Conformational equilibrium in dimethyl vinyl fluorosilane studied by infrared and Raman spectroscopy

Conformational equilibrium in dimethyl vinyl ¯uorosilane studiedby infrared and Raman spectroscopy

A. Horna, P. Klaeboea, V. Aleksaa,b, A. Gruodisa,b, C.J. Nielsena, Y.E. Nashedc,G.A. Guirgisc,1, J.R. Durigc,*

aDepartment of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, NorwaybDepartment of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania

cDepartment of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA

Received 12 April 2000; accepted 30 May 2000

Abstract

The Raman spectra (3500±20 cm21) of liquid with depolarization values and solid, as well as the infrared spectra of the gas,

the sample isolated in argon and nitrogen matrices at ca. 5 K and solid dimethyl vinyl ¯uorosilane, CH2yCHSi(CH3)2F, have

been recorded. Both gauche and syn rotamers have been identi®ed in the ¯uid phases but only the syn conformer remains in the

solid. Variable temperature (255 to 21508C) studies of the infrared spectra (4000 and 400 cm21) of dimethyl vinyl ¯uorosilane

dissolved in liquid xenon and krypton have been recorded. From the xenon and krypton data, the enthalpy differences have been

determined to be 53 ^ 9 cm21 (0.64 ^ 0.10 kJ/mol) and 44 ^ 7 cm21 (0.53 ^ 0.09 kJ/mol), respectively, with the gauche

conformer being the more stable form. The intensity variations with temperature of the Raman spectrum of the liquid gave

an enthalpy difference of 25 ^ 15 cm21 (0.30 ^ 0.18 kJ/mol) also with the gauche conformer being the more stable form.

Vibrational assignments are provided for both conformers. Complete equilibrium geometries have been determined for both

rotamers using ab initio calculations employing the 6-31G(d), 6-3111G(d,p) and 6-3111G(2d,2p) basis sets at the levels of

restricted Hartree±Fock (RHF) and/or with full electron correlation by the perturbation method, Moller±Plesset (MP), to

second order. The syn conformer is predicted to be the more stable conformer from all ab initio calculations except those of

MP2/6-31(d) which predict the gauche form being the more stable conformer by 54 cm21 (0.65 kJ/mol) although the values

favoring the syn form are all very small. These results are compared to the corresponding quantities of some similar molecules.

q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Conformational stability; FT-IR spectra; Ab initio calculations; Dimethyl vinyl ¯uorosilane

1. Introduction

A number of silanes in which the silicon atom is

attached to a sp2 hybridized carbon atom have been

investigated by infrared and Raman spectroscopy.

When the silicon atom has different substituents

attached, these molecules will have possibilities for

conformational equilibria. The vinyl silanes

CH2yCHSiX2Y in which X and Y are different groups

will, like the corresponding propenes, exist in a syn

conformer with a plane of symmetry and in two

equivalent gauche conformers. Thus, from the

Journal of Molecular Structure 554 (2000) 251±269

0022-2860/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.

PII: S0022-2860(00)00677-3

www.elsevier.nl/locate/molstruc

* Corresponding author. Tel.: 11-816-235-1136; fax: 11-816-

235-5191.

E-mail address: [email protected] (J.R. Durig).1 Permanent address: Analytical Research and Development

Department, Bayer Corp., P.O. Box 118088, Charleston, SC

29423, USA.

vibrational spectral data [1,2] and microwave investi-

gations [3] it was reported that vinylsilylchloride is

present in syn and gauche conformers in the vapor and

liquid states, whereas the gauche conformer is the

stable form in the crystal. However, for the vapor

state these investigators ®rst reported [1] the gauche

as the more stable form but from a later investigation

[2], Si-d2 isotopomer, the syn rotamer was reported as

the more stable conformer. Although the gauche

rotamer was calculated to be the more stable

conformer from ab initio RHF/6-31G(d) calculations,

the syn conformer was determined to be more stable in

the liquid by variable temperature Raman studies

[4,5]. However, it should be noted that the conformer

that is the most stable form in the liquid, may not be

the most stable rotamer in the gas. Therefore, we [6]

carried out a variable temperature FT-IR investigation

of rare gas solutions of vinylsilyl chloride. These

studies indicated that the gauche was more stable by

78 ^ 11 cm21 (0.93 ^ 0.13 kJ/mol).

Infrared and Raman studies combined with ab initio

calculations have also been carried out for dimethyl

vinyl chlorosilane [7] and for methyl vinyl dichloro-

silane [8]. Since the methyl group and the chlorine

have approximately the same size, the conformational

preference is by no means obvious for these mole-

cules. In the former molecule the gauche conformer

was more stable and was present in the crystal [7]

whereas in the latter the syn conformer had lower

energy and was the sole conformer present in the

crystal [8]. These results indicate a preference of the

methyl group eclipsing the double bond over

the chlorine atom.

When these studies are extended to ¯uorine substi-

tuents it was observed that in methyl vinyl di¯uorosi-

lane [9,10] the gauche conformer has a lower energy

in the gas and liquid whereas the syn form is present in

the crystal. In order to obtain more information on the

relative stability of the conformers of dimethyl vinyl

halosilanes we have recorded the infrared and Raman

spectra to determine the conformational stability of

dimethyl vinyl ¯uorosilane, CH2yCHSi(CH3)2F,

(DVFS), and the results of this study will be described

in the present paper.

2. Experimental

The sample was prepared by the reaction between

the chloro derivative CH2yCHSi(CH3)2Cl, and freshly

sublimed antimony tri¯uoride at room temperature

without solvent for 1 h. The sample was puri®ed by

a low temperature, low pressure fractionation column

and the purity was checked by mass spectrometry.

The Raman spectra of the liquid, amorphous solid

and crystal were obtained at different temperatures in

a capillary tube of 2 mm inner diameter, surrounded

by a Dewar, cooled by gaseous nitrogen evaporated

A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269252

Fig. 1. Raman spectra of dimethyl vinyl ¯uorosilane in two polarization directions, 1700±800 cm21 with ordinate scale 0±3000 counts, 700±

100 cm21 with 0±30,000 counts.

from a reservoir [11]. These spectra were employed

for calculating the enthalpy difference DH between

the conformers in the liquid. DVFS has a pronounced

hysteresis (undercooling) and it was possible to study

the liquid far below the freezing point. The crystal-

lization often occurred spontaneously at ca. 21238Cand the anisotropic crystal containing only one

conformer was obtained. Independently, the vapor

of DVFS was condensed on a copper ®nger at

21968C, and the Raman spectrum of the amorphous

phase was recorded. Subsequently, the amorphous

solid was annealed to temperatures slightly below

the melting point, the sample turned crystalline from

visual inspection and was recooled to 21968C before

the spectrum was obtained. The Raman spectra were

recorded digitally using a Dilor RTI-30 spectrometer

(triple monochromator, with a Peltier cooled detector)

coupled to a PC. An argon ion laser from Spectra

A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269 253

Fig. 2. Raman spectra (1050±50 cm21) of amorphous (solid line) and annealed (dotted line), crystalline solids of dimethyl vinyl ¯uorosilane.

Fig. 3. Infrared spectrum of a gas of dimethyl vinyl ¯uorosilane.

Fig. 4. Far infrared spectra of dimethyl vinyl ¯uorosilane: (A) gas;

(B) amorphous (solid line) and annealed solid (dotted line).

Physics (model 2000) was employed with perpendi-

cular illumination using the 514.5 nm line for excita-

tion. The Raman spectra are shown in Figs. 1 and 2.

The infrared spectra (Figs. 3±8) were recorded with

various Fourier transform spectrometers: Bruker

models IFS-88 and IFS-66 (4000±450 cm21), a

Nicolet model 800 (4000±450 cm21), a Perkin±

Elmer model 2000 (4000±450 cm21) and on

two vacuum benches: Bruker IFS-113v spectro-

meter (600±50 cm21) and Bomem model DA

3.002 (600±50 cm21). The latter instrument had


Fig. 5. Infrared spectra (1100±400 cm21) of amorphous (solid line) and crystalline (dotted line) solids of dimethyl vinyl ¯uorosilane.

Fig. 6. Infrared spectra (1350±800 and 860±500 cm21) of dimethyl

vinyl ¯uorosilane in an argon matrix unannealed (solid line) and

annealed (dashed line).

Fig. 7. Infrared spectra (1025±950 cm21) of dimethyl vinyl ¯uor-

osilane in a nitrogen matrix unannealed (solid line) and annealed.

a helium cooled Bolometer as detector, the other

instruments had detectors of DTGS. Beamsplitters

of Ge substrate on KBr were used in the mid-

infrared regions (MIR) whereas beamsplitters of

Mylar of thickness 3.5 and 12 m as well as one

of a metal mesh were employed in the far infrared

(FIR) region. The spectrum of the vapor was

recorded with the sample contained in cells with

KBr windows and path length 10 cm in MIR and

in cells of 20 cm and 1 m path lengths with poly-

ethylene windows in the FIR region. The spectra

of the amorphous and crystalline solids were

obtained by depositing the vapor on a CsI window

and on a wedge shaped window of silicon, cooled

with boiling liquid nitrogen, for the MIR and FIR

regions, respectively.

The sample was diluted with argon and nitrogen

(1:500 and 1:1000) and deposited on a CsI window

of a three stage Displex cryostat from APD (model

HS-4) at either 2268 or 22588C. The matrices were

subsequently annealed to various temperatures from

2253 to 22368C (argon) and from 2253 to 22398C(nitrogen) in periods from 10 min to 1 h and the

window was recooled to 22688C and the spectra

recorded (Figs. 6 and 7).

The mid-infrared spectra (Fig. 8) of the samples

dissolved in lique®ed xenon (255 to 21008C) and

krypton (2105 to 21508C) as a function of tempera-

ture were recorded on a Bruker model IFS 66 Fourier

transform interferometer equipped with a Globar

source, a Ge/KBr beamsplitter and a TGS detector.

In all cases, 100 interferograms were collected at

1.0 cm21 resolution, averaged and transformed with

a boxcar truncation function. For these studies, a

specially designed cryostat cell was used. It consisted

of a copper cell with a path length of 4 cm with

wedged silicon windows sealed to the cell with

indium gaskets. It was cooled by boiling liquid

nitrogen to 21968C. The temperature was monitored

with two Pt thermoresistors. The complete cell was

connected to a pressure manifold, allowing the ®lling

and evacuation of the cell. After the cell had cooled to

the desired temperature, a small amount of the

compound was condensed into the cell. Next the pres-

sure manifold and the cell were pressurized with the

noble gas, which immediately started to condense in

the cell, allowing the compounds to dissolve. All

observed infrared and Raman bands with signi®cant

intensities are listed in Table 1.

3. Ab initio calculations

The LCAO±MO±SCF calculations were

performed with the gaussian-94 program [12] with

Gaussian-type basis functions. The energy minima

with respect to the nuclear coordinates were obtained

by the simultaneous relaxation of all of the geometric

parameters, except for the symmetry restrictions for

the gauche and cis conformers, using the gradient

method of Pulay [13]. The structural parameters

were determined from RHF/6-31G(d) (restricted

Hartree±Fock), MP2/6-31G(d) (full electron correla-

tion by the perturbation method to the second order),

MP2/6-3111G(d,p) and MP2/6-3111G(2d,2p)

calculations and the results are given in Table 2.

The energy difference that resulted from these various

calculations ranged from 46 cm21 (0.55 kJ/mol) from


Fig. 8. Infrared spectra of dimethyl vinyl ¯uorosilane: (A) experi-

mental spectrum of the liquid Krypton (21258C); (B) calculated

spectrum of the mixture (DH� 44 cm21); (C) calculated spectrum

of the syn conformer; (D) calculated spectrum of the gauche

conformer.

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6Table 1

Infrared and Raman spectral data (abbreviations used: s, strong, m, moderate; w, weak; v, very; bd, broad; sh, shoulder; p, polarized; d, depolarized. A, B and C denote vapor

contours; asterisks denote band vanishing in the crystal spectra; arrows pointing upwards and downwards signify matrix bands which increase and decrease in intensities after

annealing; and P, Q, and R refer to the rotational±vibrational branches) for dimethyl vinyl ¯uorosilane (CH2yCHSi(CH3)2F)

Infrared Raman

Solid

Vapor Ar matrix (5 K) N2 matrix (5 K) Amorphous (80 K) Crystalline (80 K) Liquid Amorphous (80 K) Crystalline (80 K) Interpretation

3071 R m n 10

3065 Q,C 3074 w 3071 w 3059 m 3059 m 3064 m,br,P n 1

3058 P 3063 m 3062 m3027 w 3026 w

3025 w 3018W 3019 w 3018 w/m 3024 w 3022 w,D? n 2

3018 w3000 w 3000 w 2993 w,sh n 3

2988 vw 2986 w/m,sh 2981 vs,P2986 R m 2982 vw 2974 m/w,sh2973 Q,C 2976 m 2975 m 2963 s 2963 m 2960 m,sh,D n 4

2961 P 2963 w 2954 m2970 w n 5,n 6

2957 w 2957 w 2954 s n 7

2915 max w 2916 vw 2916 vw 2907 w 2907 w 2911 vs,P n 8,n 9

2888 w 2876 vw 2882 vw1937 R 1941 vw

1930w1928 Q,A

w1929 vw 1939 w 1930 w

19171919 P 1923 vw1608 R 1605 w1601 Q,A 1603 m

1601 m #1597 m/s 1596 w/m 1601 vs,P 1597 m 1601 m n10

1595 Pm

1598 w " 1599 m "1596 w/m 1601 vs,P 1597 m 1595 m n10

0

1446 w1447 m 1446 w n11, n12

1442 w1423 w 1425 vw n13

1421 vw " 1421 w # n14

1420 Q m

1414 Q m1413 m

1415 w1412 m

1415 m n151411 Q m1408 m

1413 s 1409 s1410 m

1416 vs,P1412 s

1413 m n1501405 sh m 1409 m

1403 m "1398 m # 1401 m # 1398 m 1397 m 1402 w,sh 1405 w,sh

1297 sh w 1293 w 1294 vw 1299 w 1305 w1277 w 1277 w #

1272 R 1274 m1266 Q m m 1262 vs 1263 s 1275 w 1271 w 1279vs,P 1272m 1270 m n 16

1263 Q m m 1258 vs 1263 w1254 sh m 1257 vs " 1253 s " 1254 vs,br 1257 m 1260 vw 1258 vw 1258 w n 17

1250 s 1252 m

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Table 1 (continued)

Infrared Raman

Solid


1249 w # 1244 w 1244 m 1252 vw 1250 vw n 18

1016 m " 1018 w " 1017 vw1013 max m 1014 m # 1015 1012 s 1012 s 1014 w,D 1008 w 1009 w n 19

1011 m # m # 1011 m1007 max m 1008 m

1007 s 1008 s # 1005 s 1002 s 1004w 1002 w n 20 n 200

1005 w,sh 1006 w n 210

974 R969 Q,C m 967 m " 969 vs " 970 s 959 s 967 w,D 971 w 957 w n 21

966 P 966 s,sh "966 R 964 s #964 Q,C m 963 s 966 s # p 967 w,D 963 w p

955 961 vs958 m890w 890 m " 893 w,sh

904 R p p

895 Q vs 866 vs 883 vs # 876 s 883 w,D 874 vw n 22

886 sh vs 884 vs 879 vs # n 220

876 w # 873 m # 865 s 862 s,sh 864 w855 R850 Q,C vs 847 vs " 850 vs " 845 vvs 846 vs 855 vw 850 w 848 w n 23

848 Q,C vs 845 vs # 848 vs # n 230

841 P839 m "

805 m806 R 801 s " 799 vs 797 vs 796 m 799 m,D 806 w 805 w n 24

799 Q vs 796 vs # 793 s 795 w 800 w789 P 788 m/w 792w772 Q m 770 M # 772 m # 769 m 770 m 767 w 769 vw V25

768 M " 766 w " 766 s 765 m733 W # 736 vw # 739w 741 vw 743 vw 742 vw n 26

716 R 714 W "710 Q,C m 712 vs " 711 s " 711 s p 712 m,p 714 m p n 27

703 P 710 m "694 sh m 692 m # 694 m # 695 m 696 m 696 m,P 695 m 695 m n 28,n 27

0

692 w616 R vw 607 w 606 w # 607 m p 613 vs,P 621 m,sh 613 vs n 29

0607 Q 601 w #

611 vs n 29

521 max m 528 s " 527 m " 526 w,D 530 w n 30

526 s # 525 m #515 max m 523 s # 522 w # 532 s 524 s 519 w 523 vw n 30

0

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8

Table 1 (continued)

Infrared Raman

Solid


520 m " 515 m " 519 s 519 m 517 vw400 R m394 Q, A/C 394 s 394 s 396 m,P 397 m 395 m n 31

0

388 P365 max m 366 m p 371 m,P 368 p n 31

273 max s 275 s 275 s 275 m,D 286 w 285 w n 32,n 330

272 s 277 w267 sh 263 s 261 s 265 m,P 263 m 264 m n 33,n 32

0

258 m218 vw,sh 228 w p 225 w,sh 230 w p n 34

208 w 209 w,sh 214 m180 w 201 w 209 w 195 s,D 201 m 209 m n 35,n 34

0

175 vw 179 vw 178 vw 187 w,sh 190 w,sh 187 w n 36,n 350

170 w 178 vw 178 w 178 w 175 w n 37

164 vw 151 vw 151 vw n 360

101 w 104 m,br 110 m n 39

95 vw 102m64 m47 m lattice

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Table 2

Structural parameters (bond distances in AÊ , bond angles in (8), rotational constants in MHz, and dipole moments in Debye), rotational constants, dipole moments and energy for

dimethyl vinyl ¯uorosilane

RHF/6-31G(d) MP2/6-31G(d) MP2/6-3111G(d,p) MP2/6-3111G(2d,2p)

Parameter gauche syn gauche syn gauche syn gauche syn

rC1±C2 1.325 1.325 1.344 1.344 1.347 1.347 1.340 1.340

rSi±C2 1.869 1.868 1.859 1.859 1.859 1.858 1.857 1.856

rF±Si3 1.609 1.608 1.634 1.632 1.637 1.636 1.621 1.621

rC1±H5 1.078 1.078 1.088 1.087 1.087 1.087 1.080 1.081

rC1±H6 1.077 1.076 1.087 1.086 1.088 1.087 1.081 1.080

rC2±H7 1.081 1.082 1.091 1.091 1.091 1.091 1.084 1.084

rSi±C8 1.876 1.876 1.866 1.867 1.861 1.861 1.859 1.859

rSi±C9 1.876 1.876 1.866 1.867 1.861 1.861 1.859 1.859

rC8±H10 1.087 1.088 1.094 1.094 1.094 1.095 1.087 1.087

rC8±H11 1.086 1.086 1.093 1.093 1.093 1.093 1.086 1.086

rC8±H12 1.087 1.087 1.094 1.094 1.094 1.094 1.087 1.087

rC9±H13 1.087 1.088 1.094 1.094 1.094 1.095 1.087 1.087

rC9±H14 1.087 1.086 1.093 1.093 1.094 1.093 1.086 1.086

rC9±H15 1.087 1.087 1.094 1.094 1.094 1.094 1.087 1.087

/SiC2C1 124.5 123.3 123.4 122.5 123.4 122.9 123.2 122.7

/FSiC2 107.4 106.4 107.7 106.2 107.3 106.2 107.5 106.3

/H5ClC2 122.3 122.3 122.6 122.6 122.1 122.0 122.1 122.0

/H6ClC2 122.4 121.9 121.9 121.3 121.7 121.3 121.5 121.1

/H7C2C1 117.4 117.8 117.5 117.8 117.2 117.4 117.0 117.3

/C8SiF 107.5 107.9 107.7 108.1 107.4 107.6 107.8 107.8

/C9SiF 106.7 107.9 107.2 108.1 106.7 107.6 106.8 107.8

/H10C8Si 111.7 111.1 111.3 110.8 111.4 110.6 111.2 110.6

/H11C8Si 111.8 111.2 111.8 111.0 111.4 110.8 111.4 110.8

/H12C8Si 110.3 111.4 110.2 111.4 110.1 111.2 110.1 111.1

/H13C9Si 111.2 111.1 110.7 110.8 110.7 110.6 110.6 110.6

/H14C9Si 111.3 111.2 111.3 111.0 111.0 110.8 110.9 110.8

/H15C9Si 111.3 111.4 111.3 111.4 111.1 111.2 111.1 111.1

tFSiC2C1 117.9 0.0 115.0 0.0 120.7 0.0 121.6 0.0

tH5ClC2Si 180.0 180.0 180.0 180.0 180.0 180.0 180.0 180.0

tH6C1C2H5 180.0 180.0 180.0 180.0 180.0 180.0 180.0 180.0

tH7C2C1Si 180.0 180.0 180.0 180.0 180.0 180.0 180.0 180.0

tC8SiFC2 120.9 119.9 120.5 119.6 119.9 119.7 119.8 119.7

tC9SiFC2 2119.0 2119.9 2119.2 2119.6 2119.4 2119.7 2119.4 2119.7

tH10C8SiF 178.7 181.8 178.8 182.1 178.4 181.7 178.2 181.9

tH11C8SiH10 121.1 119.8 121.0 119.7 121.0 119.6 120.9 119.7

tH12C8SiH10 2119.4 2119.9 2119.4 2119.9 2119.5 2119.8 2119.4 2119.9

tH13C9SiF 2179.3 2181.8 2179.3 2182.1 2180.6 2181.7 2179.7 2181.9

tH14C9SiH13 2119.9 2119.8 2119.8 2119.7 2119.7 2119.6 2119.8 2119.7

RHF/6-31G(d), 254 cm21 (20.64 kJ/mol) from

MP2/6-31G(d) and 22 cm21 (0.26 kJ/mol) from

MP2/6-3111G(d,p) and 10 cm21 (0.12 kJ/mol)

from MP2/6-3111G(2d,2p) calculations with the

syn rotamer the more stable conformer from each

calculation except from the MP2/6-31G(d) calcula-

tion which predicted the gauche rotamer being more

stable.

