- Email: [email protected]
Microwave spectrum of vinyl silane; structure, dipole moment and internal rotation of the silyl group
Journal of Molecular Structure, 78 (1982) Elsevier
Scientific
Publishing
Company,
MICROWAVE SPECTRUM MOMENT AND INTERNAL
YOSHIKO
SHIKI,
Department
of Chemistry,
Naka-ku, Hiroshima (Received
AKINORI
185-195 Amsterdam
-
Printed
in The Netherlands
OF VINYL SILANE; STRUCTURE, DIPOLE ROTATION OF THE SILYL GROUP
HASEGAWA
and MICHIRO
HAYASHI
Faculty of Science, Hiroshima University Higc ahi-sendamachi,
730 (Japan)
21 July 1981)
ABSTRACT Microwave spectra of vinyl silane and its twelve isotopically substituted species have been measured. The r, structure of the molecule has been determined from the observed moments of inertia. Dipole moments for the normal and SiD,CH=CH, species are determined from Stark-effect measurements and the direction of the dipole moment in the molecule is discussed. From the A -E splittings of the observed spectra for five species, the barrier to internal rotation of the silyl group has been determined. INTRODUCTION
Twenty years ago, O’Reilly and Pierce [ 11 reported the microwave spectrum and the structure of vinyl silane. Since the b dipole component of this molecule was so small that b-type transitions were fairly weak, these were only measured for the normal and SiD$H=CH* species. Since the molecule is close to a prolate symmetric top, the A rotational constant cannot be determined from the cz-type transitions alone. The structure proposed by them can only be very tentative due to the lack of A rotational constants for many species. This re-investigation of the microwave spectrum of vinyl silane was originally undertaken for the purpose of confirming the r, structure of this molecule which is considered to be a suitable reference for the structures of a series of compounds, such as methyl vinyl silane 123 and vinyl halogenosilanes. The a- and b-type transitions of the normal and isotopic species were accurately measured and the F, structure established. The dipole moment and its direction in the molecule and the barrier to internal rotation of the sibyl group were also determined. Our secondary aim was to determine accurately the barrier to internal rotation of the silyl group from the A -E splittings of the spectra, in order to compare the results with those of propylene reported by Hirota [ 31. In the course of the measurements of the spectra in the excited torsional states, we found a series attributable to the first excited SiC=C bending state. 0022-2860/82/0000-0000/$02.75
0 1982 Elsevier
Scientific
Publishing
Company
186
Some of these spectra exhibited unexpectedly
large A --fr: splittings.
Similar splittings of the spectra in the first excited skeletal bending state were found for ethyl fluoride and its deuterated species [4], and were interpreted as being due to the Fermi resonance between the second excited torsional state and the first excited skeletal bending state. The situation is probably similar for vinyl silane. In this paper, we restrict our results to the structure, dipole moment and the barrier to internal rotation obtained from the ground state alone. The results for the excited states will be reported separately since measure-
ment of the far-IR spectra of the molecule will be necessary at some stage in the future. ESPERIMENTAL
Samples of the normal and deuterated species were prepared by the reactions described by O’Reilly and Pierce [l] except for SiH,DCH=CHz which was prepared by chlorinating SiH&H=CH, with AgCI and then reducing the resulting SiH,CiCH=CH, with Iithium aluminum deuteride in dibutyl ether. A conventional 100-kHz Stark modulation spectrometer was used for
the me~~ements
of the spectra at dry-ice temperature. The spectra of the
‘gSi 3oSi and “C species were measured using samples which contained theie isoiopes in natural abundance. Very weak spectra were obtained on a strip recorder using the computer-controlled spectrometer at the National Chemical Laboratory for Industry. RESULTS
Microwave
AND DISCUSSION
spectra
For the normal and SiD3CH=CH2 species, about twenty EL-and b-type transitions with J G 10 and R, G 2 were measured. The &-type transitions exhibited doublet structures due to the internal rotation of the silyl group. Since the centrifugal contributions could not be neglected, the rotational constants were obtained by two methods, (a) all the observed A component frequencies were fitted to a modffied rigid-rotor formula which included the dJ, dm and dEz terms of a Watson centrifugal disto~ion expression and (b) the observed frequencies of a-type R-branch transitions with J G 3 and K, d 1 and of b-type Q-branch transitions with J G 6 and K, S 1 were fitted to a modified rigid-rotor formula which included only the dd term of the centrifugal distortion expression. The l1 1 + Ooo transition was also included in the lactations whenever it could be measured. The rotational constants obtained by methods (a) and (b) were essentially identical to each other, within experimental error.
187
For SiH2DCH=CH2 prefixes s- and a- denote the species in which the deuterium atom is situated in the symmetry plane and out of the symmetry plane, respectively and for SiH,CH=CHD, suffixes -c and -t denote the species in which deuterium atom is situated at the cis and tram positions relative to the SiH3 group, respectively. For the deuterated species, the rotational constants were obtained by method (b), described above. For a-SiH,DCH=CH,, b-type transitions exhibited doublet structures, assumed to arise from the existence of two equivalent minima in the internal rotation potential curve. The averages of two-component frequencies of the doublets were used in the calculation of the rotational constants. For “Si, 3oSi, and 13C species, b-type transitions with low values of J could not be measured of samples containing the isotopes in natural abundance. For 2gSiH,CH=CH, and 30SiH3CH=CH,, three b-type Q-branch transitions with 6 d J < 8 were observed. The rotational constants were then obtained from a-type R-branch transitions and the 6,5 + 606 transition by method (b). For 2gSiD,CH=CH2 and “OSiD3CH=CH,, three b-type Q-branch transitions with 10 G J G 12 were observed. In this case the rotational constants were obtained from a-type R-branch transitions and the lOI + lOolo transition. For SiH3’%H=CH, and SiH3CH=13 CH,, measurement of b-type transitions of samples containing the isotopes in natural abundance was impossible. Rotational constants A were therefore assumed’such that the difference between the values of P,[=(I, + Ib - &)/2] for the normal and 13C species was zero. The values of B and C were obtained by method (b). The rotational constants are listed in Table 1. Details of the observed transition frequencies have been deposited with the British Library Lending Division at Boston Spa, Yorkshire, U.K., as Supplementary Publication No. SUP 26207 (6 p.). r, Structure The data obtained are sufficient to determine the rS coordinates of the atoms in the molecule by the substitution method. The atom-labelling scheme of the molecule is given in Fig. 1. It is desirable that the rotational constants of the species used in the structural calculation are obtained from a set of transitions common to all species. The rotational constants obtained by method (b) for the normal and SiD3CH=CH2 species were therefore used in the calculation. For the silicon atom, there are four pairs of parent and substituted species from which four sets of coordinates can be derived by independent solutions of the Kraitchman equations. The coordinates obtained from the two pairs 2gSiD3CH=CH2 and SiD$H=CH,, 3oSiD3CH=CH2 require to SiD,CH=CH,, be transformed to those in the principal axes system of the normal species. Although this transformation depends on the coordinates of hydrogen atoms H, and H, on the silicon atom, two cyclic iterations of the calculation were found to be sufficient to fix the coordinates of the silicon atom.
