Journal of Molecular Structure 922 (2009) 83–87
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Microwave spectra and structural parameters of equatorial-trans cyclobutanol
Wei Lin a,b,1, Arindam Ganguly c,2, Andrea J. Minei b, Glen L. Lindeke b, Wallace C. Pringle b,
Stewart E. Novick b, James R. Durig c,*
a
Department of Natural Sciences and Mathematics, University of Saint Mary, Leavenworth, KS 66048, USA
Department of Chemistry, Wesleyan University, Middletown, CT 06459, USA
c
Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA
b
a r t i c l e
i n f o
Article history:
Received 26 December 2008
Received in revised form 18 January 2009
Accepted 19 January 2009
Available online 30 January 2009
Keywords:
Microwave spectroscopy
r0 structural parameters
rs structural parameters
Ab initio calculations
Cyclobutanol
a b s t r a c t
The microwave spectra of the three singly substituted 13C isotopomers and 18O isotopomer of equatorialtrans cyclobutanol, c-C4H7OH, have been observed in natural abundance by a pulsed-jet Fourier transform
microwave spectrometer. The fit for the normal species was improved from the previous study. The rotational constants for the a-13C, b-13C, c-13C and 18O isotopic species were determined. The five quartic centrifugal distortion constants were determined for the first time. These experimental values are compared
to those obtained from ab initio and density functional theory calculations. By utilizing the previously
reported microwave rotational constants for the O–D species along with the 15 constants determined
from this study combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r0 parameters have been obtained. The determined heavy atom distances in Å are:
r0(C1–C4,5) = 1.547(3), r0(C6–C4,5) = 1.556(3), r0(C–O) = 1.412(3) Å and angles O2 C1 C4;5 ¼ 120:2ð5Þ,
C4 C1 C5 ¼ 88:9ð5Þ and puckering angle sC6C5C4C1 = 31.3(10)°. The parameters are compared to the similar
ones for some other monosubstituted cyclobutanes as well as to those for cyclobutane.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
The geometry of four-membered rings is governed by the competition between ring strain that favors a planar ring, and the torsional force, which drives the molecule towards non-planarity.
Cyclobutane, c-C4H8, is a puckered molecule of D2d symmetry with
a microwave determined [1,2] puckered angle r0 of 28.32 ± 0.23°
and theoretically predicted values of 29.59° [3] and 29.68° [4].
Monosubstitution of cyclobutane can, therefore, lead to both axial
and equatorial conformations. Initially there was controversy
regarding the presence of the axial form for the cyclobutyl halides.
The initial analysis of the microwave spectra of the chloride and
fluoride [5] failed to detect the axial form even though there was
vibrational data [6] for the chloride which clearly showed the presence of the second conformer. From more recent microwave investigations of the fluoride [7] and chloride [8] the axial forms have
been identified so the substitution of a group with an asymmetric
rotor such as OH, SH, NH2, PH2, etc., could lead to the presence of
four conformers. However, in both the previous microwave investigations of the cyclobutanol [9] and the cyclobutylamine [10,11],
* Corresponding author. Tel.: +01 816 235 6038; fax: +01 816 235 2290.
E-mail address: durigj@umkc.edu (J.R. Durig).
1
Taken in part from the thesis of Wei Lin, which has been submitted to Wesleyan
University in partial fulfillment of the Ph.D. degree.
2
Taken in part from the thesis of Arindam Ganguly, which will be submitted to
UMKC in partial fulfillment of the Ph.D. degree.
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.01.040
only one conformer was identified. For the cyclobutanol the spectrum of the equatorial-trans form was predicted from the previously reported parameters for the cyclobutyl chloride [5] and
methanol [12]. Since only the normal and O–D isotopomers were
investigated [9] very little structural information was obtained in
this initial microwave investigation of cyclobutanol. In fact, there
is limited structural information on the monosubstituted cyclobutanes which could be part of the reason why the second or even the
third conformers of monosubtituted cyclobutanes containing an
asymmetric rotor have not be identified in the microwave spectra.
