Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

Article abstract of DOI:10.1016/j.molstruc.2007.08.017

The infrared spectra of 3-pentyn-2-ol, CH₃C{triple bond, long}CCH(OH)CH₃, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000-50 cm^-1 range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer in which OH is oriented anti to C₁(a_Me). From various bands assigned to a second conformer in which OH is oriented anti to H_gem(a_H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol^-1. The highest energy conformer with OH anti to C₃(a_C) was not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a_H conformer had slightly lower energy, all the calculations revealed that a_Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a_Me conformer as more stable by 0.8 and 8.3 kJ mol^-1 relative to a_H an a_C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.

Full text of DOI:10.1016/j.molstruc.2007.08.017

Available online at www.sciencedirect.com
Journal of Molecular Structure 879 (2008) 102–112
www.elsevier.com/locate/molstruc
Vibrational spectra, conformations, ab initio calculations
and vibrational assignments of 3-pentyn-2-ol
a
a,
a
b
Anne Horn , Peter Klaeboe *, Claus J. Nielsen , Gamil A. Guirgis
a
Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway
Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29403, USA
b
Received 30 July 2007; received in revised form 21 August 2007; accepted 21 August 2007
Available online 28 August 2007
Abstract
The infrared spectra of 3-pentyn-2-ol, CH3C„CCH(OH)CH3, have been recorded as a vapour and liquid at ambient temperature, as
a solid at 78 K in the 4000–50 cmꢀ1 range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were
obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid.
Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an
amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative inten-
sity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made
for all the bands of the most stable conformer in which OH is oriented anti to C1(aMe). From various bands assigned to a second con-
former in which OH is oriented anti to Hgem(aH), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ molꢀ1
The highest energy conformer with OH anti to C3(aC) was not detected.
.
Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis
set calculations for which the aH conformer had slightly lower energy, all the calculations revealed that aMe was the low energy con-
former. The B3LYP/cc-pVTZ calculations suggested the aMe conformer as more stable by 0.8 and 8.3 kJ molꢀ1 relative to aH an aC,
respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from
B3LYP/cc-pVTZ calculations.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Conformational stability; 3-pentyn-2-ol; Infrared and Raman spectra; Ab initio calculations
1. Introduction
The sample of PEOL was studied by infrared and
Raman spectroscopy in various phases. As far as the
authors know, the sample of PEOL is subjected to spectro-
scopic investigations for the first time, but the compound
has recently been mentioned as an intermediate in the syn-
thesis of C-centered chirial compounds [1]. The isomer, 4-
pentyn-1-ol, however, is commercially available and has
been investigated by various methods. A large series of
alcohols with a triple bond in a and b position relative to
the OH group were studied by infrared spectroscopy by
van der Maas and co-workers with special reference to
the OAH stretching band. These studies [2–4] demon-
strated an interaction between the OH group and the p-
electrons of the triple bond. In the present investigation
we want to investigate the conformational preference of
The present molecule 3-pentyn-2-ol, CH3C„CCH
(OH)CH3 – abbreviated PEOL, has a central C atom
attached to four different groups. Thus, there is no symme-
try plane in the molecule, it is optically active, and the com-
pound has three different conformers as is apparent from
Fig. 1. The conformers have been designated aMe, aH and
aC, indicating that the OH group is oriented anti to C1,
Hgem and C3, respectively.
*
Corresponding author. Tel.: +47 22855678; fax: +47 22855441.
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.08.017

A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
103
H10
PEOL have any symmetry, it was not found worth while
to record polarization spectra of the liquid.
H8
H9
C2
C3
C1
O5
C4
H11
H13
Raman spectra of the liquid were primarily recorded at
room temperature, but additional spectra were obtained at
13 temperatures between 295 and 143 K in a capillary tube
of 2 mm inner diameter. The tube was surrounded by a
Dewar, cooled by gaseous nitrogen evaporated from a res-
ervoir [6]. From these spectra the enthalpy difference DH
between the conformers in the liquid were calculated. The
sample did not freeze by this technique. Instead the vapour
of PEOL was condensed on a copper finger in a Raman
cryostat at 78 K, cooled with liquid nitrogen. A solid phase
was formed, but the spectra were quite similar to those of
the liquid. Annealing to temperatures between 110 and
130 K and subsequent cooling to 78 K did not change the
spectra significantly.
H7
H14
C6
H12
aC
aH
aMe
Fig. 1. The three conformers aMe, aH and aC of 3-pentyn-2-ol (PEOL).
PEOL employing infrared and Raman spectroscopy in var-
ious phases combined with quantum chemical calculations.
2.3. Infrared spectral measurements
2. Experimental
The spectra in the middle infrared region (MIR, 4000–
450 cmꢀ1) were recorded on a Bruker model IFS-66 and
a Perkin-Elmer model 2000, while the far infrared region
(FIR, 600–50 cmꢀ1) was covered by a Bruker IFS-113v
vacuum spectrometer. Beamsplitters of Ge on KBr sub-
strate were employed in the MIR, and beamsplitters with
3.5 and 12 l thickness of Mylar and of metal mesh were
used in the FIR. The vapour was studied at full pressure
of ca. 3 mbar in cells of 10 (KBr windows) and 18 cm (poly-
ethylene windows) path lengths in the MIR and FIR
regions, respectively. In MIR the most intense bands were
also studied with 1 mbar vapour pressure. MIR spectra of
the liquid were recorded as a capillary film between two
KBr windows. The solid was deposited on a cryostat with
CsI windows for the MIR region, cooled with liquid nitro-
gen. Only small changes were detected between the spectra
of the solid and the liquid, and annealing to temperatures
between 110 and 120 K made negligible changes.