TABLE
1
Rotational
( MHz)~
constants
SiH,CH=CH,
F
SiD,CH=CH,
f
SiH,CD=CH, SiH,CH=CHD-c SiH,CH=CHD-t s-SiH ,DCH=CH: a-SiH ,DCH=CH z ‘9SiH,CH=CHz “SiH,CH=CH z z9SiD,CH=CH 2 ‘OSiD,CH=CH 1 Si H 1’ %H=CH . SiH 3CH=’ %ZH,
A
B
C
34144.30(14) 34144.48(35) 24703.37(29) 24703.46(17) 29215.81(27) 30377.87(23) 34135.84(34) 29414.80(32) 30960.71(20) 34130.56(55) 34117.25(73) 24694.18(12) 24689.28( 28) 33524.85(assumed) 33946_81(assumed)
5275.26(l) 5275.27(5) 4773.28(2) 4773.25(3) 5232.30(4) 5137.12(3) 4888.94(5) 5214.83(5) 4024.24( 3) 5214.41(g) 5157.02(6) 4730.21(O) 4689.35(l) 5249.88(g) 5114.89(g)
4820.76(l) 4820.77(5) 4404.51(2) 4404.52(2) 4673.39(4) 4625.41(3) 4496.19(4) 4666.29(4) 4669.87(3) 4769.64(4) 4721.32(5) 4367.66(O) 4332.64(l) 4787.09(8) 4682.73(8)
dJ X
lozb
-1.73(4) 0.30( 43) -1.52(48) 0.23(23) 0.27( 34) 0.37(27) 0.25( 39) 0.32( 39) 0.34(25) 0.31(28) O-24(37) 0.27(5) 0.21( 14) O-36(29) 0.36( 30)
NC
RMS
22 13 18 14 13 12 12 13 13 8 8 8 8 7 7
0.04 0.09 0.09 0.05 0.07 0.06 0.09 0.09 0.06 0.08 0.10 0.02 0.04 0.08 0.08
“Figures in parentheses indicate the uncertainties attached to the last significant figures calculated from 2.5 times the standard deviation. bThe coefficient of the [J(J + l)]’ term of the centrifugal distortion formula. =The number of observed frequencies used for the least-squares calculation. dThe root-mean-sq uare deviation of the calculated frequencies from the observed ones. =Rotational constants obtained using method (a) (see text). The dJK and dEJ coefficients obtained are -0.140(5) and 4.06(7) X 10m6 MHz for the normal species and -O-085(6) and 3.95(18) X low6 MHz for the SiD,CH=CH,. fRotational constants obtained using method (b) (see text). b
t
b I
oasi
H
Fig. 1. Atom-labelling Except
for
the
scheme coordinates
and the r, structural
parameters.
of the two carbon is assumed, four
atoms where AP,
= 0
different sets of coordinates for the atoms in the molecular plane are obtained by four different choices [APc
= P,( IT)
-P,(normal)]
189
of the three moments of inertia. The averages of the four values were used as the coordinates and the errors replaced by half the difference between the maximum and minimum coordinate values, whenever this value exceeded the experimental error. Attempts were made to fix the two unreliable Kraitchman coordinate values, xb(Ht) and xb(Si). For zcb(Ht), application of the first moment equation is not helpful since the error in the coordinates of heavy atoms imparts a large uncertainty on the coordinates of lighter atoms such as hydrogen. Therefore, the x~(H*) value was fixed by the structural assumption, r(CH,) = r(CH,). For xb(Si), however, the first moment equation could be usefully applied to fix this value. The coordinates and structural parameters are given in Tables 2 and 3, respectively. The differences between the observed and calculated moments of inertia for all the species are listed in ‘Table 4. The root-mean-square deviation of these values is 0.3252 amu A2 while the ZU-Z~(X,)~and I,,, values for the normal species are -0.0445 amu A and 0.0967 amu A*, respectively. Since we used a structural assumption for xb(Ht), the differences between the observed and calculated 1, values are in some cases negative. This would not normally be expected for an r, structure which is well established. In order to ascertain the planarity of the vinyl group in the molecule, the AP, values of three deuterated species were compared with those of the TABLE
2
Atomic
coordinates
Atom
Group
HS
SiH, SiH,
3 C H C Hc
KC
(A )a -yb
1.5591( 32) -0.3926(20) 0.0806(32) -0.5289(52) -1.5995(36) 0.3016(98) 1.3861(31) -0.0904( 209)
-1.0404( 47 ) -l-7874( 5) -1.0659( 11) 0.6842( 34) O-8827(62) l-7442( 15) 1.5974(26) 2.7660(6)
CH CH i=H, CH, CH,
x, @iI
xb(Si)
Parent
and isotopic
-1.0666(g) -1.0666(21) -1.0651(7) -1.0655(g)
0.0778(104) 0.0790(236) O-0965(87) 0.0894(104)
(SiH,CH=CH,, (SiH,CH=CH,, (SiD,CH=CH,, (SiD,CH=CH,,
0.0 -t1.2001(9) 0.0 0.0 0.0 0.0 0.0 0.0 species
‘9SiH,CH=CH,) ‘OSiH,CH=CH “SiD,CH=CH,) ‘OSiD,CH=CH,)
pair
,)
“Figures in parentheses indicate the uncertainties attached to the last significant figures. bFor x,, the average of the four values obtained from the four pairs of parent and isotopic species. For Q, the value obtained from the first moment equation_ The x, and ~b values solved by the Kraitchman equations are also given above. ‘The Xb value was fixed by the structural assumption, r(CH,) = r(CHt).