Jonvik and Boggs [13] calculated the structure parameters by ab
initio Hartree–Fock gradient calculation with a 4–21 basis set augmented by d-functions on the O and N atoms. Gunde [14] et al. attempted to analyze the pucker-internal rotation modes of
cyclobutanol using a semi-rigid two-dimensional model. Due to
the lack of structural information, they used the results from the
extended SCF computations (4-31G level) to fit the experimental
rotational constants from the previous microwave investigation.
Recently, Durig [15] et al . studied the temperature dependent
infrared spectra of cyclobutanol in xenon solutions and carried
out ab initio calculations combined with the previously reported
[9] rotational constants to obtained the r0 structural parameters.
In order to determine more detailed structural information on
the four-membered ring of cyclobutanol, a microwave study was
initiated using a pulsed-jet Fourier transform spectrometer where
the three 13C isotopomers and 18O isotopomer should be identifiable in natural abundance owing to the high sensitivity of FTMW.
84
W. Lin et al. / Journal of Molecular Structure 922 (2009) 83–87
To assist in obtaining the complete structure for cyclobutanol, ab
initio calculations have been carried out particularly to predict
the carbon–carbon distances. The results of this spectroscopic
and theoretical study are reported herein.
2. Experiment and ab initio calculations
The cyclobutanol sample was purchased from Aldrich Chemical
Co., Milwaukee, WI with a stated purity of 99%, and the sample was
used without further purification. The microwave data were obtained on the pulsed-jet Fourier transform microwave spectrometer [16,17] of the Southern New England Microwave Consortium
located at Wesleyan University. The Fourier transform microwave
spectrometer consists of two large coaxial concave mirrors in an
evacuated Fabry–Perot cavity which allows gas to be pulsed from
a nozzle (about 1 ms pulse width) and forms the supersonic expansion. The rapid adiabatic expansion of the gas allows the molecules
to be cooled to very low effective temperatures where only the
lowest vibrational states and rotational levels are populated. One
advantage of the FTMW spectrometer is time averaging of signals
from many pulses on the same molecules. This increases signalto-noise ratio and allows weak intensity transitions to be successfully measured. The frequency range of the FTMW spectrometer is
5–25 GHz and the frequencies for the measured transitions are
listed in Table 1.
The ab initio calculations were performed with the Gaussian-03
program [18] by using Gaussian-type basis functions. The energy
minima with respect to nuclear coordinates were obtained by the
simultaneous relaxation of all the geometric parameters using the
gradient method of Pulay [19]. Calculations were carried out with
full electron correlation by the perturbation method [20] to second
order and only the 6-311+G(d,p) basis set was utilized for predicting the structural parameters. Density functional theory (DFT) calculations utilizing the B3LYP method were also carried out with
the 6-311+G(d,p) basis set.
3. Results
In order to assign the microwave spectra of the four singly
substituted 13C (1.1%) and 18O (0.2%) isotopomers (Fig. 1), the rotational constants from the parent species were utilized. Rotational
spectra of the parent and OD isotopomers were reported by Macdonald et al. [9]. We first improve the fit of the normal species
(all 12C atoms) by remeasuring the reported transitions more precisely due to the high resolution of FTMW spectrometer and measuring some new transitions. From these data, we improved the
values of the rotational constants and determined the quartic centrifugal distortion constants. By utilizing the data from the normal
species the rotational constants for each of the 13C isotopomers
were calculated by recalculating the center of mass and principal
axes based on the masses of all of the atoms. It was assumed that
the centrifugal distortion constants of the isotopomers would be
nearly the same as those of the normal isotopomer. Since the
intensities of b-carbon transitions are twice that of the other two
Table 1
Measured rotational frequencies (MHz) and rotational constants of five isotopomers of cyclobutanol.