2.1. Sample preparation
A sample of 3-pentyn-2-ol was prepared by adding
diisopropylamide drop wise to 1,2-dibromopropane at
ꢀ60 ꢁC in dry tetrahydrofurane followed by the addition
of acetaldehyde in one portion according to the method
of Gribble et al. [5]. The sample of PEOL was purified sev-
eral times by trap-to-trap distillation under controlled low
temperature and reduced pressure. Initially, no apparent
impurities were observed in the vibrational spectra, but
when the sample was stored in an evacuated ampoule in
a deep freeze at ꢀ20 ꢁC the spectra revealed new bands
which increased in intensities with time. The compound
turned slightly yellow on storing (see below).
2.2. Raman spectral measurements
The sample was diluted with argon (1:1000) and depos-
ited on a CsI window at 5 K of a Displex cryostat from
APD (model HS-4) with a three stage cooling system.
The spectra of the unannealed matrix were first recorded.
Subsequently, the matrix was annealed to temperatures
between 15 and 20 K to remove site effects. Then they were
annealed in steps of 3–5 K to a maximum of 35 K in argon
in periods of 10 min. Higher temperature annealing is not
feasible since the inert gas has a too high pressure in the
cryostat and the matrices turn ‘‘soft’’ followed by diffusion
of the solute. After each annealing the window was
recooled to 5 K and the spectra were recorded.
The Raman spectra were obtained using two spectrom-
eters; a Dilor RTI-30 spectrometer (triple monochromator)
with an argon ion laser from Spectra Physics (model 2000)
adjusted to give ca. 150 mW on the sample. The laser was
employing the 514.5 nm line with 90ꢁ and 180ꢁ excitation.
Subsequently the Raman spectra were recorded with a mul-
tichannel spectrometer from Horiba (Jobin Yvon) model T
64000 using a single monochromator with a CCD detector
cooled to 78 K, using 90ꢁ scattering geometry. These spec-
tra were excited by a Milennia Pro diode-pumped
(Nd:YVO4 crystal) laser from Spectra-Physics (Model J
40) adjusted to give approximately 60 mW of the 532 nm
line on the sample. The high sensitivity of the CCD detec-
tor gave spectra with very good signal/noise ratio. There-
fore, narrow slits could be employed, and these spectra
were recorded at ca. 2 cmꢀ1 resolution. Mostly, 4–5 accu-
mulations of 5 s each were made and a complete Raman
spectrum in the range 4000–100 cmꢀ1 was obtained within
ca. 5 min. Since none of the three possible conformers of
3. Results
3.1. Raman spectral results
A Raman spectrum of the liquid in the range 3200–
100 cmꢀ1 is presented in Fig. 2, whereas a Raman spectrum
of the solid, which was annealed to ca. 120 K and recorded

104
A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
3000
2500
2000
1500
1000
500
1200
1000
800
600
400
Wavenumber / cm-1
Wavenumber / cm-1
Fig. 2. The Raman spectrum of PEOL as a liquid at room temperature in
the range 3200–100 cmꢀ1
Fig. 4. Raman spectra of PEOL as liquids at 295 K (solid curve) and at
143 K (dashed curve) in the range 1300–300 cmꢀ1
.
.
definite spectral changes upon solidification, the attribution
of the Raman bands into conformer pairs is no trivial mat-
ter. The ab initio and DFT calculations of the two low
energy conformers aMe and aH (see below) reveal that most
of the wavenumber shifts between the three conformers
were small, but in nine cases the shifts were 10 cmꢀ1 or lar-
ger. Obviously, the chance of observing conformer band
pairs is largest when the bands of the different conformers
are well separated.
The observed intensity variations indicate the existence
of separate conformer bands with fairly low enthalpy dif-
ferences. The Raman bands which increase on cooling
(equipped with arrows pointing upwards in Table 1)
should belong to the more stable conformer, and they
were paired with other bands (arrows pointing down-
wards in Table 1) which diminished upon cooling. The
two low energy conformers, aMe and aH, have, according
to the quantum chemical calculations, an enthalpy differ-
ence around 1.0 kJ molꢀ1 (see below). The aC conformer
was calculated to have an enthalpy difference ca.
8.0 kJ molꢀ1 above aMe and can therefore be neglected
at room temperature.
In order to determine the enthalpy difference from the
variable temperature spectra the bands of the two possible
conformers of PEOL should both have reasonably high
intensities, they should be situated on a flat background
and not overlap other bands in the spectra. Moreover, they
must be ‘‘pure’’, meaning that their intensities should be
due to one conformer only, with no contribution, neither
from fundamentals nor from combination bands of the
other conformer. For this reason, it is highly recommended
to measure more than one pair of bands and carry out
independent calculations of DconfH for each pair. Consider-
able discrepancies might occur between the DconfH values
obtained from different pairs of bands, indicating either
poor signal to noise ratio, overlapping bands or an ill-
defined background. Eventually, one or both bands of
the pair may be contaminated with the other conformer,
meaning a superposition between a fundamental of one
3000
2500
2000
1500
1000
500
Wavenumber / cm-1
Fig. 3. The Raman spectrum of PEOL as a solid at 78 K which was
annealed to 120 K, in the range 1500–90 cmꢀ1
.
at 78 K, is presented in Fig. 3 (1500–90 cmꢀ1). The sample
of PEOL condensed on the Cu-finger at 78 K looked crys-
talline from visual observation immediately after deposit.
However, the spectra were quite similar to those of the
liquid, and none of the Raman bands of the liquid vanished
in the spectra recorded at 78 K. The sample was annealed
to temperatures in the range 110–130 K and recooled to
78 K, but these spectra were not significantly different from
those of the unannealed solid. A number of low tempera-
ture spectra were recorded, but the sample was apparently
never crystalline in agreement with the observations in the
infrared region (see below).
Raman spectra of the liquid were recorded at 13 differ-
ent temperatures between 295 and 143 K. Fig. 4 displays
the liquid spectra in the 1300–300 cmꢀ1 region obtained
at the two extreme temperatures. Fairly small intensity
variations with temperature of certain peaks relative to
neighbouring bands were observed, and they were inter-
preted as a displacement of the conformational equilib-
rium. However, with three conformers present and no

A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
105
Table 1
Infrared and Raman spectral data for 3-pentyn-2-ol, PEOL (CH3C”CACH(OH)CH3)
Infrared
Vapour
295 Ka
Raman
Liquid
295 Ka
Assignment
Matrix
Liquid
273 Ka
Solid
Crystal
aMe
aH
aC
5 Ka
78 Ka
140 Kb
78 Ka
140 Kb
3409 vw?
