2
90
P. Klaeboe et al. / Journal of Molecular Structure 1047 (2013) 282–291
Slow cooling of the compound from 293 to ca. 143 K, contained
vapor. In the Raman spectra some of these modes were among
the most intense in the spectra in agreement with the calculated
scattering intensities (Tables 2 and 3). Moreover, we expect 20
in a capillary tube, leads to a supercooled liquid which changed to
an apparent plastic solid still containing both conformers when an-
nealed to ca. 150 K. Further annealing to 165 K leads to an aniso-
tropic crystal characterized by shifted Raman peaks and a few
vanishing bands belonging to the e-conformer.
Similar results were obtained when the vapor of CMSC was
sprayed upon a Cu-finger in Raman or a CsI window in IR, both
cooled to 78 K. The sample changed to a solid when annealed to
vibrational modes involving CH
three involving CH deformation. These vibrational modes are ex-
pected in the range 1500–700 cm . However, some skeletal
stretches and bending modes are intermingled with the CH and
CH bending modes. The unexpectedly high wavenumber derived
for the CH scissoring ( ) in the anharmonic calculation of the e-
2
scissor, wag, twist and rock and
3
ꢁ
1
3
2
2
m
9
1
31–156 K, but the spectra were essentially unchanged, suggesting
conformer has been discussed above.
a possible plastic phase. The glassy solid converted to a plastic
phase and subsequently to a real crystal after annealing to 155–
As is apparent from Table 1 three bands vanishing in the crystal
(m
47
,
m
21 and
four additional modes (
signed to separate e-conformer bands. However, only two of the
possible e-bands 21 and 22 were employed in the H measure-
m
27) are described as separate e-fundamentals, but
1
1
70 K, supporting the results obtained in a capillary. Between
65 and 170 K the sample appeared crystalline, and after cooling
m
22
,
m
30
,
m
31 and 57) were tentatively as-
m
to 78 K some Raman bands vanished and others were shifted rela-
tive to those of the unannealed sample recorded at 163 K (Fig. S2).
m
m
D
ments (see above). It appears from Tables 2 and 3 that in the range
ꢁ1
ꢁ1
The Raman bands at 1027, 863, 776, 449 and 281 cm (having
below 700 cm we expect vibrational fundamentals for the e and
ꢁ1
asterisks in Table 1) vanished and that at 772 cm was strongly
reduced in intensity compared to the spectrum of the unannealed
sample. The MIR spectra supported the findings made in the Ra-
man spectra. The sample apparently passed from an amorphous
phase to a plastic phase, and finally to an anisotropic crystal.
a conformers involving Si–Cl stretch, bending of the C–C–C, C–Si–C,
C–C–Si, C–Si–Cl moieties and also ring deformation modes.
The Si–Cl stretches give rise to intense bands both in IR and in
ꢁ1
Raman and were observed at 514 (e) and 439 (a) cm in CMSC
whereas in the related 1-chloro-1-silacyclohexane [14] they were
ꢁ1
1
-Chloro-1-methylcyclohexane is closely related to CMSC and
found at higher wavenumbers, 547 (e) and 475 (a) cm . In cyclo-
propylmethyl dichlorosilane (c-C ) SiCl CH ) [39] the SiCl anti-
symmetric and symmetric stretching modes were observed at 561
from calorimetric and spectroscopic measurements the phase tran-
sitions of this molecule have been reported [23]. This cyclohexane
exists in a plastic crystalline state in a 20° interval between 214.4
and 234.5 K and is present with an axial chlorine and an equatorial
methyl group in the crystal below 214.5 K [23] fairly similar to
CMSC. It should be noted that also fluoro- and chlorocyclohexane
form plastic phases upon cooling from room temperature [1,2].
H
3 5
2
3
2
ꢁ1
and 467 cm , respectively, while in 3,3,3-trifluoropropyltrichlo-
rosilane (CF CH –CH SiCl ) [40] the three SiCl stretches were ob-
3
2
2
3
3
ꢁ1
served at 607, 582 and 470 cm . It is significant that the Si–Cl
stretch of the e-conformer is observed at considerably higher
wavenumbers than in the a-conformer, both for CMSC and 1-
chloro-1-silacyclohexane [14]. Similarly, in chlorocyclohexane the
ꢁ
1
3.7. Spectral interpretation
two C–Cl stretches were found at 741 (e) and 691 cm (a) [2,37].
