Conformational stability, r0 structural parameters, and vibrational assignments of mono-substituted cyclobutanes: Fluorocyclobutane

Article abstract of DOI:10.1016/j.saa.2010.08.082

Variable temperature (-55 to -100 °C) studies of the infrared spectra (3500-400 cm^-1) of fluorocyclobutane, c-C₄H₇F, dissolved in liquid xenon have been carried out as well as the infrared spectra of the gas. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 496 ± 40 cm^-1 (5.93 ± 0.48 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 8 ± 1%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 732 ± 47 cm^-1 (9.04 ± 0.44 kJ/mol) and the average value of 602 ± 20 cm^-1 from density functional theory predictions by the B3LYP method are significantly larger than the experimentally determined enthalpy value. By utilizing previously reported microwave rotational constants for the equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r₀ parameters have been obtained. The determined heavy atom structural parameters for the equatorial [axial] conformer are: distances () C-F = 1.383(3) [1.407(3)], C_α-C_β = 1.543(3) [1.546(3)], C _β-C_γ = 1.554(3) [1.554(3)] and angles (°) ∠C_αC_βC_γ = 85.0(5) [89.2(5)], ∠C_βC_αC_β = 89.3(5) [89.2(5)], ∠F-(C_βC_αC _β) = 117.4(5) [109.2(5)] and a puckering angle of 37.4(5) [20.7(5)]. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some other monosubstituted cyclobutanes with halogen and pseudo-halogen substituents.

Full text of DOI:10.1016/j.saa.2010.08.082

Spectrochimica Acta Part A 79 (2011) 831–840
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Conformational stability, r0 structural parameters, and vibrational assignments
of mono-substituted cyclobutanes: Fluorocyclobutane
Arindam Gangulya,1, Joshua J. Klaassena,2, Gamil A. Guirgisb, Todor K. Gouneva, James R. Duriga,∗
a Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA
b Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424, USA
a r t i c l e i n f o
a b s t r a c t
Variable temperature (−55 to −100 ◦C) studies of the infrared spectra (3500–400 cm−1) of fluorocy-
clobutane, c-C4H7F, dissolved in liquid xenon have been carried out as well as the infrared spectra
of the gas. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy differ-
ence between the more stable equatorial conformer and the axial form has been determined to be
496 40 cm−1 (5.93 0.48 kJ/mol). The percentage of the axial conformer present at ambient temper-
ature is estimated to be 8 1%. The ab initio MP2(full) average predicted energy difference from a variety
of basis sets is 732 47 cm−1 (9.04 0.44 kJ/mol) and the average value of 602 20 cm−1 from den-
sity functional theory predictions by the B3LYP method are significantly larger than the experimentally
determined enthalpy value. By utilizing previously reported microwave rotational constants for the
equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural
values, adjusted r0 parameters have been obtained. The determined heavy atom structural parame-
Article history:
Received 20 November 2009
Received in revised form 11 July 2010
Accepted 26 August 2010
Keywords:
Infrared spectrum
Xenon solutions
Conformational stability
r0 structural parameters
Fluorocyclobutane
˚
ters for the equatorial [axial] conformer are: distances (A) C–F = 1.383(3) [1.407(3)], C␣–C␤ = 1.543(3)
[1.546(3)], C␤–C␥ = 1.554(3) [1.554(3)] and angles (◦) ∠C␣C␤C␥ = 85.0(5) [89.2(5)], ∠C␤C␣C␤ = 89.3(5)
[89.2(5)], ∠F–(C␤C␣C␤) = 117.4(5) [109.2(5)] and a puckering angle of 37.4(5) [20.7(5)]. The conforma-
tional stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and
vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calcu-
lations and compared to experimental values where available. The results are discussed and compared
to the corresponding properties of some other monosubstituted cyclobutanes with halogen and pseudo-
halogen substituents.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
values [3]. However, from a more recent microwave investigation of
both the equatorial and axial conformers of chlorocyclobutane [4] it
The cyclobutane molecule is puckered in the ground vibrational
state which results from the steric forces being larger than the
ring strain forces. The structural parameters of cyclobutane have
been determined from a microwave investigation of four different
deuterated isotopomers and the puckering angle was determined
[1] to be 29.55(9)◦. Prior to this study only the structural parame-
ters of the equatorial chlorocyclobutane had been reported [2] from
a microwave study but the reported A rotational constants were
found to differ significantly from those parameters obtained by the
weighted least-squares adjusted method from ab initio predicted
has been possible to obtain the complete structural parameters [5]
from the revised rotational constants of the equatorial conformer
along with those for axial conformer combined with those pre-
dicted from the ab initio MP2(full)/6-311+G(d,p) calculations. By
utilizing the ab initio predicted parameters in combination with
previously reported microwave rotational constants it has been
possible to obtain complete r0 structural parameters in addition to
chlorocyclobutane for cyclobutylamine [6], bromocyclobutane [7],
cyanocyclobutane [8], and cyclobutanol [9]. All of these molecules
have at least two conformers present at ambient temperature. By
variable temperature infrared investigations of rare gas solutions
[5–9] the enthalpy differences have been determined after con-
fident vibrational assignments have been made by using ab initio
predicted frequencies, infrared band contours and intensities, along
with Raman activities and depolarization values. From far infrared
and low frequency Raman spectra of the gases it has been possi-
ble to obtain the potential function governing the conformational
interchange for many of these molecules. A comparison of the dif-
∗
Corresponding author. Tel.: +1 816 235 6038; fax: +1 816 235 2290.
E-mail address: durigj@umkc.edu (J.R. Durig).
Taken in part from the thesis of Arindam Ganguly, which will be submitted to
1
UMKC in partial fulfillment of the Ph.D. degree.
2
Taken in part from the thesis of Joshua J. Klaassen, which will be submitted to
UMKC in partial fulfillment of the Ph.D. degree.
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.08.082

832
A. Ganguly et al. / Spectrochimica Acta Part A 79 (2011) 831–840
ferent values of the heavy atom structural parameters and the
barrier to inversion and enthalpy differences has been made along
with predicted energy differences.
As a continuation of our conformational and structural deter-
mination of monosubstituted cyclobutanes with halogen and
pseudo-halogen substituents we have investigated the temper-
ature dependent infrared spectra of fluorocyclobutane in xenon
solutions to obtain the enthalpy difference between the two
conformers. In the initial study [2] of the microwave spec-
trum of fluorocyclobutane the structural parameters could not
be obtained from the three experimentally obtained rotational
constants. Therefore, corresponding parameters from chlorocy-
clobutane were used for fluorocyclobutane along with an estimated
˚
C–F bond distance of 1.37 A. These parameters were sufficient to
provide rotational constants which satisfactorily agreed with the
experimentally determined ones [2].