190 TABLE
3
Structural
parametersa Present work
O’Reilly and Pierce
1.853(4) 1.347(7) 122’43’(25’)
1.853 1.347 122”53’
1.479(5) 1.4’ ‘8( 2) 10S012’(25’) 110:50’(15’) 109”10’( 18’) 108”34’(7’) 109’57’( 18’) 1”45’(27’)
lOS”25’ 109”20’ 108”42’ lOS”42’ 109”2’ l”49’
Skeleton
r(SiC) (A ) r(C=C) (4 ) a( SiC=C) SiHJ
r(SiH,) (A )
r(SiH,) (A ) ==(HsSiC) a(H&C) a(HSSiH,) cr(H,SiH,)
CH
r(CH) (A 1
a(SiCH) cu(HC=C)
r(CHc) (A 1 WHt)
(A 1 a (C=CH,) o(C=CHt)
a(HCH)
1.089(6) 119”42’(36’)
117”34’( 1”) 1.094(10) l.O94(assumed) 120”21’(56’) 120’55’(52’) 118”42’(2”)
1.094 119% 117”59’
1.097
120’18’ 120”38 119”4’
aFigures in parentheses indicate the uncertainties attached to the last significant figures. bHvpothetical untilted cr(IiSiC) angle defined by r = [a(H$iC) + 2a(HaSiC)I/3- ‘Tilt angle defined by 6 = (2/3)[a(H,SiC) - a(H,SiC) 1.
corresponding species of 79Br-substituted vinyl bromide which has already been confirmed to be planar [5]. As shown in Table 5, the Ap, values of the vinyl silane species follow a similar trend to those of the vinyl bromide species, although the absolute values of the silanes are slightly higher. This slight increase in AP, values can be explained by coupling between the CH out-of-plane bending and silyl rocking modes, where the latter is an additional vibrational mode to those in vinyl bromide. Hence, the planarity of the vinyl group in vinyl silane is as plausible as that in vinyl bromide. Our proposed structure is surprisingly close to that reported by O’ReUy and Pierce [ 11, within experimental error. The tilt of the silyl group is very clear from the present r, structure, i.e., angles ar(H,SiHa and a(H,SiH,) are similar, while a(H,SiC) is smaller than a(H,SiC) by 2”38’. This arrangement produces a 1’45’ tilt towards the C=C bond. O’Reilly and Pierce reported a tilt for the sibyl group of l”49’.
14,88761(assumed)
SiH,CH="CH,
a$
APcd
0.00009(234) 2.91393(118)e O.O(sssumed) 2.91381(135$ O.O(assumed)
-
307.92344(194) 0.40378
98.80486(168) 0.38357
2.88452(106) 5.79708(61) 2.91257(167) 2.87338(87) -0.01114(193) 2.67638(69) -0.00814(175) 2.88753(111) 0.00301(217) 2.89431(100) 0.00980(205) 2.88474(94) 4.23414(69) 0.00022(200) 1.34962(176)
pee
5.79846(12) 2.88461(128) -5.79833(29)
104.83306(102) 0.40642 114.74026(55) 0.40663 106,13911(83) 0.40988 109.26092(66) 0.40758 112.40099(108) 0.39431 108.30362(95) 0.40380 0.40863 105.95682(87) 108.22048(65) 0.40890
1,
107.04132(118) 0.41191 115.70848(8) 0.41011 116.64392(19) 105.57072(180) 0.41159 0.40538
0.38502 0.40293 0.37173 0.38130 0.38206 0.38998 0.38967 0.38801
61bb
97.99762(106) 0.39071 106.83999(7) 0.40352 107.77113(16) 96.26430(166) 0.40456 0.38313
95.80097(94) 106.87675(53) 96.58763(74) 98.37736(59) 103.37120(99) 96.91123(86) 100.36562(62) 96.91916(78)
lb
aFigures in parentheses indicate the uncertainties attached to the last significant figures, b6 IB = IJobs) - Zg(calc.). Ig(calc.) was computed from the coordinates given in Table 2. “PC = (I,, + Ib - 1,)/2. d AP, = PJisotopic) -PJnormal). ‘AP, = PJz9SiD,CH=CH,) -P,(SiD,CH=CH,) = 0.00136(73). fAPc = P,(‘OSiD,CH=CH,) -P,(SiD,CH=CH,) = 0.00125(90).
14.81292(32) -0.01531 20.46539(10) 0.00094 20.46945(24) 15.07546(assumed) 0.00114
'9SiD,CH=CH1 "SiH,CH=CH, SiHi'*CH=CH, "'SiDpH=CHI
-0.01531 -0.00269 -0.02083 -0.02669 -0.01842 -0.00329 -0.01673 -0.01537
14.80110(15) 20.46767(15) 17.29803(16) 16.63632(13) 14.80485(15) 17.18101(18) 16.32314(11) 14.80714(24)
ar,b
SiH,CH=CH, SiD,CH=CH, SIH,CD=CH, SiH,CH=CHD-c SlH,CH=CHD-t s-SlH,DCH=CH, a-SlH$CH=CH, a9SiH,CH=CH,
IfI
Moments of inertia (amu A’)8
TABLE 4
192 TABLE
5
Planarity
of the vinyl group
Isotopic
( APc)
XCH=CHD-c XCH=CHD-t XCD=CH, “X = “Br
SiH,CH=CH,
79BrCH=CHI
species=
-0.00814 + 0.00301 +.01114
-0.00519 + 0.00269 -0.00539
or SiH,.
Dipole moment Stark-effect measurements were carried out on 10 and 9 low J transitions of the normal and SiD3CH=CH, species, respectively. The spectrometer was calibrated with OCS [6] before and after each measurement. Results are given in Table 6. TABLE
6
Stark coefficientsa Transition
and dipole !I\11
moments
-
00,
-
lo,
212 - l,, 2 II -
110
3 03 3 13 -
2,,
3 II -
21,
1 10
10,
-
2 II 3 II -
202
20, 30;
P,(D) pb(D) rtotal(D) a(lu-u)b p(H-SiC)c
0 0
SiD&H=CH,
SiH$H=CH, Obs.
1 01 2 02
of vinyl silane
Calc.
Obs.
2.31 l(4) -0.665( 1) 0.531(l)
-0.666 0.532
1 0 1 2 1 2 1 2 1 2 3
23.134(56) 0.516(2) -24.041(283) O-259( 1) O-857( 1) 3.322(7)
23.251 0.521 -22.634 0.262 0.855 3.308
-2.909( 6) 23.521(94) 3.613( 10) 1.053(2)
-2.934 24.001 3.599 1.044
This work
0 ‘Reilly
0.644(2) 0.130(7) 0.657(2) 11” 25’( 50’) 7’50’ or 30”36’
0.648(8) O-133(27) 0.662(12) 11°(20) 10” or 32”
2.461
2.493(3)
2.298
ii
Calc.
