Journal of Molecular Structure 550–551 (2000) 199–215 www.elsevier.nl/locate/molstruc
夽
A. Nilsen a
, P. Klaeboe a, *, C.J. Nielsen a
, G.A. Guirgis b
, V. Aleksa
1,a a
Department of Chemistry, University of Oslo, P. O. Box 1033, 0315 Oslo, Norway b
Bayer Corporation, Bushy Park Plant, Research and Development, P. O. Box 118088, Charleston, SC 29423-8088, USA
Received 9 August 1999; accepted 21 September 1999
Abstract
Bromomethyl dimethyl chlorosilane (CH
2
Br(CH
3
)
2
SiCl) was synthesised and the infrared spectra of the vapour and of the amorphous and crystalline states at liquid nitrogen temperature were obtained. Additional spectra of the compound, isolated in argon, nitrogen and xenon matrices were recorded at 5 and 15 K. Raman spectra of the liquid were obtained at various temperatures between 295 and 173 K, and spectra of the amorphous and crystalline solids were recorded.
The compound is present as anti and gauche conformers in the vapour and in the liquid states. Various infrared and Raman bands present in these phases vanished upon crystallisation. Raman temperature studies in the liquid gave a gauche–anti value of 1
:
0 ^ 0
:
3 kJ mol
⫺
1 ;
anti was the low energy conformer and was also present in the crystal. The infrared bands diminishing in the argon, nitrogen and xenon matrix spectra after annealing to 28–60 K suggested that the anti conformer also had a slightly lower energy than gauche in all the matrices. The conformational barrier was estimated to be 8–10 kJ mol
⫺ 1
.
Ab initio calculations on different levels of approximation gave optimised geometries, infrared and Raman intensities and vibrational frequencies for the anti and gauche conformers. All calculations predicted anti as the low energy conformer. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the two rotamers was obtained.
䉷
2000 Elsevier Science B.V. All rights reserved.
Keywords: Silanes; Ab initio calculations; Conformations; Infrared; Raman, Matrix isolation spectroscopy
1. Introduction
Bromomethyl dimethyl chlorosilane, CH
2
Br(CH
3
)
2
SiCl, to be abbreviated as BDCS, was synthesised for the first time. The molecule can exists in two conformations, anti and gauche as shown in Fig. 1.
夽
Dedicated to Professor James R. Durig on the occasion of his
65th birthday.
* Corresponding author. Tel.:
⫹
47-22-855678; fax:
⫹
47-22-
855441.
1
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe).
Permanent address: Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania.
The infrared and Raman spectra of BDCS were investigated among a series of five halomethyl dimethyl halosilanes, CH
2
X(CH
3
)
2
SiY (X
ˆ
Cl, Br; Y
ˆ
H, F, Cl) and these results have recently been reported. Various silanes with conformational equilibria containing one silicon atom have been studied [1–6] and molecules with two [7–9] and three [10–12] silicon atoms have recently been investigated by Hassler and coworkers and their vibrational spectra reported.
The vapour, amorphous and crystalline samples of
BDCS were recorded in the middle (MIR) and far infrared (FIR) regions. Infrared matrix isolation technique was employed to obtain spectra of the
0022-2860/00/$ - see front matter
䉷
2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 2 8 6 0 ( 0 0 ) 0 0 3 8 7 - 2

200 A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215 compound trapped in argon, nitrogen and xenon matrices. The narrow bandwidths observed in the matrix spectra are an advantage for interpreting conformational possible to
Fig. 1. The anti and gauche conformers of bromomethyl dimethyl chlorosilane (BDCS).
equilibria. They often distinguish neighbouring make it bands of different conformers which overlap in spectra of the vapour and liquid.
annealing the conformational barriers can frequently be estimated from the matrix spectra.
Raman spectra of the liquid, including polarisation measurements were obtained. Spectra of the liquid were recorded at different temperatures, some of them far below the melting point, and Raman spectra of BDCS as a crystal were observed. Moreover, the conformational energies, the structure, the force constants and the infrared and Raman intensities were calculated by ab initio methods. Our complete results for BDCS are given in the present article while some spectroscopic features and results from quantum chemistry were reported in a preliminary communication [13].
2. Experimental
2.1. Sample preparation
Moreover, by appropriate
The sample of BDCS was prepared for the first time by reaction of chlorotrimethyl silane with bromine following the procedure given by Speier [14]. The compound was distilled in a low temperature, low pressure fractionation column and the purity was checked by mass spectrometry.
No apparent impurities were observed in the mass spectra or in the vibrational spectra except for the presence of
HCl which is clearly visible in the infrared vapour spectra by the series of vibrational–rotational bands around 2700 and the rotational transitions around
200 cm
⫺
1
.
2.2. Raman spectral measurements
The Raman spectra were obtained using a Dilor
RTI-30 spectrometer (triple monochromator) and recorded digitally. An argon ion laser from Spectra
Physics (model 2000) was employed using the
514.5 nm line for excitation. Spectra of the liquid were obtained at room temperature and at 11 temperatures between 293 and 173 K in a capillary tube of 2 mm inner diameter surrounded by a
Dewar, cooled by gaseous nitrogen evaporated from a reservoir [15]. Additional spectra were recorded at five temperatures between 293 and 163 K with 5 s integration time in order to improve the signal/noise ratio for the relatively weak bands pairs. These spectra were employed for calculating the enthalpy difference
D conf
H between the conformers in the liquid. BDCS and the related halomethyl dimethyl halosilanes all had large hysteresis (undercooling) and it was sometimes possible to study the liquid 60–80
⬚ below the freezing point (ca.
237 K). The crystallisation occurred spontaneously around 165 K and an