Later by weighted least-squares adjustment method the C–F dis-
tance and the angle of the fluorine atom to the plane of the ring were
determined [10], by utilizing the three experimentally determined
rotational constants from the initial microwave study [2] along with
the remaining parameters, to be the same as the corresponding
parameters obtained for chlorocyclobutane [3]. Therefore, to obtain
a more complete structural parameter determination for fluorocy-
clobutane we have combined the ab initio MP2(full)/6-311+G(d,p)
predicted parameters with the six experimentally determined rota-
tional constants [4] for the two forms to obtain the complete
structural parameters for both conformers of fluorocyclobutane.
2. Experimental
The fluorocyclobutane sample was prepared by replacing the
chlorine atom in chlorocyclobutane (Sigma–Aldrich Chemical Co.,
with stated purity of 97%) with a fluorine atom by allowing the
chlorocyclobutane to drip onto powdered AgF2 which had been
dried under vacuum at 110 ◦C for 24 h. The sample was purified
by a low-temperature, low-pressure fractionation column and the
purity of the sample was verified by comparing the infrared spec-
trum with that previously reported [11].
Fig. 1. Comparison of experimental and predicted mid-infrared spectra of fluorocy-
clobutane: (A) simulated spectrum of equatorial conformer; (B) observed infrared
spectrum of the gas, asterisk denotes an impurity and (C) Xe solution at −70 ◦C.
simultaneous relaxation of all geometric parameters using the gra-
dient method of Pulay [13]. A variety of basis sets as well as the
corresponding ones with diffuse functions were employed with the
Møller–Plesset perturbation method [14] to the second order MP2
with full electron correlation as well as with the density functional
theory by the B3LYP method. The predicted conformational energy
differences are listed in Table 3.
In order to obtain descriptions of the molecular motions
involved in the fundamental modes of fluorocyclobutane, a nor-
mal coordinate analysis was carried out. The force field in Cartesian
coordinates was obtained with the Gaussian 03 program at the
MP2(full) level with the 6-31G(d) basis set. The internal coordi-
nates used to calculate the G and B matrices are given in Table 4
with the atomic numbering shown in Fig. 2. By using the B matrix
[15], the force field in Cartesian coordinates was converted to a
force field in internal coordinates. Subsequently, 0.88 was used as
the scaling factor for the CH stretches and CH2 scissors whereas
0.90 was used as the scaling factor for all other modes to obtain
The mid-infrared spectrum of the gas (Fig. 1B) was obtained
from 4000 to 250 cm−1 on a Perkin-Elmer model 2000 Fourier
transform spectrometer equipped with a Ge/CsI beamsplitter and
a DTGS detector. Atmospheric water vapor was removed from the
spectrometer housing by purging with dry nitrogen. The theoreti-
cal resolution used to obtain the spectrum of the gas was 0.5 cm−1
.
One hundred twenty-eight interferograms were added and trans-
formed with a boxcar truncation function. The frequencies for the
predicted and observed fundamentals are listed in Tables 1 and 2.
The mid-infrared spectra (4000–400 cm−1) of the sample dis-
solved in liquefied xenon (Fig. 1C) were recorded on a Bruker
model IFS-66 Fourier transform spectrometer equipped with a glo-
bar source, a Ge/KBr beamsplitter and a DTGS detector. In all cases,
100 interferograms were collected at 1.0 cm−1 resolution, aver-
aged and transformed with a boxcar truncation function. For these
studies, a specially designed cryostat cell was used. It consists of a
copper cell with a path length of 4 cm with wedged silicon windows
sealed to the cell with indium gaskets. The copper cell was enclosed
in an evacuated chamber fitted with KBr windows. The tempera-
ture was maintained with boiling liquid nitrogen and monitored by
two Pt thermo resistors.
3. Ab initio calculations
The ab initio calculations were performed with the Gaussian-
03 program [12] using Gaussian-type basis functions. The energy
minima with respect to nuclear coordinates were obtained by the
Fig. 2. Fluorocyclobutane showing atomic numbering.

Table 1
Observeda and calculatedb frequencies for the equatorial conformer of fluorocyclobutane.
Fundamental
Ab initio
Fixed scaledc
IR int.
Raman act.
dp ratio
Gasd
Gasf
Gas
Xe
P.E.D.h
Band contour
A
B
C
Aꢀ
ꢀ1
ꢀ2
ꢀ3
ꢀ4
ꢀ5
ꢀ6
ꢀ7
ꢀ8
␤-CH2 antisymmetric stretch
␥-CH2 antisymmetric stretch
␥-CH2 symmetric stretch
␤-CH2 symmetric stretch
CH stretch
␤-CH2 scissors
␥-CH2 scissors
CH in-plane bend
␤-CH2 wag
␤-CH2 twist
␤-CH2 rock
C–F stretch
Ring breathing
Ring deformation 1
␥-CH2 rock
Ring deformation 2
C–F in-plane bend
Ring puckering
3214
3199
3141
3138
3131
1578
1553
1449
1314
1279
1199
1168
1016
955
3015
3001
2947
2944
2937
1480
1457
1375
1247
1213
1137
1108
964
38.3
5.3
18.7
8.9
27.1
6.4
4.7
44.8
16.4
3.1
25.2
58.9
14.4
1.8
41.5
81.8
142.9
117.4
83.7
6.4
15.4
8.3
2.5
9.2
1.9
8.0
15.2
1.3
0.74
0.51
0.10
0.21
0.28
0.66