Transition
12
212
312
111
827
524
312
101
726
413
202
523
625
514
515
524
110
212
423
303
202
413
211
322
221
616
615
624
220
404
313
303
211
322
321
312
5063.514
5165.607
5836.838
6392.824
6701.391
6747.231
7703.999
8483.269
8579.000
8620.681
9475.276
10666.826
12770.710
12775.064
12777.203
14402.731
14546.089
14678.565
15114.491
15319.411
15631.318
16269.867
16276.082
17510.444
17524.224
17636.694
18819.672
20184.689
20749.493
21766.327
22763.686
22968.596
23111.888
23460.141
24346.251
202
313
101
817
432
220
000
716
414
110
431
615
515
423
514
000
111
413
211
101
321
110
312
211
524
616
532
212
312
212
202
101
221
220
211
C
10119.793(1)
4282.9531(5)
3421.0510(4)
A
B
C
a
O–Ca
a-13C
O–Ca
b-13C
0
5
1
4
3
0
2
2
1
0
1
3
8
4
3
1
1
2
7
0
2
1
0
2
3
4
4
5
7
4
1
2
0
0
1
5055.448
0
5821.801
0
5616.469
7684.843
0
8607.270
O–Ca
c-13C
O–Ca
18
O–Ca
7336.604
3
O
5180.025
0
1
5928.161
0
7662.803
1
7554.863
0
1
8727.855
1
8271.347
1
14360.117
14516.192
14659.755
1
1
0
14164.024
14440.827
13955.505
0
2
1
14312.191
14280.540
15050.353
1
1
0
14176.305
13890.994
15580.650
1
2
0
15282.547
15581.712
16223.136
16242.625
17465.336
1
0
3
1
1
15229.075
16145.027
16210.328
15586.161
16849.338
0
0
1
2
2
15028.678
14705.564
15938.869
16594.008
17784.411
2
0
0
0
0
14602.235
1
15455.381
17045.661
18172.431
2
2
3
19600.106
0
21603.862
22609.688
22711.551
22988.287
23366.939
24251.469
1
1
0
0
0
1
21372.395
22345.409
22696.198
22664.488
22983.611
23854.891
0
0
0
1
0
0
20793.959
21729.709
1
1
23136.484
4
21722.333
22711.841
22898.406
23054.419
23397.048
24277.132
10090.9682(8)
4269.1624(8)
3415.6832(7)
Observed minus calculated frequencies in kHz.
0
0
1
0
0
0
9890.2553(9)
4273.7829(8)
3389.0218(6)
10120.1857(5)
4192.0196(5)
3362.8462(3)
10116.9112(8)
4059.4080(4)
3277.2019(4)
85
W. Lin et al. / Journal of Molecular Structure 922 (2009) 83–87
Fig. 1. Equatorial-trans cyclobutanol showing atomic numbering and the plane of
symmetry for the Cs molecule and for the two equivalent C1 equatorial-gauche
forms the \C1 O2 H7 is rotated approximately ±120°.
isotopic substitutions due to the plane of symmetry (molecular
symmetry Cs) the search for its transitions based on the predicted
spectrum was made first. Scans of 10 MHz from which many
pulses of each increment were obtained over the expected frequencies and candidates with appropriate expected intensities
were identified. Four transitions that were exceptional strong for
the normal species were used for the search of the isotopomers.
Based on the assignments for the b-carbon, the a-carbon and ccarbon isotopomers the transitions for the 18O isotopomer were
identified and assigned. The intensities of the 13C transitions are
about 1% of the corresponding transitions of the parent species.
Within the frequency range of 5–25 GHz, thirty five transitions,
both a-type and c-type, were measured for the parent isotopomer.
These transitions were fit to a Hamiltonian in an Ir representation,
Watson A reduction using SPFIT program developed by Pickett
[21]. The rms of the fit is 2 kHz, approximately same as the experimental error. Transitions for each of the singly substituted isotopomers were then fit to the same Hamiltonian. The measured
transition frequencies of all five isotopomers, the differences from
the calculated frequencies, and the fitted rotational constants are
collected in Table 1. Including the normal species there are total
of 15 rotational constants all from the substitution of the ring carbons and oxygen, which should provide excellent structural
parameters for the ring.
