ELEMENT EFFECTS ON THE HOMOLYTIC BOND-DISSOCIATION ENTHALPIES OF THE ACIDIC H-A BONDS IN HA(CENTER-DOT+) RADICAL CATIONS AND THEIR NEUTRAL PRECURSORS

Estimates of the homolytic bond dissociation enthalpies for reactions of the type HA(.+) --> H-. + A(+) (BDE(HA.+) values) have been made by using eq 1, which has been derived from a thermodynamic cycle (Scheme 2). The BDE(HA.+) values for the radical cations derived by loss of an electron from three N-H and four O-H weak acids, where N-H or O-H bonds are cleaved, have been found to be smaller than the BDE(HA) values for cleavage of those bonds in their parents by only 3-12 kcal/mol. In contrast, the BDE(HA.+) values for radical cations where an acidic C-H bond is cleaved have been shown to be typically 30-50 kcal/mol smaller than the BDE(HA) values for cleavage of the corresponding acidic C-H bonds in their parents. An analysis in terms of eq 1 has shown that the difference in energies for N-H or O-H vs C-H cleavage in HA(.+) radical cations lies in the relatively small energies required to remove an electron from the lone pairs on the nitrogen and oxygen heteroatoms and in the relatively low stabilities of the hypovalent divalent nitrogen and the monovalent oxygen cation products formed on N-H or O-H cleavage compared to those of the trivalent carbon cation products formed on C-H cleavage.