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Vibrational spectra of trimethyl gallium species in relation to the force field and methyl group internal rotation
Spectrochlmica .4cta, Vol, 43A, No, 11, pp, 1405-1411, 1987. Printed in Great Britain.
0584--8539/87 $3.00+ 0,00 9 1987PergamonJournal~ Lid,
Vibrational spectra of trimethyl gallium species in relation to the force field and methyl group internal rotation D. C. MCKEAN,* G. P. McQUILLAN,* J. L. DUNCAN,* N. SHEPHARD,* B. MUNRO,* V. FAWCETT'["and H. G. M. EDWARDSt *Department of Chemistry, University of Aberdeen, Aberdeen AB9 2UE, U.K. and t School of Studies in Chemistry, University of Bradford, Bradford BD7 IDP, U.K. (Received 23 March 1987)
Abstract--Infrared and Raman spectra are reported for Ga(CH3)~, Ga(CD~)~ and Ga(CHD2)a in the gas phase. These were also examined in the i.r. spectrum in the solid phase at 78 K. The new Raman spectra of the CHD2 species strongly support earlier: i.r. evidence for CH force constant variation during free internal rotation of the methyl groups, from the presence of two bands at 2940 (vs) and 2922era- 1 (w) identifiedas due to v~vand vi~respectively. The observed a' and e' frequencies of the do and d 9 speciesare used to obtain a force field in which three interaction constants are well defined. The best value of the Ga-C stretching force constant is 2.356(28) mdyn A - 1. In the crystal phase at 78 K, the e' modes due to 6sMe and v~GaCa are split, indicating a site group symmetry lower than Ca. Gallium and carbon isotope frequency shifts are predicted.
INTRODUCTION
In this work we report i.r. and Raman spectra of trimethyl gallium species, which bear on the structure of the methyl group, its internal rotation, and on the magnitude of the G a - C stretching force constant. Most previous work on this molecule is summarised in a recent paper by DURIG and CHATTERJEE [1], who report also good quality i.r. and Raman spectra of Ga(CH3)3 in the gas, liquid and solid phases, together with a rudimentary force field. However the only study so far of isotopic species is that of MCKEAN et al. [2] who published and analysed i.r. vCH and vCD spectra of Ga(CH~)a, Ga(CDa)a and Ga(CHD2)3. The appearance of the bands seen was interpreted in terms of freely rotating methyl groups, with a variation in vCH force constant with internal rotation angle, the CH bond being weakest out of the skeleton plane, as earlier diagnosed in BMe3 [3]. The strong desirability of confirming these conclusions by use of the Raman spectrum is established by the work of CAVAGNATand LASCOMaE on similarly internally rotating methyl groups in CHD2NOz and CHD2C6D5 [4]. Our interest in the G a - C stretching force constant lies in anticipating the extension of ab initio treatments to the calculation of force constants for bonds involving metal and carbon atoms, which are of fundamental interest in chemistry. While the success of ab initio treatments of molecules involving atoms such as hydrogen, boron and carbon has been established by recent very precise experimental force fields, e.g. for C2H4., C2H6, B2H6 [5-7], their application to heavier atoms necessitates close monitoring of the validity of the approximations needed for so many electrons, and for this good quality experimental force fields are essential. It is of course not to be hoped that in a molecule as large as GaMe3 that the same quality of experimental
force field is likely to be forthcoming as is found for instance in ethane [6]. The principal coupling effect however which will influence the value of the metal--carbon stretching force constant, is that between G a - C stretching and methyl symmetric deformation motion, and hopefully, this should be well fixed by the frequencies of CH3 and CD3 species. EXPERIMENTAL
Samples of Ga(CH3)3, Ga(CDj)3, Ga(CHD2)3 and Ga(CDz)~ (CHD2)3-x were made by reacting excess GaCI3 with the appropriately labelled zinc dimethyl, prepared from methyl iodide. For the mixed (CD3) (CHD2) compound, the starting material was a 1:3 mixture of CHD2I and CDaI. The do and d9 samples prepared for recording the Raman spectra contained traces of the zinc dimethyl starting material, judging from the presence of weak polarised lines coinciding with the known values of v2 and v3 in Zn(CH3)2 [8]. No i.r. bands arising from zinc dimethyl species could however be identified in the samples used for i.r. work. Gas phase i.r. spectra were recorded on a Nicolet 7199 FT-i.r. instrument at a resolution of 0.25 cm-', down to 500 cm-l. Annealed, polycrystalline films were studied at 78 K down to 200 cm- t on a Perkin-Elmer 225 spectrometer at a resolution of about 1.5 cm -1. Raman spectra were recorded on a Spex 1401 system at a resolution of ~ 2 c m - t with neon calibration and excitation by the 488nm argon line, except in the 2000 em-1 region, where 458 nm excitation gave freedom from a spurious line. RESULTS AND DISCUSSION
The frequencies recorded are listed in Tables 1 and 2. Figures 1-3 show features of the Raman spectra in the vCH and vCD regions. Agreement with the earlier data for Ga(CH3)3 [1] is in general good. However our solid state spectra show more structure in the ll00--1200cm -1 region, which suggests a higher degree of annealing in the case of our films. This region also constitutes one of two in which our assignments
