Molecular structure of tetramethylgermane from gas electron diffraction

Journal of Molecular Structure, 245 (1991) 349-355 Elsevier Science Publishers B.V., Amsterdam

349

MOLECULAR STRUCTURE OF TETRAMETHYLGERMANE FROM GAS ELECTRON DIFFRACTION

fiVA CSAKVARI,

BfiLA ROZSONDAI

and ISTVAN HARGITTAI*

Structural Chemistry Research Group of the Hungarian Academy of Sciences, ELituGs University, P.O. Box 117, Budapest, H-1431 (Hungary) (Received 30 August 1990)

ABSTRACT The molecular structure of Ge (CH,), has been determined from gas-phase electron diffraction augmented by a normal coordinate analysis. Assuming tetrahedral symmetry for the germanium bond configuration, the following structural parameters are found: r,(Ge-C) = 1.958 kO.004 A, r,(C-H)=l.lll?O.O03Aand L(Ge-C-H)=110.7?0.2” (R=4.0%).Themethyltorsionalbarrier V, is estimated to be 1.3 kJ mol-’ on the basis of an effective angle of torsion 23.0 & 1.5”, from the staggered form, yielded directly by the analysis. The Ge-C bond length of Ge(CH,), is the same, within experimental error, as that of Ge (CSH5)4 and is in agreement with the prediction of a modified Schomaker-Stevenson relationship.

INTRODUCTION

Tetramethylgermane, as have most of the. other tetramethyl derivatives of main Group IV elements, has been the subject of several structural investigations. It has been thoroughly studied by spectroscopic methods [l-lo], including repeated reinvestigations of Ge (CH,), and its partially and entirely deuterated derivatives [ 3-71. The title compound has also been previously studied by electron diffraction, firstly by the visual technique in 1936 [ 111 and later, in 1975, by the sector-microphotometer method [ 121. A reinvestigation of Ge (CH,), was warranted for the following reasons. (1) There were discrepancies in the gas-phase electron diffraction and X-ray crystallographic Ge-C bond lengths in the series E (CH,), [ 11,121 and E (C,H,), [ 13,141, (E= C, Si, Ge, Sn) and between the E-C-H angles within the series of gas-phase molecules E (CH,), [ 12-211 (see Tables 3 and 4). (2 ) Differences existed between the quantum chemically computed r( Ge-C ) values [ 22,231. (3) There were some inconsistencies between the electron diffraction Ge-C *Author to whom correspondence

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0 1991-

Elsevier Science Publishers

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bond lengths [ 11,121 and those estimated venson relationship [ 241. EXPERIMENTAL

from a modified

Schomaker-Ste-

SECTION

A commercial Ge (CH,), sample (Alfa Inorganics) was used. Its purity was checked by gas chromatography in an HP 5880 A unit, and was found to be 99.88%. Electron diffraction patterns were recorded at 50 and 19 cm camera distances and 60 kV nominal accelerating voltage in a modified [ 251 EG-100A unit with a needle-valve nozzle [ 261 at room temperature. The electron wavelength was calibrated with a TlCl polycrystal sample [ 271. Experimental conditions are given in Table 1. Our usual procedures of data reduction [28,29] were applied, with an empirical blackness correction for Kodak electron image plates. The total experimental intensities and final backgrounds are available TABLE 1 Experimental conditions Camera distarye (cm) Wavelength (A) Nozzle temperature (K ) Exposure time (s ) Number of plates Scattering variable ranges, s (A-‘) As (A-‘)

50

19 0.049071 293

25-30 5 1.875-13.500 0.125

35-60 6 9.00-34.00 0.25

I

0

5

Fig. 1. Experimental ence curve (A=E-T)

10

15

20

25

30 s,i-1 35

(E) and theoretical (T) molecular intensities of Ge(CH,), for the final model with 7=23.0” (see Table 2).

and the differ-

351

----E -T

f(r)

