A comparison of the reactivity of germylene and dimethylgermylene with some methylgermanes. Direct kinetic and quantum chemical studies

Abstract

Time-resolved studies of germylene, GeH₂, and dimethygermylene, GeMe₂, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to try to obtain rate coefficients for their bimolecular reactions with dimethylgermane, Me₂GeH₂, in the gas-phase. GeH₂ + Me₂GeH₂ was studied over the pressure range 1–100 Torr with SF₆ as bath gas and at five temperatures in the range 296–553 K. Only slight pressure dependences were found (at 386, 447 and 553 K). RRKM modelling was carried out to fit these pressure dependences. The high pressure rate coefficients gave the Arrhenius parameters: log(A/cm³ molecule⁻¹ s⁻¹) = –10.99 ± 0.07 and E_a =−(7.35 ± 0.48) kJ mol⁻¹. No reaction could be found between GeMe₂ + Me₂GeH₂ at any temperature up to 549 K, and upper limits of ca. 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ were set for the rate coefficients. A rate coefficient of (1.33 ± 0.04) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ was also obtained for GeH₂ + MeGeH₃ at 296 K. No reaction was found between GeMe₂ and MeGeH₃. Rate coefficient comparisons showed, inter alia, that in the substrate germane Me-for-H substitution increased the magnitudes of rate coefficients significantly, while in the germylene Me-for-H substitution decreased the magnitudes of rate coefficients by at least four orders of magnitude. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) supported these findings and showed that the lack of reactivity of GeMe₂ is caused by a positive energy barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper.