Journul of Molecular Shwcture, 72 (1981) 85-97 Eisevier Scientific Publishing Company, Amsterdam - Primted in The Netherlands

RAMAN, INFRARED AND NMR SPECTRA AND STRUCTURE OF DIVINYLMETHYLBORANE

J. R. DURIG, S. A. JOHNSTON*, T. F. MOORE and J. D. ODOM

Department of Chemistry, U~i~er~~ty of South Carolina, Columbia, South Car 29208 (U.S.A.)

(Received 21 July 1980)

ABSTRACT

The F&nan spectra of gaseous, liquid and solid divinyimethylborane have been recorded from 20-3500 cm-’ and the IR spectra of gaseous and solid divinyimethyl- horane recorded over the range 30-3500 cm-l. A variable temperature study of the Raman spectrum of the liquid phase has been carried out. A complete vibrational assign- ment is presented. In the solid phase the molecule appears to have a planar heavy atom skeleton (Cs symmetry), From analysis ofthespectraofthe fluid phases, the presence of

a second isomer, in which one or both of the vinyl groups are twisted slightly out of the BC, plane (C, symmetry), is proposed. Variable temperature “C NMR studies have been csrried out. A comparison of the ‘% chemical shift of Cp of the vinyl group with the corresponding value in other vinyiboranes indicates that relatively little delocaiization of the n-electron density occurs in this molecule. Low temperature (-115%) “C NMR data are consistent with a iow barrier to rotation about the boronlrinyi carbon bond.

The possibility of interaction between the empty p orbital on the boron atom and an unsaturated organic moiety in tricoordinate organoborane com- pounds has been the subject of several experimental and theoretical investi- gations [l] . Most of the attention has focused on vinylboranes and phenyl- boranes. Trivinylborane, the vinylhaloboranes and dimethylvinylborane have been studied by IR and Raman [2-6], microwave [S, 73, photoelectron [8] and NMR 16, 9,101 spectroscopy. Theoretical studies have considered the above molecules as well as div~y~ethylbor~e and the unknown vinylborane moltecule [11--X6].

Roth theoretical and experimental investigators agree that in almost all cases some interaction between the r system of the vinyl group and the boron p orbital does exist. The extent of the interaction is still under discussion and appears to vary from molecule to molecule. Optimization of orbital overlap involved in a mesomeric interaction requires a planar geometry

*Taken in part from the thesis of S. A. Johnston which will be submitted to the Depart- ment of Chemistry in partial fuifiient of the Ph.D. degree.

0022-2860/81/0000-00001$02.50 0 1981 Eisevier Scientific Publishing Company

and ~yld~uorobor~e and d~ethyl~ylbor~e are planar molecules, aIthough the B-C bond lengths appear to be normal and the barriers about these bonds do not seem to be unusually high [ 5-71. Trivinylborane exists as two conformers [3] with the form in the solid phase possessing a planar structure. Photoelectron spectra of this molecule have been interpreted [ 83 as indicating little or no mesomeric interaction between the vinyl groups and the boron atom. Carbon-13 chemical shifts have been shown to be sensitive to and indicative of the extent of n-delocalization in phenyl- [ 173 and vinyl- boranes [9,11] _

In a recent theoretical investigation Allinger and Siefert [16] reported an INDO study of the geometries of the ground states and the rotational barriers for a number of vinylboranes. One of the molecules studied was divinyl- methylborane. A conclusion of this report was that CH3 B(C2 I& jZ was probably not planar. It was speculated that the vinyl groups are twisted “30” or so” with a rotational barrier of “4 kcal or so”.

Of course, the ability to predict experimental data is a critical test of any theory. Because there have been no complete vibrational, rotational or NMR data reported for CH3B(CZH 3 ) *, we report herein an IR, Raman and 13C NMR study of this molecule designed to determine its geometry and the barrier to rotation about the boron-vinyl carbon bond.