35 Kb
3649 m
3641 m
3656 sc
3648 m
3351 vs,br
3250 vs, br
m1
3391 vw?
3374 vw
m1
3337 s,br
3266 vw
3250 vw,br
3204 vw,br
3049 vw
3001 s,sh
2999 m
2989 vs
2981 w
m2
m3
3002 s
m3
2994 s
2982 vs
2980 vs
2984 m
2960 w
2990 m
2974 w
m4
m5
2989 m
2982 vw
2976 vw
2946 m
2939 m
2934 m
2897 w
2886 m
2867 w
2943 s
2938 s
2933 s
2957 w,sh
2932 s
2921 s
2944 m,sh
2923 vvs
2946 m,sh
2932 vvs
m6
m7
2932 w,sh
m7
m8
2884 m
2877 w
2876 m
2861 w,sh
2877 m, br
2855 w
2879 w
2863 w
2882 m
2861 w
m8
2972 vw
2753 vw,br
2740 vw
2621 vw
2566 vw
2455 m
2752 vw,br
2628 m,br
2575 m,br
2740 w
2610 vw
2740 w
2454 m
2445 w
2327 m
2260 m
2229 w
2155 w
2321 m
2255 vsfl
2221 vs›
2158 vw
2120 w
2333 m
2267 vs
2234 s
2258 m
2228 m
2264 w
2238 w
2254 m
2227 w
2150 w
m9
m9
2167 w
imp
2058 w
1963 vw
1892 vw
1715 m
1702 w
2051 m
1967 m
1891 m
1706 s
1710 m
1699 m
1718 m
1673 vw
1658 vw
1672 m
1659 m
imp
imp
m10
1459 w
1447 w
1457 m
1445 s
1448 s
1439 vs
1451 s
1451 s
m11–m13
1437 w
1419 w
1382 s
1364 s
1322 vw
1426 w,sh
1424 vw
m14
m15
m16
m17
1411 m
1415 m,sh
1382 s
1369 s
1370 s
1382 s
1383 s
1362 m,sh
1333 vw
1260 vw
1245 m
1160 m
1370 s
1363 vw
1324 w
1290 vw
1361 vw
1325 w
1303 vw
1333 m
1288 m
1256 w,sh
1160 s
1333 s
m17
m18
1301 m
1261 vw
1162 vs
m18
m19
m20
m21
1253 s
1161 w
1056 vw
1089 s
1086 s
1069 s
1162 m
1165 m
1078 s
1086 s
1087 vs
1068 w,sh
1079 vs
1036 vw
1000 s
1080 vs
1040 m
1032 m
1081 s›
1083 s
1033 m
1003 s
m22
1032 w
1033 w›
1029 vw,sh
1002 s›
966 wfl
950 vw,sh
m22
m23
m24
999 s
954 vw
990 vs
951 vw
1003 vs
964 vw,br
947 w,br
910 w
910 vw
887 s
842 vw
m25
888 m
893 m
832 vw
889 s
889 sfl
842 w
821 vw
738 w
891 s
m25
imp
imp
m26
732 w
(continued on next page)

106
A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
Table 1 (continued)
Infrared
Raman
Assignment
Vapour
Matrix
Liquid
728 m
Solid
Liquid
Crystal
723 s
aMe
aH
aC
724 m
726 m
719 w
727 m
730 sfl
m26
664 m
584 w
563 wfl
535 w›
529 sfl
397 m
329 s›
305 s
imp
imp
m27
567 vw
556 vw
541 w
522 w
538 vw,sh
528 m
534 w
527 s
397 m
332 s
305 s
m28
531 m
390 m
324 m
530 m
m28
m29
m30
m31
m32
m33
m34
m35
m36
257 m
204 w
135 m
127 m
120 m
145 m,br
a
Recording temperature.
Annealing temperature.
b
c
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; arrows pointing upwards and downwards signify bands which increase
and decrease in intensities at decreasing temperature in the Raman spectrum of the liquid.
conformer and a fundamental, combination band or over-
tone belonging to the other conformer.
The intensities of each band pair were fitted to the
equation ln(aMe/aH) = DconfH/RT + constant, where the
constant term includes the conformational entropy differ-
0
ence, and it is assumed that DconfH is constant with tem-
perature. The data are plotted as van’t Hoff plots, and
peak heights and integrated band areas were both
attempted for determining the band intensities employed.
-0.6
The Raman band pairs which were selected for the quan-
titative calculations were the following: 1081/889, 998/
889, 998/526 and 998/722 cmꢀ1, the bands in the nomina-
tor increased and those in the denominator decreased in
intensities upon cooling. It was assumed that the low
-1.2
enthalpy conformer was aMe and that the high enthalpy
conformer band was due to aH in concord with the results
of the calculations. It can be excluded that the conformer
0.003
0.004
0.005
1/T
0.006
0.007
Fig. 5. van’t Hoff plots of the following band pairs from top to bottom,
1081/889 cmꢀ1; 998/526 cmꢀ1, 998/889 cm ꢀ1 and 998/722, employing
peak heights.
aC, which according to the various MP2 and B3LYP cal-
culations had ca. 8 kJ molꢀ1 higher enthalpy than aMe
,
was not present in detectable concentrations at these tem-
peratures. However, it cannot be excluded that because of
the large number of fundamentals (36) for each con-
former, small contributions of the third conformer inter-
fere with the results.
The van’t Hoff plots are presented in Fig. 5. Employing
peak intensities, the band pairs 1081/889, 998/526, 998/
889 and 998/722 cmꢀ1 gave the DconfH values: 0.51, 0.42,
0.57 and 0.82 kJ molꢀ1, respectively, leading to a mean
value of DconfH(aH ꢀ aMe) = 0.6 kJ molꢀ1 in the liquid.
