Journal
of
MOLECULAR
STRUCTURE
Journal of Molecular Structure 41 O-41 I (1997) 47 I-475
The vibrational spectra and ab initio calculations of tram- and
cis- 1,3-dibromo- 1,3-dimethylcyclobutane
Andrk Nilsena, Peter Klaeboea’*, Claus J. Nielsena, David L. Powellb
“Department of Chemistry, University of Oslo, P.O. Box 1033, 03I5
Oslo, Norway
hDepartment of Chemistty, The College of Wooster, Wooster, OH 44691,
USA
Received 26 August 1996; accepted 24 September 1996
Abstract
Infrared spectra of the title compounds were recorded as vapours, as solutes in various solvents, and as solids at ambient
temperature and at 80 K. Raman spectra of the solids were obtained at various temperatures between ambient and 80 K. While
rrans-1,3-dibromo-l,3-dimethylcyclobutane
exists in one puckered conformer only, the cis compound can be present as a
mixture of diequatorial and diaxial bromine conformers.
In the trans compound a large majority of the IR and Raman bands of the crystal did not coincide, suggesting CZh molecular
symmetry. Accordingly, this cyclobutane ring appears to be planar or very close to planar in the crystal, as observed for certain
cyclobutanes. However, no significant changes between the IR spectra of the vapour, melt and crystal were detected.
No additional IR or Raman bands of the cis compound were detected in the vapour phase, in the melt or in saturated solutions
compared to the solid state spectra, suggesting the presence of negligible amounts of the diaxial bromine conformer.
The IR and Raman spectra of the trans and cis compounds were assigned and compared with the results of ab initio quantum
chemical calculations employing the basis sets STO-3G, HF/3-21G* and HF/6-3lG’. 0 1997 Elsevier Science B.V. 0 1997
Elsevier Science B.V.
Keywords: Ab initio; Conformations;
Cyclobutanes;
Vibrational
1. Introduction
It is well documented
that in monosubstituted
cyclobutanes the equatorial is highly preferred to the
axial position and hence the conformational
equilibrium lies strongly towards the equatorial conformer,
making it difficult to identify the existence of the
unstable (axial) conformer. However, in geminally
disubstituted cyclobutanes with two different substituents C4H6XY, the X and Y atoms will compete for
the more favourable
equatorial
position.
Small
* Corresponding author.
spectra
enthalpy
differences
between
the conformers
in
these cyclobutanes
are observed
for 1-chloro- lfluoro- [ 11, 1-chloro- 1,2,2-trifluoro- [l] and 1,1,2-trichloro-2,3,3-trifluorocyclobutane
[2].
The two title compounds rruns- 1,3-dibromo- 1,3dimethylcyclobutane
(TBMCB)
and
cis- 1,3dibromo- 1,3-dimethylcyclobutane
(CBMCB)
have
previously been studied by gaseous electron diffraction (GED) [3]. While the trans compound exists in
one confomer only, the cis compound can be present
as a mixture of diequatorial and diaxial bromine. It
was concluded [3] that the mixture in the vapour at
313 K was 79% diequatorial
and 21% diaxial
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.
PII SOO22-2860(96)09457-4
A. Nilsen et al.Nournal of Molecular Structure 410-41 I (1997) 471-475
472
bromine; moreover, the puckering angle was given as
24” for diequatorial and only 6” for diaxial bromine.
It was decided to study the two title compounds by
spectroscopic methods. Since TBMCB and CBMCB
are both doubly geminally substituted in the 1 and 3
positions, a comparison with other geminally disubstituted cyclobutanes [1,2] would be of considerable
interest.
samples were recorded in saturated solutions of
CC14 and CS2, and the solids studied at different temperatures in a capillary cooled with cold nitrogen gas
]41.
3. Results and discussion
3.1. trans-I,3-Dibromo-1,3-dimethylcyclobutane
(TBMCB)
2. Experimental
The samples were both left over from the earlier
electron diffraction
study [3]; the purities were
checked by gas chromatographic analyses.