EXPERIMENTAL

The Raman spectra were recorded on a Gary 82 spectrophotometer equipped with a Model 171 Spectra Physics argon ion laser operating on the 514.5 nm line. The spectra of the gaseous phase were recorded using a standard Gary multipass accessory. The sample was contained in a quartz cell at its room temperature vapor pressure. The laser power at the head was 4 W. The spectra of the liquid phase were obtained at various temperatures using a Miller-Harney [18] type ceI1 with 1.0 or 1.5 W at the head. The spectra of the solid phase were obtained at 20 K using a Spectrim Spectrometric Sample Conditioner Controller controlled by a Lake Shore Cryogenics Temperature Indicator/Controller. The 514.5 nm line was again used for excitation with 500 mW of power at the head. The sample was deposited on a brass plate and annealed until no further change in the spectrum was obsenred. Raman frequencies are expected to be accurate to 22 cm-’ and typical spectra are shown in Fig. 1. The instrument was calibrated with the plasma lines of the laser.

The mid-IR spectra from 400-3500 cm-1 were recorded on a Perkin Elmer Model 621 Spectrophotometer and are shown in Fig. 2. A 10 cm cell with CsI windows was used to obtain the spectrum of the gaseous phase. The spectrum of the solid phase was recorded using the same cryostat as previously described. The sample was deposited on a cesium iodide plate and annealed until no further change in the spectrum was observed, The spectrum of the solid was obtained in three segments; 400-700 cm-‘, 700-2700 cm-l

87

WAVENUMBER WI’d)

--ifi- A

WAVENUMBER Ccmi’> -

Fig. 1. Raman spectra of gaseous (A), liquid (B) and solid (C) divinylmethylborane.

Fig. 2. Infrared spectra of gaseous (A) and solid (B) divinylmethylborane.

and 2700-3500 cm-l, varying the attenuation of the reference beam for each segment due to decreasing transmittance with decreasing frequency. The instrument was calibrated with the frequencies of atmospheric water vapor [19] and standard gases [20].

The far IR spectra were recorded on a Digilab FTS-15B interferometer using 6.25~ and 25~ Mylar beamsplitters. The data on the gas phase were obtained from 30-450 cm+ _ Interferograms were obtained from 3000 scans each of the sample and empty cell and transformed using a boxcar apodiza- tion function. Resolution of ratioed spectra was 0.5 cm-’ _ ‘T’he gas cell was equipped with polyethylene windows. The solid phase data were obtained from 80 to 500 cm-l using the cryostat described above. The sample was deposited on a silicon plate and annealed until no further change in the spectrum was observed. Interferograms obtained from 1000 scans of the sample and the empty cryostat were transformed using a boxcar apodization function. Resolution of ratioed spectra was 2 cm-l.

NMR spectra (“B, 32.1 MHz; 13C, 20 MHz) were obtained on Varian Associates XClOO-15 and CFT-20 spectrometers. Spectra were obtained in a 30% (v/v) solution in CDC13 which provided the deuterium lock signal. The

*‘B chemical shift was measured relative to external BF3 .0(C2 HS )* and the 13C chemical shifts were measured relative to external TMS. Low temperature 13C NMR spectra (50.3 MHz) were obtained on a Bruker WP200 spectro- meter in a 20% (v/v) solution in a 1: 1 (v/v) mixture of CD,Cl, and CD,_

All preparative manipulations were conducted using a standard high vacuum system fitted with greaseless stopcocks. Tetravinyltin (Columbia Organic Chemical Co.) and methyldibromoborane (Alfa) were obtained commercially. The CH,BBr2 was contaminated with a substantial amount of elemental bromine which was removed by shaking the material with elemental mercury. Divinylmethylborane was prepared by the reaction of Sn(C*H,), and CH3BBr2. In a typical reaction Sn(C2HS)4 (0.89 ml, 4.97 mmoles) was added to a 100 ml tube which was fitted with a greaseless stopcock. The tube was attached to the vacuum system and the Sn(C,H,), was freeze-thaw degassed. Methyldibromoborane (4.92 run-roles) was condensed into the tube which was then removed from the vacuum system, immersed in a 0” bath and agitated by a mechanical shaker. The liquid mixture quickly began turning a pinkish-brown color. After agitation for approximately 4 h, the tube was reattached to the vacuum system and all volatile contents were pumped into the system and fractionated on a low temperature vacuum fractionation column. The yield of pure CH3 B(C2H3)2 was 4.61 mmol (94%). Purity was checked by vapor pressure measurement [ 213, “B NMR spectrum [ 221 and mass spectrometry [Zl] .

