Microwave spectrum, structure, dipole moment and internal rotation of ethyl fluorosilane

Journal of Molecular Structure, 74 (1981) 97-l 10 Elsevier Scientific Publishing Company, Amsterdam

-

Printed in The Netherlands

MICROWAVE SPECTRUM, STRUCTURE, DIPOLE MOMENT INTERNAL ROTATION OF ETHYL FLUOROSILANE

MICHIRO Department

HAYASHI,

MISAKO

of Chemistry,

IMACHI

and MIKIYA

AND

OYAMADA

Faculty of Science, Hiroshima University Higashi-sendamachi,

Naka-ku, Hiroshima 730 (Japan) (Received 2 January 1981)

ABSTRACT Microwave spectra of the trans and gauche isomers of ethyl fluorosilane and their eleven isotopically substituted species have been measured. The rs structures of the two isomers were determined from the observed moments of inertia. The molecular structures found for the two isomers in the present study are compared with those of analogous molecules. Dipole moments of the two isomers were determined by Stark-effect measurements and are also compared with those of analogous molecules. The energy difference between the trans and gauche isomers was obtained from the relative intensity measurements of the spectra and the barrier to internal rotation of the methyl group for the gauche isomer was obtained from the A-E splittings of the spectra in the fit excited methyl torsional state. The V, value was 2775 c 25 cal mol-‘.

INTRODUCTION

Molecular structures of propyl fluoride [ 11 and ethyl chlorosilane [ 21 have been investigated by means of microwave spectroscopy. For these molecules, trans and gauche rotational isomers, exist in the gaseous state. We investigated the microwave spectra of ethyl fluorosilane [3] in order to compare its molecular structure with thsse of propyl fluoride and ethyl chlorosilane. Our interest in ethyl fluorosilane related primarily to the dihedral angle of the gauche isomer, the angles between the dipole moment and the SiF bond, the energy difference between the two isomers and the barrier to internal rotation of the methyl group. However, in the course of the study, we found that the SiCC angles of the two isomers are very different (see later). This fact can be explained on the basis of the tilt phenomenon of the ethyl group as in the case of ethyl mercaptan [ 8]_ This tilt phenomenon is also found for the methyl group of methyl fluorosilane [ 71 and in the ethyl group of ethyl chlorosilane. Since a similar fact cannot be found for propyl fluoride, this phenomenon is considered to be important in relation to the chemical properties of organosilicone molecules.

0022-2860/81/0000-0000/$02.50

0 1981 Elsevier Scientific Publishing Company

98

EXPERIMENTAL

Samples of ethyl fluorosilane, CH3CH2SiD2F and CH3CH2SiHDF were prepared as follows. Ethyl chlorosilane was reduced by lithium aluminium hydride or deuteride (or mixed hydride and deuteride) in dibutyl ether and the resulting ethyl silane was chlorinated by silver chloride. Antimony trifluoride was used to convert the resulting monochloride into ethyl fluoros&me. Samples of 13CH3CH2SiH2F, CH313CH2SiH2F, CH2DCH2SiH2F, CH,CD2SiH2F and CH3CHDSiH2F were prepared from the appropriate isotopic species of ethyl silane which were prepared by the Grignard reaction of iodosilane and the appropriate isotopic species of ethyl bromide (95 at.% 13C and 98 at.% D, Prochem. BOC/Ltd.). Iodosilane was prepared by the method of Fritz and Kummer [ 43. The “Si species was measured with the sample which contains this species in natural abundance_ The microwave spectra of these samples were measured in the range from 8500 to 34000 MHz with a conventional Stark modulation spectrometer at dry ice temperature. MICROWAVE

SPECTRA

The tram isomer of ethyl fluorosilane is a slightly asymmetric prolate top molecule (K = -0.965) having cc, and & dipole moment components. However, b-type transitions are actually so weak that the dipole moment of this isomer is nearly parallel to the a-inertial axis. The a-type transitions alone could be assigned. Several groups of weak spectra attributed to the excited vibrational states exist around the ground state spectra. For the normal species, from relative intensities of the spectra, they are estimated to arise from the first excited methyl torsional, skeletal torsional and CSiF bending vibrational states, respectively_ All the a-type spectra of these groups are singlets. The gauche isomer has the asymmetry parameter K = -0.838 and &, pb and pLrdipole moment components. However, c-type transitions are actually so weak that the dipole moment of this isomer is nearly on the a-b inertial plane. Only the a- and b-type transitions could be assigned. Two groups of the spectra attributed to the excited vibrational states could be assigned. They are estimated to arise from the first excited methyl torsional and

skeletal torsional states, respectively. The b-type Q branch transitions with

5fKlO

and K,
exhibit doublet structures for the first excited methyl

torsional state. The trans isomer of ethyl fluorosilane is so close to a symmetric top molecule that the A rotational constant cannot be determined with sufficient accuracy from the observed a-type transitions. Twenty a-type transitions with JG6 and I&62 were measured for the normal species. In order to find the effective number of transitions for the determination of the rotational