IA~'~
D. C. MCKEANet al.
1406
Table 1. Vibrational wavenumbers (cm-1) and assignments for Ga(CH3)3 and Ga(CD3)5 Oa(CH3)3 i.r., gas
R, gas
i.r., cryst
3053 sh 3002 s, b d )
2994 bd, dp
2236"~ 2958 m
2919 s, bd 2919 2850 bd, sh 2390 w 1736 w 1732 w ~1440 w, bd
2913.3 vs, p 2857 w, bd
Ga(CD3)3 i.r., gas i.r., cryst
R, gas
2898 w 2848sh
2240 m'~ w, bd, dp
2195.3 2117.7 vs, p
2205 m J 2117 w 2062 sh
2222sh ) 2214 m 2205 sh 2104 wm
1465 w 1415 s ~ ) 1050 1007 J"
1456w t 1417 1196.5 m 1193 sh 1184 w
936.4 s ~934 sh 924.4 rn
935.0 s, p
vasMe vsMe 2~asMe va~GaC3 + 6sMe vsGaC3 +6sMe
1355 w 1208.9 vs, p
Assignment
6as Me 774+ 578'? 6sMe (a'l)
1213 R "1
1205,4 Q~. 1199 p J
1181.5 w 1172.5 m 1164 sh 1160 sh
947s
~922 sh 916.3 s 907 m 902 sh 896 sh
6sMe (e')
1057 w 774 vs 725 s 675 vw, sh
800 VW 602 vs
787 sh 1 709 sh ~ 744.2 s, bd 662 w, bd 646 sh 603 vw
578.4w, bd, dp
584 571 t
526.8 vs, p
610sh 599 sh 582.5 s, bd 555vw } 541 w, sh
574.3s }560.7vs
526w, bd, dp
525.5 m
477.8 vs, p
154.7 w, bd, dp
132.7 w, bd, dp
528s
523 s } 515 sh 502 sh 490 vw, sh 474 w, sp 418 vvw
pMe (e') pMe (a~) pMe vtL~GaC3
vsGaC3 6GaC3 (e')
Also observed in Raman spectra: 511.6, 1179.4 (Zn(CH3)~); 464.8, 911.7 (Zn(CD3h).
Table
2. Vibrational wavenumbers Ga(CH D2)a
i.r., gas 3012 sh 2982 sh ~2971 sh 2950 max. m 2940 q?m 2932 max.m 2921 q, m 2899 sh 2849 sh ~2231 w, bd 2147 mw, as 1267 w, vbd 1151 wm 990 m 660 s, bd 547 s
R, gas
(cm-~)
Assignment CH2D2
2969 bd, vw 2940.3q, p
ranCH
2922 q, b 2919 bd, vw
v LCH
2229 vw, bd 2141.3vw, p
vsCD~
1149 w, bd 988.6 s, p 645 w, vbd 535 m, bd 481.7 s, p 139 m, bd
6CH(a")* ~3CH(a')* ~CD2 ~5CD2 vasGaC3 vsGaC3 6asGaC~
* Local symmetry.
Va~CD2
in
disagree with those in [1]. All the features here undoubtedly derive from 6sCH3 modes, 6asCH3 bands occurring only in the 1400 era- 1 region. The pattern in the solid state spectra is repeated in the Ga(CD3)3 spectrum between 936 and 896 c m - t where 6sCD3 modes may be expected to lie.
Assionments in the vCH and vCD region In the earlier discussion of the i.r. spectra in the gas phase E2"I,the weak sharp maximum at 2940 era- 1 in the spectrum of Ga(CHD2)3, with accompanying wings at 2950 and 2932 c m - 1 was taken to be the centre of a band due to transitions resulting from the component of (ap/arcH) parallel to the group axis, which represented the average frequency over the cycle of a bond whose momentary frequency in fact varied with internal torsional angle. The sharp peak at 2921 cm -~ was then interpreted as the transition between levels where the CH bond was at right angles to the skeletal plane, in accordance with the analysis of
Vibrational spectra of trimethyl gallium species
2700
2800
3000
2900
1407
3100
3200
A~" (c~ i )
Fig. 1. Raman spectrum of Ga(CH3)~ in vapour phase, 3100-28000m-1; A---1 (n/2); B--Ill (n/2),
I
C~o
2050
.
I
I
2100
2150
I
2200 .,'V~ (cm"~)
/
2250
I
I
2300
2350
Fig. 2. Raman spectrum of Ga(CD3)3 in vapour phase, 2350-2000cm-~; A--• (n/2); B---~I (n/2).
1408
D.C. MCKEANet al.
i
A
2BOO
I 2900
I 30O0
I 3100
Z~" (cm-i )
Fig. 3. Raman spectrum of Ga(CHD2)3 in vapour phase, 3100-2800cm-t; A J-i (n/2)', B--ll (~/2).
CAVAGNAT and LASCOMBE in the situations of CHD2NO2 and CHD2C6Ds [-4-]. The new spectra confirm this interpretation in three ways. Firstly, the contour of the presumed "average" band is quite similar to that due to the fi~CH3 mode at 1205.4 erain Ga(CH3)3, and therefore plausibly associated with an in-plane vibration.* Secondly, the same CHD2 i.r. spectrum has been observed in a sample of Ga(CHD2)x (CD3)3-;, in which the ratio of CD3 to CHD2 groups was 3:1, randomly distributed. This suggests the absence of coupling effects between adjacent CHDz groups in the original material, such that the spectrum seen in Ga(CHD~)3 characterises a single methyl group. Thirdly, the Raman spectrum of Ga(CHD2)3, Fig. 3, shows a strong, narrow, polarised line at 2940.3 cm -~, in exact agreement with the postulated average i.r. band centre at 2940 cm -1 together with a much weaker line at 2922 cm-1;1" coincident with the i.r. peak at 2921 cm-~. The resemblance to the Raman spectra of CHD2NOz and CHDzPh I-4-]is indeed striking.
*However the "PR" separations of both these bands are not in accord with the predictions from the GERHARDand DENNISONrelation [9]. Calculations of the type quoted in [-4] (Ref. 17) are clearly to be desired. "i'Thepolarisation orthis weaker line is in keeping with the analysis in [4] applied to the corresponding lines in CHD2NO2 and CHD2Ph. :l:In making a comparison between the Raman and i.r. spectra due to vCH3 bands it has to be remembered that v~CH3is probably weak or even missing in the i.r., as in BMe3 13].