Employing integrated areas of the band pair 998/
899 cmꢀ1, the value DconfH = 0.4 kJ molꢀ1 was obtained
from the van’t Hoff plot (not included in Fig. 5). Fairly large
uncertainties were involved in these plots and an estimated
value of DconfH = 0.5 0.4 kJ molꢀ1 was obtained.
3.2. Infrared spectral results
A MIR vapour spectrum of PEOL between 3750 and
450 cmꢀ1 at a full pressure of ca. 3 torr is presented in
Fig. 6. In spite of treating the sample with molecular
sieves, the spectra reveal the presence of residual water
vapour which disturb the fine structure in the characteris-
tic water vapour regions. Possible rotational PQR fine
structure was observed around 2940 cmꢀ1. However, fur-
ther IR spectra in the argon matrix, liquid and solid
strongly suggest that these bands were not due to rota-
tional transitions, but to close-lying vibrational modes of
the two low energy conformers. A FIR spectrum in the

A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
107
100
100
50
0
80
4000
3500
3000
2500
2000
1500
1000
500
3500
3000
2500
2000
1500
1000
500
Wavenumber / cm-1
Wavenumber / cm-1
Fig. 8. Infrared spectra (MIR) of PEOL as a solid annealed to 120 K in
the range 4000–400 cmꢀ1, recorded at 78 K in a cryostat with windows of
CsI.
Fig. 6. Infrared vapour spectrum (MIR) of PEOL in the range 3750–
450 cmꢀ1 at 3 torr pressure, path length 10 cm.
1.0
0.9
0.26
0.30
0.24
0.28
0.22
0.26
600
500
400
300
200
100
0.20
Wavenumber / cm-1
3660
3640
3000
2950
2900
2850
Wavenumber / cm-1
Fig. 7. Infrared vapour spectrum (FIR) of PEOL in the range 600–
50 cmꢀ1 at 3 torr pressure, path length 18 cm.
Fig. 9. Infrared spectra (MIR) (3670–3625 and 3025–2850 cmꢀ1) of PEOL
in an argon matrix (1:1000) deposited and recorded at 5 K as an
unannealed matrix.
range 600–50 cmꢀ1 is given in Fig. 7, but because of the
low vapour pressure and weak bands the signal to noise
ratio is fairly low.
A spectrum of PEOL as a liquid capillary between two
KBr plates is dominated by the intense, broad band at
3350 cmꢀ1 due to strong hydrogen bonding. Since this
spectrum was very similar to that studied in a cryostat at
liquid nitrogen temperature, it is not presented for the sake
of brevity. Low temperature spectra were recorded of the
unannealed sample and of samples annealed to various
temperatures between 110 and 140 K and subsequent cool-
ing to 78 K. Only negligible changes were detected between
these spectra. The low temperature spectrum is given in
Fig. 8, and apparently the sample did not crystallize.
The infrared spectra of PEOL in argon matrices was
mixed in the ratio 1:1000 and deposited and recorded at
5 K. The 3670–3625 and 3025–2850 cmꢀ1 region is pre-
sented in Fig. 9, covering the OH and CH stretching
regions. A spectrum covering the region 1400–550 cmꢀ1 is
given in Fig. 10. The matrix spectra have sharp bands,
0.8
0.6
0.4
1400
1300
1200
1100
1000
900
800
700
600
Wavenumber / cm-1
Fig. 10. Infrared spectra (MIR) (1400–550 cmꢀ1) of PEOL in an argon
matrix (1:1000) deposited and recorded at 5 K, as an unannealed matrix.

108
A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
12
10
8
and close-lying infrared modes can frequently be detected
in these spectra. Also, a transition from the high energy
to low energy conformers can frequently be observed after
annealing the matrices. The conformational equilibrium of
the vapour phase is generally maintained when the gas mix-
ture is quickly frozen on the CsI window at 5 K, provided
that the barrier to conformational equilibrium is high
enough to prevent conversion. However, in PEOL, the very
low barrier of ca. 2.0 kJ molꢀ1 between the aH and aMe
conformers, which was derived from the calculations (see
below), makes it uncertain if the aH conformer could be
trapped in the argon matrix at 5 K. In accordance with this
assumption, only negligible spectral changes occurred
when the sample was annealed to 33 K.
aC
6
4
2
aH
aMe
0
100
τC≡C-O-H /degree
-300
-200
-100
0
200
300
3.3. Quantum chemical calculations
Fig. 11. The torsional potential of PEOL around the CAOH bond as
determined by the density functional theory MP2/6-311G(d,p) giving the
conformer aMe as most stable, followed by aH and aC.
Ab initio (MP2) and DFT (B3LYP) calculations were
performed using the Gaussian 03 programs [7] employing
a variety of basis sets (cc-pVDZ, aug-cc-pVDZ, cc-pVTZ,
aug-cc-pVTZ and cc-pVQZ). The minima on the potential
surface were found by completely relaxing the geometry,
observed values. There are only very small differences
between the results obtained with the triple- and the qua-
druple-zeta basis sets.
and the conformational enthalpy difference DconfH(aH ꢀ aMe
)
was calculated to be in the region from 0.8 to 1.2 kJ molꢀ1
from the B3LYP and from 1.2 to 1.6 for the MP2 calcula-
tions, respectively. We note that both MP2 and B3LYP
calculations with the small cc-pVDZ basis set predicts the
reverse order of the aH and aMe conformers. The most reli-
able value for DconfH is probably obtained by the complete
basis set extrapolation CBS-QB3 method [8] favouring
aMe by 1.4 kJ mol ꢀ1. For the high energy conformer
Because the assignments of the spectral bands to the
individual conformers rely entirely on the results from
the quantum chemical calculations we decided to carry
out similar calculations (B3LYP/cc-pVTZ) for related
molecules for which reliable vibrational assignments
exists, namely 2-propanol (isopropanol, CH3-CHOH-
CH3) [9] and dimethylacetylene (CH3-C„C-CH3)
[10,11]. Normal coordinate calculations, based upon har-
monic force constants derived from the DFT calculations,
was carried out as described below. First, the DFT calcu-
lated force fields of dimethylacetylene and 2-propanol
were transformed from Cartesian to internal coordinates,
derived from a suitable set of valence coordinates. Then, a
scaling of the internal force fields was performed to
aC
D
confH(aC ꢀ aMe) was calculated to be from 7.7 to
9.1 kJ molꢀ1. The dipole moments are about 2D for both
aH and aMe, and the difference between the conformers
should not cause any significant displacements of the confor-
mational equilibrium from the vapour to the fluid states.