0.75
0.66
0.74
0.74
0.67
0.22
0.11
0.74
0.32
0.49
0.49
0.02
3006
2992
3003
2991
2947
2910
2892
1473
1454
1362
1244
1222
1141
1099
961
2995
–
52S1,47S2
50S2,44S1
90S3
92S4
91S5
56S6,38S7
60S7,39S6
49S8,27S17
61S9,16S14
72S10,12S15
41S11,16S17,15S12,15S15,11S18
39S12,40S13
46S13,17S12,14S16
40S14,19S16,17S15,12S9
44S15,25S14,16S10
62S16,14S11,12S12
32S17,27S8,14S18,13S11
72S18,13S8,11S17
–
93
88
27
17
84
–
100
96
85
83
96
79
99
13
94
80
28
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
100
7
2992
2950
2912
2861*
1472
1453
1361
1242
1140
1098
1079*
959
783*
750
599
456
166
2970
2961
2950
2934
1471
1453
1361
1242
1221
1140
1098
959
2950
2895
2878
1466
1446
1358
1240
1218
1136
1090
954
12
73
83
16
100
–
ꢀ9
4
ꢀ10
ꢀ11
ꢀ12
ꢀ13
ꢀ14
ꢀ15
ꢀ16
ꢀ17
ꢀ18
15
17
4
21
1
87
6
20
72
906
751
586
450
851
750
599
456
914
750
599
911
750
599
454
792
618
474
202
3.5
2.3
7.2
1.6
1.9
3.0
0.4
0.1
455
192
166
(166)g
–
Aꢀꢀ
ꢀ19
ꢀ20
ꢀ21
ꢀ22
ꢀ23
ꢀ24
ꢀ25
ꢀ26
ꢀ27
ꢀ28
ꢀ29
ꢀ30
␤-CH2 antisymmetric stretch
␤-CH2 symmetric stretch
␤-CH2 scissors
CH out-of-plane bend
␥-CH2 wag
␥-CH2 twist
␤-CH2 wag
Ring deformation 1
␤-CH2 twist
Ring deformation 2
␤-CH2 rock
3204
3135
1537
1359
1294
1282
1221
1102
974
3006
2941
1442
1289
1228
1216
1158
1045
924
20.1
29.4
3.1
1.4
1.5
0.3
0.3
8.5
2.8
0.5
0.3
3.4
61.6
0.1
7.8
3.7
0.1
5.0
8.6
1.5
1.1
12.6
1.2
1.1
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
2988
2966*
1441
1263
1221
1161
1049
1057e
921
2988
2914*
1441
1246
1263
1216
1161
1026
921
2993
2941
1441
1260
1223
1218
1159
1050
926
2986
2940
1435
1257
1221
1215
1161
1048
921
99S19
99S20
99S21
–
–
–
–
–
–
–
–
–
–
–
–
100
100
100
100
100
100
100
100
100
100
100
100
–
–
–
–
–
–
–
–
–
–
–
–
60S22,16S23
58S23,17S27,16S22
42S24,43S25
46S25,21S24
24S26,29S28,26S27,10S30
42S27,18S24,17S22,14S26
57S28,26S26
79S29,13S24
81S30,12S26
957
821
374
908
779
355
899
835*
371
843
783*
371
897
781
–
893
774
–
C–F out-of-plane bend
a
Observed spectra: gas and Xe are IR.
b
c
d
e
f
4
˚
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (A /u), depolarization ratios (dp) and potential energy distributions (P.E.D.s).
Scaled frequencies with scaling factors of 0.88 for the CH stretches, ␤-CH2 and ␥-CH2 scissors and 0.90 for all other modes.
Frequencies listed are taken from Ref. [11], values marked with an asterisk (*) are from the Raman spectrum, all others are from the IR spectrum of the gas.
Frequency taken from the IR spectrum of the solid.
Frequencies listed are taken from Ref. [10].
Frequency taken from far infrared of the gas.
g
h
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.

Table 2
Observeda and calculatedb frequencies for the axial conformer of fluorocyclobutane.
Fundamental
Ab initio
Fixed scaledc
IR int.
Raman act.
dp ratio
Gasd
Gas
Xe
P.E.D.f
Band contour
A
B
C
Aꢀ
ꢀ1
ꢀ2
ꢀ3
ꢀ4
ꢀ5
ꢀ6
ꢀ7
ꢀ8
␥-CH2 antisymmetric stretch
␤-CH2 antisymmetric stretch
CH stretch
␥-CH2 symmetric stretch
␤-CH2 symmetric stretch
␥-CH2 scissors
␤-CH2 scissors
CH in-plane bend
␤-CH2 wag
␤-CH2 twist
␤-CH2 rock
Ring breathing
Ring deformation 2
Ring deformation 1
C–F stretch
␥-CH2 rock
C–F in-plane bend
Ring puckering
3217
3198
3152
3146
3131
1566
1538
1425
1336
1276
1194
1095
970
3018
3000
2957
2951
2937
1469
1443
1352
1267
1211
1133
1039
920
32.2
4.9
45.1
41.8
3.0
41.3
71.6
179.1
62.1
121.6
5.8
18.2
2.6
1.4
16.4
0.9
19.4
5.5
4.0
4.4
0.74
0.62
0.07
0.15
0.37
0.65
0.75
0.72
0.55
0.74
0.43
0.16
0.09
0.42
0.50
0.60
0.14
0.19
69S1,30S2
67S2,30S1
79S3,17S5
95S4
80S5,16S3
64S6,34S7
64S7,35S6
47S8,29S17,11S10
78S9,14S13
63S10,11S16
35S11,23S15,15S17,15S16,12S18
70S12,13S15
36S13,19S15,15S12,10S10
26S14,26S16,20S13
27S15,28S14,20S11,10S13
39S16,27S14,16S11
30S17,18S8,16S18,15S11,13S14
69S18,13S8,10S17
53
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
47
100
81
44
2
–
70
2
19
40
62
11
28
2
55
90
–
19
56
98
100
30
98
81
60
38
89
72
98
45
10
100
60
3.1
3.9
27.6
11.7
5.8
24.1
17.5
14.4
7.5
16.2
1.7
1.8
1348
1348
ꢀ9
ꢀ10
ꢀ11
ꢀ12
ꢀ13
ꢀ14
ꢀ15
ꢀ16
ꢀ17
ꢀ18
1039
832
1037
832
927
895
708
403
879
849
672
382
825
131
0.6
0.3
0.1
180
171
1.5
(131)e
40
Aꢀꢀ
ꢀ19
ꢀ20
ꢀ21
ꢀ22
ꢀ23
ꢀ24
ꢀ25
ꢀ26
ꢀ27
ꢀ28
ꢀ29
ꢀ30
␤-CH2 antisymmetric stretch
␤-CH2 symmetric stretch
␤-CH2 scissors
CH out-of-plane bend
␥-CH2 wag
␤-CH2 wag
␥-CH2 twist
␤-CH2 twist
3204
3131
1523
1344
1303
1292
1217
1098
979
3006
2937
1429
1275
1236
1226
1155
1042
929
9.3
26.7
4.4
0.8
2.4
0.3
5.2
2.3
0.8
5.8
0.2
3.2
72.4
0.8
6.5
1.5
6.4
0.1
5.5
6.1
14.6
0.6
0.2
0.7
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
97S19
97S20
99S21
–
–
–
–
–
–
–
–
–
–
–
–
100
100
100
100
100
100
100
100
100
100
100
100
–
–
–
–
–
–
–
–
–
–
–
–
46S22,29S23,19S28
31S23,33S25,15S26,11S22
65S24,15S23,11S22
31S25,20S27,16S24,14S23
63S26,10S25
Ring deformation 1
Ring deformation 2
␤-CH2 rock
64S27,13S24
953
779
397
904
739
377
57S28,10S22,10S29
63S29,19S28,11S25
83S30,10S29
C–F out-of-plane bend
a
Observed spectra: gas, Xe, are IR.
b
c
d
e
f
4
˚
MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (A /u), depolarization ratios (dp) and potential energy distributions (P.E.D.s).
Scaled frequencies with scaling factors of 0.88 for the CH stretches, ␤-CH2 and ␥-CH2 scissors and 0.90 for all other modes.
Frequencies listed are taken from Ref. [10].
Frequency taken from far infrared of the gas.
Symmetry coordinates with P.E.D. contribution less than 10% are omitted.

A. Ganguly et al. / Spectrochimica Acta Part A 79 (2011) 831–840
835
Table 3
Calculated energies in H and energy differences (cm−1) for the two conformers, and transition state of fluorocyclobutane.
Method/basis set
Energya, E
Energy differences, ꢁ
Equatorial
Axialb
Planar ringb
1189
Planar ringc
RHF/6-31G(d)
MP2(full)/6-31G(d)
0.952938
1.661929
1.682782
1.719525
1.739104
1.906145
1.915779
1.976511
1.984410
2.065639
2.073066
2.087353
–
583
658
668
722
712
779
793
694
730
531
MP2(full)/6-31+G(d)
MP2(full)/6-31G(d,p)
MP2(full)/6-31+G(d,p)
MP2(full)/6-311G(d,p)
MP2(full)/6-311+G(d,p)
MP2(full)/6-311G(2d,2p)
MP2(full)/6-311+G(2d,2p)
MP2(full)/6-311G(2df,2pd)
MP2(full)/6-311+G(2df,2pd)
MP2(full)/aug-cc-pVTZ
Average MP2(full)
1377
584
762
794
740
1279
1236
516
443
732 47
1270 69
659
518 50
15
B3LYP/6-31G(d)
B3LYP/6-31+G(d)
2.447881
2.465403
2.524013
2.531097
2.533492
2.539708
2.539411
2.545645
2.554573
–
643
585
600
596
577
587
612
612
605
B3LYP/6-311G(d,p)
B3LYP/6-311+G(d,p)
B3LYP/6-311G(2d,2p)
B3LYP/6-311+G(2d,2p)
B3LYP/6-311G(2df,2pd)
B3LYP/6-311+G(2df,2pd)
B3LYP/aug-cc-pVTZ
Average B3LYP
555
41
584
566
29
46
602 20
591 47
33 14
a
Energy of conformer is given as −(E + 254) H.
b
Difference is relative to equatorial form and given in cm−1
.
c
Difference is relative to axial form and given in cm−1
.
the fixed scaled force field and resultant wavenumbers. A set of
symmetry coordinates was used similar to those used for chloro-
cyclobutane [5] to determine the corresponding potential energy
distributions (P.E.D.s). A comparison between the observed and
calculated wavenumbers, along with the calculated infrared inten-
sities, Raman activities, depolarization ratios and potential energy
distributions for the equatorial and axial conformers of fluorocy-
clobutane, is given in Tables 1 and 2, respectively.
Table 4
˚
Structural parameters (A and degrees), rotational constants (MHz) and dipole moments (Debye) for equatorial and axial fluorocyclobutane.
b
Structural parameters
Int. coor.
MP2(full)/6-311+G(d,p)
B3LYP/6-311+G(d,p)
Microwavea
Equatorial
Adjusted r0
Equatorial
Axial
1.398
Equatorial
Axial
Equatorial
Axialc
1.407(3)
rC␣–F
R1
R2
R3
r1
r2
r3
1.386
1.530
1.551
1.094
1.093
1.091
1.090
1.093
117.3
89.4
1.398
1.536
1.557
1.093
1.092
1.091
1.090
1.092
116.6
90.0
1.407
1.541
1.557
1.091
1.091
1.092
1.091
1.091
1.386(2)
1.535
1.548
1.092
1.094
1.093
1.092
1.094
116.9
89.7
87.1
88.7
112.5
107.7
109.9
109.7
118.2
117.9
109.3
116.1
111.2
108.6
20
10250.412(7)
4274.930(3)
3402.614(3)
1.870(5)
0.000
1.383(3)
1.543(3)
1.554(3)
1.094(2)
1.093(2)
1.091(2)
1.090(2)
1.093(2)
ꢀ
rC␣–C␤,C␤
1.534
1.552
1.092
1.092
1.094
1.091
1.091
109.6
88.9
88.0
1.546(3)
1.554(3)
1.092(2)
1.092(2)
1.094(2)
1.091(2)
1.091(2)
109.2(5)
89.2(5)
89.2(5)
ꢀ
rC␥–C␤,C␤
rC␣–H
ꢀ
rC␤–H1, C␤ –H1
ꢀ
rC␤–H2, C␤ –H2
rC␥–H1
rC␥–H2
r4
r5
∠C␤
C␣F
ꢂ1
ꢃ1
ꢃ2
ꢃ3
ꢄ1
ꢄ2
ꢅ1
ꢅ2
ꢅ3
ꢅ4
ꢅ5
ꢆ1
ꢆ2
ꢆ3
ꢇ1
111.2
89.9
89.3
117.4(5)
89.3(5)
85.0(5)
∠C␤C␣C␤
∠C␥C␤C␣
∠C␤C␥C␤
∠HC␣C␤
86.4
87.4
87.8
87.6
88.4
88.7
88.6(5)
88.6(5)
112.1
107.7
109.3
110.4
118.8
119.6
110.2
117.7
111.4
109.4
34.0
120.2
107.2
116.3
119.1
110.4
111.4
109.9
111.3
117.8
109.6
29.2
113.2
106.7
110.0
111.3
118.0
119.0
109.4
117.1
112.2
108.8
28.0
118.8
106.4
114.8
117.6
111.9
113.1
109.0
112.7
116.5
108.6
17.6
112.1(5)
107.7(5)
109.3(5)
112.8(5)
118.8(5)
118.5(5)
110.2(5)
116.4(5)
112.4(5)
109.4(5)
37.4(5)
120.4(5)
107.2(5)
116.3(5)
123.3(5)
110.4(5)
105.8(5)
109.9(5)
108.3(5)
120.1(5)
109.6(5)
20.7(5)
∠HC␣
F
∠H1C␤C␣
∠H1C␤C␥
∠H2C␤C␣
∠H2C␤C␥
∠H1C␤H2
∠H1C␥C␤
∠H2C␥C␤
∠H1C␥H2
ꢇC␥C␤C␤C␣
A (MHz)
B (MHz)
C (MHz)
|ꢈa|
10392.071
4271.121
3411.111
1.968
0.000
0.553
2.044
8463.98
4848.58
4109.91
1.819
0.000
1.314
2.244
10204.376
4247.106
3380.445
2.066
0.000
0.599
2.151
8762.20
4619.90
3815.67
1.802
0.000
1.116
2.120
10250.620
4275.419
3402.568
8628.79
4726.22
3939.20
1.61(5)
|ꢈb|
0.0000
1.22(22)
2.02(17)
|ꢈc|
0.52(2)
1.94(1)
|ꢈt|
a
Proposed structural parameters from Ref. [10], equatorial dipole moments from Ref. [2] and rotational constants [4].