For the normal coordinate analysis, the force ®eld

in Cartesian coordinates was obtained with the gaus-sian-94 program [12] from the MP2/6-31G(d) calcu-

lations. Internal coordinates were de®ned as shown in

Fig. 9, which were used to form the symmetry coor-

dinates listed in Table 3. The Cartesian coordinates

obtained from the optimized geometry were used to

calculate the B-matrix elements with the G matrix

program of Schachtschneider [14]. These B-matrix

elements were used to convert the ab initio force

®eld in Cartesian coordinates to a force ®eld in the

desired internal coordinates. The resulting force ®elds

for the gauche and cis conformers are available from

the authors. These force ®elds were used in a mass-

weighted Cartesian coordinate calculation to repro-

duce the ab initio vibrational frequencies and to deter-

mine the potential energy distribution (PED) which is

given in Table 4 for the two conformers. All the

elements of the force ®eld in internal coordinates

from the MP2/6-31G(d) calculation were then

assigned scaling factors of 0.9 for the stretches and

bends and 1.0 for the torsions and the calculation

repeated to obtain the ®xed scaled force ®eld and

scaled vibrational frequencies.

To aid in the vibrational assignment for the

CH2CHSi(CH3)2F molecule, the infrared and Raman

spectra were calculated using frequencies, Raman

scattering activities (RHF/6-31G(d)), and infrared

intensities (MP2/6-31G(d)) determined from the ab

initio calculations. The evaluation of the Raman

activity by using the analytical gradient method has

been developed [15,16]. The activity Sj can be

expressed as:

Sj � gj�45a2j 1 7b2

j �

where gj is the degeneracy of the vibrational mode j,

a j the derivative of the isotropic polarizability and b j

the derivative of the anisotropic polarizability. The

Raman scattering cross sections, 2sj=2V; which are

A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269260T

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04

2.0

34

2.1

52

1.8

38

2(E

15

45

)0

.142

37

10

.142

58

00.9

43956

0.9

43711

1.3

48615

1.3

48714

1.4

50188

1.4

50232

DE

(cm

21)

46

54

22

10

proportional to the Raman intensities, can be calcu-

lated from the scattering activities and the predicted

wavenumbers for each normal mode using the rela-

tionship [17,18]:

2s j

2V� 2 4p4

45

! �n0 2 nj�4

1 2 exp2hcnj

kT

� �0BBB@

1CCCA h

8p2cnj

!Sj

where n 0 is the exciting frequency, n j the vibrational

frequency of the jth normal mode and Sj the corre-

sponding Raman scattering activity.

To obtain the polarized Raman scattering cross-

sections, the polarizabilities are incorporated into Sj

by Sj��12�j�=�11�j�� where r j is the depolarization ration

of the jth normal mode. The Raman scattering cross-

sections and calculated frequencies were used

together with a Lorentzian function to obtain the

calculated spectrum. The experimental and predicted

Raman spectra of dimethyl vinyl ¯uorosilane are

shown in Fig. 10. The predicted spectra are compared

to the experimental Raman spectrum of the liquid,

which is shown in Fig. 10A. These spectra were

very useful for making the vibrational assignments

to the correct bands for the two conformers.

Infrared intensities were also calculated based on

the dipole moment derivatives with respect to the

Cartesian coordinates. The derivatives were taken

from the ab initio calculations MP2/6-31G(d) and

transformed to normal coordinates by

2mu

2Qi

� ��X

j

2mu

2Xj

!Lij

where Qi is the ith normal coordinate, Xj the jth Carte-

sian displacement coordinate and Lji the transforma-

tion matrix between the Cartesian displacement

coordinates and normal coordinates. The infrared

intensities were then calculated by

Ii � Np

3c2

2mx

2Qi

� �2

12my

2Qi

� �2

12mz

2Qi

� �2" #

In Fig. 8, the predicted infrared spectra are shown for

the pure gauche (Fig. 8D), pure syn (Fig. 8C) and the

mixture (Fig. 8B). The experimental infrared spec-

trum of the normal species dissolved in liquid krypton

at 21258C is also shown for comparison in Fig. 8A.

Excluding the overtones or combination bands, the

calculated spectra have some differences from the

experimental one especially in comparing the relative

intensities of the bands in the 1300 cm21 region.

Nevertheless, they provide support for the assign-

ments of the observed bands to the indicated funda-

mentals for each conformer.

4. Conformational stability

There are a few fundamentals which show

conformer doublets in the infrared and Raman spectra


Fig. 9. Internal coordinates of dimethyl vinyl ¯uorosilane.

of the ¯uid phases. The two bands at 521 and

513 cm21 in the infrared spectrum of the gas which

are assigned as the C±H out-of-plane bending modes,

demonstrate the presence of conformers, where by

repeated annealing of the amorphous solid only the

lower frequency band remains. Other bands which are

observed in the infrared and Raman spectra of the

¯uid phases and amorphous solid but not in the spec-

trum of the annealed solid are observed at 962, 710,

607, 366, 272, 228 and 201 cm21.

These data clearly indicate that there are two

conformers present in the ¯uid phases at ambient

temperatures but only one rotamer remains in the

polycrystalline solid. The band at 710 cm21 is the

only band predicted by ab initio calculations in this

region (703 cm21) and it can de®nitely be assigned to

the Si±C stretch of the gauche conformer. Since this

band is drastically decreased in the intensity in the

infrared spectra of the argon and nitrogen matrices

and annealed solid, it can be concluded that the syn


Table 3

Symmetry coordinates (not normalized) for dimethyl vinyl ¯uoro-

silane

Description Internal coordinate

CH2 antisymmetric stretch S1 � r1 2 r2

CH3 antisymmetric stretch S2 � r5 2 r6 2 r8 1 r9

CH3 antisymmetric stretch S3 � r1 1 r2

CH3 antisymmetric stretch S4 � 2r4 2 r5 2 r6 12r7 2 r8 2 r9

CH2 symmetric stretch S5 � r1 1 r2

CH3 antisymmetric stretch S6 � 2r4 2 r5 2 r6 22r7 1 r8 1 r9

CH stretch S7 � r3

CH3 symmetric stretch S8 � r4 1 r5 1 r6 1 r7 1r8 1 r9

CH3 symmetric stretch S9 � r4 1 r5 1 r6 2 r7 2r8 2 r9

C� C stretch S10 � QCH2 deformation S11 � 2a 2 b1 2 b2

CH3 antisymmetric deformation S12 � 2d1 2 d2 2 d3 22d4 2 d5 2 d6

CH3 antisymmetric deformation S13 � d1 2 d3 2 d4 1 d6

CH3 antisymmetric deformation S14 � 2d1 2 d2 2 d3 22d4 1 d5 1 d6

CH3 antisymmetric deformation S15 � d1 2 d3 1 d4 2 d6

CH3 symmetric deformation S16 � d1 1 d2 1 d3 2f1 2 f2 2 f3 1 d4 1d5 1 d6 2 f4 2 f5 2 f6

CH3 symmetric deformation S17 � d1 1 d2 1 d3 2f1 2 f2 2 f3 2 d4 2d5 2 d6 1 f4 1 f5 1 f6