Fig. 11 shows the potential energy curve for the torsion
around the central CAO bond of PEOL obtained from the
B3LYP/aug-cc-pVDZ calculations, in which the complete
geometry was relaxed for each value of the C3AC2AOAH
torsional angle. The barrier to internal rotation from aH, to
the low energy form aMe is calculated to be around
2.0 kJ molꢀ1. The barrier from the high energy conformer,
ac, which is of less importance, was calculated to be
3.0 kJ molꢀ1. This has important consequences for the
interpretation of the matrix isolation experiments; it also
explains why it was never possible to crystallize PEOL at
liquid nitrogen temperatures.
achieve the best fit of the vibrational frequencies in the
pffiffiffiffiffiffiffiffiffiffiffi
scaled
i;j
calc:
i;j
two molecules according to: F
¼ F
ꢁ
ai ꢁ aj, where
calc:
i;j
scaled
F
and F
are the unscaled and scaled internal force
i;j
constants, respectively, and ai and aj are scaling parame-
ters for the internal coordinates i and j. The scale factors
corresponding to internal coordinates involving the same
class of valence coordinates were lumped to reduce the
number of variables. From a fit to 70 wavenumbers of
4 isopropanol isotopologues [8] the following 9 scale fac-
tors were derived: 0.93 (OH), 0.93 (CAH), 1.00 (CAO),
0.98 (CAC), 0.98 (COH), 1.00 (CCC, CCO), 0.96
(OCH, CCH), 0.89 (Me-torsion) and 0.94 (OH-torsion).
A fit to 15 wavenumbers of dimethylacetylene [10,11]
resulted in the following 5 scale factors: 0.93 (CAH),
1.00 (CAC), 0.88 (C„C), 0.96 (CCH) and 0.92 (C„CC).
These scale factors were then applied to the force fields of
the conformers aMe and aH of PEOL. The internal coor-
dinates of PEOL are collected in Table 2 while the results
of the normal coordinate calculations are listed in Table 3
3.4. Normal coordinate calculations
The wavenumbers of PEOL were initially predicted in
B3LYP/6-311G* calculations which generally result in val-
ues a few percent higher than the observations. However,
for PEOL the discrepancies from the observed values were
considerable. Applying the much larger basis sets cc-pVTZ
and aug-cc-pVQZ, the wavenumbers agreed better with the

A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
109
Table 2
Symmetry coordinates for 3-pentyn-2-ol, CH3CHOHC„CCH3 (PEOL)
a
Description
Definition
OH stretch
S1 = DR5,11
CH stretch
S2 = DR4,10
pffiffiffi
ꢀ
ꢁ
(”CA)CH3 sym stretch
(”CA)CH3 asym stretch
(”CA)CH3 asym stretch
(HOHCA)CH3 sym stretch
(HOHCA)CH3 asym stretch
(HOHCA)CH3 asym stretch
CAO stretch
CC sym stretch
CC asym stretch
C„C stretch
(C”)CAC(H3) stretch
COH bend
S3
S4
S5
S6
S7
S8
¼
¼
¼
¼
¼
¼
DR1;7 þ DR1;8 þ DR1;9
=
3
6
pffiffiffi
pffiffiffi
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ
2DR1;7 ꢀ DR1;8 ꢀ DR1;9
=
pffiffiffi
ꢁ
DR1;8 ꢀ DR1;9
=
2
pffiffiffi
ꢁ
DR6;12 þ DR6;13 þ DR6;14
2DR6;12 ꢀ DR6;13 ꢀ DR6;14
=
3
6
ꢁ
=
pffiffi
ꢁ
DR6;13 ꢀ DR6;14
=
2
S9 = DR4,5
S10
S11
S12 = DR2,3
S13 = DR1,2
pffiffi
ꢀ
ꢁ
¼
¼
DR3;4 þ DR4;6
=
=
2
pffiffi
ꢀ
ꢁ
DR3;4 ꢀ DR4;6
2
S14 = Da4,5,11
pffiffiffi
ꢀ
ꢁ
CH bend
CH twist
CCC deformation
CCC deformation
S15
S16
S17
S18
S19
S20
S21
S22
S23
S24
S25
S26
S27
S28
S29
S30
S31
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
¼
Da5;4;10 ꢀ Da3;4;10 ꢀ Da6;4;10
=
3
pffiffiffi
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ
Da3;4;10 ꢀ Da6;4;10
=
2
pffiffiffi
ꢁ
Da3;4;6 þ Da3;4;5 þ Da5;4;6
=
3
pffiffiffi
ꢁ
2Da3;4;6 ꢀ Da3;4;5 ꢀ Da5;4;6
=
6
pffiffiffi
ꢁ
CCC deformation
Da3;4;5 ꢀ Da5;4;6