The earlier microwave study on cyclobutanol [9] did not determine the centrifugal distortion constants for this molecule. With
measurement of the new transitions and the improved precision
of the previously measured transitions, it is possible to include
the five quartic centrifugal distortion constants in the fit. Since
there are 18 transitions for each of the 13C isotopomers measured,
made it possible to obtain meaningful values for the distortion
constants for these isotopomers. The fitted centrifugal distortion
constants are listed in Table 2. In the fit of the 18O isotopomer,
we utilized the five quartic centrifugal distortion constants of the
normal species to obtain the value for the rotational constants.
From the force constants obtained from the ab initio MP2(full)/
6-31G(d) and MP2(full)/6-311+G(d,p) calculations the values of the
centrifugal distortion constants have been predicted. The values of
the predicted constants are in very good agreement with the
experimentally determined values for the parent species. Also,
the results from the B3LYP/6-31G(d) predicted force constants
for obtaining the values for the centrifugal distortion constants
are within a few percentages to the experimental values for the
normal species. Thus, the B3LYP calculations were used to predict
the centrifugal distortion constants of the other isotopomers. The
natural abundance of the isotopomers are about 1.1% and 0.2%
for 13C and 18O, respectively. Consequently only the low K transitions with the relatively large predicted intensities could be measured for these singly substituted species. Thus, the low
concentration of the 18O isotope restricted the number of transitions that could be measured so that meaningful distortion constants could not be determined from these small numbers. As
can be seen from the small predicted differences of the distortion
constants for the various isotopomers, the distortion constants
for these isotopomers must be nearly the same as those for the
normal species. The calculated values are listed for each of the isotopomers along with the experimental values for the normal species in Table 2. Overall the measured centrifugal distortion
constants are of small magnitude, which indicates fairly rigid
structure for cyclobutanol. The DJ value for the parent species of
cyclobutanol is 0.692(2) kHz, whereas the corresponding value
for cyclobutane-d1 (equatorial) [1] is 6.8(25) kHz. Substituting
the hydrogen in the equatorial position by hydroxyl group reduces
the centrifugal distortion constant by almost one order than
substituting by deuterium atom. A similar effect can be found for
chlorocyclobutane [22], where the reported DJ value is
0.391(8) kHz.
Since we have measured the rotational constants the normal
isotopomer of cyclobutanol along with those of the three singly
substituted 13C isotopomers and one 18O isotopomers, we calculate
a heavy-atom Kraitchman substitution structure, rs, for the molecule using the structure program first written by Schwendeman
and modified by Hillig [23]. In this calculation we also included
the previously measured cyclobutanol-OD constants. We fit only
the parameters that concerns the substituted species, while keeping the remaining ones (the parameters of the other seven hydrogen atoms) at the values from the ab initio values. The fitted
puckering angle is 29.8(8)°. The resulting structure is listed in Table 3 along with the other structural parameters.
Durig et al. [24] have shown that ab initio MP2(full)/6311+G(d,p) calculations predict the ro structural parameters for
more than 50 carbon–hydrogen distances better than 0.002 Å compared to the experimentally determined values from isolated CH
stretching frequencies which were compared [25] to previously
determined values from earlier microwave studies. Thus, all of
the carbon–hydrogen distances can be taken from the MP2(full)/
6-311+G(d,p) predicted values for cyclobutanol. Also, we have
found [26] that we can obtain good structural parameters by
adjusting the structural parameters obtained from the ab initio calculations to fit the rotational constants obtained from the microwave experimental data. In order to reduce the number of
Table 2
Comparison of the quadratic centrifugal distortion constants (kHz) for isotopomers of equatorial-trans cyclobutanol.
