α-Substituent Effects on Si–H, P–H and S–H Bond Dissociation Energies

Abstract

CBS-Q and G3 methods were used to generate a large number of reliable Si–H, P–H and S–H bond dissociation energies (BDEs) for the first time. It was found that the Si–H BDE displayed dramatically different substituent effects compared with the C–H BDE. On the other hand, the P–H and S–H BDE exhibited patterns of substituent effects similar to those of the N–H and O–H BDE. Further analysis indicated that increasing the positive charge on Si of XSiH3 would strengthen the Si–H bond whereas increasing the positive charge on P and S of XPH2 and XSH would weaken the P–H and S–H bonds. Meanwhile, increasing the positive charge on Si of XSiH2· stabilized the silyl radical whereas increasing the positive charge on P and S in XPH· and XS· destabilized P- and S-centered radicals. These behaviors could be reasonalized by the fact that Si is less electronegative than H while P and S are not. Finally, it was demonstrated that the spin-delocalization effect was valid for the Si-, P- and S-centered radicals.