99

constants, B and C, with the accuracy necessary to establish the r, structure, the rotational constants were obtained in two ways; one from all the observed frequencies and the other from the frequencies with J<6 and K,=Gl. When a rigid rotor formula including the first term, -dJ [J(J+ l)] 2, of the centrifugal distortion formula was applied, the two sets of the rotational constants were in good agreement, within the experimental error. Then, about fifteen a-type transitions with J&6 and K,=Gl were measured for the isotopic species and the rotational constants and ClJconstants were obtained from them. For the gauche isomer, about fifty a- and b-type transitions with J] ,of the centrithe first two terms, --d, [J(J+l)] * and -d, fugal distortion terms was applied to seventeen transitions with J<4 and K, Gl. Then more than twenty a- and b-type transitions were measured for the isotopic species, and the rotational constants were obtained from the observed frequencies except for the CH,CH,SiD,F and 29Si species. Since the observed b-type transitions were those with high J values, the rotational constants of the CH3CH2SiD2F species were obtained using the formula including all the five first order centrifugal distortion terms. For the %i species, low J transitions were weak and several medium J transitions with K, G 1 were overlapped by very strong transitions for the normal species. The rotational constants were obtained from all the observed transitions, including high J and high K, transitions, so as to fit them with the formula including the five centrifugal distortion terms whose distortion constants were assumed to be identical to those obtained for the normal species. Details of these data including the data for the excited vibrational states have been deposited with the B.L.L.D. at Boston Spa, Yorkshire, U.K., as Supplementary Publication No. SUP 26193 (13 pages). The rotational constants are given in Table 1 where s-CH,DCH,SiH,F and a-CH,DCH,SiH,F for the tram isomer indicate the deuterated species whose hydrogen atoms in and out of the symmetry plane are replaced by deuterium atoms. CH2DCH2SiH2F-1 for the gauche isomer indicates the deuterated species whose No. 1 hydrogen atom is replaced by a deuterium atom and so on. Numbering of the atoms in the molecule is given in Fig-l. rs STRUCTURE

For the sake of convenience, the names of the molecules referred to are abbreviated as follows; ethyl fluorosilane, [EF] ; methyl fluorosilane, [MF] ;