Adjusted parameters using the microwave data from Ref. [4] for the given ground states.
Experimental rotational constants A = 8628.77(7) MHz, B = 4726.25(6) MHz, and C = 3939.16(8) MHz and dipole moments taken from Ref. [4].
b
c

836
A. Ganguly et al. / Spectrochimica Acta Part A 79 (2011) 831–840
The infrared spectra were predicted from the MP2(full)/6-
−100 ◦C were recorded. Only small interactions are expected to
occur between the dissolved sample and the surrounding xenon
atoms, and consequently, only small frequency shifts are antici-
pated when passing from the gas phase to the liquefied noble gas
solutions. A significant advantage of this study is that the conformer
bands are better resolved in comparison with those in the infrared
spectrum of the gas. From ab initio calculations, the dipole moments
of the two conformers are predicted to have similar values and the
molecular sizes of the two conformers are nearly the same, so the
ꢉH value obtained from the temperature dependent FT-IR study is
expected to be close to that for the gas [16–22].
31G(d) calculations. The predicted scaled frequencies were used
together with a Lorentzian function to obtain the calculated spectra.
Infrared intensities determined from MP2(full)/6-31G(d) calcula-
tions were obtained based on the dipole moment derivatives with
respect to Cartesian coordinates. The derivatives were transformed
ꢀ
with respect to normal coordinates by (∂ꢈu/∂Qi) = j(∂ꢈu/∂Xj)Lij,
where Qi is the ith normal coordinate, Xj is the jth Cartesian dis-
placement coordinate, and Lij is the transformation matrix between
the Cartesian displacement coordinates and the normal coordi-
nates. The infrared intensities were then calculated by [(Nꢆ)/(3c2)]
[(∂ꢈx/∂Qi)2 + (∂ꢈy/∂Qi)2 + (∂ꢈz/∂Qi)2]. In Fig. 1 a comparison of
the experimental and simulated infrared spectra of fluorocyclobu-
tane is shown for the spectral region from 900 to 1500 cm−1. The
predicted spectrum is in relatively good agreement with the exper-
imental spectrum which shows the utility of the scaled predicted
frequencies and intensities for supporting the vibrational assign-
ment.
As indicated earlier only the two bands at 832 and 1348 cm−1
were confidently assigned for the axial conformer. The fundamen-
tal at 1348 cm−1 could not be resolved in the spectra of the xenon
solution due to the close proximity of the ꢀ8 equatorial conformer
fundamental. Therefore, only the band at 832 cm−1 could be used
for intensity measurements for the axial conformer. The choice
of the bands to be used for the equatorial conformer was also a
little difficult since they must be sufficiently separated from any
interfering bands. Also, for best results they should have compa-
rable intensity to the band being used for the axial conformer and
preferably be in the lower frequency range to minimize interfer-
ence of overtone and combination bands. From the inspection of
the various possible bands the fundamentals at 954, 1090, 1136,
and 1240 cm−1 were selected for the equatorial conformer bands
to be use in the determination of the enthalpy difference.
4. Vibrational assignment
In order to obtain the enthalpy difference between the two
stable conformers of fluorocyclobutane it is necessary to confi-
dently identify a vibrational mode for the axial conformer which
is expected to be a very small percentage of the sample at ambient
temperature. This requires confident assignments of all the fun-
damentals of the more stable equatorial conformer. There have
been two extensive vibrational assignments [10,11] for fluoro-
cyclobutane where in the earlier study [11] the assignment was
made by using the infrared vapor phase band contours, Raman
depolarization data, expected infrared and Raman intensities and
“group frequencies”. In the more recent study [10] ab initio cal-
culations with the 3-21G(d) basis set with two scaling factors of
0.88 for stretches and 0.90 for the bends were used to predict fre-
quencies for the fundamentals and potential energy distributions.
These new data resulted in some significant changes in a few of
the descriptions of the motions contributing to some of the bands.
In particular, two ring deformations were reassigned to lower fre-
quency bands. There was also a few major changes in the vibrational
assignment where the C–H out-of-plane bend was reassigned as
The intensities of the infrared bands were measured as a
function of temperature and their ratios were determined. By appli-
cation of the van’t Hoff equation −ln K = ꢉH/(RT) − ꢉS/R, ꢉH was
determined from a plot of −ln K versus 1/T, where ꢉH/R is the
slope of the line and K is substituted with the appropriate inten-
sity ratios, i.e. Iequatorial/Iaxial. It was assumed that ꢉH, ꢉS, and ˛ are
not a function of temperature in the temperature range studied.
By combining the axial band and the four equatorial conformer
bands, four pairs were utilized for the determination of the enthalpy
difference, and the resulting values with statistical uncertainties
are listed in Table 5. As can be seen from the data in this table
there is a fair spread of values but each one has a relatively small
uncertainty. However, this spread is expected when measuring
band intensities for bands with shoulders and considerable poten-
tial for underlying overtones and combination bands. Nevertheless,
the statistical uncertainty is quite small and the statistical aver-
age obtained by using all eight pairs as a single set gives a value
of 496 9 cm−1. This error limit is derived from the statistical
standard deviation of one sigma of the measured intensity data.