RESULTS

Several conformers of divinylmethylborane with planar heavy atom skeletons are possible. These conformers are illustrated in Fig. 3. The planar heavy atom skeleton allows for maximum overlap between the empty p orbital on the boron atom and the n systems of the vinyl groups. Using bond distances and angles determined in similar molecules [ 1, 71, B-C(methyl), B-C(vi.nyl), C=C, C-H(methy1) and C-H(viny1) of l-56,1.533, 1.339,1.09 and 1.087 a, respectively, calculations of the nonbonded distances between various hydrogens were carried out. The structure shown in Fig. 3A has a nonbonded distance between the hydrogens on the vinyl groups of 0.93 A which is considerably less than twice the Van der Waals radius for hydrogen (2.4 A). The structures B and C in Fig. 3 do not contain unrealistic distances.

Assuming free rotation of the methyl group, structure B contains a CZ axis. A slight twisting of the vinyl groups out of the BC3 plane, one up and one down, preserves the C2 axis. The normal vibrations of the heavy atom skeIeton of this conformer of Cp symmetry span the representations: 6A + 6B where vibrations of both symmetry species are IR and Raman active. The in-phase motions (A) give rise to polarized Raman bands whereas the out-of-phase motions (B) give rise to depolarized Raman bands. The normal vibrations of the heavy atom skeleton of the C, conformer (see Fig. 3C) span the representations: 9A’ + 3A” where vibrations of both symmetry

89

A

B

C

Fig. 3. Illustrates the possibIe conformers with planar heavy atom skeletons.

species are Raman and IR active. The in-plane motions (A') give rise to polarized Raman bands whereas the out-of-plane motions give rise to depol- arized Raman bands. A twisting of the vinyl groups in the structure shown in Fig. 3C yields a molecule of CI symmetry. In principle, it should be possible to distinguish among the C,, C, and CL symmetries on the basis of the depolarization vahres of the Raman lines arising from the skeletal modes. The presence of two polarized Raman lines which are due to the C=C stretches in the fluid phases indicates a Ci or C, conformer and not one of C, sym- metry. Since both lines appear to remain in the spectrum of the solid, it is concluded that this main conformer remains in the solid state and possesses C, symmetry as represented in Fig. 3C. The C1 conformer is ruled out as the predominant conformer on the basis of the depolarized Raman lines at 329 and 450 cm-’ which must be due to skeletal motions. However, evidence points to the presence of a second conformer in the fluid phases and we propose that this molecule has C, symmetry with one or both of the vinyl groups twisted out of the BC3 plane. It is quite difficult to distinguish between a conformer change with a change of state from the fluid to the solid state from a change where two isomers are present in the fluid state but only one in the solid state. However, as will be explained later, the low frequency vibra- tional data appear to be more consistent with the presence of a second conformer in the fluid phases.

Vibrational assi&men t

The normal vibrations of the planar conformer (C, symmetry) of divinyl- methylborane span the representations: 26A’ + 13A” where the vibrations of both symmetry species are IR and Raman active. The in-plane motions (A’) give rise to polarized Raman bands whereas the depolarized .Raman bands correspond to out-of-plane motions (A”). Assignment of the spectra was made by using the Raman depolarization data and group frequency correlations.

The carbon-hydrogen stretching modes of the methyl group are found between 2958 and 2875 cm-’ with the lowest frequency assigned to the totally symmetric stretch. The antisymmetric deformations are visible in the IR spectrum of the solid as a shoulder at 1437 cm-‘. The symmetric defor- mation of the methyl group appears at 1297 cm-‘. The B-C(methyl) stretch occurs at approximately 848 cm-l.