=
2
pffiffiffi
ꢁ
Sym. (C„CA)CH3 deformation
Antisym. (C„CA)CH3 deformation
Antisym. (C„CA)CH3 deformation
Antisym. (C„CA)CH3 deformation
Antisym. (C„CA)CH3 deformation
Sym. (HOHCA)CH3 deformation
Antisym. (HOHCA)CH3 deformation
Antisym. (HOHCA)CH3 deformation
Antisym. (HOHCA)CH3 deformation
Antisym. (HOHCA)CH3 deformation
HOHC(AC„CA)CH3 torsion
CH3ACHOH torsion
CAOH torsion
Da2;1;7 þ Da2;1;8 þ Da2;1;9 þ Da8;1;9 þ Da7;1;9 þ Da7;1;8
=
6
pffiffiffi
ꢁ
2Da2;1;7 ꢀ Da2;1;8 ꢀ Da2;1;9
=
6
pffiffiffi
ꢁ
Da2;1;8 ꢀ Da2;1;9
=
2
pffiffiffi
ꢁ
2Da8;1;9 ꢀ Da7;1;9 ꢀ Da7;1;8
=
6
pffiffiffi
ꢁ
Da7;1;9 ꢀ Da7;1;8
=
2
pffiffiffi
ꢁ
Da4;6;12 þ Da4;6;13 þ Da4;6;14 þ Da13;6;14 þ Da12;6;14 þ Da12;6;13
=
6
pffiffiffi
ꢁ
2Da4;6;12 ꢀ Da4;6;13 ꢀ Da4;6;14
=
6
pffiffiffi
ꢁ
Da4;6;13 ꢀ Da4;6;14
=
6
pffiffiffi
ꢁ
2Da13;6;14 ꢀ Da12;6;14 ꢀ Da12;6;13
=
6
pffiffiffi
ꢁ
Da12;6;14 ꢀ Da12;6;13
=
2
pffiffi
ꢁ
Ds10;4;1;9 þ Ds6;4;1;8 þ Ds5;4;1;7
=
3
pffiffiffi
ꢁ
Ds3;4;6;13 þ Ds5;4;6;14 þ Ds10;4;6;12
=
3
S32 = Ds3,4,5,11
pffiffiffi
ꢀ
ꢁ
ꢁ
ꢁ
ꢁ
AC„CA lin bend
AC„CA lin bend
AC„CA lin bend
AC„CA lin bend
S33
S34
S35
S36
¼
¼
¼
¼
Dh1;2;3 þ Dh2;3;4
Dh1;2;3 ꢀ Dh2;3;4
Dh1;2;3 þ Dh2;3;4
Dh1;2;3 ꢀ Dh2;3;4
=
=
=
=
2
pffiffiffi
ꢀ
ꢀ
ꢀ
2
pffiffiffi
2
pffiffiffi
2
a
See Fig. 1 for numbering of the atoms.
together with the assigned fundamentals of the aMe
conformer.
4. Discussion
The PED (potential energy distribution) given in Table 3
(100F ii L2ij=kj) is expressed in terms of the internal coordi-
nates. Only PED terms larger than 10 have been included
in the table. The infrared intensities, Raman scattering
cross sections and Raman polarization ratios for the two
low energy conformers resulting from the B3LYP/cc-pVTZ
calculations are included in Table 3.
4.1. Conformations
PEOL has three possible conformers, of which the aMe
and aH have a quite low energy difference, whereas the aC
conformer has a much higher energy leading to fairly sim-
ilar concentrations of the two former and a negligible con-
centration of the latter at room temperature.
The scaled calculated wavenumbers for the aMe and
aH conformers of the same mode for PEOL (Table 3)
reveal that in 8 cases the shifts were equal to or larger
than 10 cmꢀ1, meaning that in 27 cases the shifts were
smaller than this value. Obviously, the chance for
observing separate band pairs for the aMe and aH rota-
mers is highest when the calculations suggest large sepa-
rations. Mostly, the scaled wavenumbers gave
qualitatively a reasonable agreement with the observa-
tions as demonstrated by the observed and calculated
shifts Dm(aH ꢀ aMe) (Table 3).
In spite of many attempts regarding cooling and anneal-
ing to various temperatures, no definite spectral changes
were obtained. Thus, the observations strongly indicate
that PEOL did not form crystals neither in the Raman
nor in the infrared cryostats at the conditions applied. This
result is not unexpected from the very low barrier between
the aH and aMe conformers (Fig. 11) amounting to only
2 kJ molꢀ1
.
Moreover, the enthalpy difference between these two
conformers is also very low, leading to relatively small dif-
ferences in concentrations of the two conformers also at

Table 3
Observed and calculated fundamental modes of 3-pentyn-2-ol, CH3C„CCH(OH)CH3 (PEOL) including infrared and Raman intensities and Raman depolarization ratios
aMe
aH
b
Obs.a Freq.
Scaled Freq.
Calc.c Freq.
IIR
IR
83
q
PEDd
Obs.a Freq.
Scaledb Freq.
Calc.c Freq.
IIR
IR
61
q
dne obs
dnf calc.
