Microwave Spectrum af Trimethylgermane
227
Acknowledgment. The author wishes to express his gratitude to Miss IS. Flanigen for supplying the large majority of the zeolite samples used in this work and for
helpful suggestions to the discussion. He is also grateful to M. J. O'Hara and R. W. Larson for help in the experimentalwork.
.
Spectra and Struct ure of 0rga nagermanes XV.la Microwave Spectrum of Trimethylgermane
. C)urig,* M. M. Chen,lbY. S. Li, Depattment of Chemfstry, University of South Carolina, Columbia, South Carolina 29208
anid J. B. Turner U e p r f m e n f of Chemistry, Augusta College, Augusta, Georgia 30904 (Received June 5, 1972)
The rotational spectrum of trimethylgermane has been recorded from 18.0 to 40.0 GHz. The ground-state rota,tional constants have been determined for ten different isotopic species. An r, value of 1.532 0.001 has been determined for the GeH bond distance. With this value of the GeH distance and an assumed structure for the methyl group, the following two structural parameters were obtained: r(GeC) = 1.947 i 8.006 A and LCGeH = 109.3 f 0.1". The determined structural parameters are compared to the corresponding ones for other methyl-substituted germanes.
Introduction Microwave spectra and structures of propane2a and isobutane2b have been studied by Lide who made a comparison in their structures with that of ethane and concluded that the C-C bond distance is quite c0nstan.t in the simple hydrocarbon series. The CCC angle (111.15") in isobutarie was found to be -1" smaller than the corresponding angle in propane (112.4") whereas the tertiary C-H bond distance (1.108 A) was found to be slightly longer than that in the CH2 group of propane. The microwave spectra and structures of a series of methyl-substituted silanes, which included methyl~ilane,~ dimethylsilane* and tri~ethylsilane,~ have been studied. It was found that the Si-C bond distance remained essentially constant with a value of 1.867 A in these three molecules while the CEX angle in trimethylsilane (110" 10') is smaller than that in dimethylsilane (110" 59') and the tertiary Si--H bond distance (1.489 A) can be regarded as slightly longer than those in methylsilane (1.485 A) and dimethylsilane (1.483 A ) ~These two series of comparisons seem to provide consistent results about the constant 6-M (M = C or Si) bond distance, the smaller CMC angle, and the longer M-H bond distance in tertiary compounds than those in primary and secondary compounds. An investigation of the microwave spectrum and structure of trimethylgermane is expected to provide another series o f interesting comparisons with the methyl-substituted germanes since the microwave spectra and structures of methylgermane arid dimethylgermane have been studied by Laurie6 and Thomas and L a ~ r i erespectively. ,~ The present paper gives the results of our study of the microwave spectrum and structure of trimethylgermane. Isotopic species investigated have included (CH&GeH
and (CH&GeD with the naturally abundant isotopic species of the germanium atom. Experimental and Results Samples of (CH3)GeH and (CHg)&eD were prepared by the reduction of (CH3)3GeBr with LiAlH4 and LiAlD4, respectively, in benzene at -5" followed by distillation at -0" and vacuum line distillation at -90". The products were checked by their mid-infrared spectra in the gaseous state. The microwave spectrum of trimethylgermane was investigated with a Hewlett-Packard model 8460A MRR spectrometer in the K-band (18.0-26.5 GHz) and R-band (26.5-40.0 GHz) frequency ranges. Frequency measurements were carried out while the Stark cell was covered with Dry Ice. Frequency accuracy is estimated to be 0.05 MWz for the stronger lines, but the uncertainty is larger for the weaker lines. For all of the isotopic species studied the 3 2 and 4 3 transitional frequencies were observed in the R-band frequency range and behaved like those for a rigid symmetric rotor. Assignment was made based on the relative frequency spacing and the relative intensities expected for the different isotopic species of naturally occurring abundance. The assignment was fur-
-
-