A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215 201
Fig. 2. Raman spectra (1500–100 cm
⫺
1
) of BDCS as a liquid in two directions of polarisation at ambient temperature.
anisotropic crystal containing only one conformer was obtained.
2.3. Infrared spectral measurements
The infrared spectra were recorded on a variety of
Fourier transform spectrometers: Bruker models IFS-88 and IFS-66 (4000–450 cm model 2000 (4000–450 cm
⫺
1
) and a Perkin–Elmer
⫺
1
) in MIR and on two vacuum benches: a Bruker IFS-113v (600–50 cm and a Bomem model DA 3.002 (350–50 cm
⫺
1
⫺
1
)
) in
FIR. The latter instrument had a helium cooled bolometer as detector, the other spectrometers had detectors of DTGS. The resolution was generally
1 cm
⫺
1
, except for the FIR spectra in Fig. 5 which was recorded at 0.1 cm
⫺
1 resolution. Beamsplitters of Ge substrate on KBr were employed in MIR,
Mylar beamsplitters of thickness 5 and 12 m were used in FIR in addition to a metal mesh beamsplitter.
The vapour was investigated in cells with KBr
(10 cm) and polyethylene windows (1 m and 20 cm).
The amorphous and crystalline solids were studied on a CsI window for the MIR and on a wedge shaped window of silicon in FIR, both cooled with liquid nitrogen.
The sample was diluted with argon, nitrogen and xenon (1:500 and 1:1000) and deposited on a CsI window at either 5 or 15 K on a Displex cryostat from APD (model HS-4) with a three stage cooling system. The samples were first heated to various temperatures below 20 K in order to remove site effects in the matrices. Subsequently, they were annealed in steps of 3–5 K to a maximum of 37 K in argon, 32 K in nitrogen and 60 K in xenon in periods of 15 min. At still higher temperatures the inert gases have a pressure higher than 10
⫺
3
Torr which is not feasible in the cryostat. Temperatures at which the matrices turn “soft” and diffusion starts are reported [16] to be ca. 35, 30 and 47 K for argon, nitrogen and xenon, respectively.
After each annealing the window was recooled to 5 K and the spectra were recorded.
3. Results
3.1. Raman spectral results
Raman spectra of BDCS as a liquid in two directions of polarisation at ambient temperature in the region 1550–100 cm
⫺
1 are presented in Fig. 2, while spectra of the liquid and a crystalline solid between 1500 and 40 cm
⫺
1 are compared in Fig. 3.
As is apparent, some of the bands present in the
Raman spectrum of the liquid disappear in the spectrum of the crystal. The same bands are also absent in the corresponding infrared spectrum of a crystalline solid (Table 1). These vanishing bands

202 A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215
Fig. 3. Raman spectra (1500–40 cm
⫺
1
) of BDCS as a liquid at 298 K (solid line) and as a crystalline solid (dotted line) at 80 K.
are assigned to a second conformer which is absent in the crystal. To be discussed below, it is concluded that the conformer present in the crystal is anti. The number of vanishing bands is fairly small, suggesting that most of the fundamentals of one conformer overlap those of the other. This conclusion agrees with the earlier results obtained for silanes with conformational equilibria. Both the calculations and the experimental results revealed more overlap between the conformers in BDCS than in the related molecules chloromethyl dimethyl fluorosilane [17], chloromethyl dimethyl chlorosilane [18], and bromomethyl dimethyl fluorosilane [19].
Raman spectra of the liquid were recorded at 11 temperatures between 295 and 173 K. Since some of the actual Raman bands suitable for quantitative measurements had quite low intensities, another series of measurements were recorded with 30 K intervals at
295, 263, 233, 203 and 173 K with an integration time of 5 s rather than 1 s. The intensity variations observed with temperature of certain bands relative to neighbouring bands were interpreted as a displacement of the conformational equilibrium. Particularly, the attention was focussed upon the Raman bands which vanished upon crystallisation. These bands were found to decrease in intensities at lower temperatures and belonged to one conformer. They were paired with other bands (often neighbours) which remained in the crystal. However, it is always uncertain if the corresponding liquid bands are characteristic of only one conformer or if they belong to overlapping bands of both conformers. In a number of cases the results of the force constant calculations revealed large shifts between conformer bands of the same mode (more than 5 cm
⫺
1
), suggesting that they may originate from one conformer only.
Independent calculations of
D conf
H were carried out from the three band pairs 822/802, 744/728 and
253/276 cm
⫺ 1
, in which the band of the denominator represented the conformer which vanished in the crystal. The two latter pairs will later be assigned to the modes n
22 and n
28
, while the 822/802 cm
⫺ 1 pair was attributed to the anti conformer of n
19 and the
gauche conformer of n
20
. No other band pairs were detected which were suitable for a quantitative evaluation of the conformational enthalpy difference
D conf
H
:
The intensities of each band pair were fitted to the van’t Hoff equation: ln{I
⫺ D conf anti
T
† =
I gauche
H
=
RT
⫹ constant
; where I anti
=
I gauche
T
†
}
ˆ is the ratio in peak heights or integrated areas and it is assumed that
D conf
H is constant with temperature.
Although the band pair 253/276 cm
⫺
1 had higher intensities than the other two pairs, the former bands appeared on a tilted background due to the strong band at 226 cm
⫺ 1 and the van’t Hoff plots revealed that they were not suitable for a quantitative determination of
D conf
H
:
Both peak heights and

Table 1
Observed infrared and Raman data (weak bands in the region 2500–1500 cm
⫺
1 are omitted) for bromomethyl dimethyl chlorosilane (BDCS)
Infrared Raman
Vapour (298 K) Ar-matrix (4.5 K) N
2
-matrix (4.5 K) Xe-matrix (4.5 K) Amorph (80 K) Crystal (80 K) Liquid (298 K)
2983 m sh
2980 m
2976 m, sh
2965 w
2961 w
2957 w
2954 w
2915 vw
1436 vw a
2979 vw
2945 vw
1430 w
1422 w
2973 m
2950 w, sh
1430 w
1424 vw
2979 w
2951 w
3004 m
2968 s
2944 s
2905 w
2781 vw
1432 m
3006 m
2981 m
2970 m
2965 s
2950 s
2906 w
2776 vw
1429 m
3005 w, D
2975 m, D
2947 s, P
2936 m, P
2905 vs, P
2788 vw
1414 vw
1392 w
1357 vw
1267 s
1263 s
1132 w
1129 w
1099 vw
1053 w, sh
892 vw
851 s
822 vs
820 s, sh
801 vw
1415 w
1401 vw
1395 w
1385 w
1355 vw
1278 vw
1260 s
1258 s
1253 w
1234 vw
1135 w,
#
1133 w,
#
1129 w,
"
1087 m
1073 m
#
1065 w
"
893 vw
853 s
"
849 s
#
828 w,
#
823 s
"
816 s
805 vw,
#
758 m,
"
756 m,
#
1415 w
1405 vw
1399 vw
1395 vw
1385 m
1352 vw
1278 vw
1262 s,
"
1258 s,
#
1250 w
1135 m
1133 m
1128 w,
#
1100 vw,
"
1071 w
1063 w
880 vw
854 s
849 s
830 m
824 s
814 s
805 w
758 w
755 w
1431 w
1426 m
1423 m
1415 w
1409 m
1407 m, sh
1397 w
1391 w
1384 w, sh
1380 m
1379 m, sh
1352 vw
1278 vw
1258 m,
"
1254 s,
#
1253 m
1237 w
1137 m,
#
1132m
#
1128 w,
"
1102 m
1073 w,
#
1063 m,
"
886 w,
"
881 w,
#
852 m,
"
849 m,
#
825 w,
#
818 s
812 m,
"
805m,
#
756 vw
754 w
1411 m
1396 w
1380 m
1354 w
1277 w
1256 vs
1222 vw
1140 m
1129 s
1095 vw
1050 m
903 vw
854 s
848 m
823 s
812 s
808 s
757 m
1409 m
1397 m
1392 w
1389 w
1377 s
1350 w
1275 w
1259 s
1254 s
1220 vw
1143 w
1130 s
1094 vw
1046 m
903 w
856 s
826 vs br
816 vs
*
760 m
1413 m, P?
1397 m, D
1384 m, P?
1261 m, P
1141 m, D
1130 m, P
1048 w, P?
853 w, D
822 w, D
802 w, D
Crystal (213 K)
3013 s
2984 s
2968 s
2951 vs
2910 s
2780 vw
1434 vw
1413 w
1395 m
1384 m
1259 w
1141 vw
1130 m
1046 m
852 w
818 m
*
760 m
Assignments n
5
, n
7
, n n n n n n n n n n n n n n n n n n n n n n
8 n
1
2
3
4
6
9
10
11 n
12 n
13
14a n
14g
15
16g
16a
17g
17a
18a
18g
19g
19a
20a
20g
21a
21g