The methyl rock appears as a strong band in the IR spectra. This band arising from the rocking motion appears to have fine structure, but there appears to be no regular pattern of intensity alteration, i.e., strong, weak, weak, characteristic of perpendicular vibrations of molecules having a three- fold symmetry axis. This series of Q branches at 1016, 1007, 988, 922, 915 and 909 cm-l leads us to conclude that the methyl group is nearly freely rotating at room temperature. The lack of a definite pattern may be due to the fact that the “degenerate” CH3 rocks are coupling with other normal vibrations of the molecule.

The various carbon-hydrogen stretches of the vinyl groups appear between 2960 and 3090 cm-l with the antisymmetric CHz modes being assigned to the highest frequency and the lowest frequency assigned to the symmetric CH2 stretch. The two CH in-phase and out-of-phase stretches occur at 3019 and 2998 cm-l, respectively. The two C=C! stretches can be seen clearly in the Rarnan spectra of all three phases at 1601 and 1594 cm-l. The out-of- phase C=C stretch is assigned to the higher frequency and the lower frequency is assigned to the in-phase C=C stretch. Both of these bands are polarized .which allows for the elimination of a molecular structure containing a C, axis in the fluid phases. The CH* scissoring modes are assigned to the band at 1420 cm-l for the out-of-phase motion and to the band at 1411 cm-l for the in-phase motion. The bands appearing in the spectrum of the liquid phase at 1194 and 1190 cm-l are assigned to the CH in-plane bend, out-of- phase, for the l”B and “B isotopes, respectively. The bands appearing in the spectrum of the solid phase at 1187 and 1174 cm-’ are assigned to the in-phase in-plane CH bend of the “B and “B isotopes, respectively. The out-of-plane CH bends occur at 599 cm-l. The bands at 1148 and 1120 cm-l are assigned to the ‘“B-C(vinyl) and “B-C(vinyl) stretching modes, respectively. The B-C(viny1) symmetric stretch occurs at 666 cm-l for the “B isotope and at 680 cm-’ for the l”B isotope. The CH2 twisting motions occur at 1030 cm-’ for the out-of-phase twist and at 1013 cm-* for the in-phase twist. The

91

CH2 rocking motions are assigned to the only polarized bands in this region. The out-of-phase rock occurs at 998 cm-’ and the in-phase rock occurs at 980 cm-l. The bands at 969 and 965 cm-l are assigned to the out-of-phase CH, wag and the in-phase CH2 wag, respectively. All of these motions, rocks, wags and twist occur within the region assigned to these modes in trivinyl- borane by Odom et al. [3].

The low frequency spectra are by far the most interesting. It is this portion of the spectra which should be the most sensitive to conformational changes in the molecule or the presence of more than one isomer. Of particular note is the change in the IR spectrum on going from the gaseous to the solid phase. For example both the bands at 236 and 358 cm-l which appear in the spec-

trum of the gas disappear with solidification (see Fig. 4). We believe that these two bands result from the presence of a second conformer in the gas phase. However, it should be noted that there is in general a one to one correspondence between the IR and Raman spectra of the solid. Therefore, the five skeletal bending modes (excluding the vinyl torsions) are most readily assigned based on the bands observed in the spectra of the solid phase.

~VENUYEER Cmi’,

Fig. 4. Far IR spectra of gaseous (A) and soIid (B) divinylmethylborane.

Fig. 5. Raman temperature study of liquid divinyhnethylborane -78°C (A), -53°C (B), -25°C (C), 0°C (D) and upper and lower trace at room temperature (E).