Both i.r. and Raman spectra of the v C H bands of the CHD2 species show additional broad wings, only explicable on the basis of unresolved K type structure due to free internal rotation. These are best seen in the lil (n/2) Raman spectra, at 2969 and 2919 cm-1 In the i.r. bands of Ga(CH3)3 and Ga(CD3)3 a splitting of vasCH3 into two components is explicable using the SHEPPARD and WOODMAN theory [10-] in terms of a variation of the vasCH 3 force constant with respect to the skeletal plane. The lower va~CDa band at 2205 cm -1 has indeed some C type character, in agreement with the previous evidence in CHaXY2 type molecules that the CH bond is weaker when perpendicular to the skeletal plane, than when parallel to it [3, 4]. A similar splitting is evident in the Raman spectrum of Ga(CD3)3, Fig. 2, with two broad, poorly resolved bands at 2236 and 2195 c m - t. The ~I (n/2) spectrum of Ga(CHa)3, Fig. 1, provides some evidence to suggest that a similar pair of vasCH3 bands is found there also, the shoulder at 2994cm -~ representing the high frequency component, and the lower one coinciding with v~CH3, as postulated for the i.r. bands [2].:1: The poorer resolution of the vasMe doublets in the Raman spectrum is perhaps to be associated with the presence of unresolved AK + 2 transitions as well as AK + 1 ones,
If we relate the sharp features seen in the CHD2 species in both the i.r. and Raman spectra, to the CAVAGNAT and LASCOMBE treatment, and the variation in v~CH which the latter implies, to the likely bond length variation 1-11], we consider that our earlier
Vibrational spectra of trimethyl gallium species prediction of an 0.005 A variation in r0CH with internal rotation angle to be soundly based.* FORCE FIELD
This was calculated by refinement from the l'requencies of the do and d9 species. A conventional set of real symmetry coordinates was constructed from the usual planar" XYa [14] and CHsX ones [15], as in the earlier treatment of BMe3 [16]. The relative directions o1"the symmetry coordinates involved are identified in Table 3 by stating which off-diagonal G elements involved are positive, also as in [16]. For convenience, hydrogen atom positions were fixed as appropriate to the point group C3h. Although these might have been supposed to give rise to off'diagonal terms in the G matrix between a'l, a~ and e' species, such terms were in fact not present:l For simplicity, tetrahedral HCH angles were used, although the vi~CH versus ~ correlation predicts a value of about 10T [2]. The CH and GaC bond lengths were taken to be 1.098A [2"1 and 1.967A 1,17"1 respectively. No corrections were made to the observed frequencies tbr anharmonicity. Uncertainties throughout were assumed to be _ 1%. The experimental error was in fact much less than this except in the cases of vCH, vCD, 6~CH3 and ~asCD3 modes, here due in part to Fermi resonances involving v~CH3 and vsCD 3 levels, and in part to likely ~a~CH3 and fia~CD3 splittings. For the tbrmer, rough corrections for the interactions with 2~asCHs and 26~CD~ levels were made of 19 and 7 c m - t respectively, while for the latter, and also for vasCH 3 and. raseD3 modes, "average" values were adopted. The effects of these assumptions on the lower part of the vibrational spectrum and the skeletal constants, are insignificant. It was considered desirable however to improve the representation of coupling between v~,Me and cSa~Me,v~Me and pMe, v~Me and cS~Memotions, by introducing the corresponding interaction force constants, constrained to values found in the methyl halides [15]. Refinement then showed that three further interaction force constants could be reasonably well defined, the fi~Me/vGaC3 constants in the a't (F2s) and e' (F4.6) species, and the pMe/6~GaC3 constant (F57, e'). There was also a definite preference for a negative value of F67, the %sGaCs/cSasGaC3 constant. In view of the value of - 0.23 found for this constant in BMe3 [16] it was thought best to constrain it in GaMe3 to -0.10. This choice has little effect on the value determined lbr
*The similar variation of reCH (• in MeNO2 was earlier predicted to be 0.006 A [12J or 0.004 h [4]. The latter value is favoured by a recent ab initio calculation of 0.004 ~ using the 6-31O* basis set i"13"1,in contrast to an earlier 4-3 IG value of 0.006 A [llal, also confirmed in 1,'13[. tStrictly speaking, complex symmetry coordinates are needed for the degenerate species of C3h: cf. [18-].
1409
F66, the vasGaC3 constant, but rather more on F77, the skeletal bending one. It is interesting to find negative values of the 6,Me/vGaC3 constants very similar to those in BMe3 [16], the a'~ value, - 0.24(5) mdyn/A, lower than the e' one, -0.17(4). While the difference is within the quoted errors, we could not account for the crossing over of the a'~ and e' ~sMe frequencies from the do to the d9 species without it. The values determined for F33 (a~) and F6~(e'), lead to fc~c = 2.356(28), JG,c = 0.053(17) mdyn/A. These are not too different from the values from the approximate normal coordinate treatment of [1]. The error limits on Jc,ac cover the likely effect of uncertainty due to the constraint F6~ = - 0.10 mdyn/A. Decreasing F67 to - 0.20 increases fG,c by 0.015 mdyn/A. The set of a] and e' force constants is given in Table 3, and the error vector and predictions of l3C and 7 t Ga isotope frequency shifts in Table 4. A notable feature of the latter is the small shift of 1.8 cm - t on the vasGaC3 mode at 578 cm - t. This is very much less than the separation of ~ 13 cm- ~between the two maxima observed in the i.r. gas phase spectrum, and between the two peaks in the crystal spectrum. It is therelbre out of the question to assign these features to Ga isotope splitting, as did the authors in [1]. The appearance of a minimum in the gas phase spectrum is a not uncommon result of vibration-rotation interaction, in conjunction with appropriate ~ and ~ constants, as found for instance in BF3 [ 19]. The absurdity of attributing the crystal bands at 574.3 and 560.7 cm- 1 to isotopic splitting is further emphasized by the consideration that the isotope ahundancies would make the higher band stronger than the lower frequency one, which is the reverse of what is observed. The only acceptable explanation of the splitting is in fact a site group without a C3 axis. Other tbatures of the crystal spectra which suggest low symmetry, and a significant degree ot" intermolecular interaction, are the 6,CH3 and 3sCD3 regions, near 1200 and 940 cm-1 respectively. The striking resemblance between these two suggests that there is no crossing over of the a'~ and e' modes such as is evident in the gas phase spectra (a'l 1208, 935 c m - t; e' 1205.4, ~ 947 cm-1). The complexity of the pattern seen also indicates loss of degeneracy but, without isotopic dilution, it is not possible to say whether more than one type of site is involved. At all events, the effects of the crystal field on 6~ appear to be stronger than the intramolecular coupling. With respect to the nature of the CH bonds in the solid phase, the i.r. spectrum of the Ga (CHD2)I. (CD3)3-x sample showed a broad band of about 38 cm- ~ half width, centred at 2928 cm- ~, which on annealing narrowed slightly to an asymmetric shape about 30 cm- t wide, with maximum at 2935 cm- 1 and a shoulder at 2926 cm-1. It is therefore likely that a range of CH strengths is present also in the solid.