0.493( 1) 26.335(171) 0.566(l) -26.579(30) 0.246( 1) 0.963(2) 3.896( 5) -0.913(2) -3.522(S)
and Pierce
0.491 27.879 0.559 -27.193 0.241 0.955 3.903 -0.909 -3.540 4.314 1.251
4.329(40) 1.296(8) [1]
This work 0.635(4) 0.120(15) 0.647(5) 10”41’(2O) 8”22’ or 29”42’
X 10’ MHz/(o/cm)‘. Figures in parentheses indicate the experimental uncertainties attached to the last digit. bThe angle be tween the dipole moment and the a-inertial axis. =The angle between the dipole moment and the Sic bond.
“(Au/E’)
193
The dipole moments, thus obtained, are 0.657 and 0.647 D making angles of 11’25’ and lO”41’ with the a inertial axes of the normal and deuterated species, respectively. Since the magnitude of the dipole moment for the deuterated species is slightly smaller than that foF the unsubstituted species, the negative pole of the dipole moment is concluded to be at the silicon atom. There are two possible directions of the dipole moment which are shown in Fig. 2 (directions A and B). Since the rotation of the inertial axes on going from the unsubstituted to the deuterated species is very small, the direction of the dipole moment cannot be determined uniquely from experiment alone, For ethyl silane [ 7] and trcns-propyl silane [8] , the dipole moments are reported to be 0.81 and 0.811 D and the angles between the dipole moment and the Sic bond 138 and 5”46’, respectively, These values indicate that the group moment of the silyl group is the dominant component of the dipole moment for these molecules. For &-methyl vinyl silane [Z] , the dipole moment (0.729 D) makes an angle of 11” 42’ with the bisector of cu(CSiC) and inclines its positive pole towards the methyl group. The group moment of the vinyl group in this molecule is therefore considered to be H;C=+CH. Hence, the direction of the dipole moment of vinyl silane is along A in Fig. 2 which makes an angle of 7”50’ with the Sic bond when the direction of the vinyl group moment is assumed to be equal to that in c&methyl vinyl silane. Our results for the dipole moment are roughly in agreement with those reported by O’Reilly and Pierce [ 11. Internal rotation of the silyl group As mentioned earlier, details of the analysis on the internal rotation of the silyl group will be published separately. We here report only on the results obtained from the A -E splittings of the spectra in the ground state.
Fig. 2. The dipole moment and its direction.
194
O’Reilly and Pierce [l] have also obtained the barrier V3 from the A - E splittings of a-type transitions in the ground and first excited silyl torsional states for the normal and SiDBCH=CH2 species. The A -E splittings of b-type transitions are much larger than those of a-type transitions for vinyl silane. Our data include not only the A --E splittings of the normal species but also those of the isotopic species. As shown in Table 7, the analysis was performed for the A - E splittings of five different isotopic species, using the PAM-Bootstrap method [9]. The parameters necessary for the calculation were computed from the structure described earlier. In the course of the least-squares analysis, the direction cosines of the top axis with respect to the inertial axes were adjusted so as to get the best fit of the splittings. It was found that the direction cosines caiculated Tom the rs structure gave the best fit. The silyl group tilt is thus also confirmed from the A -E splitting analysis. The values of the barrier V3 obtained for the five species are in good agreement with one another. The average value is 1488 k 24 cal mol-l which accords with that reported (1500 + 30 cal mol-‘). The results of the present analysis of the molecular structure for TABLE
7
Observed A -E
splittings (MHz)~ and barrier V, (cd mol-I) of vinyl silane
Transition
SiH,CH=CH2
615 - 616 7 Ib 8 1’19 Ia+ 101, 1 IO-
71, 81, 91, lo,,, 10,
z:: r 4 1351, 61, 7 lb 8 I? 9 181019 -
3:: 40, 50, 60, 707 80, 90, lo,,,
F(GHzjb CF PC ;: v,
(cd
0.40(-3) 0.55(4) 0.7 U-5) 0.92(-4) 1.14(-4) 5.99(B) 6.11(6) 6.12(l) 6.16(O) 6.20(-l) 6.18(-B)
133.56 -0.3683 0.0218 51.86 0.0 m0P)=
1486(5)
SiD&J-I=CHz
0.49( 2) 0.47(O) 0.49(2) 0.48(O) 0.49(O) 0.46(-3) 0.46(-4) 0.52( 1) 0.53(l) 88.80 -0.5330 0.0392 77.64 0.0 1479(20)
SiH,CD=CH,
2.55(-6) 2.54(-B) 2.65(3) 2.63(2) 2.64(4) 2.64(7)
124.62 -0.3172 0.0206 55.43 0.0 1482(10)
SiH,CH=CHD-ciP
2.63(-5) 2.73(3) 2.73(O) 2.74(-2) 2.84(5)
125.15 4.3242 0.0227 55.98 0.0 1503(7)
SiH,CH=CHD-tram
5.99(2) 5.98(-5)
6.14(2) 6.12(-4)
133.56 -0.3686 0.0200 51.97 0.0 1489(4)
‘Figures in parentheses indicate the differences between the observed and calculated A - E splittings. bThe rotational constant of the silyl top. ‘cu= ~,I,lf,,, P = hbfru/lb, is the moment of inertia of the silyl group around the top axis; (h,, hb, Y = h&/I,;Icr h,) is the direction cosine of the top axis with respect to the inertial axes. dReduced barrier, s = 4V,/9F_ =Figures in parentheses indicate the uncertainties attached to the last significant figures.
195
vinyl silane are very similar to those reported by O’Reilly and Pierce [ 1 ] However, since many additional data were added here, we believe our results to be significantly more reliable than those reported previously.
.
ACKNOWLEDGEMENTS
We thank Drs. C. Matsumura and H. Takeo of the National Chemical Laboratory for Industry for allowing us to use their computer-controlled spectrometer and for their kind assistance in measuring the weak spectra. REFERENCES 1 J. M. O’Reilly and L. Pierce, J. Chem. Phys., 34 (1961) 1176. A. Nagayama, J. Nakagawa and M. Hayashi, J. Mol. Struct., 77 (1981) 81. 3 E. Hirota, J. Chem. Phys., 45 (1966) 1984. 4 Y. Shiki, F. Yamaoka and M. Hayashi, Chem. Lett., (1980) 699. 5 Landolt-BBrnstein, Numerical Data and Functional Relationships in Science and Technology, Springer-Verlag, Berlin, 1974, Vol. 6. 6 J. S. Muenter, J. Chem. Phys., 48 (1968) 4544. 7 H. D. Petersen, Thesis, The University of Notre Dame, Indiana, 1961. 8 M. Hayashi, J. Nakagawa and Y. Aguni, Bull. Chem. Sot. Jpn., 53 (1980) 2468. 9 D. R. Herschbach, J. Chem. Phys., 31 (1959) 91.