A Raman temperature study of liquid divinylmethylborane (Fig. 5) clearly indicates a change in the spectrum with decreasing temperature. The broad band between 400 and 500 cm-l undergoes the most drastic change with temperature. At room temperature it appears as two broad bands centered at 415 and 450 cm-1 in the spectrum of the liquid and is very similar to the band shape in the spectrum of the gaseous phase. As the temperature is lowered, the band at 415 cm-l changes in shape and the band center shifts to lower frequency until it appears as a band at 386 cm-l with a shoulder at 397 cm-1 in the spectrum of the solid phase. The band centered at 450 cm-l is also present in the far IR spectrum of the gaseous phase. This band in the Raman spectrum of the liquid disappears upon solidification. The band centered at 504 cm-l in the spectrum of the solid phase slowly becomes more prominent as the intensity of the 450 cm-l band decreases. The band at 329 cm-’ is absent in the Raman spectrum of the gaseous phase. The intensity of this band increases with decreasing temperature. It appears from comparison of the data of the fluid phases with the solid phase data that there are two isomers in the fluid phases. The alternative explanation that the molecule is undergoing a conformational change with solidification is ruled out on the basis that there are too many low frequency fundamentals in the fluid phases for a single conformer. However, it should be noted that one cannot rule out completely the possibility that there are two conformers in the fluid phases with a third different one more stable at the lower temper- atures which persists in the solid state.

In the Raman spectrum of the solid there are four-clearly discernible bands at 386, 328, 287 and 215 cm-l. These bands are thus taken as four of the five skeletal bending modes, but the assignment of the molecular motions involved must be considered tentative. The highest frequency band at 386 cm-l is assigned as the B-C(methyl) in-plane bending motion. The other three bands at 328,287 and 215 cm-1 are assigned as the B-C(methy1) out-of-plane bend (A’) and the 2 B-C=C bends (A’), respectively. In the Raman spectrum of the liquid at low temperature (Fig. 5) there appears to be a one to one correspondence between the bands in the liquid and solid. In the spectrum of the liquid the 214 and 279 cm-1 bands are polarized whereas the Raman line at 329 cm-i is depolarized. Therefore this assignment is consistent with the depolarization data for the liquid phase. The polariza- tion values change with temperature. More clearly depolarized bands are present at lower temperatures which indicates that the molecule is either changing symmetry from Cl to C, upon cooling or that the concentration of the C, conformer is increasing.

The remaining skeletal bending mode, the B-C;! (vinyl) scissors, is assigned to the band at 448 cm-l in the far IR spectrum of the gas phase. It appears as a strong band in the far IR spectrum of the solid phase at 463 cm-l.

93

NMR results

The proton decoupled 13C NMR spectrum of divinylmethylbora~e obtained at ambient temperature consists of three well separated resonances, The methyl carbon resonance appears as a poorly resolved quartet at 5.5 ppm, the quartet structure arising from spin--spin coupling to the adjacent boron nucleus (Ja+ < 60 Hz), The substituted vinyl carbon resonance appears as a well resolved quartet (Ja__c = 70 Hz) at 142.2 ppm. The resonance of the fl carbon (C,) of the vinyl group is a singlet at 135.9 ppm. This is identical to the C, chemical shift observed in dimethylvinylborane [6] and is deshielded by 13 ppm relative to ethylene. This deshielding implies some removal of ‘IT electron density from the vinyl group; however, the fact that replacement of a methyl group in (CHs)2BC2H3 has no effect on the shielding of Cp indicates that the amount of n-delocahzation in these compounds is small. Indeed, these GP resonances are more shielded than that in any other vinylborane studied.

A significant amount of n-delocakation from the vinyl group to boron should lead to an increase in the barrier to rotation about the B-C bond. No vinylboranes studied to date have exhibited an unusually large barrier. Because the complex low frequency spectral region prevented definitive torsional assignments we conducted a low temperature 13C NMR study to obtain at least an upper limit to this barrier. The only change observed in the 50 MHz 13C-{1H} NMR spectrum of CH3B(CzH3)Z between ambient temper- ature and -115°C is a sharpening of the resonances as the temperature is lowered due to more efficient “B relaxation. Neither resonance doubling nor a broadening of any resonance in the spectrum was observed. Based on these data the barrier to rotation of the vinyl group about the B-C bond must be less than 4 kcaf mol-I [23] _ Thus this barrier does not indicate substantial delocalization of vinyl z electron density.

