MASSSPECTRA OF VOLATILE HYDRIDES
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Using 17.1 kcal mole-' for AHto(SizHa)2and 38.8 for AHto(GezH6),2,3AHrO(H3SiGeH3) is calculated to be 27.8. The absolute value of this figure is subject to errors, mainly from the uncertainty in AHr0(Si&) resulting from the indefinite energy of the Si produced2 relative to the standard state and, to a lesser extent, from uncertainties in AHto(GezH6). The more interesting value AH3' should however be relatively free of systematic error. Using the heats of atomization selected by Cottrel18 and our previous values for the
Mass Spectra of Volatile Hydrides.
heats of formation of monosilane2and monogermane,2, the thermochemical bond energies derived are 46.4 for E(Si-Si), 38.2 for E(Ge-Ge), and 42.5 for E(Si-Ge). Spanier and MacDiarmid observed both the boiling and melting points of silylgermane to be very close to the mean of the values for disilane and digermane. They interpreted this as evidence for a very low polarity of the molecule, consistent with the similar electronegativities of silicon and germanium. The present results are in accord with such an interpretation.
IV. Silylgermane1.2
by Fred E. Saalfeld U. S. Naval Research Laboratory, Washington, D. C .
and Harry J. Svec Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa (Received December 87, 1966)
Mass spectra of silylgermane and the related hydrides silane, disilane, germane, and digermane have been studied. Positive ion fragmentation patterns were measured using 70-v electrons. From the appearance potentials of selected ions of these compounds the following thermochemical values have been calculated (in kcal/mole) : AHto(SiGeH6) = 7.5; D(HaSi-GeH3) = 99.9; D(HaSi-SiH3) = 84; D(H3Ge-GeH3) = 75.5; AHto(SiH3) = 50.4; AHro(GeH3) = 57; e.a.(SiH3) = -39.1; e.a.(GeH3) = -32.3.
Introduction The preparation and purification of silylgermane, SiGeH6, has been described by Spanier and MacDiarmid.3 The compound was identified from its mass spectrum but neither the fragmentation pattern nor any other mass spectral properties were reported. It is the purpose of this paper to present such data and report on thermochemical properties derived from them. Experimental Section Silylgermane was prepared by passing a 1:1 mixture of silane and germane, prepared individually by standard method^,^^^ with separated isotopes %Si (99.38%)
and (96.06%), through a silent electric discharge following the procedure of Spanier and MacDiarmid. Trap-to-trap distillation was employed to separate the products of the discharge reaction. A modified Con(1) Paper 111: F. E. Saalfeld and H. J. Svec, I ~ O TQ. Chem., 3 , 1442 (1964). (2) Presented at the 13th Annual ASTM E-14 Conference on Mass Spectrometry and Allied Topics, St. Louis, Mo., May 1965. (3) E. J. Spanier and A. G. MacDiarmid, Inorg. Chem., 2 , 215 (1963). (4) A. E. Finholt, A. C . Bond, K. E. Wilebach, and H. I. Schlessinger, J . Am. Chem. SOC.,69, 2692 (1947). (5) T. 8. Piper and M. K. Wilson, J . I n ~ r g .Nucl. Chem., 4, 22 (1957).
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FREDE. SAALFELD AND HARRY J. SVEC
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Table I : Fragmentation Pattern of Silylgermane Ion type % total ionization Ion type % total ionization
GeSiHa+ 5.14 GeSi+ GeHs+ 6.82 5.76
GeSiHs+ 2.74 GeH2+ 5.37
solidated Electrodynamics Corp. 21-103c mass spectrometer which has been describedl previously was employed for determining the mass spectral data. Operating parameters were the same as those described previously’ except that for the appearance potential measurements the ionizing voltage was reduced by 0.1v instead of 0.2-v increments.
