An electron diffraction study of tetrakis(trifluoromethyl)germane, Ge(CF3)4

Journal of Molecular Structure, 51 (1979) 211-216 o Elaevier Scientific Publishing Company, Amsterdam -Printed

AN ELECTRON DIFFRACTION STUDY OF TETRAKIS(TRIFLUOROMETHYL)GERMANE,

in The Netherlands

Ge(CF&

HEINZ OBERHAMMER Znsfitut ftir Physikalische Germany)

und Theoretische

Chemie

der Universitb’t

Tiibingen

(West

REINT EUJEN FB 9 -Anorganische

Chemie,

Gesamthochschule

Wuppertal

(West Germany)

(Received 10 July 1978)

ABSTRACT The molecular structure of tetrakis(trifluoromethyl)germane was determined by electron diffraction of gases. The following geometric parameters (r: values) were obtained: C-F = 1.330 * 0.002 A, Ge-C = 1.989 * 0.005 A and LFCF = 108.0 + 0.3”. Two very crude models were used to allow for the large amplitude vibrations of the CF, groups. Both models led essentially to the same result: Td symmetry for the equilibrium configuration of the molecule, i.e. the CF, groups stagger exactly the Ge-C bonds, and a torsional force constant for the CF, groups of approximately 0.04 mdyn A. CND0/2 calculations agree surprisingly well with this experimental result. INTRODUCTION

For a structural chemist there are two interesting problems connected with a highly symmetric molecule such as Ge(CF&, i.e. the effect of fluorine substitution on the Ge-C bond and the equilibrium position of the CF3 groups together with their torsional properties. Yokozeki and Bauer [l] have pointed out that for molecules of the type A(CH3), with n = 1,2,3 the variation of the A-C bond length upon CH3/CF3 substitution is correlated to the electronegativity of the central A atom. According to this correlation, a considerable lengthening for the Ge-C bond of about 0.08-0.10 a is expected for Ge(CF& as compared to Ge(CH&. Systematic force constant calculations on CF,Ge compounds [2, 31 resulted in considerably smaller Ge-CF, stretch force constants with respect to corresponding methyl derivatives, and a bond length of 1.98-2.00 a was extrapolated for Ge(CF& [3]. For the equilibrium position of the CF3 groups we have to consider two possibilities. (1) The CF, groups stagger the Ge-C bonds, i.e. Td overall symmetry of the molecule. Because of nonbonded interactions between fluorine atoms belonging to different CF, groups, we expect a rather flat potential minimum, causing large amplitude torsional motions. (2) The CF,

212

groups are twisted away from the staggered position. The symmetry of the molecule would be T. This type of double minimum torsional potential was observed for perfluoroneopentane [ 41. EXPERIMENTAL

The sample of Ge(CF& was prepared from GeI, and excessive Hg(CF& [ 51. Separation of products was achieved by trap to trap condensation followed by glc using a 3/8 in. X 24 ft. 10% SE-30 on Chromosorb P column. The purity of the sample was proved to be better than 98% by means of glc, nmr (19F nmr: singlet at 49.2 ppm upfield from CFCIJ) and Raman spectroscopy. Melting point and vapor pressure were in good agreement with the values reported earlier [ 5b]. The diffraction intensities were recorded with Balzers Diffractograph KD-G2 [6] at two camera distances, 50 and 25 cm. Details of the experiment are listed in Table 1. The wavelength was determined from ZnO powder diffraction. For each camera distance four Kodak Electron Image plates (18 X 13 cm) were exposed. Two plates were selected for the structure determination. The standard procedures for data reduction and background refinement [ 71 were employed. The averaged molecular intensities for both camera distances are shown in Fig. 1. STRUCTURE

ANALYSIS

Comparison of the radial distribution functions of C(CF3)4 [4] and Ge( CF& (Fig. 2) indicates immediately that the torsional vibrations of the CF3 groups have much larger amplitudes in Ge(CF,)4. The exact treatment of the large amplitude motions for a molecule with four internal rotors, however, would increase the computing time enormously. Therefore we interpreted the experimental molecular intensities with two rather approximate models, performing two separate least squares analyses. Thereby a diagonal weight matrix was used with exponentially increasing elements for 1.4 < s < 4 A- l and 7 < s < 9 A-’ for the 50 and 25 cm data, respectively, and exponentially decreasing elements for 14 < s < 16.6 A-’ and 30 < s < 35 A-‘. All other elements had unit value. Intensities were used in steps of As = 0.2 a-’ TABLE 1 Details of experiment Nozzle-plate distance (cm) Sample temperature (“C) Nozzle temperature (“C) Camera pressure (torr) Nozzle diameter (mm) Exposure time (set) Electron wavelength (A ) s Range (A-‘)

50 -46 10 5 * 1o-6 0.3 20-45 0.04926( 1.4-16.6

1)

25 -46 10 1 * 1o-5 0.3 45-90 0.04925(l) 7-35

213

Fig. 1. Experimental

(0.0)

and theoretical

molecular

intensities and differences.

Ge CCF314

1 0

1

2

3

Fig. 2. Experimental

4

5

6

radial distribution

7

r(d)

function

and difference curve.