100

TABLE

1

constants

Rotational

Gauche

of

ethyl

fluorosilane

A

B

c

~~~~:~~~~~~

Ei”~?~i

E;:%;i

~:ZK~

-9-S(3) -2.5(26) -1.8(31)

10162:01(52) 10186.64(22)

3579:67(U) 3603.47(3)

3004:84(S) 3018.17(2)

3:3(22) b.Z(assumed)

-l-7(36) -S.%(assumed)

10189.36(44)

3398.77(g)

2873.36(7)

3.3<19)

-l-5(40)

9773.18<38) 9890.79(47) 9800.56(51) 9766.26(52) 9718.35(56) 9631.72(51) 9159.59(33) 10287.49(100) 10315.44(65)

3555.79(9) 3473.49(9) 3553.80(10) 3529.73(11) 3554.24(11) 3536.53(10) 3476.08(l) 3612.65(12) 3603.69(8)

2946.97(6) 2955.65(7) 2950.80(B) 3006.77(B) 2948.50(9) 3012.62<9) 2933.28(l) 3028.30(10) 3027.49(7)

3.4<18) 5.3(20) 4.5(22) 3.9(24) 3.5(24) 4.3(23) 3.1<7) 4.7<28) 4.4<18)

-1.3(40) -2.2(37) -1.8(36) -2.7(28) -l-0(34) -l-7(32) -7.7(5) -1.3<50) -l-5(32)

2871.63(4) 2788.06(4) 2856.26(2) 2867.97<6) 2717.51(7)

261X48(4) 2549.62(4) 2600.71(2) 2613.40(4) 2487.17(6) 2536.77(4) 2583.85<6) 2591.20(4) 2568.00(6) 2619.00<6) 2625.31<7)

ii:

CH313CHISiHzF CHPCH2zgSiH2Fd CH,DCH,SiH,F-1 CH,DCH,SiI&F-2 CH,DCH1SiH2F-3 CH,CHDSiH ,F-4 CH,CHDSiH,F-5 CH,CH,SiHDF-6 CH,CH,SiHDF-7 CH,CH,SiD,p CH, torsion (u=l)f Skeletaltorsion(u=l)f Trans comer CH,CH,SiHH,F “CH,CH&H,F CH,“CH,SIH,F CH,CH,=SiH,F s-CR,DCH,SiH,F a-CH,DCH,SiHZF CH,CHDSiH,F CH,CH,SiHDF CH,CH,SiD,F CH, torsion (u=l)’ Skeletal torsion <~=l)~ CSiF bendlog (u=l)f aFigures

2769.86(5)

2846.57(7) 2846.68(5) 2816.97(7)

2873.07(4) 2869.85(B) 2876.27(11)

in parentheses

2.5 times

bThe and

d’

dJKb

isomer

;$-g;{

from

(MHz)~

the

indicate

standard

first two centrifugal at lo4

including

MHz the

five

for

the

the

distortion trans

centrifugal

2619.98(10)

uncertainties

deviation. isomer, distortion

Names

attached of

respectively. terms

from

to the

last digit

calculated

species are explained in the text.

the

at 10’

constants

7.2(38) 5.4(38) 3.8(23) 6.5<45) 0.4isoj S-5(43) 10.2(61) 5.1(44) 6.4(64) 26.4(44) 10.1(73) 7.7(105)

‘(1)

and lo* Was

forty-six

MHz

for

obtained transition

by

the the

gauche

isomer

formula

frequencies_

The

constants not listed in the table are dK=-0.592(30), d~~=0.075(2), and dg~=0.652(28) in MHz. (2) Was obtained by the formula including the first two centrifugal distortion terms from seventeen transition frequencies, in a similar manner as for the other species. dThe rotational constants were obtained by transferring the five centrifugal distortion constants of the case (1) of Lhe normal species. eThe formula used for the case (1) of the normaI species was applied. The centrifugal distortion constants not listed are d,=1.804(97), d==O.O49(1), and dEK- --3.026(44) in MHz. fThe fit excited CH, torsional, skeletal torsional, and CSiF bending states for the normal species. centrifugal

distortion

ethyl silane, [EH] ; and ethyl chlorosilane, [ECI] ; the trans and gauche mers are discriminated by f and g where necessary_ Trans

iso-

isomer

As the A rotational constants are not available, the Kraitchman coordinate values of the atoms cannot be solved from the Ib and I, moments of inertia

101

H(7)

Fig-l. Identification number of atoms in the gauche and h-am isomers of ethyl fluorosilane.

alone. For the normal species, a rough A value was obtained from twenty observed transitions with J<6 and K,<2. This value (17344.84k29.85 MHz) is close to the calculated value (17355.00 MHz) from an assumed structure whose parameter values are transferred from those of [EH] [ 51 and [MF] [6]. The 1, value calculated from the rough A constant (29.13697+0.05014 amu a*) was taken as reliable although the experimental error of this vauie is about twenty times larger than those of the 1, and 1, values (175.98931 kO.00257 and 192.43004+0.00287 amu a’). Isotopic substitution of a heavy atom in the symmetry plane, e.g. C and Si atoms of [EF] , usually alters the P, [=(I, +Ib --I, )/2] value very slightly so that the AP,[P,( isotopic) - PJparent)] value is safely regarded as zero. Since the x, coordinate value of the atom is zero from symmetry, the Kraitchman equations need two of the three differences of the moments of inertia, ~U~(g=a, 6, c), between the isotopic and parent species and the Ia0 and Ibo values of the parent species. The Kraitchman coordinate values of the atoms were then solved from Nb, A&, Ia0 and Ibo values. The uncertainties of the coordinate values induced by the assumption of PC=0 are less than the experimental error since I AFc I is estimated to be less than 0.0003 amu B2 from the values for the analogous molecules. The X~ Kraitchman