CH in-plane bend S18 � y1 2 y2

CH2 twist S19 � gCH2 wag S20 � b1 2 b2

CH2 rock S21 � eSiF stretch S22 � SCH3 rock S23 � f2 2 f3 1 f5 2 f6

CH3 rock S24 � f2 2 f3 2 f5 1 f6

CH3 rock S25 � 2f1 2 f2 2 f3 12f4 2 f5 2 f6

CH3 rock S26 � 2f1 2 f2 2 f3 22f4 1 f5 1 f6

SiC stretch S27 � RSiC2 antisymmetric stretch S28 � X1 2 X2

SiC2 symmetric stretch S29 � X1 1 X2

C±H out-of-plane bend S30 � g 0

CCSi bend S31 � 2p 2 y1 2 y2

SiC2 wag S32 � v1 1 v2 2 u1 2 u2

SiC2 rock S33 � u1 2 u2 1 v1 2 v2

SiC2 deformation S34 � ��6p

1 2�S2� ��

6p

2 2�v 2 u1 2 u2 2v1 2 v2

methyl torsion S35 � t2 2 t3

CSiF bend S36 � ��6p

2 2�S2� ��

6p

1 2�v 1 u1 1 u2 1v1 1 v2

SiC2 twist S37 � u1 2 u2 2 v1 1 v2

methyl torsion S38 � t2 1 t3

Asymmetric torsion S39 � t1

Fig. 10. Raman spectra of dimethyl vinyl ¯uorosilane: (A) experi-

mental spectrum of the liquid; (B) calculated spectrum of the

mixture (DH� 44 cm21); (C) calculated spectrum of the syn

conformer; (D) calculated spectrum of the gauche conformer.

A.

Ho

rnet

al.

/Jo

urn

al

of

Mo

lecula

rStru

cture

554

(2000)

251

±269

263

Table 4

Observed and calculated wavenumbers for gauche and syn conformers of dimethyl vinyl ¯uorosilane

Description Gauche Syn

Ab

initioa

Fixed

scaledb

IR

int.c

Raman

actd

dp

ratiod

Obse PED Ab

initioa

Fixed

scaledb

IR

int.c

Raman

actd

dp

ratiod

Obse PED

A 0 n 1 CH2 antisymmetric

stretch

3282 3113 15.5 134.4 0.14 3062 99S1 3289 3120 10.4 134.0 0.14 3071 99S1

n 2 CH3 antisymmetric

stretch

3206 3042 7.1 70.6 0.67 3019 40S2,41S3,10S6 3207 3042 3.4 68.2 0.73 3019 94S2


stretch

3203 3039 8.4 76.1 0.57 3000 54S3,44S1 3207 3042 10.2 64.8 0.63 3000 96S3


stretch

3201 3037 14.6 81.1 0.75 2975 53S4,20S5,10S6 3197 3033 14.4 140.6 0.75 2975 95S4

n 5 CH2 symmetric

stretch

3200 3036 2.1 139.9 0.73 29701 54S5,19S4,18S7 3201 3037 11.7 51.0 0.75 29701 85S5,14S7


stretch

3198 3034 3.9 74.2 0.72 29701 77S6,18S4 3195 3031 0.4 146.8 0.68 29701 94S6

n 7 CH stretch 3187 3023 9.4 33.7 0.72 2957 75S7,24S5 3182 3019 7.3 14.6 0.75 2957 86S7,13S5

n 8 CH3 symmetric

stretch

3108 2948 0.4 146.2 0.01 2916 61S8,39S9 3105 2946 1.3 217.5 0.01 2916 100S8

n 9 CH3 symmetric

stretch

3105 2946 0.6 68.2 0.01 2916 61S9,39S8 3104 2945 0.4 0.4 0.75 2916 100S9

n 10 CyC stretch 1680 1594 11.2 24.8 0.14 1601 61S10,33S15 1677 1591 11.8 30.9 0.16 1599 62S10,31S15

n 11 CH2 deformation 1534 1455 9.3 3.0 0.47 14461 49S11,45S12 1525 1446 1.9 2.3 0.75 14421 55S11,40S16


deformation

1526 1447 4.1 26.8 0.74 14421 51S12,44S11 1531 1453 11.8 32.2 0.75 14461 55S12,40S18


deformation

1521 1443 1.9 17.5 0.75 1425 79S13,17S14 1519 1441 1.2 13.3 0.75 1425 43S13


deformation

1517 1439 0.7 8.9 0.71 1421 76S14,17S13 1515 1438 0.9 16.2 0.75 1421 93S14


deformation

1489 1413 23.1 31.5 0.41 1413 65S15,24S10 1486 1410 18.3 30.8 0.34 1409 68S15,24S10

n 16 CH3 symmetric

deformation

1379 1308 33.5 0.8 0.59 12631 97S16 1377 1307 30.6 0.8 0.43 12621 98S16

n 17 CH3 symmetric

deformation

1374 1303 58.0 1.2 0.75 12571 96S17 1372 1302 56.4 1.0 0.75 12571 97S17

n 18 CH in-plane

bend

1324 1256 2.9 13.3 0.37 1244 59S18,25S20 1323 1255 1.3 14.5 0.35 1244 58S18,26S20

n 19 CH2 twist 1062 1008 20.5 0.5 0.74 1013p 58S19,37S30 1063 1007 27.0 0.4 0.75 1013p 62S19

n 20 CH2 wag 1053 999 15.6 1.1 0.75 1008 60S20,29S18 1050 996 20.1 1.3 0.75 1006 57S20,30S18

n 21 CH2 rock 995 944 38.7 3.1 0.64 966 98S21 999 947 127.5 3.3 0.64 969 99S21

n 22 SiF stretch 948 900 174.7 0.5 0.74 883 45S22,36S25 947 898 129.8 0.9 0.64 879 45S22,30S25

n 23 CH3 rock 916 869 163.0 1.8 0.72 848 70S23 919 872 178.0 1.7 0.69 850 71S23

A.

Ho

rnet

al.

/Jo

urn

al

of

Molecu

lar

Stru

cture

554

(2000)

251

±269

26

4

Table 4 (continued)

Description Gauche Syn

Ab

initioa

Fixed

scaledb

IR

int.c

Raman

actd

dp

ratiod

Obse PED Ab

initioa

Fixed

scaledb

IR

int.c

Raman

actd

dp

ratiod

Obse PED

n 24 CH3 rock 842 799 114.4 0.9 0.67 799 22S24,41S28,

15S26,

11S33

846 802 147.4 1.4 0.75 799 34S24,38S28,

13S33

n 25 CH3 rock 822 779 11.6 2.1 0.74 766 46S25,45S22 823 781 13.9 2.1 0.75 772 45S25,43S22

n 26 CH3 rock 797 756 0.2 2.1 0.75 736 57S26,30S24 797 757 1.0 1.7 0.75 736 61S26,25S24

n 27 SiC stretch 741 703 37.9 3.6 0.52 711 41S27,13S23,29S29 723 686 22.9 5.4 0.75 694 41S27,36S29,11S23

n 28 SiC2 antisymmetric

stretch

727 689 2.5 4.8 0.74 694 46S28,34S24,17S26 725 688 3.1 5.4 0.43 694 50S28,22S26,26S24

n 29 SiC2 symmetric

stretch

620 588 0.7 16.6 0.06 601 56S29,30S27 626 594 0.0 14.3 0.01 606 45S29,34S27

n 30 C±H out-of-plane

bend

540 518 21.2 6.2 0.68 527 25S30,12S36,

32S19,21S38

530 507 21.8 5.4 0.75 515 33S30,30S19,

16S38

n 31 CCSi bend 373 353 10.3 3.6 0.48 365p 46S31,27S33 404 383 19.1 4.4 0.25 394p 42S31,41S36

n 32 SiC2 wag 271 257 13.6 1.0 0.67 273p 43S32,33S36 255 241 7.0 1.7 0.75 267p 71S32

n 33 SiC2 rock 257 244 6.5 1.4 0.71 267p 40S33,11S31,25S37 282 269 11.4 1.1 0.43 273p 68S33,10S37,14S38