2 M. Imachi,
Scientific
Publishing
Company,
MICROWAVE SPECTRUM MOMENT AND INTERNAL
YOSHIKO
SHIKI,
Department
of Chemistry,
Naka-ku, Hiroshima (Received
AKINORI
185-195 Amsterdam
-
Printed
in The Netherlands
OF VINYL SILANE; STRUCTURE, DIPOLE ROTATION OF THE SILYL GROUP
HASEGAWA
and MICHIRO
HAYASHI
Faculty of Science, Hiroshima University Higc ahi-sendamachi,
730 (Japan)
21 July 1981)
ABSTRACT Microwave spectra of vinyl silane and its twelve isotopically substituted species have been measured. The r, structure of the molecule has been determined from the observed moments of inertia. Dipole moments for the normal and SiD,CH=CH, species are determined from Stark-effect measurements and the direction of the dipole moment in the molecule is discussed. From the A -E splittings of the observed spectra for five species, the barrier to internal rotation of the silyl group has been determined. INTRODUCTION
Twenty years ago, O’Reilly and Pierce [ 11 reported the microwave spectrum and the structure of vinyl silane. Since the b dipole component of this molecule was so small that b-type transitions were fairly weak, these were only measured for the normal and SiD$H=CH* species. Since the molecule is close to a prolate symmetric top, the A rotational constant cannot be determined from the cz-type transitions alone. The structure proposed by them can only be very tentative due to the lack of A rotational constants for many species. This re-investigation of the microwave spectrum of vinyl silane was originally undertaken for the purpose of confirming the r, structure of this molecule which is considered to be a suitable reference for the structures of a series of compounds, such as methyl vinyl silane 123 and vinyl halogenosilanes. The a- and b-type transitions of the normal and isotopic species were accurately measured and the F, structure established. The dipole moment and its direction in the molecule and the barrier to internal rotation of the sibyl group were also determined. Our secondary aim was to determine accurately the barrier to internal rotation of the silyl group from the A -E splittings of the spectra, in order to compare the results with those of propylene reported by Hirota [ 31. In the course of the measurements of the spectra in the excited torsional states, we found a series attributable to the first excited SiC=C bending state. 0022-2860/82/0000-0000/$02.75
0 1982 Elsevier
Scientific
Publishing
Company
186
Some of these spectra exhibited unexpectedly
large A --fr: splittings.
Similar splittings of the spectra in the first excited skeletal bending state were found for ethyl fluoride and its deuterated species [4], and were interpreted as being due to the Fermi resonance between the second excited torsional state and the first excited skeletal bending state. The situation is probably similar for vinyl silane. In this paper, we restrict our results to the structure, dipole moment and the barrier to internal rotation obtained from the ground state alone. The results for the excited states will be reported separately since measure-
ment of the far-IR spectra of the molecule will be necessary at some stage in the future. ESPERIMENTAL
Samples of the normal and deuterated species were prepared by the reactions described by O’Reilly and Pierce [l] except for SiH,DCH=CHz which was prepared by chlorinating SiH&H=CH, with AgCI and then reducing the resulting SiH,CiCH=CH, with Iithium aluminum deuteride in dibutyl ether. A conventional 100-kHz Stark modulation spectrometer was used for
the me~~ements
of the spectra at dry-ice temperature. The spectra of the
‘gSi 3oSi and “C species were measured using samples which contained theie isoiopes in natural abundance. Very weak spectra were obtained on a strip recorder using the computer-controlled spectrometer at the National Chemical Laboratory for Industry. RESULTS
Microwave
AND DISCUSSION
spectra
For the normal and SiD3CH=CH2 species, about twenty EL-and b-type transitions with J G 10 and R, G 2 were measured. The &-type transitions exhibited doublet structures due to the internal rotation of the silyl group. Since the centrifugal contributions could not be neglected, the rotational constants were obtained by two methods, (a) all the observed A component frequencies were fitted to a modffied rigid-rotor formula which included the dJ, dm and dEz terms of a Watson centrifugal disto~ion expression and (b) the observed frequencies of a-type R-branch transitions with J G 3 and K, d 1 and of b-type Q-branch transitions with J G 6 and K, S 1 were fitted to a modified rigid-rotor formula which included only the dd term of the centrifugal distortion expression. The l1 1 + Ooo transition was also included in the lactations whenever it could be measured. The rotational constants obtained by methods (a) and (b) were essentially identical to each other, within experimental error.
187
For SiH2DCH=CH2 prefixes s- and a- denote the species in which the deuterium atom is situated in the symmetry plane and out of the symmetry plane, respectively and for SiH,CH=CHD, suffixes -c and -t denote the species in which deuterium atom is situated at the cis and tram positions relative to the SiH3 group, respectively. For the deuterated species, the rotational constants were obtained by method (b), described above. For a-SiH,DCH=CH,, b-type transitions exhibited doublet structures, assumed to arise from the existence of two equivalent minima in the internal rotation potential curve. The averages of two-component frequencies of the doublets were used in the calculation of the rotational constants. For “Si, 3oSi, and 13C species, b-type transitions with low values of J could not be measured of samples containing the isotopes in natural abundance. For 2gSiH,CH=CH, and 30SiH3CH=CH,, three b-type Q-branch transitions with 6 d J < 8 were observed. The rotational constants were then obtained from a-type R-branch transitions and the 6,5 + 606 transition by method (b). For 2gSiD,CH=CH2 and “OSiD3CH=CH,, three b-type Q-branch transitions with 10 G J G 12 were observed. In this case the rotational constants were obtained from a-type R-branch transitions and the lOI + lOolo transition. For SiH3’%H=CH, and SiH3CH=13 CH,, measurement of b-type transitions of samples containing the isotopes in natural abundance was impossible. Rotational constants A were therefore assumed’such that the difference between the values of P,[=(I, + Ib - &)/2] for the normal and 13C species was zero. The values of B and C were obtained by method (b). The rotational constants are listed in Table 1. Details of the observed transition frequencies have been deposited with the British Library Lending Division at Boston Spa, Yorkshire, U.K., as Supplementary Publication No. SUP 26207 (6 p.). r, Structure The data obtained are sufficient to determine the rS coordinates of the atoms in the molecule by the substitution method. The atom-labelling scheme of the molecule is given in Fig. 1. It is desirable that the rotational constants of the species used in the structural calculation are obtained from a set of transitions common to all species. The rotational constants obtained by method (b) for the normal and SiD3CH=CH2 species were therefore used in the calculation. For the silicon atom, there are four pairs of parent and substituted species from which four sets of coordinates can be derived by independent solutions of the Kraitchman equations. The coordinates obtained from the two pairs 2gSiD3CH=CH2 and SiD$H=CH,, 3oSiD3CH=CH2 require to SiD,CH=CH,, be transformed to those in the principal axes system of the normal species. Although this transformation depends on the coordinates of hydrogen atoms H, and H, on the silicon atom, two cyclic iterations of the calculation were found to be sufficient to fix the coordinates of the silicon atom.
