The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

The effects of protonation on the geometries and stabilization energies of prototypical • CH 2 X radicals (X = NH 2, OH, OCH 3, PH 2, SH, F, Cl, Br, CN, CHO, and NO 2 ) have been studied with the use of ab initio molecular orbital calculations at the G2 level. The proton affinities at X of the • CH 2 X radicals and the analogous substituted methanes, CH 3 X, are compared and the corresponding heats of formation calculated. For π-donor substituents (X = NH 2, OH, OCH 3, PH 2, SH, F, Cl and Br), protonation at X leads to considerable + C:XH character for both CH 3 XH + and • CH 2 XH +, resulting in substantially lower heterolytic bond dissociation enthalpies and longer C−X bonds. Protonation also strengthens the C−H bonds in CH 3 XH + and, in combination with the reduced interaction of the lone pair on X with the singly-occupied orbital at the radical center in the radicals, results in negative radical stabilization energies for • CH 2 XH + . For the π-acceptor substituents (CN, CHO, and NO 2 ), protonation enhances hyperconjugative electron donation from the methyl group to the π* orbital of X, thereby resulting in C−X bonds in CH 3 XH + and • CH 2 XH + that are shorter than those in the unprotonated species. This also leads to weaker C−H bonds and, together with enhanced delocalization of the unpaired electron in the radical, leads to positive radical stabilization energies for • CH 2 XH + . The proton affinities of the radicals with π-donor substituents are 30−70 kJ mol - 1 lower than those of their closed-shell counterparts. This may be attributed to the decreased availability of the lone-pair orbital(s) on X, resulting from interaction with the singly-occupied orbital at the radical center. The • CH 2 CHO and • CH 2 NO 2 radicals have proton affinities that are similar to those of their closed-shell counterparts because protonation takes place in a plane almost orthogonal to the singly-occupied orbital on C, and so there is less effect in going from CH 3 X to • CH 2 X.