D. C. MCKEAN et al.
1410
Table 3. Force constants for GaMe3 a'~ and e' species*
e~
St = vsCH3, $2 = ~sCH3, $3 = vsGaC3; G23 positive F~I = 4.9625(422); F22 = 0.4554(127); F33 = 2.4627(330) FI2 = 0.15; F13 = 0.10; F23 = -0.2383(483). Sl = vasCH3, S 2 = vsCH3, $3 = 6asCH3, S,I. = ~sCH3, $5 = pCH3, $6 = va,GaC~, $7 = 6asGaC3; G46, G57, positive. FI 1 = 4.7777(576), F22 = 4.9625(422), F33 = 0.5357(66) F44 = 0.4435(125), Fss = 0.3656(159), F66 = 2.3023(393) F77 = 0.3358(423). El3 = --0.15, Fis = 0.10, F24 = 0.15, F26 = 0.10, F3s = -0.01, F,~6 = -0.1664(426), F~7 = 0.1518(353) F67 = --0,10.
G67
*Computed with MGa = 69.72, rGac ----1.967, rCH = 1.098 A, = tetr. In brackets, the standard deviation. Constrained force constants were borrowed from methyl halides [,15], except for Fo7 (see text). Stretching constants in mdyn/A, bending ones in m d y n / h / r a d 2.
Table 4. Observed and calculated wavenumbers (cm-1) lbr a't and e'. species robs*
~t
Av(71Ga)~
Av(J~C~)w
2900 1209 526.8 2110 935 477.8
- 23.8 -0.8 -0.4 + 17.8 +0.7 + 0.4
0 0 0 0 0 0
2.5 7.8 17.1 3.9 12.8 11.6
Ga(CH3)3
2961 2900 1440 1205.4 774 578 154.7
- 18.6 - 23.8 + 13.3 - 3.5 + 1.7 - 1.8 + 0.6
0 0 0 0 0.4 1.8 0.3
10.9 2.5 3.3 8.9 5.9 13.9 3.2
Ga(CD3 )3
2222 2110 1030 947 602 526 132.7
+ 13.7 + 17.8 -7.9 + 3.1 - 1.8 + 2.1 - 0.5
0 0 0 0. i 1.1 1.3 0.3
15.6 3.9 4.2 14.2 5.5 8.4 2.0
Ga(CH3 )3 Ga(CD3)3
eI
*This work, gas phase. All uncertainties + 1 ~o. robs - talc. ~.Av(69Ga- 7tGa). w - 13Ca).
Acknowledgements--We thank the SERC for the F.T.-i.r. facility, Dr. C. HUNTER for assistance in recording the i.r. gas phase spectra and a referee for helpful comments. REFERENCES
[,1] J.R. DURloand K. K. CHATTERJEE,.]. Raman Spectrosc. 11, 168 (1981). [2"] D.C. MCKEAN, G. P. McQUJLLAN, I. TORTO and A. R. MORRISSON, J. molec. Struct. 141,457 (1986). [3] D. C. McKEAN, H. J. BECHER and F. BRAMSIEPE, Spectrochim. Acta 33A, 951 (1977). [4] D. CAVAGNATand J. LASCOMBE,J. molec. Spectrosc. 92, 141 (1982).
[5] J. L. DUNCANand E. HAMILTON, J. molec. Struct. 76, 65 (1981). [-6] J. L. DUNCAN, R. A. KELLY, G. D. NZVELLINI and F. TULLINI, d. molec. Spectrosc. 98, 87 (1983). [7] J. L. DUNCAN, J. HARPER, E. HAMILTON and G. D. NIVELLINL J. molec. Spectrosc. 102, 416 (1983). [8] I. S. BUTLER and M. L. NEWBURY, Spectrochim. Aeta 33A, 669 (1977). [-9"] S.L. GERHARDand D. M. DENNISON, Phys. Rev. 43, 197 (1933). ['I0] N. SHEPPARDand C. M. WOODMAN,Proc. R. Soc. A313, 149 (1969). 111] D. C. McKr~N (a) Chem. Soc. Reo. 7, 399 (1978); (b)d. molec. Struct. 113, 251 (1984).
Vibrational spectra of trimethyl gallium species [12] D.C. MCKEANand R. A. WATT,J. molec. Spectrosc. 61, 184 (1976). [13] G. A. JEFFREY, J. R. RUBLE, L. M. WINGERT, J. H. YATESand R. K. McMULLAN, J. Am. chem. Soc. 107, 6227 (1985). 1-14] J. L. DUNCAN,d. molec. 3pectrosc. 13, 338 (1964). 115] J. L. DUNCAN,A. ALLANand D. C. MCKEAN, Molec. Phys. 18, 289 (1970).
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[16] H. J. BECHERand F. BRAMSIEPE,Spectrochim. Acta 35A, 53 (1979). The bending force constants here are scaled with the CH or BC bond lengths (unlike ours). 1"17] B. BEAGLEY,D. G. SCHMIDLmGand I. A. STEER,J. molec, gtruct. 21, 437 (1974). [18] A. ROOSTAD,B. N. CYvxn,S. J. CYVmand J. BRUNVOLL, J. molec. Struct. 35, 121 (1976). [19] J. L. DUNCAN,J. molec. Spectrosc. 22, 247 (1967).