value of the C atom in the methyl

group is so small

that the value is unreliable. Actually, the calculated C-C bond length is 1.542 a, which is too large. The 3tb(C) value for the present molecule was calculated on the assumption of r(CC)=1.534 a, which is the value for the gauche isomer.

102

Substitution of an in-plane hydrogen atom usuaLly produces a positive A.P~ value which is small but appreciable. Since the Kraitchman equations need only two of the three differences of the moments of inertia, the Kraitchman coordinate values depend on which pair of the three differences are used or whether all the three differences are used. For the H, hydrogen atom in [EFJ , as AI= is not available, the Kraitchman values were solved in two ways. First, the values were obtained on the assumption of AP,=O, that is, they were obtained from AI,, AI,, I,” and Ibo values. Second, they were obtained by the assumption of APp,=0.00131 amu A2 which is the value for the corresponding species of [EH] [ 51. The coordinate values of the four sets arising from the choices of the pair of the moments were averaged. The values for the second case were used and the uncertainties of the values were regarded as the differences of the values between the first and second cases. The assumption of A.Pc-0.00131 amu A2 is considered to be quite plausible because all the APc values for the analogous molecules were found to be between 0.0015 and 0.0008 amu A’. Isotopic substitution of an atom out of the symmetry plane produces, in general, a large AP= value. The Kraitchman coordinate values cannot be obtained unless all the three differences of the moments of inertia are available. However, when one of the three coordinate values is given from the other source, the situation is analogous to isotopic substitution of an atom in the symmetry plane. For the hydrogen atom in the SiJ!iZFgroup, the x, value was assumed to be identical to that of [MF] [6]. This assumption is considered to be equivalent to the assumption that the AP= value is common to the CH&H2SiD2F and CH$iJ&F species, since the x, value is obtained from the nP, value of the di-deuterated species by the use of the relation of Ix,] = (AP,/2Am)“*. For the hydrogen atom in the CHz group, the x, value was assumed to be identical to that of [ EH] [ 51. The x, value of the H, hydrogen atom in the methyl group was, first, assumed to be identical to the corresponding value of [ EH] [ 51. However, this assumption produces unreasonable r(CH, ) and ar(CCH,) values so that the x, vabx v!as shifted so as to give r(CH,)=r(CH,). Care was taken to reduce uncertainties arising from the propagation of the experimental error in the calculation of the coordinate values. The details of the calculation devices will be published separately [7] . The coordinate values of the fluorine atom were obtained from the first moment equations. The coordinate values are given in Table 2. The root mean square deviation of the calculated moments from the observed ones for al! the measured species (hereafter referred to as RMS) is satisfactorily small (0.0973 amu A*). The I,, value is -0.4645 amu A’, which is fairly large. The zero Iti condition was applied to the xb value of the carbon atom in the methyl group in order to avoid the assumption on the C-C bond length mentioned earlier but the resulting C-C bond length value was unacceptable. The differences between the observed and calculated Ib and I,

103 TABLE

2

Atom coordinates (A)= Atomb

xa

Xb

Trams isomer

Ske1eton

F Si C( CH, )

SiH, CH.

H H

CH,

IHs

L

Ha

Gauche isomer F Si Skeleton C(CH, )

SiH, CH,

C(CH,) H, (out) { H, (in) H,(P) I H,

Hi

CH,

H,(P) H,

-1.8414(120) -0.4768(32) O-9763( 11) 2.3075( 52) -O-4832(81) O-9257(78) 3.1648(18) 2.3995(143)

0.3809(192) d-4768(63) O-6727(32) -0.0901(64) -l-3183( 38) l-3206(115) 0.5660(16) -0.7257(205)

-l-3396( 127) -0.6512(48) 1.0970(45) 1.9594( 23) -0.6797(73) -l-4575(36) l-5137(33) 1.0573(48) 1.5120(28) 2.0087( 24) 2.9832(16)

O-8574( 223) -0.5485(57) -0.5150(98) O-5786(83) -0.7100(65) -l-5983(35) -l-5067(35) -0.4494(107) 1.5835( 28) O-4275(111) 0.5602(94)