n 34 SiC2 deformation 221 210 0.6 1.6 0.52 218 28S34,33S32 199 189 0.4 1.3 0.73 1958 82S34

n 35 methyl torsion 189 189 0.4 2.6 0.75 1958 51S35,21S34,14S36 177 174 0.1 4.1 0.75 175p 76S35,10S36

n 36 CSiF bend 174 172 0.0 1.4 0.69 175p 18S36,29S34,34S35 155 149 0.5 0.5 0.75 151X 36S36,23S35,27S31

n 37 SiC2 twist 166 159 0.2 0.4 0.66 164p 40S37,18S31,13S35,

13S33,14S38

175 167 0.1 0.3 0.72 170p 74S37

n 38 methyl torsion 158 158 0.0 0.1 0.69 ± 76S38,14S37 152 152 0.0 0.0 0.75 ± 92S38

n 39 asymmetric torsion 70 70 0.1 7.7 0.75 95# 68S39,28S30 78 78 0.1 8.4 0.75 101# 62S39,28S30

a Calculated with the MP2/6-31G(d) basis set.b Scaling factors 0.9 for stretching and bending coordinates and 1.0 for torsional coordinates.c Calculated infrared intensities in km/mol from the MP2/6-31G(d) calculation.d Calculated Raman activities in AÊ 4/amu and dp ratios, from RHF/6-31G(d) calculation.e Frequencies are obtained from the annealed spectrum of the nitrogen matrix except for the ones with (p), (X), (1), (8), and (#) signs are taken from infrared gas, infrared solid, argon

matrix, Raman liquid and Raman solid, respectively.

conformer remains in the annealed solid with a minor

amount of the gauche conformer (Figs. 5±7). Further

support for this conclusion is found from the assign-

ments for the CH2 rock and CCSi bend which are

observed at 966 and 370 cm21 for the gauche

conformer. Similarly, the other listed bands, which

disappear upon solidi®cation and annealing, are all

assigned to the gauche conformer and they will be

discussed latter. Therefore, all the spectral data indi-

cate that the syn form is the stable conformer in the

annealed solid.

Variable temperature studies of the infrared spectra

of DVFS dissolved in liquid xenon (Table 5) and

krypton (Table 6) were conducted to determine the

enthalpy difference between the two stable confor-

mers. An important advantage to this temperature

study is that the conformer peaks are better resolved

and the area under them is more easily measured than

bands observed in the infrared spectrum of the gas.

Infrared spectral data from 4000 to 400 cm21 were

obtained at different temperatures between 255 to

21008C for the xenon solution and between 2105

and 21508C for the krypton solution. The spectral

changes in lique®ed krypton of the pairs at 962/965,

710/695 and 522/513 cm21 are shown in Fig. 11.

From all spectral data from lique®ed xenon and

krypton solutions we observed increases in the inten-

sity of the infrared bands assigned to the gauche

conformer as the temperature decreases. This clearly

con®rms the stability of the gauche rotamer over the

syn conformer in these rare gas solutions.

In order to obtain the enthalpy difference, ten

spectral data points were obtained over the tempera-

ture range 255 to 21008C for the xenon solution and

2105 to 21508C for the krypton solution. The inten-

sities of each conformer pair were ®t to the equation

2ln K � �DH=RT�2 �DS=R� where K is the intensity

ratio (Ig/Ic) and it is assumed that DH is not a function

of temperature. Using a least squares ®t of the slope of

the line, a DH value of 53 ^ 9 cm21 was obtained

from the 710/695 cm21 bands from the xenon data.

The pair at 962/965 cm21 was not resolved suf®-

ciently to be measured in the temperature range of

the xenon measurements and, therefore, not utilized

in the calculation. Since the signal-to-noise ratio was

relatively low in the range of 600±500 cm21, we were


Table 5

Temperature and intensity ratios for conformational study of

dimethyl vinyl ¯uorosilane dissolved in liquid xenon

T (8C) 1000/T (K) I710/I695 2ln K

255 4.58 2.8438 21.1647

260 4.69 2.9173 21.1651

265 4.80 2.8899 21.1774

270 4.92 2.9631 21.1725

275 5.05 3.0203 21.1398

280 5.18 3.0587 21.1794

285 5.31 3.0263 21.1999

290 5.46 3.0630 21.12601

295 5.61 3.1828 21.2459

2100 5.78 3.0814 21.2813

DHa 53 ^ 9

a DH� 53 ^ 9 cm1 (0.64 ^ 0.10 kJ/mol) with the gauche

conformer the more stable form.

Table 6

Temperature and intensity ratios for conformational study of dimethylvinyl ¯uorosilane dissolved in liquid krypton

T(8C) 1000/T (K) I962/I965 2ln k I695/I710 2ln k I51/I513 2ln k

2105 5.95 2.5385 20.9316 3.2050 21.1647 1.9853 20.6858

2110 6.13 2.7410 21.0083 3.2062 21.1651 2.0637 20.7245

2115 6.32 2.7051 20.9951 3.2460 21.1774 2.1420 20.7617

2120 6.53 2.8240 21.0382 3.2300 21.1725 2.3272 20.8447

2125 6.75 2.7941 21.0275 3.1262 21.1398 2.4425 20.8930

2130 6.99 2.8300 21.0403 3.2525 21.1794 2.4469 20.8948

2135 7.23 2.7710 21.0192 3.3200 21.1999 2.4657 20.9025

2140 7.51 2.8501 21.0473 3.5256 21.12601 2.3350 20.8480

2145 7.80 2.8876 21.0604 3.4760 21.2459 2.4022 20.8764

2150 8.12 2.9420 21.0791 3.6014 21.2813

DHa (cm21) 32 ^ 7 39 ^ 8 69 ^ 21

a Average DH� 44 ^ 7 cm21 (0.53 ^ 0.09 kJ/mol) with the gauche conformer the more stable form.

unable to measure DH from the pair at 522/513 cm21.

Utilizing the krypton data for the above three

conformer pairs (Fig. 11), DH values of 32 ^ 7,

39 ^ 21 and 69 ^ 21 cm21 were obtained with an

average value of 44 ^ 7 cm21 (0.53 ^ 0.09 kJ/mol)

with the gauche form the more stable conformer.

Additional infrared spectra of DVFS were recorded

in argon and nitrogen matrices (1:500 and 1:1000)

deposited at 5 and 15 K; the spectra in the argon

matrices are given in Fig. 6, whereas detailed spectra

in a nitrogen matrix are presented in Fig. 7. Suppo-

sedly, the conformational equilibrium of the vapor

phase is maintained when the gas mixture is shock

frozen on the CsI window at 5 or 15 K, provided the

barrier to conformational equilibrium is above 3 and

5 kJ mol21, respectively. When the matrices were

annealed below 20 K some small spectral changes

occurred, which were interpreted as a relaxation of

DVFS in the matrix lattice. The samples were subse-

quently annealed for 10 min at every 3 K between 20

and 37 K for argon and 20 and 34 K for the nitrogen

matrices before being recooled to 5 K and the spectra

recorded.