0584--8539/87 $3.00+ 0,00 9 1987PergamonJournal~ Lid,
Vibrational spectra of trimethyl gallium species in relation to the force field and methyl group internal rotation D. C. MCKEAN,* G. P. McQUILLAN,* J. L. DUNCAN,* N. SHEPHARD,* B. MUNRO,* V. FAWCETT'["and H. G. M. EDWARDSt *Department of Chemistry, University of Aberdeen, Aberdeen AB9 2UE, U.K. and t School of Studies in Chemistry, University of Bradford, Bradford BD7 IDP, U.K. (Received 23 March 1987)
Abstract--Infrared and Raman spectra are reported for Ga(CH3)~, Ga(CD~)~ and Ga(CHD2)a in the gas phase. These were also examined in the i.r. spectrum in the solid phase at 78 K. The new Raman spectra of the CHD2 species strongly support earlier: i.r. evidence for CH force constant variation during free internal rotation of the methyl groups, from the presence of two bands at 2940 (vs) and 2922era- 1 (w) identifiedas due to v~vand vi~respectively. The observed a' and e' frequencies of the do and d 9 speciesare used to obtain a force field in which three interaction constants are well defined. The best value of the Ga-C stretching force constant is 2.356(28) mdyn A - 1. In the crystal phase at 78 K, the e' modes due to 6sMe and v~GaCa are split, indicating a site group symmetry lower than Ca. Gallium and carbon isotope frequency shifts are predicted.
INTRODUCTION
In this work we report i.r. and Raman spectra of trimethyl gallium species, which bear on the structure of the methyl group, its internal rotation, and on the magnitude of the G a - C stretching force constant. Most previous work on this molecule is summarised in a recent paper by DURIG and CHATTERJEE [1], who report also good quality i.r. and Raman spectra of Ga(CH3)3 in the gas, liquid and solid phases, together with a rudimentary force field. However the only study so far of isotopic species is that of MCKEAN et al. [2] who published and analysed i.r. vCH and vCD spectra of Ga(CH~)a, Ga(CDa)a and Ga(CHD2)3. The appearance of the bands seen was interpreted in terms of freely rotating methyl groups, with a variation in vCH force constant with internal rotation angle, the CH bond being weakest out of the skeleton plane, as earlier diagnosed in BMe3 [3]. The strong desirability of confirming these conclusions by use of the Raman spectrum is established by the work of CAVAGNATand LASCOMaE on similarly internally rotating methyl groups in CHD2NOz and CHD2C6D5 [4]. Our interest in the G a - C stretching force constant lies in anticipating the extension of ab initio treatments to the calculation of force constants for bonds involving metal and carbon atoms, which are of fundamental interest in chemistry. While the success of ab initio treatments of molecules involving atoms such as hydrogen, boron and carbon has been established by recent very precise experimental force fields, e.g. for C2H4., C2H6, B2H6 [5-7], their application to heavier atoms necessitates close monitoring of the validity of the approximations needed for so many electrons, and for this good quality experimental force fields are essential. It is of course not to be hoped that in a molecule as large as GaMe3 that the same quality of experimental
force field is likely to be forthcoming as is found for instance in ethane [6]. The principal coupling effect however which will influence the value of the metal--carbon stretching force constant, is that between G a - C stretching and methyl symmetric deformation motion, and hopefully, this should be well fixed by the frequencies of CH3 and CD3 species. EXPERIMENTAL
Samples of Ga(CH3)3, Ga(CDj)3, Ga(CHD2)3 and Ga(CDz)~ (CHD2)3-x were made by reacting excess GaCI3 with the appropriately labelled zinc dimethyl, prepared from methyl iodide. For the mixed (CD3) (CHD2) compound, the starting material was a 1:3 mixture of CHD2I and CDaI. The do and d9 samples prepared for recording the Raman spectra contained traces of the zinc dimethyl starting material, judging from the presence of weak polarised lines coinciding with the known values of v2 and v3 in Zn(CH3)2 [8]. No i.r. bands arising from zinc dimethyl species could however be identified in the samples used for i.r. work. Gas phase i.r. spectra were recorded on a Nicolet 7199 FT-i.r. instrument at a resolution of 0.25 cm-', down to 500 cm-l. Annealed, polycrystalline films were studied at 78 K down to 200 cm- t on a Perkin-Elmer 225 spectrometer at a resolution of about 1.5 cm -1. Raman spectra were recorded on a Spex 1401 system at a resolution of ~ 2 c m - t with neon calibration and excitation by the 488nm argon line, except in the 2000 em-1 region, where 458 nm excitation gave freedom from a spurious line. RESULTS AND DISCUSSION
The frequencies recorded are listed in Tables 1 and 2. Figures 1-3 show features of the Raman spectra in the vCH and vCD regions. Agreement with the earlier data for Ga(CH3)3 [1] is in general good. However our solid state spectra show more structure in the ll00--1200cm -1 region, which suggests a higher degree of annealing in the case of our films. This region also constitutes one of two in which our assignments
