Issue 13, 2004

An investigation of the germylene addition reaction, GeH2 + C2H2: 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 acetylene, C2H2. 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 297–553 K. The reaction showed a very slight pressure dependence at higher temperatures. The high pressure rate constants (obtained by extrapolation at the three higher temperatures) gave the Arrhenius equation: log(k/cm3 molecule−1 s−1) = (−10.94 ± 0.05) + (6.10 ± 0.36 kJ mol−1)/RT ln 10These Arrhenius parameters are consistent with a fast reaction occurring at approximately 30% of the collision rate at 298 K. Quantum chemical calculations (both DFT and ab initio G2//B3LYP and G2//QCISD) of the GeC2H4 potential energy surface (PES), show that GeH2 + C2H2 react initially to form germirene which can isomerise to vinylgermylene with a relatively low barrier. RRKM modelling, based on a loose association transition state, but assuming vinylgermylene is the end product (used in combination with a weak collisional deactivation model) predicts a strong pressure dependence using the calculated energies, in conflict with the experimental evidence. The detailed GeC2H4 PES shows considerable complexity with ten other accessible stable minima (B3LYP level), the three most stable of which are all germylenes. Routes through this complex surface were examined in detail. The only product combination which appears capable of satisfying the observed lack of a strong pressure dependence is Ge(3P) + C2H4. C2H4 was confirmed as a product by GC analysis. Although the formation of these products are shown to be possible by singlet–triplet curve crossing during dissociation of 1-germiranylidene (1-germacyclopropylidene), it seems more likely (on thermochemical grounds) that the triplet biradical, ˙GeCH2CH2˙, is the immediate product precursor. Comparisons are made with the reaction of SiH2 with C2H2.

Article information

Article type
Paper
Submitted
16 Dec 2003
Accepted
03 Mar 2004
First published
16 Apr 2004

Phys. Chem. Chem. Phys., 2004,6, 3370-3382

An investigation of the germylene addition reaction, GeH2 + C2H2: Time-resolved gas-phase kinetic studies and quantum chemical calculations of the reaction energy surface

R. Becerra, S. E. Boganov, M. P. Egorov, V. I. Faustov, I. V. Krylova, O. M. Nefedov, V. M. Promyslov and R. Walsh, Phys. Chem. Chem. Phys., 2004, 6, 3370 DOI: 10.1039/B316465A

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