An investigation of the prototype germylene addition reaction, GeH2 + C2H4: Time-resolved gas-phase kinetic studies and quantum chemical calculations of the reaction energy surface

Abstract

Time resolved studies of germylene, GeH₂, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with ethylene, C₂H₄. The reaction was studied in the gas-phase over the pressure range 1–100 Torr, with SF₆ as bath gas, at 5 temperatures in the range 293–555 K. The reaction shows the characteristic pressure dependence of a third-body assisted association reaction. The high pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k^∞/cm³ molecule⁻¹ s⁻¹) = (−10.61 ± 0.08) + (5.37 ± 0.56 kJ mol⁻¹)/RT ln10. These Arrhenius parameters are consistent with a fast reaction occurring at approximately half the collision rate at 298 K. RRKM modelling based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure dependent curves for a choice of the critical energy, E₀ = 130 kJ mol⁻¹. Quantum chemical calculations (both DFT and ab initio G2//QCISD) of the GeC₂H₆ potential energy surface (PES), show that GeH₂ + C₂H₄ initially form a π-complex, which can either collapse to germirane or isomerise by a 1,2 H-shift to ethylgermylene with a relatively low barrier. This indicates that the observed pressure dependence must correspond the formation of two products, of which ethylgermylene is the more stable. It also shows that germiranes with 1-H substituents will thermally rearrange to ethylgermylenes with very low barriers. A detailed examination of the PES shows that other potential reaction products are unlikely to be formed. Thermochemical considerations show that germirane is less strained than silirane, and that divalent state stabilisation energies (DSSE) for germylenes are hardly greater than those for silylenes.