The antisymmetric and symmetric C–Si–C ring stretches appear
ꢁ1
CMSC has 21 atoms, the e and a conformers both have a plane of
as characteristic bands around 600–640 cm in the silacyclohex-
anes. The symmetric mode 25 is invariably very intense in the Ra-
symmetry (C
s
symmetry) and each conformer has 57 modes of
m
vibration, The fundamentals of both conformers will divide into
man spectra and well suited for quantitative Raman
measurements. Unfortunately, in the three compounds: CMSC, 1-
chloro-1-silacyclohexane [14] and in 1-methyl-1-silacyclohexane
[21] the symmetric C–Si–C stretches in the e and a conformers
coincide. In the Raman spectra of 1-fluoro-1-silacyclohexane, how-
ever, the C–Si–C symmetric stretches were observed at 630 (a) and
0
00
3
2 A and 25 A modes and give rise to polarized and depolarized
Raman bands, respectively. Since the order of the fundamentals
sometimes change between the harmonic and anharmonic calcula-
tions (Tables 2 and 3), the latter are made the basis for the number-
ing. The assignments are in fair agreement with the results of the
B3LYP/cc-pVTZ anharmonic calculations, but in some cases the
deviations between the observed and calculated modes are
significant.
ꢁ
1
659 cm (e) and were ideally suited for
DH determinations [8].
It is immediately apparent from Table 1 that among the 57
vibrational modes expected for each conformer, the majority (34)
consists of overlapping e and a modes. This is supported by the fact
that only 4 of the 57 e-modes vanish in the crystal spectra since
most of them coincide with a-modes. The results of the anhar-
monic calculations reveal that 38 of the corresponding e and a
4. Conclusions
A disubstituted silacycohexane, 1-chloro-1-methyl-1-silacyclo-
hexane CMSC was synthesized, and the compound was investi-
gated by infrared and Raman spectroscopic techniques at
different temperatures and in various states of aggregation. The
compound can exist in two stable conformers, and both of these
were observed in the vibrational spectra. In the vapor, fluid and
amorphous low temperature phases it was observed that the Cl
(a) was more stable than Cl (e) and dominated in the equilibrium.
At low temperatures the following phases: (1) supercooled liquid,
(2) amorphous solid, (3) plastic solid and (4) anisotropic crystal
were observed, the state depending upon the temperature and
the thermal history of the sample. From variable temperature Ra-
man spectra of the liquid in the range 293–163 K, peak intensities
and/or integrated band areas from the band pair 785 (a)/776 (e)
ꢁ1
modes are situated closer than 10 cm apart, strongly suggesting
coinciding fundamentals. Therefore, IR vapor contours or Raman
polarization measurements have limited significance for the spec-
tral interpretation. The calculated polarization ratios of Tables 2
and 3 show that in 5 cases for the e and 3 instances for the a-con-
0
former, the A modes have depolarization ratios equal to 0.75 and
some additional fundamentals have
q values between 0.72 and
0
.75 (Tables 2 and 3). As is apparent from Tables 1–3 the three e
0
12
and a conformer modes m , m13 and m17 are all attributed to be A
modes although they are measured to have depolarized (
q
= 3/4)
ꢁ1
Raman bands (Table 1) in agreement with the calculated
q
values.
cm were treated in van’t Hoff plots to determine the enthalpy
In CMSC 10 hydrogens are attached to the ring carbons and 3 to
the methyl group, giving rise to 13 C–H stretches of each con-
difference
D
H
(e(Cl–a(Cl),
giving
an
average
value
1.5 ± 0.5 kJ mol 1.
ꢁ
0
00
former. The 8 A and 5 A modes of the e and a conformers assigned
to CH and CH antisymmetric and symmetric stretches give rise to
highly overlapping modes between 3000 and 2850 cm in the
The spectral results were compared with the results of quantum
chemical calculations including MP2/6-31G(d) and DFT (B3LYP)
calculations with cc-pVDZ, cc-pVTZ, cc-pVQZ basis sets and G3
3
2
ꢁ1