T=OO CCCH

C-H

H H Ge-C

Ge H

C.‘C?‘H

CH

HH

C H T=OO

A 0

T=230

1

2

3

4

5cA

6

Fig. 2. Experimental (E) and theoretical (T) radial distributions of Ge (CH,), and the difference curves (d=E-T) for the 7=23.0” and 7=O” models; a=0.002 k. Contributions of some important internuclear distances are indicated.

as supplementary material.* The molecular intensities and radial distributions are shown in Figs. 1 and 2. STRUCTURE

ANALYSIS

The GeC, skeleton and the methyl groups were assumed to have Td and C,, symmetries, respectively. Fully staggered models were suggested for C (CH, ), and even for Ge(CH,), [ 121, while Nagashima et al. 1151,Si(CHsJ4 1161, [171found the best fit for Sn (CH,), with freely or nearly freely rotating methyl groups. Their so-called free-rotation model was constructed by mixing three equally weighted hindered-rotation models. In our model all methyl groups were allowed to rotate by the same angle of torsion z, which was taken as 0” for the staggered form (see Fig. 3 ) . Four independent parameters were chosen as follows: r (Ge-C ), r(C-H), L (GeCH), and z. Initial values of the geometrical parameters and the vibrational amplitudes (1) were obtained from the experimental radial distribution and from earlier electron diffraction studies [ 121. A normal coordinate analysis was carried out applying Christen’s program [ 301 extended to 18 atoms, using experimental frequencies for Ge (CH, )4, Ge (CH,),CD,, and Ge ( CD,)4 *Tables of total experimental electron diffraction intensities and final backgrounds, full listing of parameters and correlation matrix are available as Supplementary Publication No. SUP 26417 (6 pages) from the British Library Lending Division, Boston Spa, Wetherby, Yorkshire LS23 7BQ, U.K.

352

Fig. 3. Molecular model of Ge (CH,), and the numbering of atoms. TABLE 2 Molecular parameters of Ge(CHs)4 from electron diffraction (ED) and normal coordinate analysis (NCA)”

Tse

Independent

C-H Ge-C GeCH CGeC 7

Dependent

H-C-H Hll**.H12 Cl***C2 Ge.**H Cl..*H22 Cl.*.H23 Cl...H21

ED

ED

NCA

r,or L

1

0,

1

K

0.082 0.057”

0.003 0.002

0.079 0.050

0.074 0.006

0.147” 0.119d 0.121 0.231d 0.198 0.126

0.003 0.004 0.003 0.006 0.019 0.011

0.134 0.120 0.116 0.232 0.251 0.154

0.125 0.004 0.036 0.026 0.020 0.021

b

parameters

1.111 0.003 1.958 0.004 110.7 0.2 109.471 (assumed) 23.0 1.5 parameters

108.2 1.802 3.198 2.570 3.244 3.616 4.123

0.2 0.006 0.006 0.006 0.012 0.016 0.010

“Distances ( rg) , vibrational amplitudes (1) and perpendicular vibrational amplitudes (K) are in 8, angles in degrees. Numbering of atoms in Fig. 3. bEstimated total errors [ 281. c*dRefinedtogether.

from Biedermann et al. [ 61 completed by the a, and fi CH3 torsional frequencies given by Durig et al. [ 31. Calculated vibrational amplitudes were used as initial values and differences were assumed for groups of amplitudes (Table 2) in least-squares refinements, based on molecular intensities [ 311.Coherent and incoherent scattering factors were interpolated for our experimental ac-

353

celerating voltage from tabulated values [ 321. Unit weights were used for all data points. Contributions from all interactions, including all H* * *H, were taken into account. Models with z= 0 and r# 0’ have been tested. The best fit of theoretical to experimental curves was obtained with ~=23.0? 1.5” (R=4.0%). The z=O’ model could even be rejected at a 99.5% significance level using Hamilton’s formal statistical test [ 331. RESULTS AND DISCUSSION