Results and Discussion Fragmentation Pattern. The monoisotopic fragmentation pattern of the positive ions of silylgermane is given in Table I. These values have been corrected for the isotopic impurities in the silicon and germanium used in the preparation. This pattern agrees well with the fragmentation pattern calculated6 from the data obtained for silylgermane prepared with silicon and germanium of normal isotopic abundances. Monoisotopic spectra obtained for silane, disilane, germane, and digermane, all prepared using separated isotopes, 28Si and 7aGe, agree with our previously published Appearance Potentials. The appearance potentials of selected ions from the compounds studied and the postulated ion source processes are presented in Table 11. These values are based on the average of seven measurements for each ion and the uncertainty cited is the standard deviation. Krypton was used as an internal standard to calibrate the electron voltage scale. In order to estimate the accuracy of the measured appearance potentials, argon was always introduced with the krypton and the hydride and the ionization potential of argon was then determined with the voltage scale correction obtained from the krypton measurement. By this method the absolute accuracy was found to be 1 0 . 2 v. The relationship between appearance potentials and heats of formation and bond energy is well known, as are the errors in this type of measurement.’rg The measured appearance potential is assumed to be the heat of reaction and Hess’s law is applied to calculate the heat of formation provided appropriate ancillary data are available. Supporting data used in these calculations, taken from the work of Gunn and Green,lO are (in kcal/mole): AHro(SiH4) = 7.3, AHto(GeH4) = 21.6, AHfo(Si2H6)= 17.1, and AHtO(GezH6) = 38.7. The Journul of Physical Chemistry
GeSiHr + 19.32 GeH+ Ge+ 3.56 6.61
G&iH3 + 6.03 &HaC 3.77
GeSiHz+ 15.01 SiHz+ SiH+ 2.59 3.88
GeSiH + 9.47 Si + 4.02
Table I1 : Appearance Potentials and Postulated Ion Source Processes Appearance potential, ev (1) 10.20 f 0.03 (2) 10.26 3t 0.10 (3) 10.80 i.0.07 (4) 11.31 f 0.12 (5) 11.81 =t0.09 (6) 11.32 f 0.14 (7) 12.01 =t0.09
It has been assumed that the measured appearance potentials given in Table I1 do not contain excess kinetic energy. This assumption has not been tested adequately; therefore, the value for AHtO(SiGeH6) reported here should be taken as a lower limit since the reported appearance potentials may be somewhat greater than the true ones. However, the agreement between the values for the heat of formation of silylgermane calculated from the appearance potentials and postulated processes 6 and 7 of Table 11, 7.6 and 7.4 kcal/mole, respectively, is some support for the assumption of negligible excess kinetic energy. Thermochemical values for silylgermane and the silyl and germy1 radicals, calculated using equations similar to those described previously,l are shown in Table 111. The electron affinities (e.a.) of SiH, and GeH3 were computed by Neale’s method.” The negative value of the e.a. for these radicals indicates that they form negative ions nonspontaneously. The excellent agreement with our previous results’ for AHfO (SiH3+), aHfo(SiH3),I(SiH3), D(H3Si-SiH3), and D(H3Si-H) is gratifying and may be fortuitous, but it gives us greater confidence in the measurements. (6) J. H. Beynon, “Mass Spectrometry and its Application to Organic Chemistry,” Elsevier Publishing Co., Amsterdam, 1960, p 556. (7) F. E. Saalfeld and H. J. Svec, Inorg. Chem., 2 , 46 (1963). (8) F. E. Saalfeld and H. J. Svec, ibid., 2, 50 (1963). (9) F. H.Field and J. L. Franklin, “Electron Impact Phenomena and the Properties of Gaseous Ions,” Academic Press Inc., New York, N. Y.,1967,pp 80-88. (10) 9. R. Gunn and L. G. Green, J. Phya. Chem., 65, 779 (1961). (11) R. S. Neale, ibid., 68, 143 (1964).
MASSSPECTRA OF VOLATILE HYDRIDES
Table 111: Thermochemical Results (kcal/mole) AHfo(GeH3+)= 218 AHfo(GeH3)= 57 AHr0(SiH3+)= 227 AHf'(%&) = 50.4 AHio(GeSiHf,) = ' 7 . P I(GeH3) = 161 (7.0 ev) I(SiH3) = 176 (7.6 ev) I(GeSiH3) = 235 (10.2 ev)
D(H3Ge-SiHa) = 99.9" D(HsSi-SiH3) = 84 D(H3Ge-GeH3) = 75.5 D(H3Si-H) = 95.2 D(H3Ge-H) = 87.2 e.a.(SiHa) = -39.1 ( - 1 . 7 ev) e.a.(GeHt) = -32.2 (-1.4 ev)
Average of the values for processes 6 and 7 cited in Table 11.