Prominent changes were observed when the

samples were annealed to ca. 34 K for argon and

32 K for nitrogen. Certain bands were enhanced,

others diminished in intensities after annealing

which are interpreted as a displacement of the confor-

mational equilibrium. Qualitatively, the same inten-

sity changes occurred in the spectra of both matrices,

but they were more prominent in the spectra obtained

from the argon matrix. As we shall see, the infrared

bands which vanished or were reduced in intensity in

the spectra of the crystal were enhanced in the

matrices after annealing, whereas those present in

the crystal spectra were reduced in intensities. These

bands are indicated with arrows pointing upwards or

downwards, respectively, in Table 1. The observed

annealing temperatures (34 and 32 K) suggested that

the conformational barrier is ca. 9 kJ mol21 from the

curves given by Barnes [19]. However, it can be seen

from the spectral data shown in Figs. 5±7 that no

infrared bands disappeared completely in the spectra

of the matrices after annealing. Therefore the enthalpy

difference between the conformers must be quite low

in both matrices since an equilibrium was maintained

at the annealing temperatures at 32±34 K. A rough

estimate using a simple Boltzmann distribution

suggests an enthalpy difference of 17±33 cm21 in

the matrices which is in agreement with the results

from the xenon and krypton solutions in con®rming

that the gauche conformer is the more stable form.

Upon crystallization, bands at 969, 876, 711, 607,

366, 228 and 201 cm21 presented in the Raman

spectra of the liquid and amorphous phases signi®-

cantly diminish and/or disappeared. These bands are

due to the second conformer and seven spectra data

points of the liquid were recorded between 19 and

21208C in order to obtain the enthalpy difference.

Band pairs at 712/696 and 371/396 cm21 were


Fig. 11. Temperature dependent infrared spectra of dimethyl vinyl ¯uorosilane.

selected to obtain DH. The band at 696 cm21 might

have a contribution from the other conformer. A series

of van't Hoff plots based on measured peak heights

were obtained giving DH values of 17 and 33 cm21,

respectively, for the pairs mentioned above with the

average value of 25 ^ 15 cm21 (0.30 ^ 0.18 kJ/mol)

also with the gauche form being the more stable

conformer.

5. Vibrational assignment

The conformational analysis of DVFS shows that

the molecule exists in two stable conformations in the

¯uid phase. The syn conformer has Cs symmetry and

the 39 fundamentals will span the irreducible repre-

sentation 23 A 0 and 16 A 00, whereas for the gauche

conformer with C1 symmetry all the fundamentals

belong to species A. Since most of the observed

bands in the infrared spectrum of the vapor and the

Raman bands of the liquid are common to both the syn

and gauche conformers, the band contours and the

polarization ratios are of limited help in the spec-

tral interpretation. For the sake of similarity, the

fundamentals of both the gauche and syn confor-

mers have been numbered consecutively, instead

of the conventional numbering of the modes

belonging to species A 0 before those of A 00 in

the syn conformer. Guided by the assignments of

the similar normal modes for dimethyl vinyl chlo-

rosilane [7] and also by the calculated spectral

intensities and predicted wavenumbers from ab

initio calculations, we propose the vibrational

assignments listed in Table 1.

The assignments of the carbon±hydrogen modes

have been previously reported [7] for dimethyl vinyl

chlorosilane, and with only minor wavenumber shifts,

they remain essentially the same for DVFS. The

spectra of these two compounds look similar down

to about 1000 cm21. In the region below 1000 cm21,

a few features were observed in the infrared and

Raman spectra of the gas or liquid which disappear

upon crystallization. The C-type Q-branches located

at 969 and 964 cm21 in the infrared spectrum of the

gas are assigned to the CH2 rocks with the syn

conformer having the higher wavenumber. The inten-

sity of this band increases with decreasing the

temperature of the krypton (Fig. 11) solution

con®rming that the gauche form is the more stable

conformer in the gas phase. The Si±C stretch is

assigned to the C-type Q-branch at 710 cm21 for the

gauche form and to the shoulder at 694 cm21 for the

syn conformer where the former has almost disap-

peared from the spectrum of the crystalline solid.

The later one is also assigned to the SiC2 antisym-

metric stretch for the both conformers. It should be

noted that the gauche conformer has a minor contri-

bution to this band but we used it as a conformer band

(Fig. 11) for the enthalpy determination. The CH out-

of-plane bending conformer pair is observed in the

infrared spectrum of the liquid krypton solution at

521 and 513 cm21 for the gauche and syn conformers,

respectively. The CCSi bending mode observed at

365 cm21 for the gauche form is evident in the far

infrared spectra of the gas and amorphous solid but

disappears from the spectrum upon annealing (Fig. 6)

the sample.

In the amorphous and crystalline states the infrared

and Raman bands from 258 to 286 cm21 are assigned

to SiC2 wag and SiC2 rock. The ab initio calculations

predict very weak infrared bands as well as weak

Raman lines for the normal modes below 225 cm21

and the assignments of these remaining ®ve funda-

mentals n 34±n 38, are less certain as shown in Table

1. The Raman bands at 225 and 209 cm21, appearing

as shoulders and their corresponding very weak

infrared counterparts, are assigned to the SiC2 defor-

mation for the gauche. However, the intense peak at

195 cm21 in the Raman spectrum of the liquid is

assigned to both the SiC2 deformation for the syn

conformer and the methyl torsion for the gauche

form. The n 36 and n 350 modes are observed as a very

weak infrared band at 175 cm21 in the spectrum of the

vapor for the gauche and syn conformers, respec-

tively. The weak infrared band of the vapor at

170 cm21 corresponds to the infrared and Raman

bands around 178 cm21 in the condensed phases and

is attributed to n 37 of the syn rotamer and the very

weak infrared band at 164 cm21 (spectrum of the

gas) is assigned to the gauche for the same normal

mode. There are no bands observed for n 38, the methyl

torsion. The asymmetric torsional mode is assigned to

the two bands at 101 and 95 cm21, which were

observed in the Raman spectrum of the liquid and

calculated at 78 and 70 cm21 for the syn and gauche

conformers, respectively.


6. Discussion

In the present study of DVFS, neither the infrared

band conformer of the gas nor the depolarization ratio

of the liquid was helpful in the determination of the

conformational stability of this molecule. Addition-

ally, the calculated energies for the syn and gauche

rotamers derived from the ab initio calculations have

large uncertainties, but the force constants and the

wavenumbers after appropriate scaling usually give

good agreement with those of the observed fundamen-

tals. Within the group frequency regions for the CH3

and CH2 stretching and deformation vibrations, the

calculated wavenumbers for the syn and gauche

fundamentals are usually separated by less than

5 cm21 (Table 2). Below 1450 cm21 there are 12

instances in which the fundamentals of the syn and

gauche conformers are separated by more than

5 cm21. The largest shifts are calculated for the six

fundamentals n 27, n 28, n 30, n 31, n 32 and n 37 in which

they should be larger than 10 cm21. As expected, most

of the observed syn/gauche conformer separations

predicted pairs are located approximately with the

wavenumber.

The following observed band pairs from the Raman

spectrum of the liquid: 971/963, 883/864, 712/696,

621/613, 371/396, 275/265, 225/209 and 195/

187 cm21 in which the high frequency bands vanished

(or were reduced in intensity) in the infrared and

Raman spectra of the annealed solid are correlated

with the calculated scaled wavenumbers of the syn

and gauche conformers, respectively (Table 4). It

was found from all eight of these pairs that the high

frequency bands can qualitatively be ®tted with the

predicted wavenumbers of the gauche form and the

remaining bands to those of the syn conformer

whereas the opposite interpretation is not feasible.

The experimental and calculated frequencies are

also in reasonable agreement for the n 21, n 27, n 28,

n 31, n 33, n 34 and n 35 fundamentals. Two band pairs

have less convincing assignments where for n 22 the

observed difference is 19 cm21 and that calculated

only 1 cm21 and n 30 which was assigned to overlap-

ping bands at 515 cm21 although a difference of

11 cm21 was predicted. However, we feel that there

are compelling reasons to attribute the vanishing

bands to the gauche conformer, which means that

the syn conformer remains in the crystal even though

the gauche conformer has lower energy than the syn

rotamer in the rare gas solutions, the liquid state and

also in the matrices.