Table 1 (continued)
Infrared Raman Assignments
Vapour (298 K) Ar-matrix (4.5 K) N
2
-matrix (4.5 K) Xe-matrix (4.5 K) Amorph (80 K) Crystal (80 K) Liquid (298 K) Crystal (213 K)
747 w
681 vw
654 w
577 s
494 s
275 w
254 m
243 m sh
180 w
159 w
130 vw
98 m
94 m
81 m
746 m,
"
741 m
738 m
720 vw
682 vw,
#
653 m,
"
650 m,
"
643 vw
617 vw
580 s
574 m
567 vw
494 m,
"
492 s
487 m
746 w,
"
745 w,
#
739 w,
#
716 vw
680 vw,
#
652 w
650 w
642 vw
625 vw
582 s
580 m
577 s
567 vw
492 s
490 s
487 s,
"
485 s
747 w
744 vw,
#
735 w
717 vw
678 vw,
#
652 vw
638 vw
580 w,
#
578 w,
"
570 vw
491 m
483 m
728 w
702 w
678 m
650 m
613 w
574 s
479 vs
276 s
251 s
236 m
225 s
192 s
162 m
151 w
117 s
99 vw
81 w, sh
74 w
747 s
742 w
739 w
*
702 w
*
656 m
649 s
643 m
614 vw
572 s
556 w
489 m
480 s
473 m
*
254 s
243 m sh
228 w
186 m
172 m
146 w sh
119 s
95 w sh
86 m
*
66 w
58 w
45 w
35 vw
28 vw
744 m, P
728 m, P
706 w, D
677 vw
650 vs, P
623 w, P
573 vvs, P
481 vs, P
276 m, P
253 m, P
239 m, D
226 s, D
188 m, D
179 w, D
161 s
110 w
87 b w
76 b w
747 s
*
705 w
684 vw
660 w, sh
650 vs
622 vw
574 vs
557 w
473 vs
*
255 s
245 s
230 s
193 s
169 s
160 s
117 vw
90 s
78 s
*
53 s
38 m
27 m n n n n n n n n n
22a
22g
23
24g
24a
25
26g
26a
27 n n n n n n n n
28g n
28a
29
30
31
32
33
34
35
Lattice n
36a n
36g
Lattice
Lattice
Lattice
Lattice
Lattice a
Abbreviations: w, weak; v, very; m, medium; s, strong; br, broad; P, polarised; D, depolarised; asterisks denote bands disappearing in the IR and Raman spectra of the crystal;
" band intensities increase after annealing;
# band intensities decrease after annealing.
b
Appear in the R( n
) function.

A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215 205
295
263
233
203
173
Table 2
Temperature and intensities for the anti and gauche conformations of bromomethyl dimethyl chlorosilane
D
H
ˆ
0
:
95 kJ mol
1
:
06 kJ mol
⫺
1 †
⫺
1 ; D
H
ˆ
T 1/T I
823
I
802
I
746
I
728
0.00339
0.0038
0.00429
0.00493
0.00578
9440
24729
20941
26843
25309
9582
22835
17555
21882
19248
22835
56091
51124
58466
58355
32411
73650
61873
65622
60739 ln (I
823
/I
802
)
⫺
0.01493
0.07968
0.17637
0.20434
0.27375
ln (I
746
/I
728
)
⫺
0.3502
⫺
0.27235
⫺
0.19083
⫺
0.11547
⫺
0.04004
integrated band areas were attempted for determining band intensities. However, in spite of careful curve resolution and determination of band areas with advanced computer programs, the calculations based upon band areas invariably showed a large scatter. Table 2 summarises the results of the van’t
Hoff plots based upon peak height measurements; the actual plots have been presented in the earlier communication. The band pairs 822/802 and 744/728 cm
1.06 kJ mol
D conf
⫺
1
⫺
1 gave the values 0.95
and
H
ˆ
1
:
0
, respectively, giving the average value
^ 0
:
3 kJ mol
⫺ 1 with anti being the low energy conformer in the liquid.
3.2. Infrared spectral results
An infrared survey spectrum of BDCS as a vapour in the 3100–450 cm
⫺
1 range at the full pressure of
7 Torr is presented in Fig. 4. A vapour spectrum in
FIR (380–30 cm
0.1 cm
⫺
1
⫺
1
) in a cell of 1 m path length and resolution is given in Fig. 5. The infrared spectra of the amorphous and crystalline solids at
80 K are found in Fig. 6 (1600–400 cm
Fig. 7 (450–50 cm
⫺ 1
⫺
1
) and in
). The infrared bands present in the amorphous solid, but vanishing in the crystal spectra after annealing, are the same as those disappearing in the crystal phases in the Raman spectra (see above). An exception is the very weak
Raman band at 677 cm
⫺
1 which seemed to remain in the crystal although the band is assigned to a
gauche band n
24
(These bands also diminish in intensity at lower temperatures and, therefore, belong to the high-energy conformer in the liquid.).
Additional infrared spectra of BDCS were recorded in argon and nitrogen matrices and also extended to xenon matrices (mixing ratios 1:500 and 1:1000) deposited both at 5 and 15 K. Infrared spectra
(1500–400 cm
⫺
1
) of BDCS in an argon matrix, deposited at 5 K of the unannealed and annealed sample are shown in Fig. 8, while spectra in nitrogen and xenon matrices are given in Figs. 9 and 10, respectively. The conformational equilibrium of the
Fig. 4. MIR spectrum (4000–400 cm
⫺
1
) of BDCS as a vapour, path length 10 cm at 7 Torr pressure.