This error limit does not take into account small associations with
the liquid xenon or the interference of overtones and combina-
tion bands in near coincidence with the measured fundamentals.
The variations in the individual values are undoubtedly due to
these types of interferences, but by taking several pairs, the effect
of such interferences should cancel. Nevertheless, we expect the
error is probably significantly larger than the statistical uncertainty
nearly accidently degenerate with the ␤-CH2 wag at 1242 cm−1
.
The more accurately predicted fundamental frequencies along with
the predicted infrared intensities and Raman activities as well as
the relatively sharp bands in the spectrum from the xenon solu-
tion provides considerably more spectral information for making
the vibrational assignment.
With the confident assignment for the fundamentals of the
equatorial conformer a search was made for bands arising from the
axial conformer. As expected the C–F stretch should be the most
intense band in the spectral “fingerprint” region. Unfortunately
there is mixing of this mode with both ring deformations along
with the ␤-CH2 rock. Nevertheless, the band at 832 cm−1 is quite
pronounced and it can be assigned as a fundamental of the axial
conformer. There is also a band at 1348 cm−1 which undoubtedly
is due to the axial conformer. There is another band at 1037 cm−1
which is probably due to the axial conformer but it is partly over-
laid by the equatorial band at 1048 cm−1. Therefore, there are only
two bands of the axial conformer which could be considered for the
determination of the enthalpy difference for the two conformers.
obtained so we believe the determined ꢉH value is 496 40 cm−1
.
Thus, the abundance of the less stable axial conformer present at
ambient temperature is 8 1%.
6. Structural parameters
We have [21] shown that ab initio MP2(full)/6-311+G(d,p) cal-
culations predict the r0 structural parameters for more than 50
˚
5. Conformational stability
carbon–hydrogen distances better than 0.002 A compared to the
experimentally determined values from isolated CH stretching fre-
quencies which are compared [22] to previously determined values
from earlier microwave studies. Thus, all of the carbon–hydrogen
r0 parameters can be taken from the MP2(full)/6-311+G(d,p) pre-
To determine the enthalpy difference between the two con-
formers, the mid-infrared spectra of fluorocyclobutane dissolved
in liquefied xenon as a function of temperature from −55 to

A. Ganguly et al. / Spectrochimica Acta Part A 79 (2011) 831–840
837
Table 5
Temperature and intensity ratios of the equatorial and axial bands of fluorocyclobutane.
T (◦C)
1/T (×10−3 K−1
)
I1240/I832
I1136/I832
I1090/I832
I954/I832
Liquid xenon
−55.0
−60.0
−65.0
−70.0
−75.0
−80.0
−85.0
−90.0
−95.0
−100.0
4.584
4.692
4.804
4.923
5.047
5.177
5.315
5.460
5.613
5.775
28.047
30.335
34.427
37.342
42.126
45.832
51.851
55.804
64.312
69.057
21.816
22.479
25.001
26.659
30.737
31.483
36.427
37.080
42.029
44.885
73.347
82.453
91.191
20.196
21.944
24.540
25.717
29.125
32.373
35.535
39.539
43.091
47.951
99.334
107.541
120.126
132.261
144.495
156.716
176.726
ꢉHa (cm−1
)
536 14
439 18
499 14
508 11
a
Average value ꢉH = 496 9 cm−1 (5.93 0.11 kJ mol−1) with the equatorial conformer the more stable form and the statistical uncertainty (1␴) obtained by utilizing all
of the data as a single set.
dicted values for fluorocyclobutane. Also, we have found [23] that
we can obtain good structural parameters by adjusting the struc-
tural parameters from the ab initio calculations to fit the rotational
constants obtained from the microwave experimental data. In order
to reduce the number of 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, i.e. C–C and C–F, for fluorocyclobutane.
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. Thus, it is possible
to reduce the number of heavy atom independent structural param-
eters to five with the CC and CF bond lengths, and the angles of FCC,
CCC and the ring puckering angle. Therefore, if the CH parame-
ters are taken from the MP2(full)/6-311+G(d,p) calculations, then
it should be possible to obtain “adjusted r0” structural parameters
for the equatorial and axial conformers of fluorocyclobutane by uti-
lizing the previously reported [4] six rotational constants from the
microwave study.
These determined adjusted r0 parameters are listed in Table 4
and the final fit of the rotational constants is shown in Table 6. The
differences are quite small with those for the B and C rotational
constants fit to better than 0.5 MHz. The largest adjustment for the
angles is associated with the ring puckering angle which changes
from 34.0◦ to 37.4◦ for the equatorial conformer and from 29.2◦
to 20.7◦ for the axial conformer and the largest distance change is
˚
ꢀ
the C␣–C␤ (C␤ ) bond which increased from 1.530 to 1.543 A for the
˚
equatorial conformer and 1.534 to 1.546 A for the axial conformer.
The C–F bond distance in the axial conformer also exhibits a signif-
˚
icant change from 1.398 to 1.407 A. The remaining changes were
relatively small. It is somewhat difficult to estimate the uncertain-
ties in these structural parameters, but it is believed those for the
˚
CH distances should be no more than 0.002 A and for the heavy
˚
atom distances 0.003 A, whereas the uncertainties in the angles is
estimated to be 0.5◦. These parameters are expected to be more
Table 6
Comparison of rotational constants (MHz) obtained from modified ab initio,
MP2(full)/6-311+G(d,p) structural parameters and the experimental values from
the microwave spectra of equatorial and axial fluorocyclobutane.
Isotopomers
Rotational
constant
Experimentala
Calculated
|ꢁ|
eq-C4H7F
A
B
C
10250.412(7)
4274.930(3)
3402.614(3)
10250.620
4275.419
3402.568
0.208
0.489
0.046
ax-C4H7F
A
B
C
8628.77(7)
4726.25(6)
3939.16(8)
8628.79
4726.22
3939.20
0.02
0.03
0.04
Fig. 3. Low frequency spectra of gaseous fluorocyclobutane: (A) background; (B) far
a
infrared and (C) Raman.
Ref. [4].

838
A. Ganguly et al. / Spectrochimica Acta Part A 79 (2011) 831–840
Table 7
Observed and calculated frequencies (cm−1) for the ring-puckering vibrations of fluorocyclobutane.
Conformer
Equatorial
Transition
Observeda
Infrared
Calculatedb
Obs. − Calc.