IA~'~
D. C. MCKEANet al.
1406
Table 1. Vibrational wavenumbers (cm-1) and assignments for Ga(CH3)3 and Ga(CD3)5 Oa(CH3)3 i.r., gas
R, gas
i.r., cryst
3053 sh 3002 s, b d )
2994 bd, dp
2236"~ 2958 m
2919 s, bd 2919 2850 bd, sh 2390 w 1736 w 1732 w ~1440 w, bd
2913.3 vs, p 2857 w, bd
Ga(CD3)3 i.r., gas i.r., cryst
R, gas
2898 w 2848sh
2240 m'~ w, bd, dp
2195.3 2117.7 vs, p
2205 m J 2117 w 2062 sh
2222sh ) 2214 m 2205 sh 2104 wm
1465 w 1415 s ~ ) 1050 1007 J"
1456w t 1417 1196.5 m 1193 sh 1184 w
936.4 s ~934 sh 924.4 rn
935.0 s, p
vasMe vsMe 2~asMe va~GaC3 + 6sMe vsGaC3 +6sMe
1355 w 1208.9 vs, p
Assignment
6as Me 774+ 578'? 6sMe (a'l)
1213 R "1
1205,4 Q~. 1199 p J
1181.5 w 1172.5 m 1164 sh 1160 sh
947s
~922 sh 916.3 s 907 m 902 sh 896 sh
6sMe (e')
1057 w 774 vs 725 s 675 vw, sh
800 VW 602 vs
787 sh 1 709 sh ~ 744.2 s, bd 662 w, bd 646 sh 603 vw
578.4w, bd, dp
584 571 t
526.8 vs, p
610sh 599 sh 582.5 s, bd 555vw } 541 w, sh
574.3s }560.7vs
526w, bd, dp
525.5 m
477.8 vs, p
154.7 w, bd, dp
132.7 w, bd, dp
528s
523 s } 515 sh 502 sh 490 vw, sh 474 w, sp 418 vvw
pMe (e') pMe (a~) pMe vtL~GaC3
vsGaC3 6GaC3 (e')
Also observed in Raman spectra: 511.6, 1179.4 (Zn(CH3)~); 464.8, 911.7 (Zn(CD3h).
Table
2. Vibrational wavenumbers Ga(CH D2)a
i.r., gas 3012 sh 2982 sh ~2971 sh 2950 max. m 2940 q?m 2932 max.m 2921 q, m 2899 sh 2849 sh ~2231 w, bd 2147 mw, as 1267 w, vbd 1151 wm 990 m 660 s, bd 547 s
R, gas
(cm-~)
Assignment CH2D2
2969 bd, vw 2940.3q, p
ranCH
2922 q, b 2919 bd, vw
v LCH
2229 vw, bd 2141.3vw, p
vsCD~
1149 w, bd 988.6 s, p 645 w, vbd 535 m, bd 481.7 s, p 139 m, bd
6CH(a")* ~3CH(a')* ~CD2 ~5CD2 vasGaC3 vsGaC3 6asGaC~
* Local symmetry.
Va~CD2
in
disagree with those in [1]. All the features here undoubtedly derive from 6sCH3 modes, 6asCH3 bands occurring only in the 1400 era- 1 region. The pattern in the solid state spectra is repeated in the Ga(CD3)3 spectrum between 936 and 896 c m - t where 6sCD3 modes may be expected to lie.
Assionments in the vCH and vCD region In the earlier discussion of the i.r. spectra in the gas phase E2"I,the weak sharp maximum at 2940 era- 1 in the spectrum of Ga(CHD2)3, with accompanying wings at 2950 and 2932 c m - 1 was taken to be the centre of a band due to transitions resulting from the component of (ap/arcH) parallel to the group axis, which represented the average frequency over the cycle of a bond whose momentary frequency in fact varied with internal torsional angle. The sharp peak at 2921 cm -~ was then interpreted as the transition between levels where the CH bond was at right angles to the skeletal plane, in accordance with the analysis of
Vibrational spectra of trimethyl gallium species
2700
2800
3000
2900
1407
3100
3200
A~" (c~ i )
Fig. 1. Raman spectrum of Ga(CH3)~ in vapour phase, 3100-28000m-1; A---1 (n/2); B--Ill (n/2),
I
C~o
2050
.
I
I
2100
2150
I
2200 .,'V~ (cm"~)
/
2250
I
I
2300
2350
Fig. 2. Raman spectrum of Ga(CD3)3 in vapour phase, 2350-2000cm-~; A--• (n/2); B---~I (n/2).
1408
D.C. MCKEANet al.
i
A
2BOO
I 2900
I 30O0
I 3100
Z~" (cm-i )
Fig. 3. Raman spectrum of Ga(CHD2)3 in vapour phase, 3100-2800cm-t; A J-i (n/2)', B--ll (~/2).
CAVAGNAT and LASCOMBE in the situations of CHD2NO2 and CHD2C6Ds [-4-]. The new spectra confirm this interpretation in three ways. Firstly, the contour of the presumed "average" band is quite similar to that due to the fi~CH3 mode at 1205.4 erain Ga(CH3)3, and therefore plausibly associated with an in-plane vibration.* Secondly, the same CHD2 i.r. spectrum has been observed in a sample of Ga(CHD2)x (CD3)3-;, in which the ratio of CD3 to CHD2 groups was 3:1, randomly distributed. This suggests the absence of coupling effects between adjacent CHDz groups in the original material, such that the spectrum seen in Ga(CHD~)3 characterises a single methyl group. Thirdly, the Raman spectrum of Ga(CHD2)3, Fig. 3, shows a strong, narrow, polarised line at 2940.3 cm -~, in exact agreement with the postulated average i.r. band centre at 2940 cm -1 together with a much weaker line at 2922 cm-1;1" coincident with the i.r. peak at 2921 cm-~. The resemblance to the Raman spectra of CHD2NOz and CHDzPh I-4-]is indeed striking.
*However the "PR" separations of both these bands are not in accord with the predictions from the GERHARDand DENNISONrelation [9]. Calculations of the type quoted in [-4] (Ref. 17) are clearly to be desired. "i'Thepolarisation orthis weaker line is in keeping with the analysis in [4] applied to the corresponding lines in CHD2NO2 and CHD2Ph. :l:In making a comparison between the Raman and i.r. spectra due to vCH3 bands it has to be remembered that v~CH3is probably weak or even missing in the i.r., as in BMe3 13].