c-C4H7OH
DJ
DJK
DK
dJ
dK
a
a-13C cyclobutanol
b-13C cyclobutanol
c-13C cyclobutanol
18
O cyclobutanol
MP2/
6-31G(d)
MP2/
6-311+G(d,p)
B3LYP/
6-31G(d)
Exp. this
study
B3LYP/
6-31G(d)
Exp. this
study
B3LYP/
6- 31G(d)
Exp. this
study
B3LYP/
6-31G(d)
Exp. this
study
B3LYP/
6-31G(d)
This
studya
0.637
2.22
2.69
0.112
1.69
0.646
2.29
2.66
0.113
1.71
0.670
2.04
3.60
0.119
1.66
0.692(8)
2.08(3)
3.68(8)
0.127(2)
2.05(20)
0.662
1.98
3.69
0.112
1.62
0.620(3)
2.13(7)
3.60(2)
0.144(1)
1.09(31)
0.665
2.05
3.47
0.121
1.66
0.679(3)
2.22(7)
3.48(2)
0.134(1)
1.68(29)
0.641
1.92
3.76
0.112
1.60
0.598(4)
1.90(4)
4.32(3)
0.156(7)
1.02(19)
0.620
1.93
3.82
0.106
1.61
0.692
2.08
3.68
0.127
2.05
The values were held constant to the values obtained for the normal species.
86
W. Lin et al. / Journal of Molecular Structure 922 (2009) 83–87
Table 3
Structural parameters (Å and degree), rotational constants (MHz) and dipole moment (Debye) for cyclobutanol equatorial-trans (Cs) conformer.
Structural parameters
rC1–O2
rC1–C4, C5
rC6–C4, C5
rO2–H7
rC1–H3
rC4–H8, C5–H10
rC4–H9, C5–H11
rC6–H12
rC6–H13
O2 C 1 C 4 ; C 5
C4 C1 C5
C6 C4 ðC5 ÞC1
C4 C6 C5
C 1 O2 H 7
H3 C1 C4 ; C5
H3 C1 O2
H8 C4 C1 ; H10 C5 C1
H8 C4 C6 ; H10 C5 C 6
H9 C4 C1 ; H11 C5 C1
H9 C4 C6 ; H11 C5 C6
H8 C4 H9 ; H10 C5 H11
H12 C6 C4 ; C5
H13 C6 C4 ; C5
H12 C6 H13
sH7O2C1H3
sC6C5C4C1
A (MHz)
B (MHz)
C (MHz)
|la|
|lb|
|lc|
|lt|
MP2(full)/6-311+G(d,p)a Eq-trans
MWb Eq-trans
Ab initio/MWc
rs parameters
Adjusted r0 parameters for Eq-trans
1.408
1.542
1.548
0.961
1.093
1.096
1.092
1.091
1.093
120.6
88.3
86.8
87.9
107.0
110.2
105.9
109.8
110.1
118.8
119.6
109.6
118.0
111.1
109.3
180.0
28.5
10246.7
4290.1
3438.8
1.69
0.00
1.00
1.96
1.420*
1.416
1.547
1.552
0.961
1.093
1.095
1.091
1.091
1.093
120.2
88.9
86.6
88.5
107.2
110.4
105.9
109.8
109.2
118.8
120.7
109.6
118.1
110.6
109.3
180.0
30.8
10119.6
4284.0
3422.7
1.410(4)
1.547(3)
1.554(2)
0.956(6)
1.412(3)
1.547(3)
1.556(3)
0.961(3)
1.093(2)
1.096(2)
1.092(2)
1.091(2)
1.093(2)
120.2(5)
88.9(5)
87.1(5)
88.3(5)
107.8(5)
110.4(5)
105.9(5)
109.8(5)
107.9(5)
118.8(5)
121.4(5)
109.6(5)
118.6(5)
110.0(5)
109.3(5)
180.0(10)
31.3(10)
10120.1
4283.1
3421.2
1.525
1.550
0.956
1.100
1.085
1.085
1.100
1.100
121.7*
90.8
82.3
89.2
108.0
112.3
97.0*
114.0
114.0
114.0
115.1
112.0
120.1
108.3
110.0
180.0
19.9
10119.6(10)
4282.9(4)
3421.0(4)
1.39(5)
0.0
0.81(5)
1.62(5)
119.5(4)
88.8(1)
87.6(2)
88.3(1)
108.3(6)
29.8(8)
a
Calculated using MP2(full)/6-311+G(d,p).