Trivinylborane [S] exists as two isomers in the gas and liquid phases. It is therefore not surprising to find two isomers in divinylmethylborane. The isomer that is present in the solid phase of trivinylborane [3] is the planar form with CSh symmetry. The corresponding planar form of divinylmethyl- borane has C, symmetry and is shown in Fig. 3C. The proposed assignment given in Table 1 is based on this form. The symmetry of the second conformer cannot be determined from the vibrational study. One might expect that the second conformer would have the vinyl groups twisted slightly out of plane. The nonbonded distances between the hydrogens of the CH, group and those of the adjacent CH group were found to be 2.00 a which is somewhat less than twice the Van der Waals’ radius of hydrogen (2.4 A ). The slight twisting of the vinyl groups would alleviate any nonbonded H - * * H repul- sions of these groups assuming from our model that calculations on the planar form are correct.

94

TABLE 1

Observed IR and Raman frequencies (cm-* ) of divinyhnethylboranea

Infrared RanKUl Assiimen t

Solid Liquid Solid

3067m

303Ow,sh

297 5m

2915m

1938w

1605s

1423s

1305s

1177m

1126s

1095s 1026sh

1015m

3085m

3056m

3027m

2970m

2948m 2918sh

1945w 1895w

1598s

1437sh

1433sh 1419s 1368~

1297s

1208sh

1200m

1187sh

1174s

1148m

1120s

1087s

1018s

32OOw,p 3068m,p

3019m,p 2998sh,p 2992sh,p

2989us,p

2977sh,p

2905m,p

1624w,p 1603m,p 1597s,p

1418m,p

1342w,p

1295m,p 1282sh,p

1192w,p

1122vw

3183w,p

3050m,dp

303Ow,p

297 5s,p

2953w,p

2881m,p

16OOs,p 1593s,p 1578w,p

1412sh,p 1407m,p

1303sh,p

1288m,p

1216w,p 1194sh,p

119ow,p

1177sh,p

1149w,p

1125w,p

1086w,p 1028w,p

1015w,dp

3066m

3057m

3031w

2979vs

2973sh 2958m 2922m 2895sh 2875m

1601s 1594s 1577w

1420sh 1411m

1303sh 1297sh 1288m 1284sh 1216~~

1193vw

118Ovw

1126~~

103ovw 1022vw 1013w

VI

V2

V3 V4

V5

V6

v27

V7

V8

V-3

VI0

2 x 1605 CH, antisymmetric stretch, out-of-phase CH, antisymmetric stretch, in-phase CH stretch in-phase CH stretch cut-of-phase CH, symmeiric stretch, out-of-phase CH, symmetric stretch, in-phase

CH, asymmetric stretch CH, asymmetric stretch

CH, symmetric stretch

Impurity 12C=C stretch out-of-phase i2C=C stretch in-phase “C=C stretch in-phase

VII P26 CH, antisymmetric

VIZ

VI3

VI4

VI5

vI6

VI7

v29

V30

deformations CH, scissors out-of-phase CH, scissors in-phase

Impurity 2 x 666

CH, symmetric deformation

CH in-plane bend, out-of- phase, ‘OB CH in-plane bend, out-of- phase, l*B CH in-plane bend, in-phase, “B CH in-plane bend, in-phase, “B l”B-vi.nyl antisymmetric stretch “B-vinyl antisymmetric stretch

CH, twist out-of-phase

CH, twist in-phase 1003w,dp 1004sh

95

TABLE 1 (continued)