Both i.r. and Raman spectra of the v C H bands of the CHD2 species show additional broad wings, only explicable on the basis of unresolved K type structure due to free internal rotation. These are best seen in the lil (n/2) Raman spectra, at 2969 and 2919 cm-1 In the i.r. bands of Ga(CH3)3 and Ga(CD3)3 a splitting of vasCH3 into two components is explicable using the SHEPPARD and WOODMAN theory [10-] in terms of a variation of the vasCH 3 force constant with respect to the skeletal plane. The lower va~CDa band at 2205 cm -1 has indeed some C type character, in agreement with the previous evidence in CHaXY2 type molecules that the CH bond is weaker when perpendicular to the skeletal plane, than when parallel to it [3, 4]. A similar splitting is evident in the Raman spectrum of Ga(CD3)3, Fig. 2, with two broad, poorly resolved bands at 2236 and 2195 c m - t. The ~I (n/2) spectrum of Ga(CHa)3, Fig. 1, provides some evidence to suggest that a similar pair of vasCH3 bands is found there also, the shoulder at 2994cm -~ representing the high frequency component, and the lower one coinciding with v~CH3, as postulated for the i.r. bands [2].:1: The poorer resolution of the vasMe doublets in the Raman spectrum is perhaps to be associated with the presence of unresolved AK + 2 transitions as well as AK + 1 ones,
If we relate the sharp features seen in the CHD2 species in both the i.r. and Raman spectra, to the CAVAGNAT and LASCOMBE treatment, and the variation in v~CH which the latter implies, to the likely bond length variation 1-11], we consider that our earlier
Vibrational spectra of trimethyl gallium species prediction of an 0.005 A variation in r0CH with internal rotation angle to be soundly based.* FORCE FIELD
This was calculated by refinement from the l'requencies of the do and d9 species. A conventional set of real symmetry coordinates was constructed from the usual planar" XYa [14] and CHsX ones [15], as in the earlier treatment of BMe3 [16]. The relative directions o1"the symmetry coordinates involved are identified in Table 3 by stating which off-diagonal G elements involved are positive, also as in [16]. For convenience, hydrogen atom positions were fixed as appropriate to the point group C3h. Although these might have been supposed to give rise to off'diagonal terms in the G matrix between a'l, a~ and e' species, such terms were in fact not present:l For simplicity, tetrahedral HCH angles were used, although the vi~CH versus ~ correlation predicts a value of about 10T [2]. The CH and GaC bond lengths were taken to be 1.098A [2"1 and 1.967A 1,17"1 respectively. No corrections were made to the observed frequencies tbr anharmonicity. Uncertainties throughout were assumed to be _ 1%. The experimental error was in fact much less than this except in the cases of vCH, vCD, 6~CH3 and ~asCD3 modes, here due in part to Fermi resonances involving v~CH3 and vsCD 3 levels, and in part to likely ~a~CH3 and fia~CD3 splittings. For the tbrmer, rough corrections for the interactions with 2~asCHs and 26~CD~ levels were made of 19 and 7 c m - t respectively, while for the latter, and also for vasCH 3 and. raseD3 modes, "average" values were adopted. The effects of these assumptions on the lower part of the vibrational spectrum and the skeletal constants, are insignificant. It was considered desirable however to improve the representation of coupling between v~,Me and cSa~Me,v~Me and pMe, v~Me and cS~Memotions, by introducing the corresponding interaction force constants, constrained to values found in the methyl halides [15]. Refinement then showed that three further interaction force constants could be reasonably well defined, the fi~Me/vGaC3 constants in the a't (F2s) and e' (F4.6) species, and the pMe/6~GaC3 constant (F57, e'). There was also a definite preference for a negative value of F67, the %sGaCs/cSasGaC3 constant. In view of the value of - 0.23 found for this constant in BMe3 [16] it was thought best to constrain it in GaMe3 to -0.10. This choice has little effect on the value determined lbr
*The similar variation of reCH (• in MeNO2 was earlier predicted to be 0.006 A [12J or 0.004 h [4]. The latter value is favoured by a recent ab initio calculation of 0.004 ~ using the 6-31O* basis set i"13"1,in contrast to an earlier 4-3 IG value of 0.006 A [llal, also confirmed in 1,'13[. tStrictly speaking, complex symmetry coordinates are needed for the degenerate species of C3h: cf. [18-].
1409
F66, the vasGaC3 constant, but rather more on F77, the skeletal bending one. It is interesting to find negative values of the 6,Me/vGaC3 constants very similar to those in BMe3 [16], the a'~ value, - 0.24(5) mdyn/A, lower than the e' one, -0.17(4). While the difference is within the quoted errors, we could not account for the crossing over of the a'~ and e' ~sMe frequencies from the do to the d9 species without it. The values determined for F33 (a~) and F6~(e'), lead to fc~c = 2.356(28), JG,c = 0.053(17) mdyn/A. These are not too different from the values from the approximate normal coordinate treatment of [1]. The error limits on Jc,ac cover the likely effect of uncertainty due to the constraint F6~ = - 0.10 mdyn/A. Decreasing F67 to - 0.20 increases fG,c by 0.015 mdyn/A. The set of a] and e' force constants is given in Table 3, and the error vector and predictions of l3C and 7 t Ga isotope frequency shifts in Table 4. A notable feature of the latter is the small shift of 1.8 cm - t on the vasGaC3 mode at 578 cm - t. This is very much less than the separation of ~ 13 cm- ~between the two maxima observed in the i.r. gas phase spectrum, and between the two peaks in the crystal spectrum. It is therelbre out of the question to assign these features to Ga isotope splitting, as did the authors in [1]. The appearance of a minimum in the gas phase spectrum is a not uncommon result of vibration-rotation interaction, in conjunction with appropriate ~ and ~ constants, as found for instance in BF3 [ 19]. The absurdity of attributing the crystal bands at 574.3 and 560.7 cm- 1 to isotopic splitting is further emphasized by the consideration that the isotope ahundancies would make the higher band stronger than the lower frequency one, which is the reverse of what is observed. The only acceptable explanation of the splitting is in fact a site group without a C3 axis. Other tbatures of the crystal spectra which suggest low symmetry, and a significant degree ot" intermolecular interaction, are the 6,CH3 and 3sCD3 regions, near 1200 and 940 cm-1 respectively. The striking resemblance between these two suggests that there is no crossing over of the a'~ and e' modes such as is evident in the gas phase spectra (a'l 1208, 935 c m - t; e' 1205.4, ~ 947 cm-1). The complexity of the pattern seen also indicates loss of degeneracy but, without isotopic dilution, it is not possible to say whether more than one type of site is involved. At all events, the effects of the crystal field on 6~ appear to be stronger than the intramolecular coupling. With respect to the nature of the CH bonds in the solid phase, the i.r. spectrum of the Ga (CHD2)I. (CD3)3-x sample showed a broad band of about 38 cm- ~ half width, centred at 2928 cm- ~, which on annealing narrowed slightly to an asymmetric shape about 30 cm- t wide, with maximum at 2935 cm- 1 and a shoulder at 2926 cm-1. It is therefore likely that a range of CH strengths is present also in the solid.