The IR spectra were recorded with FT-IR spectrometers from Bruker using models 88 (4000-400
cm-‘) and 113~ (vacuum bench, 700-50 cm-‘), and
from Perkin-Elmer,
model 2000 (4000-400 cm-‘).
The samples were recorded as pellets in KBr and
polyethylene,
as melts and as crystallized
solids
between KBr plates. Low temperature IR spectra of
the amorphous and annealed solids were recorded on a
CsI window at 80 K. Finally, saturated solutions in
Ccl4 and CS2 were recorded.
Raman spectra of the powders and larger single
crystals formed in ampoules during storage in a
deep freeze, were recorded with a Dilor RT 30 spectrometer, interfaced to a PC, and excited by an argon
ion laser model 2000 from Spectra-Physics, using the
514.5 nm line. Additional
Raman spectra of the
Fig. 1. The structure of tram-
1,3-dibromo1,3-dimethylcyclobutane
The structure of TBMCB is shown in Fig. 1, drawn
according to the reported [3] puckering angle of 18”.
With C, symmetry, the fundamentals
should divide
themselves between species a’ and a”, which are all
active in both the infrared and the Raman spectra.
Infrared and Raman spectra of the solid sample are
given in Figs. 2 and 3, respectively. A planar cyclobutane ring with CZh symmetry would lead to 14 as, 10
a,, 10 b, and 14 b, modes, of which the g and u modes
are Raman and IR active, respectively. The mutual
exclusion between the IR and Raman bands should
be easily observed, except in cases of accidental
degeneracy which are particularly frequent in the
3000 and 1450 cm-’ regions where the various CH
stretching and deformation modes overlap.
A complete list of the fundamentals
cannot be
shown here for the sake of brevity, but the observed
IR and Raman bands of the solid below 1000 cm-’ are
given in Table 1. As is apparent, approximately five
instances of coinciding IR and Raman bands (written
(TBMCB); puckering
angle 18” (left) and planar (right).
473
A. NilserdJoumal of Molecular Structure 410-41 I (1997) 471-475
Table
I
Observed IR and Raman bands of trans- I ,3-dibromocyclobutane
(TBMCB)
as a solid” below 1000 cm-’
IR
Raman
982h VW
986 v
IR
433s
915 s
398 vs
311 VW
894 w
873 s
363 m
863 s
iO0
at ambient temperature.
on the same line) were observed below 1000 cm-‘,
while 21 bands (on separate lines) had no counterpart
in the other spectra. Therefore, the spectral activities
alone suggest CZh symmetry and a planar or nearly
planar structure of TBMCB in the solid. Although
the cyclobutanes are generally puckered in all phases,
there are examples of substituted cyclobutanes that
are planar in the crystal: tram- 1,3-cyclobutanedicarboxylic acid [5] and its methyl ester [6], and cis,truns,cis- 1,2,3,4-tetracyanocyclobutane
[7]. In all these
cases, the molecule exhibits a symmetry centre in
the case of a planar cyclobutane ring. It should be
noted that according to recent results, tram- 1,3-dibromocyclobutane
[8] can exist in three different crystalline forms, in which one (metastable) undoubtedly is
planar, as revealed by the infrared and Raman spectra.
t
L
1500
500
1000
Wavenumberkm-1
Fig. 3. Infrared spectrum of solid BMCB
797 VW
769 VW
768 VW
713hw
Wavenumberkm-1
Fig. 2. Raman spectrum of solid BMCB
192 VW
353 w
325 VW
298 w
157 w
750
at ambient temperature.
Raman
269 m
713 s
160 s
630 VW
146 w
522 w
133 VW
138 VW
SO5 m
59 w
458 VW
42 VW
*Ambient temperature.