0.0 0.0 0.0

0.0 t 1.2099(

50) +0.8748(50) 0.0 +0.8677(50)

-O-1278(548) O-1634(194) -0.4317(118) O-2121(227) 1.6464( 32) -0.4900( 124) -O-2825( 199) -l-5153(35) 0.0249(139) 1.3038(39) --0.1985(265)

aFigures in parentheses indicate the uncertainties attached to the last significant figures. bIdentification numbers of atoms are given in Fig-l. (out) and (in) indicate atoms in and out of the skeletal plane (CCSi), respectively. (p) indicates the atom beIonging to the CH bond nearly parallel to the SiF bond.

moments given in Table 3, are negative in contrast to the fact that the differences are usually positive for those molecules whose r, structures are well established. Since the hydrogen atoms give much smaller contributions to

the moments than those of the heavy atoms, the negative differences

of the

calculated moments are considered to arise from a remaining slight inadequacy of the coordinate values for the heavy atoms. The structural parameters calculated from the coordinate values are given in Table 4. Gauche isomer The Kraitchman coordinate values for all the atoms can be obtained except for those of the fluorine atom whose coordinate values have to be solved from the first moment equations. However, the absolute xc, Kraitchman values of two of three hydrogen atoms in the methyl group [H( 5) and H( 7)] and those of the hydrogen atom H(3) in the CH, group are small and unreliable. Several trials were made for the solutions of these three values from the co-

176.98031(267)-0.03658 181.26446(260)-0.03617 176.93634(149) -0.04066 176.21402(360)-0.03800 186.97038(462)-0.03696 182.46643(296)-0.02563 177.63871(412)-0.04497 177.53163(293)-0.04421 179.40411(433)-0.04466

1.01666 1.02196 1.02742 1.01047 0.09461 1.00773 1.04839 1.03396 1.06710 1.06376 0.94696 1.00376 -0.13183 198.21666(288)-0.12902 194.32247(164)-0.13182 193,37881(289)-0.13188 203,10286(~82)-0.13801 199.22068(380)-0.13207 196.60045(464)-0.13118 196.03668(324)-0.12997 196.79766(468)-0.13183

192.93004(287)

166,72863(308) 0.66192 170.86636(392) 0.66071 168.18721(430) 0.64381 167.44446(133) 0.66662 ~76,88363(428) 0.65904 171.40016(343) 0.66069 170.98660(416) 0.66304 171.26767(468) 0.63764 168.07960(441) 0.59836 171,40090(499) 0.63221 167,76304(473) 0.71686 172.29041(59} 0.67680

P 0

X1,18111(226) 11,22374(286) 0,04264(364) 11.36208(324) 0.18008(396) 11.20708(99) 0.02608(247) 1~,20307~308) 0.02197(382) 11.17404(261) 0.21427(432) 12,80204(308) 0.07183(404) 11,26293(336) 2.24126(408) 13.42236(341)-O.O0706(34[i) 11,39638(368) 1.62094(382) 13,80928(337) 0.02107(382) 14,13544(100) 2.96433(219)

aFigures in p~entheses indicate the uncertainties attached to the last si~ificant figures, Identification number of the hydrogen atom attached to the dueterated species name is given in Fig. 1, b61,=l,(obsd) - Ig(catc.), (g=u, b, and c). I,(calc.) was computed from the r, coordinate values listed in Table 2. cPC=(I,+lb - ZC)/2. dAP,=P,(isotopic) -c P,(parent). eFor the trans isomer, I, was not available, Thus, PC and AP, values cannot be calculated.

Tmnsisomer CH,CH,SlH,F 13CH,CH,SiH,F CH,"CH,SiH,F CH,CH,"SiHIF ~CH~DCH~SiH~F a-CH,DCH$iH,F CH,CHDSiH~F CH,CH,SiHDF CH,CHISiDzF

Qoucheisomer 49.28980(173) 0.16267 139.80104(282) CH,CH,SiH,F "CH,CH,SIH,F 49.66118(210)0.14760 143.66166(86B) CH,'%HISiHIF 49.73188(264)O.lti891141.17940(414) 49,61166(108)0.16416 140.24696(101) CH,CH,%iH,F CH~DC~~SiH~F-1 49.68697(213)OS4699 148,69370(889) CH,DCH,SiH,F-2 61.71049(198)0.14488 142.12776(339) CH,DCH,SiH,F-3 61.06664(244)0.11873 146.40604(366) CH,CHDSiH,F-4 61.66606(266)0.15866 142,20737(412) CH$HDSIH,F-6 61.74716(278)0.17472 143.17716(442) CH,CH,SiHDF.6 62,00226(296)0.14417 142.18949(462) 62,46990(277)0.16832 142.90162(391) CH$H#HDF-7 66‘17462(~90~0.16779 145,38676(42) CH,CH*SiD~F

Moments of inertia of ethyl. fluorosilane (amu Rz)’

TABLE 3

105 TABLE

4

Structural

parametersa

skeleton r(CC) r(CSi) r(SrF) c(CCSi) ol(CSiF) ~(CCSIF)

(A) (A) (a)

SiH, group r(SiH)g r(SrH)o,t cu(FSrH)i, W=H),,t a
l-534(14) 1.847(10) 1.592(24) 114?1'(47') 109°31'(1022') 5S043'(1020')

110°57'(29') I

110°21'(36')

1.090(S) 109"25'(54')

I

I I

CH~group CrVX3)Pl

(A) b% (A)

109"37'(58') 106O47'(54')

1.080(6) 1.080<-med) I

112O45'(28')

~(CCH,).C~(CCH)PI c(CCHa) a(H,CH& N+,CH,) NHaCHa)

1_534(assumed) 1.853(5) 1.612(15) 111°50'(29') 109°30'<54') 1800(-med)

107°28'(1012')

c(SiCH) MCCH)P ol(CCH) c(HCH)

r(CHs) r(CHa). r
gauche

I

(A) (A.)I

or
tmns

l-474(6)

%

[~
I

111°30'(1029') 106'52'(l"42') 106°58'(1015')

CH,CH$H,

CH,SiH,F

CH$H,SiH,F

1.540(2) l-866(2)

l-849(5) l-597(5)

113O11'(12') 108"53'(30')

l-476(12) l-477(5) l-492(20) 108°6'(1052') lo83o’b 105O36'(2O53') 112O47'(44') 112"28'(l"30') 109053'(1°2') I 110°0'(30') l10°38'(1016')

1.086(-med) 1.086(12) 107O35'(51') 106'43'(l"15') 112°11'(1018') 113?9'(1°53') 101°54'(1037')

1.103(13) 1.116ilOj 1.103(23) 110°42'(lo12') 110°15'(1"13') 110°2'(lo36') 108°56'(2031') 10S056'(lo7') 107'53'(l"46')

l-097(2)

I I

108°46'b

llo=ho’b 105O46'(24')

1.093<-med) l-093(2)

I

111°57"?o) 111°2'(30') 107"15'(1°) 106O59'(30')

aFigures in parenthesesindicatethe uncertainties attached to the la&significant The parametersdenoted as r(SiH),, r(SiH),,, parallel to the SIF bond.respectively, the gauche isomermbThe uncertaintieswerenotdescribed bytheauthor.

figures.

for

ordinate values of the other atoms. It was found that (1) the zero 1, and I,, conditions and (2) either the zero 1, or I,, condition with an assumption concerning the structure, gave unacceptable values of these three x, values so that the zero inertia product condition could not be used although the absolute values of Inc and Ib, ;re fairly large. The values obtained by the assumptions of r[CH(G)]=r[CH(7)], cr[H(5) CH(7)]=a[H(6) CH(7)] and were chosen as plausible ones after several test calcular[CH(3)] =r[CH(4)] tions were carried out. The coordinate values obtained are given in Table 2. The RMS value (0.7054 amu A2 ) is larger than that for the Pans isomer and the Iab, I, and I,, values (-0.0148,0.7931 and -0.7656 amu A* ) are fairly

106

large while the differences between the observed and calculated moments of inertia given in Table 3 are positive, as is expected for the rs structure. The structural parameters calculated from the coordinate values are given in Table 4. DISCUSSION

ON

MOLECULAR

STRUCTURES

In Table 4, structural parameters of the tram and gauche isomers of ethyl fluorosilane are compared with each other and with those of methyl fluorosilane [ 61 and ethyl silane [ 51. Though comparison with those of ethyl chlorosilane is interesting, the rs structure cannot be obtained from the reported rotational constants by Typke et al. [2]. Therefore, we predicted the reported rotational constants assuming that the parameter values are identical to those of the present molecule except for some values relating to the chlorine atom which were adjusted so as to reproduce rotational constants_ The RMS values between the predicted rotational constants and the reported ones are 0.07 and 0.19% for the tram and gauche isomers, respectively, so that the present parameter values of ethyl fluorosilane are considered to be close to those of ethyl chlorosilane. The adjusted parameter values are given in Table 5 where the corresponding values reported by Typke et al. 123 are also added. For the sake of convenience, the parameters for the trans and gauche isomers will hereafter be denoted as, for example, r(SiC), and r(SiC),, respectively, if necessary. For the gauche isomer, the parameters in and out of the CCSi skeletal plane will be denoted as r(SiH), and r(SiH),,, and those nearly parallel to the SiF bond will be denoted as r(CH), and so on. TABLE

5

Skeletal

structural

parameters

Trans

for ethyl

Present

Vpke

1.534 1.853

1.534 1.859

2.066

1,532 1.869

2.060

111” 50’ 109”30’ 180”

lll”50’b 109030’= 180”

111” 18’ 109” 54’ 180”

1.612

isomer

CH,CH,SiH2F

CH,CH,SiH,Cl

work r(SiC) (A) r(SiX) (A) a(CC3i) a(CSiX) T(CCSiF)

and chlorosilane Gauche

isomer

CH,CH,SiHIF

r(CC) (Ala

fluorosilane

CH,CH,SiH,Cl Present

Typke

1.534 1.847

1.534 1.859

2.066

1.532 1.869

2.060

114”ll’ 109”31’ 58”43’

113”51’b 109O3O’C

113”O 109”54’ 59” 50’

work

et a.Ld

1.592

60”13’

et aLd

aFor the trans isomer of ethyl fluorosilane, the r(CC) value was taken to be equal to that of the gauc;le isomer. For ethyl chlorosilane, we assumed that the r(CC) value is equal to that of ethyl fluorosilane. ba(HCH) was used to balance with a(CCSi). Ca(HSiH) was used to balance with a(CSiC1). dTypke et al. assumed that r(CC), r(SiC), r(SiC1) and a(CSiC1) values do not change between the buns and gauche isomers.