All parameters were assumed except the CO distance and two angles indicated with an asterisk with other parameters transferred from the corresponding ones in
cyclobutyl chloride and methanol [9].
c
Ref. [15].
b
independent variables, the structural parameters are separated
into sets according to their types. Bond lengths in the same set
keep their relative ratio which results in only two heavy atoms distances for cyclobutanol. Also, the bond angles, and torsional angles
in the same set keep their differences in degrees. This assumption
is based on the fact that the errors from ab initio calculations are
systematic. Therefore, it should be possible to obtain ‘‘adjusted
r0” structural parameters for the equatorial-trans conformer of
cyclobutanol utilizing the microwave rotational constants from
the reported values for these five isotopomers. Additionally the
microwave spectrum of the O–D isotopomer, c-C4H7OD, was reported earlier and the three rotational constants determined. Thus,
18 rotational constants were used to determine r0 the structural
parameters of cyclobutanol. The determined parameters are listed
in Table 3 along with those previously assumed for initially assigning the microwave spectrum [9] of normal species. The puckering
angle in the adjusted ro structure is 31.3(10)°, which agrees with
the value determined from the structure within the experimental
uncertainties. These values are close to the reported puckering angle of 28.58(9)° for cyclobutane, as expected. The fitted dihedral
angle sH7O2C1C6 is 0° in the adjusted ro structure. The adjusted
ro structure reproduces the rotational constants of the observed
isotopomers to within 0.6 MHz except for the A constant of the
b-13C isotope which differs by 1.5 MHz. The observed and calculated rotational constants for all the isotopomers and their differences are listed in Table 4.
There have been interesting discussions about the effects of
different substituents on cyclopropane in terms of the r and p
electrons withdraw and donation between the substituent and
the ring [27–30]. It has been shown that the C–C bond length adjacent to the site of the substituent and that of the opposite bonds
are sensitive indicators of these effects. Nevertheless, for the
four-membered rings, there will be little change of the structural
Table 4
Comparison of the fit of the rotational constants from the r0 parameters obtained
from ab initio MP2(full)/6-311+G(d,p) predicted parameters adjusted with the 18
microwave determined rotational constants.
Conformer
Isotopomer
Rotational const.
Obs.
Calc.
|D|
Eq-t
c-C4H7OH
Eq-t
c-C4H7OD
Eq-t
a-13C
Eq-t
b-13C
Eq-t
c-13C
Eq-t
18
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
10119.8
4282.9
3421.1
9911.8
4132.7
3347.3
10090.9
4269.2
3415.7
9890.3
4273.8
3389.0
10120.2
4192.0
3362.8
10116.9
4059.4
3277.2
10120.2
4283.2
3421.3
9911.6
4132.4
3347.6
10091.1
4269.2
3415.7
9888.8
4273.8
3389.0
10120.2
4192.1
3363.0
10117.5
4058.8
3276.9
0.4
0.3
0.3
0.2
0.3
0.3
0.2
0.0
0.0
1.5
0.0
0.0
0.0
0.1
0.1
0.6
0.6
0.3
O
W. Lin et al. / Journal of Molecular Structure 922 (2009) 83–87
Table 5
Structural parameters of the ring for a few four-membered ring molecules (Å and
degree).