‘Observed in the melt.
trans- 1,3-Dibromocyclobutane
is undoubtedly puckered in the melt and in solution, but in one planar form
the molecule has CZh symmetry (or very close to C&,
which apparently
gives favourable
crystal forces
counteracting
the unfavourable
dihedral angles of
the eclipsed planar form.
The molecular structure of TBMCB was derived
from ab initio quantum chemical calculations with
the basis sets 6-31G*; the optimized structure converged in a puckered conformer with diequatorial bromine. When constrained to a planar structure, the
calculated wavenumbers
for the fundamentals
gave
one negative value for the lowest vibrational mode,
characteristic of a transition state. A complete list of
the scaled fundamentals (scaling factor 0.90) for C,
and CZh symmetry generally gave small shifts between
the fundamentals of less than 5 cm-‘, but one case of
16 cm-’ and two instances of 13 cm-’ shifts were
calculated for these structures.
No significant shifts were observed between the
IR or Raman frequencies of the solid and those of
the melts or solutions, suggesting no drastic change
in the structure between these phases. We tend to
assume that the cyclobutane ring in TBMCB may be
planar or very close to planar, not only in the solid
state but possibly also in the melt. This conclusion
is at variance with the results in the vapour phase
[3], which were interpreted with a puckering angle of
18”.
A. Nilsen et al/Journal of Molecular Structure 4/O-4/ I (1997) 471-475
474
Fig. 4. The structure of cis-1,3-dibromo-1,3-dimethylcyclobutane
(CBMCB)
3.2. cis-1,3-Dibromo-1,3-dimethylcyclobutane
(CBMCB)
The two possible conformers of CBMCB are shown
in Fig. 4. Infrared spectra of the solid sample in the
ranges 1500-450 cm-’ and 500- 100 cm-’ are given
Wavenumber/cm-1
I
in the aa (left) and ee (right) conformers (relative to Br).
in Fig. 5, and a Raman spectrum is shown in Fig. 6.
However, no additional IR or Raman bands were
detected in the vapour phase, in the melt or in saturated solutions of CBMCB relative to the solid state
spectra. In variance with the GED results [3], the present spectral data suggest that negligible amounts of
the diaxial conformer are present in the fluid phases.
Quantum chemical calculations were carried out
with the basis sets STO-3G, 3-21G* and 6-31G*. It
is significant that a stable conformer was obtained for
the ee form of the two Br substituents (with a Br-Br
distance equal to 4.68 A) and that no stable minimum
was detected for a corresponding
aa form (Fig. 4),
neither when the energy was minimized with full freedom nor when the puckering angle for aa was constrained to values between 6 and 10”. Additional
calculations with larger basis sets and with electron
correlation might change this conclusion, but with two
Br atoms in the molecule such calculations would be
quite costly. Accordingly,
we believe from the
observed infrared and Raman spectra that if any aa
I
500
250
Wavenumber/cm-1
moo
Fig. 5. Infrared spectra of solid CBMCB
infrared (lower) regions.
in the mid- (upper) and far-
500
Wavenumber/cm-1
Fig. 6. Raman spectrum of solid CBMCB.
A. NilsedJoumal
of Molecular
conformer at all is present in the fluid phases, it must
be of lower concentration
than the value of 21%
reported [3] in the vapour at 313 K.
Close inspection of the intensity curves employed
in the GED study [3] suggests that the data may be
interpreted
in terms of large amplitude
motions
around the equilibrium
ee conformer rather than
involving an additional aa conformer. The reported
[3] Br-Br distances of 4.9 A for the ee and 4.5 A
for the aa conformer, which can be compared with
an intermediate value of 4.68 A from the present ab
initio calculations, support this conclusion.
Acknowledgements
The authors are grateful to S. Samdal for his help
with the ab initio calculations and interpretation of the
Structure
410-411
(1997) 471-475
475
earlier results from electron diffraction, and to T.
Jonvik and K. Griesbaum for donating the sample.
References
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[3] T. Jonvik and K. Griesbaum, J. Mol. Struct., 172 (1988) 203.
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