206 A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215
Fig. 5. FIR spectrum (330–50 cm
⫺
1
) of BDCS as a vapour, path length 100 cm at 7 Torr pressure, the rotational transitions are caused by an impurity of HCl.
vapour phase is maintained when the gas mixture is quickly frozen on the CsI window provided that the conformational barrier is not extremely low. The quality of the infrared matrix spectra are generally better when deposited at 15 K, but in the case of very low conformational barriers the 5 K deposition temperature is advantageous to prevent instantaneous conformational conversion.
Various spectral changes occur after annealing around 20 K. They are different in the various matrices and are interpreted as site effects due to relaxation of BDCS in the matrix cages. At higher annealing temperatures (20–33 K in nitrogen,
20–38 K in argon and 20–50 K in xenon) changes were observed in the matrix spectra which were correlated with conformational changes in BDCS.
However, the intensity variations observed in the matrices were small, suggesting that an equilibrium might be present even at these low temperatures. The background absorption increased significantly in
Fig. 6. MIR spectra (1600–400 cm
⫺
1
) of amorphous (dotted line) and crystalline (solid line) solids of BDCS at 80 K.

A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215 207
Fig. 7. FIR spectra (450–50 cm
⫺
1
) of amorphous (lower curve) and crystalline (higher curve) solids of BDCS at 80 K.
all the three matrices after each annealing, a phenomenon which was observed also for the related halosilanes [17–20], making the interpretation more difficult.
The bands which are reduced in intensities in the matrix spectra after annealing are the same as those which disappear on crystallisation. Consequently, both the argon, nitrogen and xenon matrices stabilise the same conformer which is the low energy conformer in the liquid and is also present in the crystal.
The same features were also observed for the closely related molecule chloromethyl dimethyl chlorosilane
[18]. The opposite observations were made for chloromethyl dimethyl fluorosilane [17] and bromomethyl dimethyl fluorosilane [19]. In these compounds the matrix bands which disappeared or were reduced in intensities after annealing and therefore belonged to the high energy conformer in the matrices, were
Fig. 8. MIR spectra (1500–400 cm
⫺
1
) of BDCS in an argon matrix deposited at 5 K, unannealed sample, (solid line) and annealed to 35 K,
(dotted line).

208 A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215
Fig. 9. MIR spectra (775–475 cm
⫺
1
) of BDCS in a nitrogen matrix, unannealed (solid line) and annealed to 25 K (dotted line).
correlated with the low energy conformer in the liquid and were also present in the crystals.
It appears from the matrix spectra that many bands diminished in intensities rather than disappeared after annealing. This indicates that conformational equilibria were established in each of the matrix spectra after annealing at the temperatures 20–50 K. Thus, the conformational enthalpy difference in BCDS must be quite low in the matrices (below 0.4 kJ mol higher
D conf
H than 0.5 kJ mol
⫺
1
⫺
1
). With simple calculations show that the conformational equilibrium would be highly displaced after annealing.
The lowest annealing temperature at which the equilibrium of the solute molecule is displaced from the high to the low energy conformation was used for estimating the conformational barrier. This temperature seemed to vary between the matrices and was ca. 30 K in the nitrogen, 34 K in the argon and 40 K in the xenon matrices. From the curves correlating annealing temperature and activation energy/barrier height given by Barnes [21] the conformational barrier was estimated to be 8–10 kJ mol
⫺
1
.
If secondary effects due to matrix viscosity or matrix– solute interactions are neglected this barrier height may also be valid for the isolated molecules in the vapour phase. The observed wave numbers for the infrared and Raman bands of BDFS in the various states of aggregation are listed in Table 1. The infrared and Raman bands which definitely vanish in the crystalline solids are equipped with asterisks.
Infrared bands of BDFS in the various matrices are fitted with arrows pointing upwards or downwards if the bands increase or decrease in intensity after annealing, respectively. However, because of the increased background scattering in these matrices after annealing, particularly in the high frequency region, some of the intensity variations were uncertain.
Fig. 10. MIR spectra (1150–450 cm
⫺
1
) of BDCS in a xenon matrix, unannealed (solid line) and annealed to 50 K (dotted line).

A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215 209
Table 3
Observed and calculated fundamentals for the anti conformer of bromomethyl dimethyl chlorsilane (BDCS)
Calculated a
Observed b
Fund.
G
Calc.
Scal.
km mol
⫺
1
A
4 amu
⫺
1
Dep cm
⫺
1
Dep I
IR
I
Raman
PED c
682
601
507
267
245
227
189
170
1186
956
923
906
849
793
760
732
163
158
107
65
1598
1593
1586
1583
1570
1450
1444
1296
3321
3258
3250
3249
3248
3246
3178
3176
A
00
A
0
A
0
A
00
A
0
A
00
A
0
A
00
A
0
A
0
A
00
A
00
A
0
A
0
A
00
A
0
A
00
A
0
A
0
A
00
A
00
A
0
A
00
A
00
A
0
A
0
A
0
A
0
A
00
A
0
A
00
A
0
A
0
A
00
A
0
A
00 n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
15.88
26.52
10.14
1.53
2.08
1.92
0.62
1.33
4.21
0.56
0.67
0.04
1.66
7.70
3.23
2.56
1.13
0.01
0.21
0.75
0.50
10.71
2.78
12.66
12.76
2.82
0.09
3.54
63.27
92.93
95.56
46.10
138.75
19.34
221.54
0.37
1438
1433
1427
1425
1413
1305
1300
1166
2989
2932
2925
2924
2923
2921
2860
2858
1067
860
2.21
84.28
831 141.60
815 93.0
764
714
684
659
12.13
18.01
8.36
0.005
614
541
15.79
47.44
456 103.75
267 11.03
245
227
189
170
6.63
5.05
1.35
0.37
163
158
107
65
0.30
0.17
4.56
3.31
5.60
10.17
2.36
0.04
18.28
21.44
45.99
10.79
1.15
13.64
13.31
7.97
24.70
0.84
2.85
5.88
0.75
3005
0.1184
2983 d
0.7499
2980
0.75
2976
0.7437
2957
0.75
2957
0.0018
2915
0.75
2915
0.6315
1436
0.7498
1414
0.75
1414
0.75
1392
0.6994
1384 d
0.0275
1258 g
0.75
1263
0.7278
1128 g
0.75
1063 g
0.3499
852 g
0.2998
818 g
0.75
0.75
816
756 g
0.4680
747
0.75
706 d
0.75
650 d
0.6963
623 d
0.0825
578 g
1.0896
494
0.4151
254
0.75
243
0.6293
226
0.75
180
0.6441
179 d
0.5024
161 d
0.75
159
0.7192
130
0.75
94
P
P
P
P
P
P
P
P
D
D
D
D
P
P
P
P
P
D
D
D
P
P
P?
D
D
P?
P?
D
P?
P w w vw vw m e m m m vw vw vw w m e m g s m m m e w m e m w g w g m g s g vs vw g w
W e m d m h w g w vw m w m m w w w m m w vs w vvs vs m m s m w s m m m m m m m m vs vs vw f w w a c e b d f g h
Calculated at the HF/6-311 G
ⴱ level in Gaussian 94 , scaled with a factor of 0.9 above and 1.0 below 400 cm
⫺
1
.
From IR bands in the vapour phase except when noted.
Potential energy distribution. Symmetry coordinates with contributions below 10% are omitted.
From Raman spectra of the liquid.
From IR spectra of the amorphous solid.
From Raman spectra of the crystal.
From IR spectra of the Xe-matrix.
From IR spectra of the crystal.
S22(100)
S5(99)
S7(66)S8(33)
S24(87)S25(12)
S8(67)S7(33)
S25(87)S24(12)
S6(100)
S23(100)
S19(85)
S18(86)
S33(86)S34(10)
S34(86)S33(10)
S13(91)
S15(95)
S30(95)
S14(91)
S28(96)
S16(75)
S17(77)
S29(32)S31(29)S21(20)
S32(73)S31(10)
S2(74)
S21(48)S31(45)
S29(52)S21(14)S32(12)
S1(62)S4(21)
S4(51)S12(16)S1(15)
S3(72)S9(10)
S9(29)S12(16)S13(6)S17(17)
S27(45)S26(32)
S11(49)S9(26)S16(20)
S26(45)S27(37)
S20(55)S10(25)
S20(35)S10(25)S12(15)
S36(98)
S12(47)S13(19)S10(15)
S35(90)
3.3. Quantum chemical calculations
Hartree–Fock quantum chemical calculations were performed using the Gaussian-94 program [22] with the basis sets; HF/3-21G
311G
ⴱ
ⴱ
, HF/6-31G
ⴱ and HF/6-
. Two minima on the potential surface were found by relaxing the geometric parameters with standard optimisation methods. The conformational