(1) (a) For part XiV, see J. Phys. Chem., 76, 1558 (1972). (b) Taken in part from the thesis' of M . M. C. which is to be submitted to the Department of Chemistry in part.ial fulfillment of the Master of Science degree. (2) (a) D. R. Lide. Jr., J. Chem. Phys., 33, 15.14 (1960); (b) ibid., 33, 1519 (1960). (3) R. W. Kilb and L. Pierce, J. Chem. Phys., 27, 108 (1957). (4) L. Pierce, J. Chem. Phys., 31, 547 (1959). (5) L. Pierce and 0.W. Petersen, J. Chem. Phys., 33, 907 (1960). (6) V. W. Laurie, J. Chem. Phys., 50, 1210 (1959). (7) E. C . Thomas and V. W. Laurie, J. Ghem, Phys., 50,3512 (1969). The Journal of Physical Chemistry, V d , 77, No, 2, 1973
J. R. Durig, M. M. Chen, Y. S.Li, and J. B. Turner
228 TABLE I: Rotational Frequencies and Rotational Constants of Trimethylgermme in the Ground Vibrational State 3
-
2
4-3
2 8 ,778.96 ~ 38 372.22 28,746.09 38,328.79 38,308.00 28,795.09 38,287.06 38,246.64 27,942.68 37 256.72 27,915.48 37,220.91 27,902.00 37,203.40 27,889.64 37,186.25 37,155.50 I
~
8, MHz
4796.53 4791.10 4788.50 4785.88 4780.83 4657.09 4652.61 4650.42 4648.28 4643.79
TABLE ll: Rotational Frequencies and Constants of Trimethylgermane in an Excited Vibrational State
-
I, amu,
A2
105.36 105.48 105.54 105.60 105.71 108.52 108.62 108.67 108.72 108.83
ther confirmed hy the substitution method8 in which the constant coordinate of the Ge atom along the molecular symmetric axis in each specific molecule was obtained by different wayis of combining the various isotopic species for a particular parent molecule. No clearly resolved Stark component was observed with the various Stark field strengths accessible with our present experimental equipment because of the weak spectrum as well as the interference caused by the surrounding lines due to the molecules in excited vibrational states. In Table I are listed the rotational frequencies for trimethylgermane in the ground vibrational state. Also listed in Table I are the rotational constants which were calculated from the 4 3 transition because of the better frequency accuracy. The 2 I transition is expected to be in the K-band spectral region but it was too weak to be measured with our present experimental equipment. Next to each ground-state line there is a rather strong satellite line (see Table 11).This line is believed to arise from the corresponding rotational transition in the first excited state of the torsional modes, since the torsional modes for this molecule have the lowest frequency vibrations9 and the rotational frequency in this excited state is usually found on the lower frequency side of the ground-state linelo for symmetric tops of the type +-
--
The W e atom has a spin quantum number of 912; however, no hyperfine structure for the 73Ge species was resolved. From the half-width of the observed lines an upper limit of 3 MHz for the quadrupole constant can, therefore, be given to the 73Ge species of trimethylgermane. Structure Our present experimental information does not allow a complete structural determination; however, the r, (GeM) bond distance and the heavy atom parameters can be well determined since the coordinates of the Ge and H atoms cam be obtained by the substitution method8 and the skeleton citructure was found to be insensitive to the locations of the hydrogen atoms in the methyl groups. For ( CXI&74GeH, hotb the germanium and the hydrogen atoms are on the C axis and the former atom is calculated to have a c coordinate of 0.238 A and the latter one is determined to have a c coordinate of 1.770 A. Both atoms are located on the same side of the center of mass. Thus, the Ge-H bond distance is calculated to be 1.770 - 0.238 = 1.532 A with im estimated uncertainty of 0.001 A. This rs (Ge-H) distance is. used along with several different assumptionis of thr methyl group parameters in order to deThe Journal ofPhysical Chemistry, Vol. 77, No. 2, 1973
3-2
4'3
E , MHz
28,777.25 38,369.1a 4796.15 28,744.6 ~ ~ 3 2 5 . 9 84790.75 38,304.85 4788.1I 28,713.47 38,284.13 4785.52 38,243.71 4780.46 37,253.85 4656.73 37,218.15 4652.28 37,200.69 4649.91 27,887.97 37,183.54 4647.94
1. amu,A2
106.37 105.49 105.55 105.61 105.72 108.53 108.63 108.69 108.73