3656
3049
3001
2989
2981
2943
2922
2877
2228
1459
1447
1447
1447
1426g
1419h
1370
1362
1260
1160
1078
1068
1033g
1029
999
3674
3007
3002
2969
2969
2934
2916
2858
2223
1465
1455
1448
1447
1387
1387
1369
1327
1260
1151
1089
1064
1035
1033
984
890
716
552
533
395
331
307
254
224
140
112
21
3816
3120
3115
3080
3080
3044
3025
2966
2349
1498
1487
1480
1479
1418
1412
1395
1355
1281
1170
1103
1080
1056
1054
997
900
727
555
537
396
334
308
267
236
141
113
22
24
20
21
8
0.24
0.73
0.70
0.75
0.75
0.01
0.04
0.21
0.27
0.74
0.66
0.75
0.75
0.39
0.28
0.40
0.72
0.72
0.23
0.50
0.15
0.55
0.64
0.70
0.20
0.29
0.54
0.42
0.70
0.72
0.71
0.75
0.72
0.74
0.75
0.69
100 S1
88 S7, 12 S8
12 S7, 88 S8
61 S4, 38 S5
38 S4, 61 S5
100 S6
100 S3
99 S2
84 S12
15 S28, 75 S29
74 S28, 14 S29
43 S23, 48 S24
48 S23, 43 S24
92 S20
12 S11, 62 S25
19 S14, 34 S16, 29 S25
75 S15, 11 S16
3648
3660
2998
2988
2969
2969
2928
2922
2916
2224
1460
1458
1448
1447
1389
1385
1373
1319
1253
1158
1082
1049
1035
1032
995
880
703
553
535
394
335
305
239
232
139
114
19
3802
3111
3100
3080
3080
3038
3032
3025
2349
1492
1490
1481
1479
1419
1410
1403
1347
1272
1178
1091
1067
1057
1054
1010
890
23
24
29
9
0.20
0.64
0.53
0.75
0.75
0.53
0.02
0.04
0.27
0.73
0.70
0.75
0.75
0.40
0.75
0.47
0.39
0.26
0.21
0.54
0.53
0.64
0.72
0.67
0.15
0.21
0.66
0.33
0.74
0.70
0.74
0.72
0.70
0.73
0.75
0.66
ꢀ8
ꢀ14
ꢀ9
ꢀ14
0
45
98
93
99
163
292
145
294
8
109
48
99
92
67
236
287
294
6
9
9
10
28
5
2
9
3
2
7
1
1
0
3
8
2
4
5
3
5
4
0
2
2
2
2999
ꢀ2
9
9
0
16
27
41
11
2
4
7
8
1
24
47
17
54
8
27
30
1
15
13
27
12
0
8
8
8
1
69
11
17
11
24
33
51
1
ꢀ6
+6
+58
+1
ꢀ5
+3
0
2938
2284
+5
+7
8
10
10
28
6
5
6
3
2
3
6
1
1
3
8
2
4
4
3
0
+2
ꢀ2
+4
ꢀ8
ꢀ7
7
ꢀ7
ꢀ15
0
ꢀ1
+11
ꢀ10
ꢀ13
+1
+2
ꢀ1
+4
ꢀ2
ꢀ15
+8
ꢀ1
+2
ꢀ2
133
1245
ꢀ29
ꢀ15
44 S14, 10 S15, 28 S16
16 S10, 15 S11, 25 S13, 13 S27
30 S9, 10 S13, 16 S26, 20 S27
18 S11, 10 S14, 28 S26
13 S21, 71 S22
1040g
888
+7
3
71 S21, 15 S22
1
53
17
6
3
7
6
18
3
81
32
2
22 S9, 20 S10, 10 S11, 18 S27
40 S9, 17 S11, 11 S26, 11 S27
20 S20, 16 S13, 25 S17, 11 S26
30 S18, 10 S19, 11 S34, 19 S36
13 S10, 32 S17, 18 S34
12 S18, 35 S19, 14 S34, 11 S36
18 S19, 53 S36
27 S18, 15 S33, 40 S34
22 S31, 69 S32
63 S31, 29 S32
47
16
9
6
6
8
20
5
10
93
2
910i
ꢀ22
ꢀ14
738g
567
714
555
539
396
338
306
251
243
535g
390
ꢀ4
324
6
3
2
1
1
2
0
305g
257
204
135
120g
12 S17, 25 S33, 46 S35
18 S18, 40 S33, 22 S35
95 S30
140
114
20
1
0
2
0
0
a
The wavenumbers are derived from infrared vapour spectra, except when noted.
b
c
d
e
f
For scaling, see text.
The calculations are based upon the DFT basis set B3LYP/cc-pVQZ.
PED terms below 10 % are neglected.
Dm(aH ꢀ aMe) obs.
Dm(aH ꢀ aMe) calc.
g
h
i
From Raman liquid.
From IR matrix.
From IR liquid.

A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
111
liquid nitrogen temperature (78 K) preventing crystalliza-
tion. Another consequence of the small barrier is that only
the lowest energy conformer will be effectively isolated in
the matrix spectra. From the curves presented by Barnes
[12] the high temperature conformer will not be trapped
above 10 K, but convert to the low energy conformer.
Accordingly, negligible annealing of the matrices above
the deposition temperature of 5 K will convert the high
energy (aH) to the low energy (aMe)conformer. Therefore,
the matrix isolation spectra will not give reliable informa-
tion, and the attribution of IR and Raman bands to the
respective conformers of PEOL is quite uncertain and relies
heavily on the results of the calculations.
The scaled wavenumbers derived from the calculations
(Table 3) support the conclusion that the aMe conformer
is the low energy conformer. As is apparent from Table 3,
the Dm (aH ꢀ aMe) of the observed and calculated shifts
are in reasonable agreement. Had it been the other way
around, and the aH conformation was the low energy form,
much larger discrepancies between the observed and calcu-
lated wavenumbers shifts would have been observed.
Except for the lowest torsional mode m36, all the aMe fun-
damentals have been assigned, whereas 13 modes in aH
have been tentatively assigned.
of m8 are calculated to be 58 cmꢀ1 instead of the observed
shift which is merely 7 cmꢀ1
.
Two very intense peaks were observed in the Raman
spectra of the liquid at 2255 and 2221 cmꢀ1 with medium
intense counterparts in IR. This position is in good agree-
ment with the group frequencies for acetylenic molecules.
The calculated shift is only 1 cmꢀ1, however, but tenta-
tively the two C„C stretching modes for the aMe and aH
conformers are described to these bands. A possible expla-
nation is to attribute the 2258 cmꢀ1 bands to a combina-
tion band or an overtone which was enhanced by Fermi
resonance to the m9 mode 2228 cmꢀ1
.
The various CH3 antisymmetric and symmetric defor-
mations and the CH deformation are expected in the range
1465–980 cmꢀ1. It is characteristic that most of these
modes (m1–m24) have overlapping fundamentals of the two
more stable conformers, aMe.and aH as seen from the spec-
tra and from the results of the calculations. Moreover, the
antisymmetric CH3 deformation modes m11–m13 seem to
coincide at 1447 cmꢀ1 while m10 is found at slightly higher
wavenumber at 1459 cmꢀ1. The two modes m14 and m15 were
found around 1426 and at 1415 cmꢀ1 and are mainly due to
CH3symmetric deformations, whereas m16 and m17 at 1370
and 1362 cmꢀ1 involve mainly CAH twist and bend. The
CAH bending supposedly gives rise to an aH fundamental
at 1433 cmꢀ1, and the predominantly CAOH bend at
4.2. Spectral assignments
1260 cmꢀ1(m18) has an aH component at 1245 cmꢀ1
.