TABLE
1
Rotational
( MHz)~
constants
SiH,CH=CH,
F
SiD,CH=CH,
f
SiH,CD=CH, SiH,CH=CHD-c SiH,CH=CHD-t s-SiH ,DCH=CH: a-SiH ,DCH=CH z ‘9SiH,CH=CHz “SiH,CH=CH z z9SiD,CH=CH 2 ‘OSiD,CH=CH 1 Si H 1’ %H=CH . SiH 3CH=’ %ZH,
A
B
C
34144.30(14) 34144.48(35) 24703.37(29) 24703.46(17) 29215.81(27) 30377.87(23) 34135.84(34) 29414.80(32) 30960.71(20) 34130.56(55) 34117.25(73) 24694.18(12) 24689.28( 28) 33524.85(assumed) 33946_81(assumed)
5275.26(l) 5275.27(5) 4773.28(2) 4773.25(3) 5232.30(4) 5137.12(3) 4888.94(5) 5214.83(5) 4024.24( 3) 5214.41(g) 5157.02(6) 4730.21(O) 4689.35(l) 5249.88(g) 5114.89(g)
4820.76(l) 4820.77(5) 4404.51(2) 4404.52(2) 4673.39(4) 4625.41(3) 4496.19(4) 4666.29(4) 4669.87(3) 4769.64(4) 4721.32(5) 4367.66(O) 4332.64(l) 4787.09(8) 4682.73(8)
dJ X
lozb
-1.73(4) 0.30( 43) -1.52(48) 0.23(23) 0.27( 34) 0.37(27) 0.25( 39) 0.32( 39) 0.34(25) 0.31(28) O-24(37) 0.27(5) 0.21( 14) O-36(29) 0.36( 30)
NC
RMS
22 13 18 14 13 12 12 13 13 8 8 8 8 7 7
0.04 0.09 0.09 0.05 0.07 0.06 0.09 0.09 0.06 0.08 0.10 0.02 0.04 0.08 0.08
“Figures in parentheses indicate the uncertainties attached to the last significant figures calculated from 2.5 times the standard deviation. bThe coefficient of the [J(J + l)]’ term of the centrifugal distortion formula. =The number of observed frequencies used for the least-squares calculation. dThe root-mean-sq uare deviation of the calculated frequencies from the observed ones. =Rotational constants obtained using method (a) (see text). The dJK and dEJ coefficients obtained are -0.140(5) and 4.06(7) X 10m6 MHz for the normal species and -O-085(6) and 3.95(18) X low6 MHz for the SiD,CH=CH,. fRotational constants obtained using method (b) (see text). b
t
b I
oasi
H
Fig. 1. Atom-labelling Except
for
the
scheme coordinates
and the r, structural
parameters.
of the two carbon is assumed, four
atoms where AP,
= 0
different sets of coordinates for the atoms in the molecular plane are obtained by four different choices [APc
= P,( IT)
-P,(normal)]
189
of the three moments of inertia. The averages of the four values were used as the coordinates and the errors replaced by half the difference between the maximum and minimum coordinate values, whenever this value exceeded the experimental error. Attempts were made to fix the two unreliable Kraitchman coordinate values, xb(Ht) and xb(Si). For zcb(Ht), application of the first moment equation is not helpful since the error in the coordinates of heavy atoms imparts a large uncertainty on the coordinates of lighter atoms such as hydrogen. Therefore, the x~(H*) value was fixed by the structural assumption, r(CH,) = r(CH,). For xb(Si), however, the first moment equation could be usefully applied to fix this value. The coordinates and structural parameters are given in Tables 2 and 3, respectively. The differences between the observed and calculated moments of inertia for all the species are listed in ‘Table 4. The root-mean-square deviation of these values is 0.3252 amu A2 while the ZU-Z~(X,)~and I,,, values for the normal species are -0.0445 amu A and 0.0967 amu A*, respectively. Since we used a structural assumption for xb(Ht), the differences between the observed and calculated 1, values are in some cases negative. This would not normally be expected for an r, structure which is well established. In order to ascertain the planarity of the vinyl group in the molecule, the AP, values of three deuterated species were compared with those of the TABLE
2
Atomic
coordinates
Atom
Group
HS
SiH, SiH,
3 C H C Hc
KC
(A )a -yb
1.5591( 32) -0.3926(20) 0.0806(32) -0.5289(52) -1.5995(36) 0.3016(98) 1.3861(31) -0.0904( 209)
-1.0404( 47 ) -l-7874( 5) -1.0659( 11) 0.6842( 34) O-8827(62) l-7442( 15) 1.5974(26) 2.7660(6)
CH CH i=H, CH, CH,
x, @iI
xb(Si)
Parent
and isotopic
-1.0666(g) -1.0666(21) -1.0651(7) -1.0655(g)
0.0778(104) 0.0790(236) O-0965(87) 0.0894(104)
(SiH,CH=CH,, (SiH,CH=CH,, (SiD,CH=CH,, (SiD,CH=CH,,
0.0 -t1.2001(9) 0.0 0.0 0.0 0.0 0.0 0.0 species
‘9SiH,CH=CH,) ‘OSiH,CH=CH “SiD,CH=CH,) ‘OSiD,CH=CH,)
pair
,)
“Figures in parentheses indicate the uncertainties attached to the last significant figures. bFor x,, the average of the four values obtained from the four pairs of parent and isotopic species. For Q, the value obtained from the first moment equation_ The x, and ~b values solved by the Kraitchman equations are also given above. ‘The Xb value was fixed by the structural assumption, r(CH,) = r(CHt).