Infrared Raman Assignment

Solid Gas Liquid Solid

97 8w 970s

949s

860w,br

970s 98Gw,br, dp

945s 862sh 848s 832~ 817sh 695w

65Ow,br 665w,p

57 8vw,br 565~

459w 448~

378vw

358w 303m

236w,br

146s

463~ 450w,br

977w 975w,dp

973w

852w,br

666w.p

589w,br

450w,br

380~ 415w,br,p 396w,p

329w 329vw,dp

289w,br 27Gw,p 279w,p

21&h 213w,p 214w,p

998w 992sh 98Osh 975sh 969m

965sh

845w

68Gw

666w

599w

528sh

504w

397sh

386~

328~

287~

223sh 215w

138s 116sh

91s 72m 58sh 55m 46m 39m 32w 22w :

VlS CH, rock out-of-phase

“I9 CH, rock in-phase

v31 CH, wag out-of-phase

V32 CH, wag in-phase v~~,v,, CH, rocksb

B-C(metbyl) stretch

‘“B-vinyl symmetric stretch “J3-vinyl symmetric stretch 287 + 328

CH out-of-plane bend, out-of-phase CH out-of-plane bend, in-phase

B-CJ vinyl) scissors

‘“B-C(methyl) in-plane bend “B-C(methyl) in-plane bend Isomer Ii B-Cfmethyl) out-of-plane bend B-C-C bend Isomer II ‘“B-C=C bend “B--&C bend Isomer II

Lattice modes

aAbbreviations used: s, strong; m, medium; w, weak; v, very; br, broad; sh, shoulder; p, polarized; dp, depolarized. bSee text for discussion of the fine structure found in this region.

In the far IR spectrum of the gas several bands appear which are assigned to the bands of a second isomer. Similar results were obtained several times on several samples. After obtaining the spectrum shown in Fig. 4, the purity of the sample was checked by IIB NMR and was found to be greater than 90% divinylmethylborane. These bands do not appear to correspond to those of dimethylvinylborane [6], trivinylborane [ 31 or trimethylborane [ 241.

The variable temperature study of the Raman liquid (Fig. 5) indicates a definite change in structure with temperature. The spectrum of the liquid at room temperature is very similar to the spectrum of the gaseous phase. The region of the spectrum below 600 cm-l undergoes the most change from room temperature (Fig. 5E) to approximately -53°C (Fig. 5B j. This further supports the far IR data that a second isomer is present. The change in de- polarization values with decreasing temperature indicates a change in mol- ecular symmetry from C1 to C, or a significant increase in the C, conformer_

Assigning the band at 138 cm- 1 to one of the vinyl torsions gives an unrea.Iistic value of approximately 8.5 kcal mol-’ for the barrier $0 internal rotation. The torsional frequencies are probably below 100 cm-l and more than likely are very weak. However, a stucly of the far IR region from 110 to 30 cm-l using the FTS-15B yielded no information on the torsions. It is possible that the intense line seen in the spectrum of the Raman solid (Fig. 1) at 91 cm-’ is one of the torsional modes. There is a very broad band in the Raman spectrum of the gas phase centered around 90 cm-*. A frequency of 90 cm-l for the torsion gives a barrier of approximately 3.4 kcal mol-’ . Low temperature (-115°C) 13C NMR data are consistent with this low barrier to rotation about the boron-vinyl carbon bond. This barrier is lower than the barriers in vinyldifluoroborane (4.17 kcal mol-‘) [7] and dimethylvinylborane (4.23 kcal mol-’ ) [6] . However, if we assume the bands at 146 and 91 cm-l are the out-of-phase and the in-phase vinyl torsions, respectively, it is possible to obtain a reasonable barrier of 5.8 kcal mol-’ by averaging the F numbers for the two vinyl groups and the frequencies. The two torsions may be “coupled” leading to an unreasonable barrier when only one of the vinyl torsions is considered.

This study indicates that in order to obtain a better understanding of the low frequency skeletal vibrations, additional molecules containing the divinyl- boryl moiety should be investigated.

ACKNOWLEDGEMENTS

The authors grztefully acknowledge the financial support of this study by the National Science Foundation under Grants CHE-77-08310 and CHE-77- 10098. The authors also thank Mr. Stan Huffstetler for preparing the initial sample of CH3 B(C1Hx)2 used in this study.

97

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