Remote substituent effects on bond dissociation energies of para-substituted aromatic silanes

UMLYP/6-31G(d) and ROMP2/6-311++G(d, 2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C6H4-SiH2X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-YC6H4-SiH3 was small, as the slope (rho(+)) of the BDE - sigma(rho)(+) regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C6H4-SiH2F was much stronger, whose rho(+) value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C6H4-SiH2Cl was also found to be strong with a rho(+) value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C6H4-SiH2Li was found to have a large and positive slope (+ 9.12 kJ/mol) against sigma(rho)(-). The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C6H4-SiH2X) as well as the product (4-Y-C6H4-SiH2. radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.