Gas-phase and solution-phase homolytic bond dissociation energies of H-N+ bonds ill the conjugate acids of nitrogen bases

The oxidation potentials of 19 nitrogen bases (abbreviated as B: six primary amines, five secondary amines, two tertiary amines, three anilines, pyridine, quinuclidine, and 1,4-diazabicyclo[2,2,2]-octane), i.e., E(ox)(B) values in dimethyl sulfoxide (DMSO) and/or acetonitrile (AN), have been measured. Combination of these E(ox)(B) values with the acidity values of the corresponding acids (PKHB+) in DMSO and/or AN using the equation: BDE(HB)(+) = 1.37pK(HB)(+) + 23.1 E(ox)(B) + C (C equals 59.5 kcal/mol in AN and 73.3 kcal/mol in DMSO) gave estimates of solution phase homolytic bond dissociation energies of H-B+ bonds. Gas-phase BDE values of H-B+ bonds were estimated from updated proton affinities (PA) and adiabatic ionization potentials (aIP) using the equation, BDE-(HB+)(g) = PA + aIP - 314 kcal/mol. The BDE(HB)(+) values estimated in AN were found to be 5-11 kcal/mol higher than the corresponding gas phase BDE(HB+)(g) values. These bond-strengthening effects in solution are interpreted as being due to the greater solvation energy of the HB+ cation than that of the B--. radical cation.