107

r(CC) of [EF] is smaller than that of [EH] but larger than those of the other molecules reported. r(SiC), is larger than r(SiC),. The average of r(SiC), and r(SiC), is close to that of [ MF] and is smaller than those of [ EH] , methyl silane (1.867 A), and disilyl methane [ 1.874 A ] . r(SiF), is larger than r(SiF),. The r(SiF) value of [MF] is between r(SiF), and r(SiF), . cy(CCSi), is much larger than cy(CCSi), . A similar tendency is also found for [ECI] . On the other hand, cr(CSiF) is common to the trans and gauche isomers_ These facts indicate that the situation about the SiCC angles is similar to that about the SCC angles in the trans and gauche isomers of ethyl mercaptan [8]. The difference of the SCC angles between the tram and gauche isomers is considered to arise from the tilt phenomenon induced by the lone pair electrons on the sulfur atom in the case of ethyl mercaptan. For ethyl halosilane, the SiCC angles of the tram and gauche isomers have the following relations if the above facts are regarded to arise from the tilt phenomenon where the ethyl group tilts towards the two hydrogen atoms in the SiHz group: (SiCC), = Q!- 6 and a(SiCC), = a + 6/2, where Q!and 6 denote the unperturbed SiCC angle and the tilt angle, respectively. (Yand 6 values are calculated tiom the observed ac(SiCC) values to be (a, 6 )=(112”24’, l”3’) for [EF] and (113”11’, l”21’) for [ECl] _ The unperturbed SiCC angles (112”24’, 113”ll’) of ethyl halosilane are nearly equal to the SiCC angle of [ EH] (113”ll’). Though similar relations are also expected for the SiCH angles, uncertainties of the coordinate values of the H(3) hydrogen atom for the gauche isomer and of the hydrogen atoms m the CH, group for the trans isomer make the effects ambiguous. The origin of the tilt phenomenon is unresolved for ethyl

halosilane

but

definitely

arises

from

the lone

pair electrons

on the sul-

fur atom for ethyl mercaptan. As will be published in a separate paper [ 71, the tilted methyl group is also found by us for [MF] . The methyl group in [MF] tilts towards the two hydrogen atoms in the SiH2 group by about l”50’. The tilt angle value and the direction of the present molecule are essentially equal to those of [MF]. For the gauche isomer, the dihedral angle r(CCSiF) is less by ca. l”20’ than 60” which is presumably expected for the gauche isomer, while the reported dihedral angles of propyl fluoride [I] and [EC11 [2] are 63” and ca. 60”, respectively. DIPOLE

MOMENT

Stark-effect measurements were carried out on several low J transitions of the normal and CH3CH2SiD2F species for the trans and gauche isomers. The spectrometer was calibrated with carbonyl sulfide [ 91 before and after the measurements on the samples. Results are given in Table 6. Dipole moment values for the two isotopic species are essentially equal for both the tram and gauche isomers, those of the gauche isomer being smaller than those

108 TABLE

6

Dipole moments of tram and gauche Transition IMI

2 01 -

lcl,

2,*-l,, 2 ,*+l,o 3 ,z + 303

Gauche

Tmns comer

isomer

CH,CH,SiH,F

0 1 0 0 1 3

tib

1.811 0.702 0.910 0.623 2.658

0.005 0.001 0.002 0.023 -0.009

l-368(22) l-025(55) 0.066(281) 1.711(51) 36O53' 53O12' 87O47' 2S033' 42Oll'

CHICH,SiHHIF

CH,CH,SiJ&F

AU/E=

Observed ad I&l !&,I
ethyl fluorosilane

AU/E'= -0.641 1.980 0.621 0.905

Observed 1.332<12) 1.056<32) 0.182(146) 1.710<31) 38O51' 51O48' 83-54' 27O28' 44O12'

sb 0.009 -0.002 0.007 0.006

PredlctedC 1.335 1.068 0.059 1.711

Au/E== -2.231 1.836 1.852 1.785

Observed

CHICH,SiDIF

iZb 0.013 0.015 -0.004 0.005

Au/E'= -2.339 1.823 1.792 1.765

Observed

Lib -0.010 0.027 -0.006 -0.033

Predicted'

1.785(g) 0.070(94)

1.776(18) 0.150(45)

1.784 0.054

1.787<11) 2Oll' 87045'

l-783(18) 4O49' 85OlO'

1.783

30"5' 40°24'

28OO' 42O30'

