possible A/B hybride or C-type
band contours and R and/or P branches separated ca. 10 cmꢀ1 were
observed with Q-branches at 1450, 1192, 1184, 1042, 996, 878, 776
and 506 cmꢀ1. The agreement between the observed and calcu-
lated fundamentals in the anharmonic approximation is generally
very good (with average relative deviations around 1%, see below.

128
P. Klaeboe et al. / Journal of Molecular Structure 1015 (2012) 120–128
Very strong IR bands with a Q-branch at 996 cmꢀ1 are attrib-
uted to m16 of both conformers, involving predominantly C-C
stretch. The vapor bands at 881, 878 and 873 cmꢀ1 form R, Q and
P branches with possible AB hybride contour, and they are, in addi-
Theoretical and Computational Chemistry (CTCC), Department of
Chemistry, University of Oslo for a stipend covering traveling and
living expenses in Norway.
tion to Si–H stretch (m7), the most intense in the IR spectra in agree-
Appendix A. Supplementary material
ment with the calculations (Tables 2 and 3). They are assigned to
m18 (FSiH bending) of the a-conformer, whereas the corresponding
e-conformer band is found at 845 cmꢀ1 in the vapor. Strong vapor
bands at 835 cmꢀ1 are attributed to Q-branches for the e and a-
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2012.02.016.
conformers of the mixed stretching and bending modes, m43
.
References
The IR vapor bands at 845 ( 18) and 808 (
m
m
19) cmꢀ1 and the cor-
[1] I. Arnason, G.K. Thorarinsson, E. Matern, Z. Anorg. Allg. Chem. 626 (2000) 853.
[2] I. Arnason, A. Kvaran, A. Bodi, Int. J. Quant. Chem. 106 (2006) 1975.
[3] F. Freeman, C. Fang, B.A. Shainyan, Int. J. Quant. Chem. 100 (2004) 720.
[4] I. Arnason, A. Kvaran, S. Jonsdottir, P.I. Gudnason, H. Oberhammer, J. Org.
Chem. 67 (2002) 161.
[5] I. Arnason, H. Oberhammer, J. Phys. Chem. 107 (2003) 243.
[6] I. Arnason, A. Kvaran, S. Jonsdottir, P.I. Gudnason, H. Oberhammer, J. Org.
Chem. 67 (2002) 3827.
[7] R. Bjornsson, I. Arnason, Phys. Chem. Chem. Phys. 11 (2009) 8689.
[8] A. Bodi, R. Bjornsson, J. Mol. Struct. 978 (2010) 14.
[9] A.J. Weldon, G.S. Tschumper, Int. J. Quant. Chem. 107 (2007) 2261.
[10] L.B. Favero, B. Velino, W. Caminati, I. Arnason, A. Kvaran, J. Phys. Chem. A110
(2006) 995.
responding Raman bands have 55 and 50 % contribution from Si–F
stretch, for the e- and a-conformers, respectively, but they are
weaker and less characteristic than the Si–Cl stretching bands in
1-chloro-1-silacyclohexane [24]. Both of these modes are mixed
with F–Si–H bend. In fluorocyclohexane the C–F stretches of the
e and a conformers are also mixed with various bending modes,
but they appear widely separated at 1065 and 954 cmꢀ1 in the va-
por [18,20] (at 1054 and 940 cmꢀ1 in the Raman spectra). It is well
known that in chloro- and bromocyclohexanes the C–X stretch is at
a higher wavenumber in the e than in the a conformer [22,23].
Two Raman bands are observed at 659 and 630 cmꢀ1, and
according to PED they are more than 70% localized to the symmet-
ric C–Si–C stretches of the e and a-conformers, respectively. Both of
these bands are highly polarized, they are among the most intense
in the Raman spectra (Tables 2 and 3 and Fig. 3), they are very
weak in IR, but well suited for the determination of DconfH(e ꢀ a).
They were employed in Fig. 6 and in the earlier studies [11].
[11] A. Bodi, A. Kvaran, S. Jonsdottir, E. Antonsson, S.Ó. Wallevik, I. Arnason, A.V.
Belyakov, A.A. Baskakov, M. Hölbling, H. Oberhammer, Organometallics 26
(2007) 6544.
[12] F.A. Miller, B.M. Harney, Appl. Spectrosc. 24 (1970) 291.
[13] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, T. Vreven, J. Montgomery, J. A., J.E. Peralta, F. Ogliaro,
M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M.
Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth,
P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman,
J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision B.01, Gaussian, Inc.,
Wallingford, CT, 2009.
[14] L.A. Curtiss, K. Ragavachari, P.G. Redfern, V. Rasslov, J.A. Pople, J. Chem. Phys.
109 (1998) 7764.
[15] G.O. Sørensen, VIBROT, Department of Chemistry, University of Copenhagen,
Copenhagen, Denmark.
[16] D. Michalska, R. Wysokinski, Chem. Phys. Lett. 403 (2005) 211.
[17] A.I. Fishman, W.A. Herrebout, B.J. van der Veken, J. Phys. Chem. A 106 (2002)
4536.
[18] J.R. Durig, R.M. Ward, K.G. Nelson, T.K. Gounev, J. Mol. Struct. 976 (2010) 150.
[19] J.R. Durig, C. Zheng, A.M. El Defrawy, R.M. Ward, T.K. Gounev, K. Ravindranath,
N.R. Rao, J. Raman Spectrosc. 40 (2009) 197.
Below 630 cmꢀ1 three overlapping e and a-modes 615 (
46) and 353 cmꢀ1
m24 and 25) were observed. The remaining IR
m45), 366
(m
(
m
and Raman bands interpreted as fundamentals have been assigned
as separate e or a-fundamentals. Exceptions are the bands around
170 cmꢀ1, tentatively interpreted as combination bands. The Ra-
man band at 395 cmꢀ1 attributed to m24 A0 of the a conformer
was combined with the e mode m25 at 311 cmꢀ1 to give DconfH (
e ꢀ a).
The average relative deviations between the observed and cal-
culated (anharmonic), unscaled modes give 1.16% for the e-con-
former and 0.92% for the a-conformer. The largest contributions
to the deviations for both conformers are given by the four modes
situated below 800 cmꢀ1
Tables 2 and 3.
: m22, m27, m44 and m47 as is apparent from
[20] S.D. Christian, J. Grundnes, P. Klaeboe, E. Tørneng, T. Woldbaek, Acta Chem.
Scand. A34 (1980) 391 (and earlier papers).
[21] P. Klaeboe, Acta Chem. Scand. 23 (1969) 2641.
[22] T. Woldbaek, Acta Chem. Scand. A36 (1982) 641.
Acknowledgements
[23] P. Klaeboe, Vibr. Spectrosc. 9 (1995) 3.
[24] V. Aleksa, G.A. Guirgis, A. Horn, P. Klaeboe, R.J. Liberatore, C.J. Nielsen, Vibr.
Spectrosc., submitted for publication.
The authors are grateful to Dr. Niels Højmark Andersen for his
valuable assistance in installing the low temperature Raman
attachment. Valdemaras Aleksa is grateful to the Centre for