Structural parameters
Cyclobutanola
Cyclobutaneb
rCa–Cb (Å)
rCb–Cc (Å)
Puckering angle (°)
1.547(3)
1.556(3)
31.3(10)
1.5555(2)
1.5555(2)
28.58(9)
a
b
c
Chlorocyclobutanec
Equatorial
Axial
1.539(3)
1.558(3)
30.7(5)
1.547(3)
1.557(3)
22.3(5)
This study.
Ref. [1].
Ref. [31].
parameters of the ring by substitution [9,31]. Particularly, the earlier microwave work on cyclobutanol had used the ring structure
of chlorocyclobutane from Kim and Gwinn [5] in their structural
fit.
Table 5 lists the puckering angles of cyclobutane, chlorocyclobutane, and cyclobutanol as determined by microwave spectroscopy. There is a small shortening effect on the bond lengths of
the carbon atoms that are adjacent to the substituent atom. The
bond lengths were reduced by 0.009 and 0.017 Å for cyclobutanol
and chlorocyclobutane, respectively. However, the bond length of
the carbon atom that are not adjacent to the substituted carbon
atom remains unchanged. Hoffmann [30] suggested that for substituted cyclopropane, the lengthening C–C bond geminal to the substituent and a corresponding shortening of the C–C bond not
involving the substituted carbon is due to the conjugation between
the unsaturated substituents and the delocalized electron system
of the ring. From a theoretical study [29] it has been shown that
the substituents on cyclopropane ring introduces significant modifications of the C–C bond lengths. In general, the larger C2–C3 and
the shorter C1–C2 bonds correspond to the more electronegative
substituents. Additional data are required for more conclusive discussions on these effects for four-membered rings.
Durig et al. [15] recently measured the temperature dependent
infrared spectrum of cyclobutanol in xenon solutions. Combining
the experimentally observed transitions with ab initio calculation,
they were able to derive the potential function for the internal torsion of cyclobutanol. The enthalpy difference is considered to be at
least 150 cm-1 between the most stable equatorial-trans and the second most stable equatorial-gauche form. There is strong indication
from the low frequency pair that the DH is about 200 cm-1. The predicted dipole moment components [9] for the gauche form are
|la| = 0.6 D, |lb| = 1.2 D, and |lc| = 0.1 D. From the observed OH torsional frequencies, the two wells corresponding to the gauche form
are predicted to be about 233 cm-1 higher in energy. The barrier between these two wells is 704 cm-1. This will possibly split each of the
vibrational energy levels in these two wells into a pair of torsional
states. The b dipole moment is antisymmetric to the torsional motion around the C–O bond. Thus, the b-type transitions will be the
ones connecting the two torsional states. If the barrier is sufficiently
high so that the energy difference between these two 0+ and 0 torsional states is small, these b-type transitions will appear as doublets. The microwave spectrum of this conformer, although more
difficult because of the coupling of possible splittings in the ground
state, could help improve the potential function.
The possibility of the even higher energy axial forms can not be
completely ruled out either. The initial analysis of the microwave
spectra of the cyclobutyl chloride and fluoride [5] failed to detect
the axial form even though there was vibrational data [6] for the
chloride which clearly showed the presence of the second conformer. From more recent microwave investigations [7,8] of these
87
molecules, the axial forms have been identified. To aid in future
search of this equatorial-gauche conformer, we calculated the
structure as well as the rotational constants using the same basis
set. The calculated rotational constants for the gauche form are
A = 10285.9 MHz, B = 4338.9 MHz, and C = 3448.5 MHz. If we assume the same scaling factors for both the trans and gauche forms,
the proposed rotational constants for the gauche form are
A = 10158.5 MHz, B = 4331.7 MHz, and C = 3430.6 MHz. Hopefully
someone will identify and assign the transitions for the gauche
form which should provide interesting scientific information on
the structural parameters and the potential governing the internal
rotation of the conformer.
Acknowledgements
JRD acknowledges the University of Missouri-Kansas City, for a
Faculty Research Grant for partial financial support of this research. WL thanks University of Saint Mary for support from Professional Development Fund.
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