In addition to the band pairs assigned to the syn and

gauche conformers on the basis of spectral changes on

crystallization, described above, the infrared spectra

in argon and nitrogen matrices can frequently give

clues to close lying conformer pairs. Thus, the band

pairs attributed to n 10, n 15, n 17, n 19, n 23, n 25, n 29 and

n 30 observed at 1601, 1411, 1254, 1013, 848, 772, 521

and 515 cm21, respectively, can all be tentatively

assigned to separate conformer bands. As apparent,

each of these pairs of bands are characterized by

intensity changes in one or both matrices after

annealing and they are indicated with arrows in

Table 1. The bands, which are enhanced after

annealing, are assigned to the gauche form, whereas

those that diminish in intensity to the syn conformer.

It is characteristic that in each of these pairs the

wavenumber difference in the matrices is small

(between 2 and 8 cm21) leading to overlapping

bands in the ¯uid phases. Since the bandwidths are

much lower in the matrices the separate bands due to

the syn and gauche conformers can be detected.

Because of matrix, effects frequently encountered

during annealing some of these assignments may be

erroneous. However, if the same general features are

observed in both matrices we can be fairly con®dent

in the experimental results. It should be emphasized

that for the additional band pairs n 21 around 964 cm21

and n 23 at 710 and 694 cm21 the bands changed both

during crystallization and on annealing the matrices

and the conclusions should be de®nitive.

The conformational energy difference in DVFS can

be compared with the corresponding value for related

silanes. In dimethyl vinyl chlorosilane [7] CH2y

CHSi(CH3)2Cl, the gauche conformer also has the

lower energy, with DH (syn±gauche) equal to

0.5 ^ 0.1 kJ mol21 (equal to that of DVFS within

the experimental uncertainty); however, in the present

study the syn form is present in the crystal which is in

contrast to the chloro analogue. The vibrational

spectra of the two conformers are strikingly similar

for DVFS and the corresponding chloro analogue. In

vinylsilyl chloride [5] (CH2yCHSiH2Cl) the syn

conformer was more stable by 1.2 kJ mol21, in the

liquid state revealing increased stability of the syn

conformer when the two methyl groups are absent.


However, the gauche conformer was present in the

crystal [5], as well as in the rare gas solution [6]

which makes this molecule quite different to the situa-

tion in DVFS.

A comparison of the structural parameters

predicted by ab initio calculations (Table 2) with all

basis sets indicates little difference in the parameters

upon conformer changes from the gauche to the syn

form. With a given basis sets, for example 6-31G(d)

and 6-3111G(d,p) with full electron correlations, the

bond distances agree within 0.005AÊ and bond angles

within 18 for the corresponding parameters of the two

rotamers. Even with the larger 6-3111G(2d,2p)

basis set, the differences become insigni®cant for

most of the parameters (Table 2).

Two halogens (F or Cl) and one methyl group

attached to the Si atom leads to methyl vinyl di¯uor-

osilane (CH2yCHSi(CH3)F2) and methyl vinyl

dichlorosilane (CHyCHSi(CH3)Cl2) both of which

the conformational stabilities have been investigated.

In methyl vinyl di¯uorosilane the gauche conformer

is more stable in the rare gas solution [10], but the syn

form is present in the crystal [9]. The synconformer

was the more stable form in the rare gas solution [20]

and this form was also present in the crystal of vinyl

dichlorosilane [8], demonstrating the effect of the

larger chlorine atom compared to the ¯uorine atom

for these similar molecules. It should be noted that

in the latter molecules the syn conformer has both

halogens in the gauche positions compared to the

CyC bond, whereas in the gauche conformer, one of

the halogens is situated in the syn position of the CyC

bond. Finally, in methyl vinyl silane (CH2yCH-

Si(CH3)H2) the single methyl group prefers the

gauche conformation [21] with a DH equal to

1.59 ^ 0.13 kJ mol21 but the syn conformer is present

in the crystal [22]. If the energy difference between

the conformers is low as observed for DVFS, the high

energy conformer, which frequently has the larger

dipole moment, can be preferred in the crystal because

of larger crystallization energies for that conformer

which can overcome the higher conformational

energy.

Acknowledgements

J.R.D. acknowledges the University of Kansas City

Trustees for a Faculty Fellowship award for partial

®nancial support of this research.

References

[1] V. Govern, L. Khristenko, Yu.A. Pentin, Vopr. Stereokhim. 2

(1972) 57.

[2] L.V. Khristenko, Yu.A. Pentin, Vestn. Mosk. Univ. Ser. 2

Khim. 31 (1976) 304.

[3] M. Imachi, J. Sci. Hiroshima Univ. Ser. A. 42 (1978) 43.

[4] J. Sullivan, M. Qtaitat, J. Durig, J. Mol. Struct. (Theochem)

202 (1989) 159.

[5] J. Durig, J.F. Sullivan, G.A. Guirgis, M.A. Qtaitat, J. Phys.

Chem. 95 (1991) 1563.

[6] J.R. Durig, Y.E. Nashed, M.A. Qtaitat, G.A. Guirgis, J. Mol.

Struct. 525 (2000) 191.

[7] G. Guirgis, Z. Shen, M. Qtaitat, J. Durig, J. Mol. Struct. 403

(1997) 57.

[8] J. Durig, G. Guirgis, Y. Kim, W. Yan, M. Qtaitat, J. Mol.

Struct. 382 (1996) 111.

[9] J.R. Durig, G.A. Guirgis, M.A. Qtaitat, J. Raman Spectrosc.

26 (1995) 413.

[10] Y.E. Nashed, G.A. Guirgis, J.R. Durig, unpublished data.

[11] F. Miller, B. Harney, Appl. Spectrosc. 24 (1970) 291.

[12] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G.

Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A.

Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-

Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J.

Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe,

C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres,

E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S.

Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-

Gordon, C. Gonzalez, J.A. Pople, gaussian 94, Revision D.

2, Gaussian, Inc., Pittsburgh PA, 1995.

[13] P. Pulay, Mol. Phys. 17 (1969) 197.

[14] J.H. Schachtschneider, Vibrational Analysis of Polyatomic

Molecules, Parts V and VI, Technical Report Nos. 231 and

57, Shell Development Co. Houston, TX, 1964 and 1965.

[15] M. Frisch, Y. Yamaguchi, J. Gaw, H. Schaefer III, J. Binkley,

J. Chem. Phys. 84 (1986) 531.

[16] R. Amos, Chem. Phys. Lett. 124 (1986) 376.

[17] A. Anderson (Ed.), The Raman Effect, vol. 1, Marcel Dekker,

New York, 1971.

[18] P. Polavarapu, J. Phys. Chem. 94 (1990) 8106.

[19] A. Barnes, J. Mol. Struct. 113 (1984) 161.

[20] G.A. Guirgis, P. Zhen, J.R. Durig, Spectrochim. Acta (2000)

(in press).

[21] Y. Jin, G.A. Guirgis, J.R. Durig, submitted for publication.

[22] J. Durig, J. Sullivan, M. Qtaitat, J. Mol. Struct. 243 (1991)

239.


Conformational equilibrium in dimethyl vinyl fluorosilane studied by infrared and Raman spectroscopy

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Transcript of Conformational equilibrium in dimethyl vinyl fluorosilane studied by infrared and Raman spectroscopy