EFFECTS OF STRUCTURAL-CHANGES ON ACIDITIES AND HOMOLYTIC BOND-DISSOCIATION ENERGIES OF THE H-N BONDS IN AMIDINES, CARBOXAMIDES, AND THIOCARBOXAMIDES

The equilibrium acidities in DMSO have been measured for acetamidine, benzamidine, N,N'-diphenylbenzamidine, N,N-diethylbenzamidine, diphenylmethanimine, guanidine, N,N'-diphenylguanidine, N,N'-diphenylurea, and N,N'-diphenylthiourea. Combination of the resulting pK(HA) values for these weak acids with the oxidation potentials of their conjugate bases gave estimates of their homolytic bond dissociation energies (BDEs). These acidities and BDEs are compared with those of the corresponding carboxamides and thiocarboxamides. The change in hybridization of nitrogen between NH3 and Ph2C = NH causes the acidities and BDEs to increase by about 14 and 9 kcal/mol, respectively. These changes are similar to the increases in gas-phase acidities and BDEs observed for the change in hybridization between CH3CH3 and CH2 = CH2 (12 and 12 kcal/mol, respectively). The BDE of the H-N bond in HN3 is about 25 kcal/mol lower than that in Ph2C = NH, despite the apparent similarities in hybridization. The acidities of the H-N bonds in acetamide, benzamide, and urea are 2.2, 4.6, and 2.2 kcal/mol higher than those of the H-N bonds in acetamidine, benzamidine, and guanidine, respectively, and their BDEs are 6, 5, and 7 kcal/mol higher. The acidities of the H-N bonds in thioacetamide, thiobenzamide, and thiourea are 9.6, 8.8, and 8.1 kcal/mol higher than those of the H-N bonds in acetamide, benzamide, and urea, respectively, and their BDEs are 17, 16, and 18 kcal/mol lower. The 6 kcal/mol lower BDEs for the H-N bonds in acetamidine and benzamidine than in ammonia point to the presence of resonance energy in HN.C(R) = NH radicals in contrast to its near absence in HN.C(R) = O radicals.