Weight
Raman
1 ← 0
2 ← 1
4 ← 2
6 ← 4
166.44
157.25
146.50
129.04
167
157
147
166.92
157.29
145.51
129.46
−0.48
−0.04
0.99
−0.42
1.0
1.0
1.0
1.0
Axial
5 ← 3
130.94
132.03
−1.09
1.0
a
The infrared were utilized preferentially over the Raman [10] in the calculations.
b
Calculated by using a potential of the form V (cm−1) = (8.46 0.12) × 105X4 − (4.05 0.18) × 104X2 + (6.01 0.11) × 104X3 with a reduced mass of 214 amu.
accurate than those that could be obtained from electron diffrac-
tion or microwave study alone or a microwave study with all atoms
substituted to obtain rs parameters.
the far infrared and low frequency Raman (Fig. 3) data the “hot
bands” series for the equatorial conformer as well as the first tran-
sition in the axial well were assigned (Table 7) and the assignment
used matches the previous spectral study [10]. With this assign-
ment the potential function (Fig. 4) was determined to have a ꢉH
of 450 cm−1 with a barrier of 774 cm−1 as compared to the previous
study where the potential function gave a barrier slightly smaller
at 713 cm−1. This ꢉH value is on the low side of the experimentally
determined value for this study. This potential function is not sig-
nificantly different from the potential function from the previous
study [10].
7. Discussion
The determined enthalpy difference of 496 40 cm−1 is a
reasonable value compared to the relative values obtained for bro-
mocyclobutane (ꢉH = 291 22 cm−1 [7]) and chlorocyclobutane
(ꢉH = 361 17 cm−1 [5]) as compared to the predicted values from
the MP2(full) calculations with a relatively large number of basis
sets. For the bromide the ꢉE value was 560 46 cm−1 and for the
chloride the ꢉE value was 646 73 cm−1 where the calculations
include basis sets up to aug-cc-pVTZ. Therefore, the ab initio cal-
culations predicted considerably large energy differences than the
experimentally determined enthalpy difference for both the chlo-
ride and bromide. For the fluoride the predicted energy difference is
732 47 cm−1 which is about 230 cm−1 larger than the experimen-
tal determined enthalpy difference. However, from the microwave
study of fluorocyclobutane [4] the energy difference was deter-
mined to be 620 30 cm−1 which is significantly larger than the ꢉH
value obtained herein. Nevertheless it is clear that the enthalpy dif-
ference for the fluoride is much larger than the value for the other
two halocyclobutanes.
The three parameters which show significant differences from
the previous study [10] of this molecule are the puckering angle,
ꢀ
ꢀ
as well as the C␣–C␤ (C␤ ) and C␥–C␤ (C␤ ) bond distances. In the
previous study these parameters were all assumed with values
◦
˚
˚
of 20 , 1.535 A, and 1.548 A, respectively; however for this study
we obtained values of 37.4(5)◦ for the puckering angle along with
˚
ꢀ
ꢀ
1.543(3) and 1.554(3) A for the C␣–C␤ (C␤ ) and C␥–C␤ (C␤ ) bond
distances in the equatorial conformer. We found that there are two
parameters which were very sensitive to the A rotational constant,
ꢀ
with those being the C␣–C␤ (C␤ ) bond distance and the puckering
angle for the equatorial and axial conformers. We also found that
the C–F distance in the axial conformer was very sensitive to even
small changes in the A rotational constant and to a lesser extent the
B and C rotational constants so that even a relatively small change
In the present study the experimentally determined enthalpy
difference was larger at 496 cm−1 than the 413 cm−1 enthalpy
difference used in the previous study [10] for determining the ring-
puckering potential function. With the higher and more reliable
ꢉH value, we have again obtained the potential function govern-
ing the conformational interchange for fluorocyclobutane. By using
˚
of 0.001 A would change the fit of the rotational constants by one to
˚
two MHz for each rotational constant. This lead to the 1.407 A C–F
bond distance where even a small change would drastically alter
the rotational constant fit. Therefore, the determined C–F distance
has a very small uncertainty.
Fig. 4. Potential function governing the ring-puckering vibration of fluorocyclobutane. V (cm−1) = (8.46 0.12) × 105X4 − (4.05 0.18) × 104X2 + (6.01 0.11) × 104X3 with a
reduced mass of 214 amu.

A. Ganguly et al. / Spectrochimica Acta Part A 79 (2011) 831–840
839
Table 8
Structural parameters (A and degrees) of cyclobutane and some fluoroalkanes and fluoroalkenes.
a
˚
Parameters
Fluoromethane
Fluoroethane
Fluoroethene
cis-3-Fluoropropene
Isopropyl fluoride
gauche-1-Fluoropropane
MP2b
1.387
Exp.c
MP2b
Exp.d
MP2b
1.346
Exp.e
MP2b
Exp.f
MP2b
Exp.g
MP2b
Exp.h
rC␣–F
1.383(1)
1.396
1.510
1.387(3)
1.512(2)
1.342(4)
1.399
1.492
1.388(4)
1.495(4)
1.406
1.512
1.398(13)
1.522(7)
1.398
1.512
1.526
1.093
1.390(5)
1.506(2)
1.526(3)
1.097(1)
rC␣–C␤
rC␥–C␤
rC␣–H
1.091
1.087(1)
1.093
1.094(1)
1.084
1.080*
1.094
1.098(2)
1.095
1.092*
Parameters
gauche-1-Fluoro-2-
eq-Fluorocyclobutane
ax-Fluorocyclobutane
Chaireq-fluorocyclohexane
Chairax-fluorocyclohexane
methylpropane
MP2b
Exp.i
MP2b
Exp.j
MP2b
Exp.j
MP2b
Exp.k
MP2b
Exp.k
rC␣–F
1.396
1.515
1.528
1.398(13)
1.525(5)
1.536*
1.386
1.530
1.551
1.094
1.383(3)
1.543(3)
1.554(3)
1.094(2)
1.398
1.534
1.552
1.092
1.407(3)
1.546(3)
1.554(3)
1.092(2)
1.403
1.515
1.532
1.399(2)
1.5346(1)
1.5358(1)
1.408
1.518
1.530
1.4021(14)
1.5348(1)
1.5359(1)
rC␣–C␤
rC␥–C␤
rC␣–H
a
Structural parameters [1]: rC–C 1.555(2), rC–H 1.0910(6), and puckering angle 29.55(3).
From ab initio MP2(full)/6-311+G(d,p).
Ref. [24]: re structural parameters.
Ref. [25]: re structural parameters.