Table 4
Observed and calculated fundamentals for the gauche conformer of bromomethyl dimethyl chlorsilane (BDCS)
Calculated a
Fund.
Calc.
Scal.
km mol
⫺
1
A
4 amu
⫺
1
Dep
Observed b cm
⫺
1
Dep I
IR
934
874
843
791
753
748
689
599
500
293
237
230
190
178
174
161
107
55
3169
1599
1591
1587
1583
1568
1450
1443
1303
1196
953
3304
3265
3247
3242
3236
3233
3173 n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
0.6438
0.6034
0.6856
0.3764
0.7111
0.6293
0.2343
0.2010
0.0529
0.4807
0.6741
0.7178
0.6750
0.6017
0.4830
0.4082
0.6495
0.7464
0.7250
0.7438
0.7478
0.1513
0.6383
0.6631
0.0189
0.0070
0.7346
0.7498
0.7490
0.7491
0.6509
0.0155
0.3035
0.7256
0.7490
0.5407
0.35
0.91
1.38
11.85
4.58
7.08
6.98
21.36
11.34
3.27
2.07
1.81
1.28
1.84
0.07
0.05
0.37
1.44
67.91
64.01
101.85
95.18
127.02
39.87
166.73
70.92
2.14
13.5
8.93
8.48
9.75
3.42
0.39
3.08
4.86
0.74
149.2
71.29
7.07
13.85
15.94
12.53
25.78
32.64
63.91
21.29
6.33
4.92
0.46
0.31
0.04
0.04
0.54
1.29
6.8
11.27
4.36
2.58
1.93
14.30
22.83
41.44
9.92
2.78
112.8
2.35
8.33
14.66
17.62
24.23
5.73
9.21
841
787
759
712
678
673
620
539
450
293
237
230
190
178
174
161
107
55
2852
1439
1432
1428
1425
1411
1305
1299
1173
1076
858
2974
2939
2922
2917
2912
2910
2856
3005 d
2983
2980
2976
2957
2957
2915
2915
1436
1414
1414
1392
1384 d
1254 d
1263
1137 g g
1073
849 g
825 g
801
754 g
728
706 d
681
623 d
580 g
494
275
243
226
180
179 d
161 d
159
130
81
P
P
P
P
P
D
P
P
P
D
D
D
D
P
P
P
P
P
D
D
D
P
P p?
D
P?
P?
D
P?
P
D a c e b d f g h
Calculated at the HF/6-311G
ⴱ level in Gaussian 94 ; scaled with a factor of 0.9 above and 1.0 below 400 cm
⫺
1
.
Bands from IR vapour phase, except when noted.
Potential energy distribution. Contributions below 10% are omitted.
Bands from Raman spectra of the liquid.
Bands from IR spectra of the amorphous solid.
Bands from Raman spectra of the crystal.
Bands from IR spectra of the Xe-matrix.
From IR spectra of the crystal.
w w vw vw vw vw vw m e m m m vw h m w g s vw w g w e w e w m s g e m m w g g g m w g w m s e w m e w vw m
I
Raman w m m w w s m m vs m m m vs vw f m m w m m w vw w vvs vs m m s m w s w w
PED c
S22(97)
S7(49)S24(47)
S24(40)S7(37)S8(13)
S5(93)
S8(74)S25(15)
S25(75)
S6(69)S23(26)
S23(70)S6(27)
S19(86)
S18(82)
S34(57)S33(35)
S33(53)S34(38)
S13(90)
S15(94)
S30(93)
S14(91)
S28(96)
S16(74)
S17(66)
S31(41)S21(28)
S32(84)
S2(41)S29(15)S4(14)S31(13)
S21(52)S31(20)S29(13)
S29(34)S2(11)S17(11)S31(11)S4
S1(58)S2(13)
S4(39)S12(20)
S3(78)
S12(23)S11(17)S10(13)S13(12)
S9(24)S26(19)S11(13)S27(10)
S9(55)S17(15)S26(11)
S11(42)S10(28)
S26(42)S27(31)
S20(82)
S36(90)
S12(48)S13(22)S27(14)
S35(87)

