Geometry of the transition state of radical abstraction reactions involving Si-H, Ge-H, and Sn-H Bonds

Transition-state interatomic distances in the reactions C˙H₃+SiH₄, Si˙H₃+SiH₄, C˙H₃+GeH₄, and C˙H₃+SnH₄ are calculated by the B3LYP density functional and intersecting parabolas methods. A semiempirical algorithm is developed for the calculation of the Y...H and C...H distances in the transition state of the radical abstraction reactions R˙+YH involving silanes, germanes, and stannanes and the reverse reactions of silyl, germanyl, and stannyl radicals with hydrocarbons. This algorithm is used to calculate interatomic distances in these reactions. An analysis of the calculated data shows that the Y...H and C...H distances in these reactions depend on the following factors: the enthalpy of reaction, the radius of the Y atom (Y = C, Si, Ge, Sn), and four-electron repulsion during the attack of a radical on the C-H bond adjacent to the double bond. Empirical equations relating the interatomic distances to the enthalpy of reaction and to the Y-R bond length are set up.