190 TABLE
3
Structural
parametersa Present work
O’Reilly and Pierce
1.853(4) 1.347(7) 122’43’(25’)
1.853 1.347 122”53’
1.479(5) 1.4’ ‘8( 2) 10S012’(25’) 110:50’(15’) 109”10’( 18’) 108”34’(7’) 109’57’( 18’) 1”45’(27’)
lOS”25’ 109”20’ 108”42’ lOS”42’ 109”2’ l”49’
Skeleton
r(SiC) (A ) r(C=C) (4 ) a( SiC=C) SiHJ
r(SiH,) (A )
r(SiH,) (A ) ==(HsSiC) a(H&C) a(HSSiH,) cr(H,SiH,)
CH
r(CH) (A 1
a(SiCH) cu(HC=C)
r(CHc) (A 1 WHt)
(A 1 a (C=CH,) o(C=CHt)
a(HCH)
1.089(6) 119”42’(36’)
117”34’( 1”) 1.094(10) l.O94(assumed) 120”21’(56’) 120’55’(52’) 118”42’(2”)
1.094 119% 117”59’
1.097
120’18’ 120”38 119”4’
aFigures in parentheses indicate the uncertainties attached to the last significant figures. bHvpothetical untilted cr(IiSiC) angle defined by r = [a(H$iC) + 2a(HaSiC)I/3- ‘Tilt angle defined by 6 = (2/3)[a(H,SiC) - a(H,SiC) 1.
corresponding species of 79Br-substituted vinyl bromide which has already been confirmed to be planar [5]. As shown in Table 5, the Ap, values of the vinyl silane species follow a similar trend to those of the vinyl bromide species, although the absolute values of the silanes are slightly higher. This slight increase in AP, values can be explained by coupling between the CH out-of-plane bending and silyl rocking modes, where the latter is an additional vibrational mode to those in vinyl bromide. Hence, the planarity of the vinyl group in vinyl silane is as plausible as that in vinyl bromide. Our proposed structure is surprisingly close to that reported by O’ReUy and Pierce [ 11, within experimental error. The tilt of the silyl group is very clear from the present r, structure, i.e., angles ar(H,SiHa and a(H,SiH,) are similar, while a(H,SiC) is smaller than a(H,SiC) by 2”38’. This arrangement produces a 1’45’ tilt towards the C=C bond. O’Reilly and Pierce reported a tilt for the sibyl group of l”49’.
14,88761(assumed)
SiH,CH="CH,
a$
APcd
0.00009(234) 2.91393(118)e O.O(sssumed) 2.91381(135$ O.O(assumed)
-
307.92344(194) 0.40378
98.80486(168) 0.38357
2.88452(106) 5.79708(61) 2.91257(167) 2.87338(87) -0.01114(193) 2.67638(69) -0.00814(175) 2.88753(111) 0.00301(217) 2.89431(100) 0.00980(205) 2.88474(94) 4.23414(69) 0.00022(200) 1.34962(176)
pee
5.79846(12) 2.88461(128) -5.79833(29)
104.83306(102) 0.40642 114.74026(55) 0.40663 106,13911(83) 0.40988 109.26092(66) 0.40758 112.40099(108) 0.39431 108.30362(95) 0.40380 0.40863 105.95682(87) 108.22048(65) 0.40890
1,
107.04132(118) 0.41191 115.70848(8) 0.41011 116.64392(19) 105.57072(180) 0.41159 0.40538
0.38502 0.40293 0.37173 0.38130 0.38206 0.38998 0.38967 0.38801
61bb
97.99762(106) 0.39071 106.83999(7) 0.40352 107.77113(16) 96.26430(166) 0.40456 0.38313
95.80097(94) 106.87675(53) 96.58763(74) 98.37736(59) 103.37120(99) 96.91123(86) 100.36562(62) 96.91916(78)
lb
aFigures in parentheses indicate the uncertainties attached to the last significant figures, b6 IB = IJobs) - Zg(calc.). Ig(calc.) was computed from the coordinates given in Table 2. “PC = (I,, + Ib - 1,)/2. d AP, = PJisotopic) -PJnormal). ‘AP, = PJz9SiD,CH=CH,) -P,(SiD,CH=CH,) = 0.00136(73). fAPc = P,(‘OSiD,CH=CH,) -P,(SiD,CH=CH,) = 0.00125(90).
14.81292(32) -0.01531 20.46539(10) 0.00094 20.46945(24) 15.07546(assumed) 0.00114
'9SiD,CH=CH1 "SiH,CH=CH, SiHi'*CH=CH, "'SiDpH=CHI
-0.01531 -0.00269 -0.02083 -0.02669 -0.01842 -0.00329 -0.01673 -0.01537
14.80110(15) 20.46767(15) 17.29803(16) 16.63632(13) 14.80485(15) 17.18101(18) 16.32314(11) 14.80714(24)
ar,b
SiH,CH=CH, SiD,CH=CH, SIH,CD=CH, SiH,CH=CHD-c SlH,CH=CHD-t s-SlH,DCH=CH, a-SlH$CH=CH, a9SiH,CH=CH,
IfI
Moments of inertia (amu A’)8
TABLE 4
192 TABLE
5
Planarity
of the vinyl group
Isotopic
( APc)
XCH=CHD-c XCH=CHD-t XCD=CH, “X = “Br
SiH,CH=CH,
79BrCH=CHI
species=
-0.00814 + 0.00301 +.01114
-0.00519 + 0.00269 -0.00539
or SiH,.
Dipole moment Stark-effect measurements were carried out on 10 and 9 low J transitions of the normal and SiD3CH=CH, species, respectively. The spectrometer was calibrated with OCS [6] before and after each measurement. Results are given in Table 6. TABLE
6
Stark coefficientsa Transition
and dipole !I\11
moments
-
00,
-
lo,
212 - l,, 2 II -
110
3 03 3 13 -
2,,
3 II -
21,
1 10
10,
-
2 II 3 II -
202
20, 30;
P,(D) pb(D) rtotal(D) a(lu-u)b p(H-SiC)c
0 0
SiD&H=CH,
SiH$H=CH, Obs.
1 01 2 02
of vinyl silane
Calc.
Obs.
2.31 l(4) -0.665( 1) 0.531(l)
-0.666 0.532
1 0 1 2 1 2 1 2 1 2 3
23.134(56) 0.516(2) -24.041(283) O-259( 1) O-857( 1) 3.322(7)
23.251 0.521 -22.634 0.262 0.855 3.308
-2.909( 6) 23.521(94) 3.613( 10) 1.053(2)
-2.934 24.001 3.599 1.044
This work
0 ‘Reilly
0.644(2) 0.130(7) 0.657(2) 11” 25’( 50’) 7’50’ or 30”36’
0.648(8) O-133(27) 0.662(12) 11°(20) 10” or 32”
2.461
2.493(3)
2.298
ii
Calc.