A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215 211
33
34
35
36
29
30
31
32
25
26
27
28
21
22
23
24
Table 5
Symmetry coordinates for bromomethyl dimethyl chlorosilane (BDCS)
A
0
15
16
17
18
9
10
11
12
13
14
19
20
7
8
5
6
3
4
1
2
Si–C symmetric stretch
Si–C antisymmetric stretch
Si–Cl stretch
C–Br stretch
CH
2 symmetric stretch
CH
3 symmetric stretch
CH
3 antisymmetric stretch
CH
3 antisymmetric stretch
Symmetric C–Si–C bend
Antisymmetric C–Si–C bend
Antisymmetric C–Si–C bend
Si–C–Br bend
CH
2 scissor
CH
2 wag
CH
3 symmetric deformation
CH
3 antisymmetric deformation
CH
3 antisymmetric deformation
CH
3 antisymmetric deformation
CH
3 antisymmetric deformation
CH
3 torsion
A
00
Si–C antisymmetric stretch
CH
2 antisymmetric stretch
CH
3 symmetric stretch
CH
3 antisymmetric stretch
CH
3 antisymmetric stretch
C–Si–Cl deformation
C–Si–C deformation
CH
2 twist
CH
2 rock
CH
3 symmetric deformation
CH
3 antisymmetric deformation
CH
3 antisymmetric deformation
CH
3 antisymmetric deformation
CH
3 antisymmetric deformation
CH
2
Br torsion
CH
3 torsion
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
1
2
3
4
5
6
S
7
S
8
S
9
10
11
12
13
14
15
16
17
18
19
20
ˆ
3
⫺
1
=
2
ˆ
6
⫺
1
=
2
ˆ
S
ˆ
T
ˆ
2
⫺
1
=
2
ˆ
6
⫺
1
=
2
2
2
2
ˆ
12
⫺
1
=
2
ˆ
ˆ
6
ˆ
6
ˆ
6
ˆ
ˆ
ˆ
1
2
⫺ d
1
=
2
2
⫺
1
=
2
V
1
2
1
2
1
2
s
7
d
1
⫺
1
=
2
ˆ
12
⫺
1 g g
⫺
1
1
1
=
=
R
1
2R
⫺ d
J
ˆ
12
⫺
1
=
2
ˆ
ˆ
12
ˆ
1
2
ˆ
2
⫺ b a
=
1
2
2
2
2
⫺
2
⫺ t
1
⫹
⫺
R
2
⫹ s
⫹
2d
⫹
⫹
1
2
3
J
F b
2 g
1
⫹ g b b a a
1
1
1
2
2
3
⫹ d
5
1
⫹
⫺
⫹
⫺
1
3
⫹
R d
8
2
2
†
J
⫺
⫺
⫹
⫺
⫹
3
† b
⫹
R
3
⫺
⫹ d
3
⫺ d
2 t
2
J
F u u b b a a
⫹
1
1
2
⫺ d
6
2
⫹
⫺
⫹
⫺
5
2
5
2
2
R
J
⫺
⫺
⫺
⫺
⫺
†
3
⫹ d
⫺ d
3 u u b b b a a
†
3
†
⫺
J
F
2
⫹
2d
4
2
†
†
3
6
6
3
3
⫺
3
†
4
†
†
⫹ d
5
F a
1
⫹
2
3
†
⫹
2
1 b a
⫺ d
5
⫺
⫺
4
4
⫹ d
6
F a
⫺
⫺
2
2 b a
†
⫺ d
⫺
⫺
5
5 a
⫺
⫺
8
F
3
†
3 b a
†
⫹
6
†
6
† b
4
⫹ b
5
⫹ b
6
⫺ a
4
⫺ a
5
⫺ a
6
†
S
25
S
26
S
27
S
28
S
29
S
21
S
22
S
23
S
24
S
30
S
31
S
32
S
33
S
34
S
35
S
36
ˆ
2
⫺
1
=
2
ˆ
2
⫺
1
=
2
ˆ
6
⫺
1
=
2
ˆ
12
⫺
1
=
2
ˆ
ˆ
2
ˆ
6
ˆ 1
2
ˆ 1
2
ˆ
12
ˆ
12
ˆ
ˆ
12
ˆ
ˆ t
1
2
1
2
ˆ
2
1
2
⫺ d
1
=
2
2
⫺
1
=
2
g g
⫺ b
⫺ a
1
1
1
1
1
=
2
1
=
⫺
1
=
2
2
=
R
2
s
7
d
1
⫺
⫺
2
2
2
2
⫺
2
⫺ t
J
F
2
⫺
R
⫺ s
⫹ d
8
2d
⫺ d b
2
2 g g b b a a
3
1
1
2
2
3
3
⫺
2
⫺ d
J
F
1
⫺
⫹
⫹
⫺
1 t
⫺
⫺
⫺
3
†
3
†
† u u b
⫹
⫺ d
5
⫺
⫺ b b a a
2
1
1
2
5
2
5 d
3
⫺ d
3
3
3
†
†
⫹ d
6
2
⫹
⫺
⫹
⫺
⫹
⫺
⫹ u u b
⫺ d
4 b b a a
2
2
3
†
†
†
⫺
⫺
6
3
†
⫺
⫺ d
5
2d a
2
6
3
†
⫺
2
1
4 b a
⫺
4
4
⫺ d
6
⫹ d
5 a
⫹
⫹
2 b a
†
⫹ d
6
⫺
5
5 a
⫹
⫹
3
† b
⫺
6
† a
6
† b
4
⫺ b
5
⫺ b
6
⫹ a
4
⫹ a
5
⫹ a
6
† energies were: 10.8, 8.3 and 7.4 kJ mol
⫺
1
, for the three basis sets, respectively. All these calculations favour anti as the low energy conformer, and the value 7.4 kJ mol
⫺
1 derived from the basis set
HF/6-311G
ⴱ was employed in comparison of BDCS with the other halomethyl dimethyl halosilanes [23].
The calculated
D
H values are much larger than those found experimentally. They were also higher than the
D
H(gauche–anti) calculated for chloromethyl dimethyl chlorosilane [18], chloromethyl dimethyl fluorosilane [17] and bromomethyl dimethyl fluorosilane [19]. Negligible variations in the bond distances and bond angles for the anti and gauche conformers were calculated. The Si–C bond was 189 pm for both the anti and gauche conformers in BDCS, and the same values were calculated for chloromethyl dimethyl chlorosilane [18]. Analytical H–F force constants were derived for each of the two conformers in BDCS, using the HF/6-311G
ⴱ basis set. In addition, infrared intensities, Raman scattering cross sections and Raman polarisation ratios, r
, were calculated and these data are contained in Tables 3 and 4.
3.4. Normal coordinate calculations
The calculated ab initio force constants were