In the first case the two bond lengths, the FCF angle and the vibrational amplitudes for all interatomic distances (except the C ** *C distance) were refined, assuming Td symmetry for the equilibrium configuration..The torsional force constant of the CF, groups was estimated from the vibrational amplitudes in the following way. Analysis of the infrared and Raman spectra resulted in a force field for this molecule [3]. No information about the torsional motions, however, could be obtained from the vibrational spectra. With the known force field and systematic variation of the torsional force constant f,, the vibrational amplitudes were calculated (Table 2). The values for some distances between fluorine atoms belonging to different CF3 groups show a strong dependency on the torsional force constant. Comparing the spectroscopic amplitudes with the respective experimental values leads to a torsional force constant f, as low as about 0.04 mdyn A. With the force field of ref. 3, completed by this torsional force constant, the vibrational corrections Ar = r, - rt [8] were calculated. The concept of perpendicular amplitudes, however, results in unrealistically large corrections

214 TABLE 2 Spectroscopic vibrational amplitudes for various torsional force constants f, (mdyn A) and experimental amplitudes in A. Error limits are three times standard deviations. For numbering of atoms see Fig. 3.

f7 = 0.0 C-F Ge-C F,,*.*F,, Ge.0.F c ...c C, * . . F,, C;.*F,, F,,..*F,, F,,...F,, F 11.” F,, F,,**.F,,

0.046 0.052 0.059 0.072 0.118 0.157 0.103 0.222 0.152 0.173 0.095

f, = 0.08

0.203 0.103 0.296 0.190 0.323 0.095

f7 = 0.06

0.216 0,103 0.317 0.202 0.359 0.095

f7 = 0.04

Experimental ~ ~

0.240 0.103 0.355 0.222 0.422 0.095

0.048(3) 0.052( 8) 0.058(4) 0.085( 3) 0.120 (assumed) 0.269( 29) 0.128(23) 0.302(47) 0.257(45) 0.426(62) 0.138(37)

for interatomic distances that do not depend on torsion. For fr = 0.04 mdyn 8, the vibrational corrections Ar would be 0.055 and 0.099 A for the C-F and F - - - F,? distances, respectively. Therefore the torsional contributions to thi perpendicular mean square amplitudes were neglected for these distances [9]. Using the vibrational corrections given in Table 3, the r”, structure was determined in a final least squares analysis (Table 3). For the second approximate model the torsional position for the CF3 groups was allowed to deviate from the staggered configuration, i.e. 7 # 0 and T symmetry for the molecule. An optimized value of 7 = 17.3 + 4.3” was determined in a least squares analysis. The molecular intensities, however, are not very sensitive towards variations of torsional position of CF3 groups. The sum of the errors squared is reduced only by about 5% when this new geometric parameter is introduced. The other geometric parameters differ by

Fig. 3. Molecular model.

215 TABLE

3

Independent and dependent interatomic distances and vibrational corrections Ar = r, - r& Error limits are three times standard deviations. For numbering of atoms see Fig. 3.

r&(A) 1.330( 2) 1.989(5) 2.152(6) 2.759(6) 3.248(8) 3.536( 9) 4.415(9) 3.306( 13) 4.761(12) 3.944(10) 5.458(11)

C-F Ge-C F . ..F 12 G;...F c;**c, C;..F,, C, .

a*F,,

F,,*.*F,, F,, ..‘F,, F,,*.*F,, F,,**.F,,

ArtA) 0.004 0.004 0.005 0.006 -0.001 0.008 0.016 0.010 0.020 -0.020 0.023

LFCF = 108.0” f 0.3 Indices R,,

of resolution

= 7.6%

R,,

= 8.2%

less than 0.001 A and 0.04”, respectively, from the values obtained previously. Only the vibrational amplitudes change considerably, the new values no longer showing trends comparable to the spectroscopic values. If we assume again Td equilibrium configuration and interpret the experimental result as thermal deviation from this equilibrium configuration, i.e. reff - “*, we estimate from this torsional mean square amplitude a force constant off, = 0.04’_‘$,~ mdyn A. Thus both crude models lead to the same result: the molecular intensities can be interpreted with T,-jequilibrium structure for Ge(CF3), with a very low torsional force constant of approximately 0.04 mdyn A. Electron diffraction, however, is not capable of distinguishing between a very flat potential function with its minimum at r = 0” and a double minimum potential function with a very low barrier at r = 0. DISCUSSION

The effect of CHJCF, substitution on the Ge-C bond length is smaller than predicted from the correlation between this substitution effect and the electronegativity of the central atom [l] . In Ge(CF,), (rg(Ge-C) = 1.995 _+0.005 A) this bond is 0.050 f 0.006 A longer than in Ge(CH3)4 [lo] (r,(Ge-C) = 1.945 + 0.003 A). The geometry of the CF, groups agrees with the general trends that have been observed for CF, groups in trifluoromethyl halides, XCF, with X = F, Cl, Br and I [7b]. Corresponding to the electropositive character of germanium, the C-F bond is relatively long