Indian Journal of Chemistry Vol. 32A, July \.993, pp. 557-560 A quantum chemical study of hydrogen abstraction from silane by methyl radical E J Padma Malar Department of Physical Chemistry, University of Madras, Guindy Campus, Madras 600 025, India Received 23 November 1992; accepted 5 January 1993 The potential profile for the hydrogen abstraction reaction from silane by methyl radical is com- puted quantum chemically. The all-valence electrons semiempirical SCF MO methods SINDOI and MNDo predict comparable activation barriers of 68.5 and 75.2 kJ mol-I respectively. The lower activation barrier of the title reaction as compared to that of the thermoneutral reaction CH) + CH4 -+ CH4 + CH) reflects the greater ease of abstracting a hydrogen atom from silane than that from methane. This is in agreement with the. lower bond dissociation energy in silane (377.5 kJ mol-I) than that in methane (431.4 kJ mol-t). Hydrogen atom abstraction from organic sub- strates by odd electron species such as inorganic and organic radicals, diradicals and excited states is a subject of many studies1•2• These reactions play an important role in the propagation of many chain reactions. A number of theoretical investi- gations of hydrogen abstraction reactions using quantum chemical techniques at varying levels of sophistication have contributed towards the un- derstanding of the potential profiles along the reaction paths, activation barriers, structures of the transition states, tunneling, etc.3 However, considerably less work has been reported4 on the abstraction of hydrogen from the silicon ana- logues of the substrates. Chatgilialoglu5 has re- cently shown that tris(trimethylsilyl)silane is a val- uable reducing agent for a variety of organic sub- strates. The key step in the reduction reactions in- volves hydrogen abstraction from the silane deri- vative: R + (Me3SihSiH -+ RH + (Me3Si)3Si' '" (1) The trialkyl silyl radicals are found to be highly reactive species towards various organic function- al groupS6. However, the corresponding silanes are rather poor hydrogen atom donors towards alkyl radicals under normal conditions 7• Substitu- tion of the silyl hydrogens results in significant improvement in the hydrogen donor ability of the organosilane8• A knowledge of the potential pro- files and activation barriers of these reactions will be helpful in the design of new radical reactions. Theoretical investigations of hydrogen abstrac- tion from silane are limited. Gordon has studied the abstraction of hydrogen by triplet methylene and silylene from silane3f. So far, there is no re- port of theoretical studies of hydrogen abstraction from silane by alkyl radicals. The aim of the pres- ent work is to investigate quantum-chemically the hydrogen abstraction reaction from the parent si- lane by methyl radical: CHj + SiH4 -+ CH4 + SiHj ... (2) Computational procedure Hydrogen atom abstraction from silane by the methyl radical has been studied using the all-val- ence electrons semiempirical SCF MO method SIND019• Our recent analysis of hydrogen ab- straction from methane by methyl radical using, the SINDOI method yields activation barrier and transition state geometry comparable to ab initio results3n. This method includes d-orbitals in the basis set of the second-row atoms9h; earlier stud- ies show good agreement with 6-31 G* and experimental results for compounds containing them 10,11. We have also studied the reaction using the widely applied MINDO/312 and MNDO\3 methods. The reaction was studied by treating the com- posite system as a supermolecule. Since the com- posite system contains an odd electron, comput- ations were performed using the unrestricted Har- tree-Fock formalism in the SINDOI and MIN- DO/3 methods. The MNDO method treats the open-shell system by the restricted Hartree-Fock formalism but the half-electron correction is in- troduced 13. The distance R between the methyl