A THEORETICAL-STUDY OF THE EFFECTS OF PROTONATION AND DEPROTONATION ON BOND-DISSOCIATION ENERGIES

Ab initio molecular orbital calculations indicate that the bond dissociation energies (BDE) for homolytic cleavage of CX bonds (X = C, N, O, F) are increased by protonation of the corresponding alkyl, amine, alcohol, or fluoride functional groups; the effect of deprotonation of these groups is rather small for saturated species, whereas for unsaturated ones deprotonation leads to large increases in the CX BDEs. The effects on the CC BDEs in CCX compounds are quite systematic: protonation of X increases the CC BDE, while the converse holds for deprotonation. Two types of correlation between bond lengths and homolytic bond dissociation energies are observed. Firstly, protonation and deprotonation lead to a normal correlation for the adjacent CC bonds: the bond length decreases as the BDE increases. Protonation, however, results in an anomalous correlation for the CX bonds: the bond length increases as the BDE increases. These observations are rationalized in terms of electronegativity, resonance stabilization, and competing heterolytic dissociation.