212 A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215
481 cm
⫺ 1
, being the most intense Raman peak in the entire spectrum. In chloromethyl dimethyl chlorosilane
[18] the Si–Cl stretches form a close-lying band pair at
490 and 481 cm
⫺ 1
, again being the most intense Raman bands in the spectrum. The C–Br stretching mode in
BDCS are delocalised and contribute to the fundamentals at 577 ( n
26
) and 623 cm conformer and 728 ( n
22
⫺
1
), 681 ( n
( n
24
25
) for the anti
) and 577 cm
⫺
1
( n
27
) for the gauche rotamer. In bromomethyl dimethyl fluorosilane [19]. The C–Br stretch is also delocalised, but contribute mainly to the fundamentals at 577 (anti) and 560 cm
⫺
1
(gauche) which are the most intense bands in the Raman spectrum of this molecule and were employed in the van’t Hoff plots.
Fig. 11. Valence coordinates for BDCS.
transformed from Cartesian to symmetry coordinates, derived from a set of valence coordinates. These diagonal and off-diagonal force constants for the
anti and gauche conformers are not given for the sake of brevity, but can be obtained from the authors.
The ab initio calculated wavenumbers are listed as
Calc. in Tables 3 and 4 and are invariably higher than the experimental values. In order to make a complete assignment of the observed infrared and
Raman bands, a normal coordinate analysis with scaled force constants was carried out.
Simple scaling factors of 0.9 for the stretching and bending modes above, and 1.0 for the modes below
400 cm
⫺ 1 were applied in agreement with the calculations carried out on the related molecules [17–20].
The potential energy distribution (PED) listed in
Tables 3 and 4 are expressed in terms of the symmetry coordinates (Table 5) and are defined from the internal coordinates for BDFS given in Fig. 11.
Only PED terms larger than 10% have been included in Tables 3 and 4. As is apparent from Table 3 the CH
2
( n
1 and n
2
), CH
3
( n
3
– n
8
) stretching modes for the anti conformer are highly localised. Moreover, the CH
3 symmetric (
(( n
( n
18
, n n
14
) and antisymmetric deformations
19
), the CH
2 scissor ( n
13
), twist (
16
) are were localised, but CH
3 rock ( n n
17
) and wag
24
) is highly mixed. As expected the fundamentals of the gauche conformer with no symmetry are less localised than those of the anti conformer (Table 4). The Si–Cl stretching modes for the anti and gauche conformers
( n
27
) should be separated 6 cm
⫺ 1 according to the calculations, but they are observed as one band at
4. Discussion
4.1. Conformations
The bands around: 802, 728, 677, 276 and 76 cm
⫺ 1 vanish during crystallisation both in the infrared and
Raman spectra as listed in Table 1. Undoubtedly, these bands belong to the conformer which disappears in the crystal and they are also reduced in intensity with temperature as observed in the Raman spectra of the liquid.
Do these bands belong to the anti or gauche conformer? The fact that the ab initio quantum chemical calculations gave enthalpy differences between 10.8 and 7.4 kJ mol
⫺
1 depending on the level of approximation (see above) with anti as the low energy conformer, will not be regarded as conclusive. In the series of five halosilanes with the structure CH
2
X(CH
3
)
2
SiY investigated so far, anti was invariably calculated [23] to be the low energy conformer at the HF/6-311G
ⴱ ranging from 7.4 to 1.3 kJ mol
⫺ 1 level with values but the anti and
gauche conformers were both found to be the more stable in the liquid state and in the matrices for the various halomethyl dimethyl halosilanes [23].
Since the contours in the infrared vapour spectra are poorly resolved (conf. Figs. 4 and 5), they cannot be employed for determining the conformers. The 36 fundamentals in the anti conformer will divide themselves between 20 of symmetry species A
0 and 16 of species A
00 of which the former will give rise to polarised, the latter to depolarised bands in the

A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215
Raman spectra. In the gauche conformer without symmetry all the 36 fundamentals will give polarised
Raman bands. With most of the anti and gauche bands overlapping the polarisation values are of limited value for the assignments.
The most reliable criterion for assigning the bands to anti and/or gauche involves correlating the infrared and Raman bands vanishing in the crystal with the scaled ab initio wave numbers for the anti (Table 3) and gauche (Table 4) fundamentals. There are all together nine instances in which the calculated anti and gauche fundamentals were separated 10 cm
⫺
1 or more, later to be assigned as n
33 and n n
1
, n
36
. In four of these cases (
5
, n n
1
,
6
, n n
5
,
20
, n
6 n
24
, and n n
28
,
33
) the observed bands were attributed to overlapping anti and gauche fundamentals. In the case of n n
20
, n
22
, n
24
,
28 and n
36
, however, band pairs were observed at
812/808, 744/728, 650/677, 253/276 and 87/76 cm
⫺ 1
(Raman spectra of the liquid) in which the bands of the denominator vanished and those of the numerator remained after crystallisation. On a qualitative basis the vanishing bands must be correlated with gauche, meaning that the anti conformer remains in the crystal.
This assumption is also qualitatively supported by n
22
, although the observed anti–gauche shifts were 19 and those calculated were only 2 cm
⫺
1 for this fundamental. If the gauche conformer should remain in the crystal, the observed wavenumber shifts would all be contradictory to those calculated.
The intensity variations of the Raman bands with temperature reveal that the bands which disappear in the crystal are enhanced at higher temperatures (see above). Accordingly, the anti and gauche conformers are the low- and high-energy conformers, respectively, in the liquid. In the other halomethyl dimethyl halosilanes recently studied in our laboratories, the related chloromethyl dimethyl chlorosilane [18] crystallised in the anti conformer which was also the low energy conformer in the liquid by 0.7 kJ mol
⫺
1
. Both chloromethyl dimethyl fluorosilane [17] and bromomethyl dimethyl fluorosilane [19] crystallised as
gauche which was also the low energy conformer in the liquid, while the anti conformer was more stable in the matrices (and probably in the vapours). Thus, it appears that the presence of C–F favoured the crystallisation into the gauche conformer compared to the C–Cl and C–Br linkages. Finally, bromomethyl dimethyl silane crystallised in an
213 apparently metastable gauche and a stable anti conformer, while the crystallisation of the monodeuterated species was less straightforward. The anti conformer was the more stable conformer both in the liquid and in the matrices of these latter molecules.
The infrared spectra of BDCS in argon, nitrogen and xenon matrices were recorded both with high and low sample thickness and subsequent annealing to temperatures in the range 20–50 K, but the results regarding conformer stability or barriers were somewhat uncertain. Increased scattering from the matrices demonstrated by higher backgrounds were observed for all the five halomethyl dimethyl halosilanes [17–20,24] after annealing, but the effect was still more pronounced for BDCS than the other silanes. It is possible that the halomethyl dimethyl halosilanes hydrolyse after annealing, due to small amounts of water on the glass and metal surfaces of the vacuum system, and BDCS apparently more than the other compounds in the series.
In addition to the band pairs assigned to anti and
gauche conformers on the basis of spectral changes on crystallisation described above ( n n
20
, n
22
, n
24
, n
28 and
36
) the infrared spectra in the matrices can frequently give clues to close lying band pairs. Thus, the band pairs attributed to n
14
, n
16
, n
17
, n
18
, n
19
, n
21 and n
26 situated around 1267, 1130, 1053, 851, 822, 757 and
577 cm
⫺
1
, respectively, can all be tentatively assigned to separate conformer bands. As is apparent, each of these band pairs are characterised by intensity changes in one or more matrices after annealing and fitted with arrows in Table 1. The bands which are enhanced after annealing are assigned to the anti, those that diminish in intensity to the gauche conformer. This is clearly demonstrated by the band pairs n
20
, n
22
, n
24
, since the bands that vanish after crystallisation attributed to the
gauche conformer are reduced in intensity after annealing in the matrix spectra. Opposite, the bands of these pairs remaining after crystallisation are enhanced in the matrix spectra after annealing.
It is characteristic that in each of the pairs n n
17
, n
18
, n
19
, n
21 and n
26
14
, n
16
, observed in the IR matrix spectra the wave number difference in the matrices is small (between 2 and 10 cm
⫺
1
) leading to overlapping bands in the fluid phases. Since the bandwidths are much lower in the matrices, the separate anti and
gauche bands can be detected. Because of the matrix effects frequently encountered during annealing, some of these attributions may be erroneous. However, if