termine the Ge-C bond distance and CGeH angle by the least-squares fit to the observed rotational constants of the ten isotopic species. For this calculation the assumption was made that the symmetric axis of each methyl group coincides with the CGe axis. This assumption is probably reasonable since the germanium atom has no nonbonded electron pairs.ll In Table 111 are summarized the results of our calculation. In column two of Table 111, the assumed methyl group structure is identical with that found for trimethylsilane. In earlier microwave studies3y4 it has been found that the methyl group parameters did not vary significantly among the various methyl-substituted silanes. Also, the methyl parameters were found to be similar between trimethylsilane and isobutane.2b Based on these data, it is assumed that the methyl groups in trimethylgermane have structural parameters similar to those found for the corresponding parameters in trimethylsilane within the stated limits of 0.005 A for the C-H bond distance and 0.2" in the HCGe angle. From the first-order approximation and the results listed in Table HI, the variation of the Ge-C distance with respect to the HCGe angle and the CH bond distance were found to be 0.003 A per degree and 0.2 A per angstrom, respectively. The HGeC angle is essentially unchanged with respect to the stated variation in either the HCGe angle or the CH distance. Thus, one can readily see that ro (C-Ge) = 1.947 f 0.006 A and L C G ~ H= 109.3 f 0.1". The errors given represent the range of values when the methyl group parameters are allowed to change within the reasonably stated ranges. Discussion For the sake of easy comparison, we have listed the essential structural parameters of the methyl-substituted germanes in Table IV. It is interesting to note that the Ge-C bond distance in trimethylgermane as determined in our present study is the same as those in methylgermane and dimethylgermane, and the CGeC angle in trimethylgermane is not significantly smaller than that in dimethylgermane. This former result is in good agreement with those in the substituted methane and silane series. Unfortunately, there is not enough information for dimethylgermane to make a comparison of the Ge-H bond
(8) J. Kraitchman, Arner. J. Phys., 21, 17 (1953). (9) J. H. Mulligan, private cammunication. (10) D. R . Lide, Jr., and D. E. Mann, J. Chem. Phys., 29,914 ('1958) (11) L. Pierce and M. Hayashi, J. Chern. Phys., 55,479 (1961).
Microwave Spoctrurn of Trimethylgermane
229
TABLE I I 1: Struiclural IParameters OS Trimethylgermane
r(C-bl)pa A L-GBC I i , a deg
r ( G e - i i ) , bA r(Cie-C), A rL. HGeG, deg a Assumed
1
2
3
4
5
1.10 110,97 1.5318 1.946 109.3
1.095 110.97 1.5318 1.947 109.3
1.09 110.97 1.5318 1.948 109.3
1.09 109.7 1.5318 1.952 109.3
1.095 111.32 1.5318 1.946 '109.3
b r a structure
TABLE IV: Structure! Parameters of Methylgerrnanes Molecule
r(Ge-C),
A
L CGeC
r(Ge-H), A
(CH3)3Gel-1 (CH3)2GeH2 CH 3GeH 3
Ref -
l l _ _ _ l _
1.947 f 0.006 1.950 f 0.003 1.9453 f 0.0005
109.6 0.1 110.0 f 0.50
distance in tri methylgermane. The difference of the Ge-H bond distance between trimethylgermane and dimethylgermane seems insignificant within the stated experimental errors. From Gn analysis of the infrared spectrum,12 the GeH bond distance in germanium hydride was found to be 1.527 9. 0.t3133 8. A direct comparison of this distance in the hydride cannot be made with the corresponding rs distance in the trimethylgermane. However, it is well known that the ro value obtained by fitting the experimental rotational constants is usually larger than the
1.532 f 0.001 1.529 f 0.005
_
_
l
~
-
-
-
_
l
Present work 7 6
corresponding rs value for the same molecule. Therefore, it can be concluded that the GeH distance is longer in trimethylgermane than in germanium hydride which is similar to the case of the silanes.
Acknowledgment. The authors gratefully acknowledge the financial support of this work by the National Science Foundation by Grants No. GP-20723 and GY9498. (12) L. P. Lindman and M. K. Wilson, J. Chem. Phys., 22, 1723 (1964).
The Journal of Physicai Chemistry, Vo/. 77, No. 2, 1973
