Theoretical Study of the Effects of Protonation and Deprotonation on Bond Dissociation Energies of Second-Row Elements: Comparison with First-Row Elements

Abstract

Ab initio MO calculations indicate that protonation of XHn to XHn+1+ increases bond dissociation energies (BDEs) for homolytic cleavage of CX bonds in CH3XHn, CH3−CH2−XHn, CH2CH−XHn, and CH⋮C−XHn compounds (X = N, O, F, P, S, and Cl). Deprotonation of XHn to XHn-1- (X = C, N, O, Si, P, and S) in saturated species generally results in small decreases in CX BDEs; in unsaturated substances, as a result of resonance, deprotonation yields large increases in the CX BDEs, X = Si being the exception. For adjacent CC bonds, protonation of XHn increases the CC BDEs because it produces larger electronegativity differences between bonded groups; deprotonation decreases the BDEs as a result of resonance effects. Two types of correlation between bond lengths and homolytic BDEs are observed, with second-row elements exhibiting bond length changes to a much lesser extent than first-row ones. Firstly, bond lengths of adjacent CC bonds increase as their BDEs decrease. Secondly, and apparently anomalously, CX bond lengths and homolytic BDEs both increase with protonation of XHn, except with X = P. The increase in BDE together with the increase in bond length is, in part, a result of the focus on homolytic BDEs: heterolytic cleavage of most protonated, and many deprotonated, species is actually preferred, in which case bond lengths generally increase as bond strengths decrease in accordance with the normally accepted trend.