0.493( 1) 26.335(171) 0.566(l) -26.579(30) 0.246( 1) 0.963(2) 3.896( 5) -0.913(2) -3.522(S)
and Pierce
0.491 27.879 0.559 -27.193 0.241 0.955 3.903 -0.909 -3.540 4.314 1.251
4.329(40) 1.296(8) [1]
This work 0.635(4) 0.120(15) 0.647(5) 10”41’(2O) 8”22’ or 29”42’
X 10’ MHz/(o/cm)‘. Figures in parentheses indicate the experimental uncertainties attached to the last digit. bThe angle be tween the dipole moment and the a-inertial axis. =The angle between the dipole moment and the Sic bond.
“(Au/E’)
193
The dipole moments, thus obtained, are 0.657 and 0.647 D making angles of 11’25’ and lO”41’ with the a inertial axes of the normal and deuterated species, respectively. Since the magnitude of the dipole moment for the deuterated species is slightly smaller than that foF the unsubstituted species, the negative pole of the dipole moment is concluded to be at the silicon atom. There are two possible directions of the dipole moment which are shown in Fig. 2 (directions A and B). Since the rotation of the inertial axes on going from the unsubstituted to the deuterated species is very small, the direction of the dipole moment cannot be determined uniquely from experiment alone, For ethyl silane [ 7] and trcns-propyl silane [8] , the dipole moments are reported to be 0.81 and 0.811 D and the angles between the dipole moment and the Sic bond 138 and 5”46’, respectively, These values indicate that the group moment of the silyl group is the dominant component of the dipole moment for these molecules. For &-methyl vinyl silane [Z] , the dipole moment (0.729 D) makes an angle of 11” 42’ with the bisector of cu(CSiC) and inclines its positive pole towards the methyl group. The group moment of the vinyl group in this molecule is therefore considered to be H;C=+CH. Hence, the direction of the dipole moment of vinyl silane is along A in Fig. 2 which makes an angle of 7”50’ with the Sic bond when the direction of the vinyl group moment is assumed to be equal to that in c&methyl vinyl silane. Our results for the dipole moment are roughly in agreement with those reported by O’Reilly and Pierce [ 11. Internal rotation of the silyl group As mentioned earlier, details of the analysis on the internal rotation of the silyl group will be published separately. We here report only on the results obtained from the A -E splittings of the spectra in the ground state.
Fig. 2. The dipole moment and its direction.
194
O’Reilly and Pierce [l] have also obtained the barrier V3 from the A - E splittings of a-type transitions in the ground and first excited silyl torsional states for the normal and SiDBCH=CH2 species. The A -E splittings of b-type transitions are much larger than those of a-type transitions for vinyl silane. Our data include not only the A --E splittings of the normal species but also those of the isotopic species. As shown in Table 7, the analysis was performed for the A - E splittings of five different isotopic species, using the PAM-Bootstrap method [9]. The parameters necessary for the calculation were computed from the structure described earlier. In the course of the least-squares analysis, the direction cosines of the top axis with respect to the inertial axes were adjusted so as to get the best fit of the splittings. It was found that the direction cosines caiculated Tom the rs structure gave the best fit. The silyl group tilt is thus also confirmed from the A -E splitting analysis. The values of the barrier V3 obtained for the five species are in good agreement with one another. The average value is 1488 k 24 cal mol-l which accords with that reported (1500 + 30 cal mol-‘). The results of the present analysis of the molecular structure for TABLE
7
Observed A -E
splittings (MHz)~ and barrier V, (cd mol-I) of vinyl silane
Transition
SiH,CH=CH2
615 - 616 7 Ib 8 1’19 Ia+ 101, 1 IO-
71, 81, 91, lo,,, 10,
z:: r 4 1351, 61, 7 lb 8 I? 9 181019 -
3:: 40, 50, 60, 707 80, 90, lo,,,
F(GHzjb CF PC ;: v,
(cd
0.40(-3) 0.55(4) 0.7 U-5) 0.92(-4) 1.14(-4) 5.99(B) 6.11(6) 6.12(l) 6.16(O) 6.20(-l) 6.18(-B)
133.56 -0.3683 0.0218 51.86 0.0 m0P)=
1486(5)
SiD&J-I=CHz
0.49( 2) 0.47(O) 0.49(2) 0.48(O) 0.49(O) 0.46(-3) 0.46(-4) 0.52( 1) 0.53(l) 88.80 -0.5330 0.0392 77.64 0.0 1479(20)
SiH,CD=CH,
2.55(-6) 2.54(-B) 2.65(3) 2.63(2) 2.64(4) 2.64(7)
124.62 -0.3172 0.0206 55.43 0.0 1482(10)
SiH,CH=CHD-ciP
2.63(-5) 2.73(3) 2.73(O) 2.74(-2) 2.84(5)
125.15 4.3242 0.0227 55.98 0.0 1503(7)
SiH,CH=CHD-tram
5.99(2) 5.98(-5)
6.14(2) 6.12(-4)
133.56 -0.3686 0.0200 51.97 0.0 1489(4)
‘Figures in parentheses indicate the differences between the observed and calculated A - E splittings. bThe rotational constant of the silyl top. ‘cu= ~,I,lf,,, P = hbfru/lb, is the moment of inertia of the silyl group around the top axis; (h,, hb, Y = h&/I,;Icr h,) is the direction cosine of the top axis with respect to the inertial axes. dReduced barrier, s = 4V,/9F_ =Figures in parentheses indicate the uncertainties attached to the last significant figures.
195
vinyl silane are very similar to those reported by O’Reilly and Pierce [ 1 ] However, since many additional data were added here, we believe our results to be significantly more reliable than those reported previously.
.
ACKNOWLEDGEMENTS
We thank Drs. C. Matsumura and H. Takeo of the National Chemical Laboratory for Industry for allowing us to use their computer-controlled spectrometer and for their kind assistance in measuring the weak spectra. REFERENCES 1 J. M. O’Reilly and L. Pierce, J. Chem. Phys., 34 (1961) 1176. A. Nagayama, J. Nakagawa and M. Hayashi, J. Mol. Struct., 77 (1981) 81. 3 E. Hirota, J. Chem. Phys., 45 (1966) 1984. 4 Y. Shiki, F. Yamaoka and M. Hayashi, Chem. Lett., (1980) 699. 5 Landolt-BBrnstein, Numerical Data and Functional Relationships in Science and Technology, Springer-Verlag, Berlin, 1974, Vol. 6. 6 J. S. Muenter, J. Chem. Phys., 48 (1968) 4544. 7 H. D. Petersen, Thesis, The University of Notre Dame, Indiana, 1961. 8 M. Hayashi, J. Nakagawa and Y. Aguni, Bull. Chem. Sot. Jpn., 53 (1980) 2468. 9 D. R. Herschbach, J. Chem. Phys., 31 (1959) 91.
2 M. Imachi,