214 the same general features are observed in different matrices we can be fairly confident in the experimental results.
4.2. Spectral assignments
A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215
The assignments of the infrared and Raman spectra of BDCS to the anti and gauche conformers appear in
Tables 1, 3 and 4 and they are very similar to those of the closely related chloromethyl dimethyl chlorosilane [18]. As expected, most of the modes are slightly shifted towards lower wave numbers from chloromethyl dimethyl chlorosilane [18] to BDCS.
The fundamentals of both anti and gauche have been numbered consecutively in order to maintain the similarity between the two conformers rather than the conventional numbering of the modes belonging to species A
00 before those of A
00 in the anti conformer.
We have assigned five band pairs to separate anti and
gauche bands based mainly upon the infrared and
Raman bands vanishing in the crystal spectra and seven band pairs to variations in the matrix spectra after annealing while the remaining 24 fundamentals are attributed to overlapping anti and gauche bands.
We expect in principle eight C–H stretching fundamentals n
1
–
3010 and 2900 cm n
⫺
8
1 of each conformer between
, with six hydrogens belonging to methyl and two to methylene groups. In this region the fundamentals of anti overlap those of gauche and the fundamentals n
5
/ n
6 and n
7
/ n
8 are assumingly accidentally degenerate.
The ab initio calculations reveal that the CH
3 deformation and the CH
2 scissoring and wagging modes should overlap considerably. The calculated, scaled wave numbers suggest (Tables 2 and 3) that five fundamentals ( n
9
– n
13
) for both conformers should be situated between 1440 and 1400 cm
⫺ 1
.
Four of them were assigned to the Raman bands with corresponding infrared peaks in this region, while the fifth fundamental n
11 was tentatively attributed to the infrared bands around 1400 cm
⫺
1 with no Raman counterpart. From the annealing experiments in the nitrogen and xenon matrices n
14
(CH
2 wag) for the anti and gauche conformers were assumed to be approximately 4 cm
⫺
1 apart although they were calculated to coincide at 1305 cm
⫺
1
.
Although the infrared and Raman bands around
1140 cm
⫺
1 remained with low intensities in the crystal spectra they are tentatively attributed to the gauche component of n
16
(CH
3 deformation) with the band of the anti conformer at slightly lower wave numbers.
Also n
17 appeared as close lying conformer bands around 1073 and 1065 cm
⫺
1 based upon the matrix data, while n
15 was tentatively assigned to the infrared bands around 1255 cm
⫺
1 with no Raman analogue.
A number of weak infrared bands from 1380 to 1200 cm
⫺
1 with no Raman counterparts are presumably combination bands or overtones.
According to the calculations and the appearance of the spectra, no fundamentals are expected between
1050 and 850 cm
⫺
1
. A CH
3 deformation mode ( is found with a vapour band at 851 cm
⫺ 1 n
18
) and the matrix spectra suggested the anti band to be at slightly higher wavenumber than the gauche.
The fundamental n
19
(CH
3 def.) is predicted from the calculations to be separated 10 cm
⫺ 1 in good agreement with the matrix data. The CH
2
( n rocking mode
20
) is assigned to the vapour bands at 820 (anti) and 801 cm
⫺
1
(gauche) in good agreement with the calculations, and the latter bands vanished in the infrared and Raman spectra of the crystal. The anti and gauche conformers for n
21
(CH
3 def.) were tentatively assigned to the matrix bands at 758 and
756 cm
⫺
1 although only one Raman band was observed in the crystal spectrum at 760 cm
⫺
1
. One of the asymmetric C–Si–C stretches ( n
22
) is correlated with the bands at 747 (anti) and 730 cm
⫺ 1
(gauche) although the calculations predict a much smaller band separation. The other C–Si–C stretch
( n
23
) had coinciding conformer peaks around
710 cm
⫺ 1
. The n
24 fundamentals were highly mixed modes at 650 (anti) and 681 cm be separated 14 cm
⫺
1
⫺
1
(gauche), they should according to the calculations.
The vibrational modes n
25
, n
26 and n
27 were all mainly connected with stretching of the heavy atoms and they involved Si–C sym. stretch, C–Br stretch and Si–Cl stretch, respectively. None of these fundamentals appeared as separate bands for the two conformers in the condensed states, but n
26 had close lying peaks in the matrix spectra. They seemed to change intensity only in the xenon spectra and have tentatively been attributed to separate gauche and anti fundamentals. The three modes were attributed to bands at 623, 577 and 481 cm
⫺ 1
, respectively, and the two latter peaks were the most intense in the entire
Raman spectra in agreement with the calculations. As

A. Nilsen et al. / Journal of Molecular Structure 550–551 (2000) 199–215 mentioned above, the Si–Cl and C–Cl stretches were both separated into anti and gauche components for chlormethyl dimethyl chlorosilane [18], while the
anti and gauche bands for the C–Cl and the Si–F stretching modes in chloromethyl dimethyl fluorosilane
[17] were superimposed. In bromomethyl dimethyl fluorosilane [19] the C–Br stretches appeared at 557
(anti) and at 560 cm
⫺
1
(gauche) while the Si–F stretching modes coincided at 877 cm
⫺
1
.
The mixed bending modes n
28 were assigned to separate bands at 275 (gauche) and 254 cm
⫺
1
(anti) in good agreement with the predicted wavenumber shifts while n
29 was assigned to overlapping anti and gauche bands around 243 cm
⫺ 1
. The two fundamentals n
30 and n
31 were all highly mixed skeletal deformation modes giving rise to strong
Raman bands in the crystal and found at 226 and
188 cm
⫺ 1 in the Raman spectra of the liquid, respectively, common to the anti and gauche conformers. In the anti conformer the modes n
32 and n
33 both contributed to symmetric methyl torsion, but in the gauche conformer this vibration was localised to
161 cm
⫺
1 n
33
. They were observed at 179 and in the liquid, respectively, common to both conformers. The asymmetrical methyl torsion was highly localised to n
34 in both conformers and assigned to the infrared bands around 150 cm
⫺
1 and in agreement with the low intensity predicted in the calculations this mode was not observed in the
Raman spectra. The Si–C–Br bend contributed to n
35 and was observed at 130 cm
⫺ 1 in the vapour. Finally, the asymmetric BrCSiCl torsional mode ( n
36
) gave distinct infrared bands in the vapour phase (Fig. 5) at 94 and 81 cm
⫺
1 for anti and gauche, respectively. They were also clearly observed in the Raman spectra of the liquid at 81 and 74 cm
⫺
1
, employing the R( n
) formalism [25].
References
Acknowledgements
The authors are grateful to Mrs Anne Horn for valuable assistance. V.S. acknowledges a grant from the Norwegian Research Council reserved for the
Baltic Countries and North West Russia.
215
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