D. C. MCKEAN et al.
1410
Table 3. Force constants for GaMe3 a'~ and e' species*
e~
St = vsCH3, $2 = ~sCH3, $3 = vsGaC3; G23 positive F~I = 4.9625(422); F22 = 0.4554(127); F33 = 2.4627(330) FI2 = 0.15; F13 = 0.10; F23 = -0.2383(483). Sl = vasCH3, S 2 = vsCH3, $3 = 6asCH3, S,I. = ~sCH3, $5 = pCH3, $6 = va,GaC~, $7 = 6asGaC3; G46, G57, positive. FI 1 = 4.7777(576), F22 = 4.9625(422), F33 = 0.5357(66) F44 = 0.4435(125), Fss = 0.3656(159), F66 = 2.3023(393) F77 = 0.3358(423). El3 = --0.15, Fis = 0.10, F24 = 0.15, F26 = 0.10, F3s = -0.01, F,~6 = -0.1664(426), F~7 = 0.1518(353) F67 = --0,10.
G67
*Computed with MGa = 69.72, rGac ----1.967, rCH = 1.098 A, = tetr. In brackets, the standard deviation. Constrained force constants were borrowed from methyl halides [,15], except for Fo7 (see text). Stretching constants in mdyn/A, bending ones in m d y n / h / r a d 2.
Table 4. Observed and calculated wavenumbers (cm-1) lbr a't and e'. species robs*
~t
Av(71Ga)~
Av(J~C~)w
2900 1209 526.8 2110 935 477.8
- 23.8 -0.8 -0.4 + 17.8 +0.7 + 0.4
0 0 0 0 0 0
2.5 7.8 17.1 3.9 12.8 11.6
Ga(CH3)3
2961 2900 1440 1205.4 774 578 154.7
- 18.6 - 23.8 + 13.3 - 3.5 + 1.7 - 1.8 + 0.6
0 0 0 0 0.4 1.8 0.3
10.9 2.5 3.3 8.9 5.9 13.9 3.2
Ga(CD3 )3
2222 2110 1030 947 602 526 132.7
+ 13.7 + 17.8 -7.9 + 3.1 - 1.8 + 2.1 - 0.5
0 0 0 0. i 1.1 1.3 0.3
15.6 3.9 4.2 14.2 5.5 8.4 2.0
Ga(CH3 )3 Ga(CD3)3
eI
*This work, gas phase. All uncertainties + 1 ~o. robs - talc. ~.Av(69Ga- 7tGa). w - 13Ca).
Acknowledgements--We thank the SERC for the F.T.-i.r. facility, Dr. C. HUNTER for assistance in recording the i.r. gas phase spectra and a referee for helpful comments. REFERENCES
[,1] J.R. DURloand K. K. CHATTERJEE,.]. Raman Spectrosc. 11, 168 (1981). [2"] D.C. MCKEAN, G. P. McQUJLLAN, I. TORTO and A. R. MORRISSON, J. molec. Struct. 141,457 (1986). [3] D. C. McKEAN, H. J. BECHER and F. BRAMSIEPE, Spectrochim. Acta 33A, 951 (1977). [4] D. CAVAGNATand J. LASCOMBE,J. molec. Spectrosc. 92, 141 (1982).
[5] J. L. DUNCANand E. HAMILTON, J. molec. Struct. 76, 65 (1981). [-6] J. L. DUNCAN, R. A. KELLY, G. D. NZVELLINI and F. TULLINI, d. molec. Spectrosc. 98, 87 (1983). [7] J. L. DUNCAN, J. HARPER, E. HAMILTON and G. D. NIVELLINL J. molec. Spectrosc. 102, 416 (1983). [8] I. S. BUTLER and M. L. NEWBURY, Spectrochim. Aeta 33A, 669 (1977). [-9"] S.L. GERHARDand D. M. DENNISON, Phys. Rev. 43, 197 (1933). ['I0] N. SHEPPARDand C. M. WOODMAN,Proc. R. Soc. A313, 149 (1969). 111] D. C. McKr~N (a) Chem. Soc. Reo. 7, 399 (1978); (b)d. molec. Struct. 113, 251 (1984).
Vibrational spectra of trimethyl gallium species [12] D.C. MCKEANand R. A. WATT,J. molec. Spectrosc. 61, 184 (1976). [13] G. A. JEFFREY, J. R. RUBLE, L. M. WINGERT, J. H. YATESand R. K. McMULLAN, J. Am. chem. Soc. 107, 6227 (1985). 1-14] J. L. DUNCAN,d. molec. 3pectrosc. 13, 338 (1964). 115] J. L. DUNCAN,A. ALLANand D. C. MCKEAN, Molec. Phys. 18, 289 (1970).
1411
[16] H. J. BECHERand F. BRAMSIEPE,Spectrochim. Acta 35A, 53 (1979). The bending force constants here are scaled with the CH or BC bond lengths (unlike ours). 1"17] B. BEAGLEY,D. G. SCHMIDLmGand I. A. STEER,J. molec, gtruct. 21, 437 (1974). [18] A. ROOSTAD,B. N. CYvxn,S. J. CYVmand J. BRUNVOLL, J. molec. Struct. 35, 121 (1976). [19] J. L. DUNCAN,J. molec. Spectrosc. 22, 247 (1967).