The ‘‘free’’ OH stretching mode (m1) appeared as weak
vapour bands at 3656 and 3648 cmꢀ1 for aMe and aH,
respectively, whereas in the matrix these fundamentals were
detected at 3649 and 3641 cmꢀ1, respectively. Since small
residual moisture was present in the sample, the vapour
and matrix spectra were affected by sharp water bands in
this region. In the liquid at ambient temperature and in
the solid at 78 K the H-bonded OH stretches were observed
The three highly mixed fundamentals m19, m20 and m21 of
aMe are attributed to the bands at 1160, 1078 and
1068 cmꢀ1 and no counterparts of the aH conformer were
detected. Three Raman bands at 1081, 1033 and
1002 cmꢀ1 were enhanced at lower temperature and the
first and the last of these were strong and employed in
the Miller–Harney calculations (see above). They were
assigned to m21, m22 and m24 of the aMe conformer, respec-
tively. A very weak shoulder at 1029 cmꢀ1 in the Raman
spectrum of the liquid was tentatively attributed to m23 of
the aMe conformer. Three Raman bands observed in the
liquid at 889, 730 and 529 cmꢀ1 all diminished in intensities
at lower temperatures and are equipped with arrows point-
ing downwards in Table 1. They are assigned to the aH con-
former of m25, m26 and m28, respectively. All these bands were
employed in the van’t Hoff calculations (see Fig. 5). Their
aMe counterparts were suggested at 910, 738 and
535 cmꢀ1, although the very weak band at 910 cmꢀ1 was
observed only in the infrared spectra of the liquid and
solid.
The remaining low frequency vibrational modes of the
aMe conformer in PEOL, m29 ꢀ m35 are attributed to IR
and/or Raman bands in the range 390–120 cmꢀ1. In accor-
dance with the small wavenumber shifts between the con-
formers, they are probably coinciding and no alternatives
for the aH conformer were detected. The only exception
is the fundamental m32 attributed to CAOH torsion, which
was calculated to be situated at 254 and 239 cmꢀ1, respec-
tively, in aMe and aH, but no indication of an aH band was
detected. The lowest torsional mode m36 was calculated at
as very broad, intense IR bands at 3351 and 3250 cmꢀ1
,
respectively, apparently coinciding for both conformers.
In the Raman spectra of the liquid the OH stretching bands
gave rise to very weak uncertain peaks.
Except when noted, the assignments are made to the low
energy conformer aMe. One CAH and six CH3 stretches
give rise to seven IR and Raman bands in the range
3050–2850 cmꢀ1 of variable intensities. The IR bands at
3049 and 3001 cmꢀ1 without Raman counterparts are
assigned to the CH3 asymmetric stretches (neighbouring
the C„CA) m2 and m3. A possible m3 mode for the aH con-
former is situated at 2999 cmꢀ1. Additional CH3 asymmet-
ric stretching modes (neighbouring HOCH) m5 and m6 are
attributed to the vapour bands at 2981 and 2943 cmꢀ1
.
The Raman band at 2923 cmꢀ1 is the most intense in the
entire spectrum and is attributed to coinciding sym. CH3
stretches (m7) for aMe and aH while the IR vapour bands
are separated by 5 cmꢀ1 and appear at 2938 and
2943 cmꢀ1 for the two conformers. The C–H stretch (m8)
of aMe and aH, respectively, are attributed to the IR vapour
bands at 2877 and 2884 cmꢀ1 and their Raman counter-
parts. However, the calculated conformer shifts (Table 3)

112
A. Horn et al. / Journal of Molecular Structure 879 (2008) 102–112
21 cmꢀ1 with very low IR and Raman intensities, and as
expected this mode was not detected.
[6] F.A. Miller, B.M. Harney, Appl. Spectrosc. 24 (1970) 291.
[7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.
Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N.
Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B.
Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.
Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg,
V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O.
Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Fores-
man, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski,
B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen,
M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision B.04,
Gaussian, Inc., Pittsburgh PA, 2003.
As is apparent from Table 1 a number of observed infra-
red and Raman bands are left unassigned. Many of these,
particularly the infrared bands, are probably due to combi-
nation bands or overtones, sometimes enhanced by Fermi
resonance. In other cases they are interpreted as impurity
bands, since their intensities – in the beginning very low –
increased with aging of the sample, kept in an evacuated
ampoule in a refrigerator. Examples are the Raman weak
bands at 2120, 1672, 1659, 842, 821, 664 and 584 cmꢀ1
which increased in intensities with storage time.
References
[1] J.T. Lowe, J.S. Panek, Org. Lett. 7 (2005) 1529.
[2] F.A.J. Singelenberg, T. Visser, J.H. van der Maas, J. Mol. Struct. 175
(1988) 239.
[8] J.A. Montgomery Jr., M.J. Frisch, J.W. Ochterski, G.A. Petersson, J.
Chem. Phys. 110 (1999) 2822.
[3] T. Visser, J.H. van der Maas, Spectrochim. Acta 39A (1983) 241;
T. Visser, J.H. van der Maas, Spectrochim. Acta 40A (1984) 959;
T. Visser, J.H. van der Maas, Spectrochim. Acta 41A (1985) 757.
[4] M.H. Langoor, J. H van der Maas, J. Mol. Struct. 403 (1997) 213.
[5] G. W Gribble, H.H. Joyner, F.L. Swizer, Syntetic Commun. 22 (1992)
2997.
[9] C. Tanaka, Nippon Kagaku Zasshi 83 (1962) 521.
`
[10] S. Sportouch, R. Gaufres, Comptes Rendues, Serie C: Sciences
Chimiques 267 (1968) 598.
[11] I.M. Mills, H.W. Thompson, Proc. Roy. Soc. (London) A226 (1954)
306.
[12] A.J. Barnes, J. Mol. Struct. 113 (1984) 161.