ax lo5 ~z/(volt/Cm)‘. b6=(Av/E* )obs-(~v/~‘)c~c__ =Predicted value from the dipole components of the normal species. dFigures in parentheses indicate the experimental uncertainties attached to the last significant figures. eThe angle between the dipole moment and the g-inertial axis &=(I, b, and c). fThe angles between the dipole moment and the SiF and SIC bonds. of the tram isomer. The dipole moment value of the gauche isomer (1.711 D) is slightly larger than that of [MF] [6] (1.700 D). That of the trans isomer is larger by 0.085 D than that of [MF]; For propyl fluoride [l],the dipole moment value of the tram isomer (2.050 D) is also larger than that of the gauche isomer (1.902 D). The difference of the dipole moments between the trans and gauche isomers for [EF] is somewhat smaller than for propyl fluoride. The angles between the dipole moment and the inertial axes are given in Table 6. Since the dipole moments of [EF] are expected to make a small angle with the SiF bond, of the two possible directions the one with a smaller angle to the SiF bond than the other is taken as the actual one. For the tram isomer, the principal axes of the deuterated species are obtained by a rotation of about 30’ from those of the normal species, while-the dipole moment of the deuterated species makes a larger angle by about 2*40’ with the a-inertial axis than the corresponding angle for the normal species. For the tram isomer, the direction of the dipole moment for the deuterated species seems to be slightly shifted. On the other hand, for the gauche isomer, the k and pb dipole moment components of the deuterated species can be explained by the rotation of the principal axes induced by the substitution of the hydrogen atoms; the p, component predicted is one-third of the observed one although the large uncertainty for the observed c(=component prevents a definite conclusion.

109

The angles between the dipole moment and several bonds in the molecule are given in Table 6. The dipole moment of the gauche isomer makes a smaller angle with the SiF bond than that of the tram isomer. The dipole moment of [MF] makes an angle of about 26”30’ with the SiF bond, while the dipole moments of both the trans and gauche isomers of [EF] make larger angles with the SiF bond. ENERGY

DIFFERENCE

BETWEEN

THE TRAiVS

AND

GAUCHE

ISOMERS

Relative intensity measurements of the spectra attributed to the two isomers were carried out at dry ice temperature. The pair of 2,2 + 1 ,,(10730 MHz) and 2,, f- 110 (11234 MHz) transitions and 4,3 + 404 (10271 MHz) transition were used for the tram and gauche isomers, respectively. The energy difference was calculated from the relative intensities of these spectra to be 55-~50 cal mol-’ , and the gauche isomer was more stable than the tram isomer. The energy differences of propyl halides were reported to be 470, 50 and 100 cal mol-* for fluoride [ 11, chloride [ 10,111 and bromide [ 111, respectively. The gauche isomers are also more stable than the trans isomer for these molecules. The energy difference of the present molecule is smaller than that of propyl fluoride and close to those of propyl chloride and bromide_ INTERNAL

ROTATION

OF THE METHYL

GROUP

For thegauche isomer, the barrier to the methyl internal rotation was calculated by the standard PAM formula from the observed A-E splittings found for b-type Q branch transitions attributed to the first excited methyl torsional state. The quantities necessary to the calculation were calculated from the r, structure described earlier. The moment of inertia of the methyl group around the internal rotation axis is 1,=3.2599 amu A2 and the direction cosines of the internal rotation axis against the inertial axes are (A, p, Y) =(0.5594, 0.7113, 0.4257). The barrier V, obtained is 2775+25 cal mol-’ which is slightly smaller than the value for the gauche isomer of propyl fluoride (2865 cal mol-‘) [l] _ REFERENCES 1 2 3 4 5 6

E. Hirota, J. Chem. Phys., 37 (1962) 283. V. Typke, M. Dakkouri and W. Zeil, Z. Naturforsch. Teil A, M. Hayashi, H. Hikino and M. Imachi, Chem. Lett., (1977) G. Fritz and D. Kummer,Z. Anorg. Allg. Chem., 304 (1960) D. H. Petersen, Thesis, The University of Notre Dame, 1961. L. Pierce, J. Chem. Phys., 29 (1958) 383; L. C. Krisher and 32 (1960) 1619. 7 M. Hayashi and Y. Shiki, to appear. 8 J. Nakagawa, K. Kuwada and M. Hayashi, Bull. Chem. Sot.

29 (1974) 1. 322.

1081.

L. Pierce, J. Chem.

Jpn.,

49 (1976)

Phys.,

3420.

110 9 J. S. Muenter, J. Chem. Phys., 48 (1968) 4544. 10 T. N. Sara&man, J. Chem. Phys., 39 (1963) 469. 11 C. Komaki, I.Ichishima, K. Kuratani, T. Miyazawa, T. Shimanouchi and S. Miiushii, Bull. Chem. Sot. Jpn., (1955) 330.