Ref. [26]: r0 structural parameters; value with asterisk is assumed.
Ref. [27]: adjusted r0 structural parameters.
Ref. [28]: r0 structural parameters; value with asterisk is from isolated C–H stretch.
Ref. [29]: r0 structural parameters.
Ref. [30]: r0 structural parameters; value with asterisk is assumed.
This study.
b
c
d
e
f
g
h
i
j
k
Ref. [31] r0 structural parameters.
Table 9
Structural parameters of the ring for a few four-membered ring molecules (A and degree).
˚
Structural parameters
Cyclobutanea
Chlorocyclobutaneb
Equatorial
Bromocyclobutanec
Equatorial
Fluorocyclobutaned
Equatorial
Axial
Axial
Axial
rC␣–X
rC␣–C␤
rC␤–C␥
Puckering angle
1.783(5)
1.539(3)
1.558(3)
30.7(5)
1.803(5)
1.547(3)
1.557(3)
22.3(5)
1.942(3)
1.541(3)
1.552(3)
29.8(5)
1.952
1.548
1.546
1.383(3)
1.543(3)
1.554(3)
37.4(5)
1.407(3)
1.546(3)
1.554(3)
20.7(5)
1.5555(2)
1.5555(2)
28.58(9)
24.0
a
Ref. [1].
Ref. [5], X = Cl.
Ref. [7], X = Br.
This study, X = F.
b
c
d
We have compared the heavy atom and C–F distances of a
number of fluoroalkanes and alkenes (Table 8) in order to demon-
strate the effect of fluorine atom on the structural parameters of a
molecule. The ab initio MP2(full)/6-311+G(d, p) predicted C–F dis-
tances for all the fluoroalkanes and alkenes is predicted to be larger
than the experimentally determined values. Whereas the C␣–C␤
˚
in fluorocyclobutane they are equal but 0.004 A shorter than those
for the chloride. The most pronounced effect of a substituent on
four membered rings can be visualized by comparing the puck-
ering angle between the conformers. The largest difference of
∼16◦ is obtained for the case of fluorocyclobutane for the halo-
ꢀ
gens. The C␣–C␤ (C␤ ) bond also exhibits interesting behavior
˚
ꢀ
(C␤ ) bond distance is on average predicted 0.010 A shorter than
in the presence of a halogen substituent as the bond is signif-
icantly shorter for the halogen substituted molecules than the
corresponding bond for the cyclobutane without substitution.
This bond also is significantly longer in the axial conformers of
chlorocyclobutane and fluorocyclobutane than in their equatorial
conformers.
its corresponding experimental value. It is also of interest that the
ꢀ
C␥–C␤ (C␤ ) distance for gauche-1-fluoropropene is predicted to be
equal to the experimentally determined value.
The heavy atom structural parameters for the halogen sub-
stituted four member rings also shows some interesting trends
ꢀ
(Table 9). The C␥–C␤ (C␤ ) distances for the conformers of chloro-
The centrifugal distortion constants obtained for the asymmet-
ric reduced Hamiltonian for both conformers are listed in Table 10
˚
ꢀ
cyclobutane differ by only 0.001 A and for the C␥–C␤ (C␤ ) distance
Table 10
Quadratic centrifugal distortion constants (kHz) for equatorial and axial conformers of fluorocyclobutane.
Equatorial
Axial
MP2(full)/
6-31G(d)
MP2(full)/
6-311+G(d,p)
B3LYP/6-
31G(d)
B3LYP/6-
311+G(d,p)
Exp.a
MP2(full)/
6-31G(d)
MP2(full)/
6-311+G(d,p)
B3LYP/6-
31G(d)
B3LYP/6-
311+G(d,p)
Exp.a
ꢁJ
ꢁJK
ꢁK
ıJ
0.68
1.93
3.4
0.122
1.8
0.68
1.967
3.4
0.124
1.8
0.73
1.59
4.8
0.131
1.8
0.76
1.388
5.4
0.135
1.8
0.72(2)
1.74(7)
4.4(2)
0.138(4)
1.7(1)
2.6
−6.0
8.9
2.6
−5.9
8.9
3.7
−12.2
18.5
5.4
−23.3
37.3
−2.(2)
10.(1)
−27.(1)
−0.02
−0.02
−0.2
−0.5
0.7(1)
ıK
1.3
1.3
1.9
3.5
−7.(3)
a
Values taken from Ref. [4].

840
A. Ganguly et al. / Spectrochimica Acta Part A 79 (2011) 831–840
along with the values obtained from the force constants that were
predicted from the ab initio MP2(full)/6-31G(d) and 6-311+G(d, p)
calculations and from the density functional theory calculations by
the B3LYP method with both the 6-31G(d) and the 6-311+G(d,p)
basis sets. In general the fit is much better with the larger basis
set from the B3LYP calculations where the differences between
the predicted and experimental values is quite small for all the
constants for the equatorial conformer. However for the axial con-
former the difference from the predicted values from the MP2(full)
and B3LYP calculations are significantly different from the values
reported from the microwave study [4]. The magnitude of the con-
stants is in fair agreement but the signs of the constants are all
different for the experimentally determined values compared to
the predicted values.
We recently obtained the enthalpy difference for the equatorial
and axial conformers of cyanocyclobutane [8] and the determined
experimental value was 254 12 cm−1 (3.03 12 kJ/mol) with the
equatorial conformer more stable, which indicates that there is
23 1% of the axial form present at ambient temperature. This
value is smaller than the corresponding values for the halocy-
clobutanes and the question arises what are the factors of the
substitute species that control the relative amounts of the two
forms. The ab initio MP2(full) calculations predicted an enthalpy
difference for the cyanide of 238 23 cm−1 which is consistent with
the experimentally determined value. Thus it would be of interest
to investigate the conformational stability of ethynylcyclobutane,
as it is isoelectronic with cyanide, to determine the enthalpy differ-
ence for the two conformers for this molecule. Another molecule
of interest is silylcyclobutane where the axial form is predicted
more stable by ab initio MP2(full) calculations using relatively large
basis sets, whereas, by using the smaller basis sets the equatorial
form is predicted more stable. Also of interest would be the iso-
cyanocyclobutane which is expected to have trans and cis forms in
addition to the equatorial and axial conformers. Therefore, the con-
formational study of these and similar molecules might be used to
determine the factors which control the relative stability of these
types of molecules.
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JRD acknowledges the University of Missouri–Kansas City, for a
Faculty Research Grant for partial financial support of this research.