However, using Franklin's method12 to calculate AHrO (GeSiHg) with pertinent data from Gunn and Greenlo for SizH6 and GezH6gives a value of 27.85 kcal/mole, which is 20.3 kcal/mole more positive than our value. Gunn and Kindsvater13 have measured AHro(GeSiH6) calorimetrically and their result agrees with the theoretical value. The discrepancy between the mass spectrometric value and the theoretical and calorimetric values may be explained by the fact that two or more of the radicals or ions involved in the processes shown in Table I1 are energetically nonequivalent due to electronic excitation effects or that the wrong ion source process was assumed. Unfortunately, with the instrument used for this study it is impossible to check either of these possible errors, although the ionization efficiency curves do not indicate a complex process occurring in the ion source. It is pointed out that the discrepancy between the mass spectral and the theoretical values can also be explained by the existence of a (d -P d)n interaction. In the case of silylgermane it is difficult to account for the enhanced stability on the basis of differences in electronegativities. However, these facts may be rationalized by the existence of (d + d)n bonds in silylgermane. As cited above, the calculated heat of formation is greater than the measured value for AHro(SiGeH6); however, Franklin's computation method'l was devised primarily for saturated hydrocarbons where (d + d)n interactions do not occur. The existence of such back-bonding should increase the stability of the molecule, therefore the actual heat of formation is expected to be less positive than the calculated value. Davidson, et uZ.,14 explain the planar structure of trisilylphosphine on the basis of a (d + d)n interaction. Goodman, et u1.,l5 also report participation of d orbitals in the Si-C bond in derivatives of phenylsilane and suggest general applicability when possible. Furthermore, Cox16 has shown by microwave spectroscopy that the Si-Ge bond length in silylgermane is 2.356 f 0.0005 A. Similar measurements by him indicate that the Si-Si bond length in monofluorodisilane, FH2Si-SiH3,
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is 2.330, showing very little evidence of double bond character. Cox16 interprets this fact as the lack of a suitable electron donor in this molecule for the silicon d orbitals. The sum of the covalent radii of silicon and germanium is 2.39 A, indicating definite bond shortening in silylgermane. The structure and thermochemical data strongly suggest that the 3d electrons of germanium are donated to the vacant 3d orbitals of silicon in silylgermane, forming partial double bonds. This bonding could account for the discrepancy between the mass spectral and calculated values for AHto(SiGeH~). The calorimetric value, although consistent with the intermediate melting and boiling points3 compared with disilane and digermane, is inconsistent with the measurement of the Ge-Si bond distance. It is difficult to assess the discrepancy between the mass spectral and calorimetric data in view of the past agreement between these two methods for the simple hydrides.7~8~10 While it is possible that one or more of the ions or radicals involved in Table I1 are energetically nonequivalent due to kinetic energy or electronic excitation effects as mentioned above, the mass spectral data do not indicate any such effects. On the other hand, the assumption relevant to the calorimetric value that the energy of the solid Sb-Si-Ge mixture produced by the explosion, relative to the separated elements, is the same for a mixture of disilane-digermane and silylgermane may not be entirely valid. The ionization potential of the germy1 radical is lower than that of the silyl radical, agreeing with the concept that the ionization potential should decrease within a chemical group as the atomic number increases. Internal consistency between the values for A H i O (SiGeH6) and D(H3SiGeH3), derived from processes 6 and 7 of Table 11, supports the assumption that the measured appearance potential values do not contain excess kinetic energy; however, there is no absolute assurance of this fact. Acknowledgment. The authors wish to thank Professor A. Peter Cox of the Department of Chemistry, The University, Bristol 8, England, who graciously furnished microwave data for silylgermane prior to its publication, and to Drs. S. R. Gunn and J. H. Kindsvater for supplying a copy of their paper on silylgermane before its publication. (12) J. L.Franklin, Ind. Eng. Chem., 41, 1070 (1949). (13) S. R. Gunn and J. H. Kindsvater, J . Phys. Chem., 70, 1750 (1966). (14) G. Davidson, E. A. V. Ebsworth, G. M. Sheldrick, and C. A. Woodward, Chem. Commun. (London), No. 7, 122 (1965). (15) L. A. Goodman, A. H. Konstam, and L. H. Sommer, J. Am. Chem. Soc., 87, 1012 (1965). (16) A. P.Cox, private communication of unpublished results, 1965.
Volume 70,Number 6 June 1966