The rg bond lengths, bond angles, nonbonded distances, experimental and calculated vibrational amplitudes, and estimated total errors are compiled in Table 2. The 1.958 If:0.004 A Ge-C bond length_(r,) lies between the results of Brockway and Jenkins (r (Ge-C) = 1.98 + 0.03 A) [ 111and Hencher and Mustoe ( rg ( Ge-C ) = 1.945 +-0.003 A) [ 121, and is in excellent agreement with the prediction of the Schomaker-Stevenson relationship as modified by Blom and Haaland, 1.96 A [ 241. We have recently reinvestigated the structure of tetraphenyltin and obtained a shorter bond distance, r,(Sn-C) =2.137 + 0.005 A [ 131 than reported earlier, 2.160 t 0.007 A [34]. Thus, with our longer r,(Ge-C) in Ge(CH,), and shorter rg( Sn-C) in SnPh4, the E-C bonds in E ( CHB )4 do not appear shorter than, but are indistinguishable, within experimental error, from those in EPh4 throughout the series E = Si, Ge, Sn (see Table 3). It is noteworthy that the gas-phase electron diffraction and X-ray crystallographic data for the tetraphenyl derivatives are in good agreement (see Table 3). Recently (1986) two quantum chemical publications reported the geometry of tetramethylgermane [ 22,231. Almlof and Faegri [ 221 used GelC3H5 basis set in high quality Hartree-Fock calculations, where relativistic effects were taken into account by the Breit-Pauli Hamiltonian and first order perturbation theory. Glidewell [23] used MNDO-UHF SCF calculations. The Ge-C bond lengths of 1.970 and 1.941 A were reported, respectively, the former being TABLE 3 Element-carbon bond lengths (A) in tetramethyl and tetraphenyl derivatives of main Group IV elements determined by gas-phase electron diffraction (ED, rg) and X-ray crystallography (XD) Molecule

ED

Molecule

ED

XD

C(CB,), Si(CI-W4 Ge(CH,), Sn(CI-W4

1.537(3)” 1.875(2)b 1.958(4)C 2.144(3)d

CPh, SiPh( GePh, SnPh,

1.871(4)e 1.960(4)” 2.137(5)’

1.553(3)f 1.878(2)g 1.957(4)h 2.144(14)’

“Ref. 15. bRef. 16. ‘This work. dRef. 17. “Ref. 13. Ref. 18. PRef. 19. hRef. 14. Ref. 20.

354 TABLE 4 Selected parameters diffraction”

of tetramethyl derivatives of main Group IV elements determined by electron

Molecule

r,(E-C)

r,(C-H)

L (E-C-H)

Ref.

C(CH,),

1.539(2) 1.537(3) 1.875 (2) 1.958(4) 2.144(3)

1.120(3) 1.114(8) 1.115(7) 1.111(3) 1.118(9)

110.0(4) 112.2(28) 109.2 (8) 110.7(2) 112.0(16)

21 15 16 This work 17

Si(CH3)4

Ge(CHd, Sn(CH,),

“Distances ( rg) are in Bngstrijms,angles in degrees. Uncertainties in parentheses refer to the last digit of the parameter and are the estimated total errors.

very close to the experimental value. (Note, however, that this comparison involves r, computed distances and rg experimental value.) The Ge-C-H angle of the title compound, 110.7 2 0.2’) is larger than that reported previously, 108.2 +-1.5’ [ 121, and is unambiguously larger than regular tetrahedral. The analogous angles in the rest of the series are rather uncertain (Table 4 ) . The methyl torsional barrier ( V,,) has been estimated by applying the method of Vilkov et al. [ 351. It is known that electron diffraction yields an effective structure as a result of molecular vibrations. Vilkov’s method estimates the torsional barrier from the effective torsional angle as compared with an assumed symmetrical equilibrium configuration. Such an assumption in our case implies r= 0”. The estimated V,,= 1.3 kJ mol-l is smaller than the torsional barrier of solid state Ge (CH,), which is reported to be 5.5 kJ mol-l [ 31. ACKNOWLEDGEMENTS

We are grateful to Dr. KLlman Ujszaszy (EGIS, Budapest) for checking the purity of the Ge (CH, ), sample and Mrs. Maria Kolonits for experimental work. Financial support was provided by the Hungarian National Scientific Research Foundation (OTKA, No. 132).

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