Substituent effect on N-H bond dissociation enthalpies of carbamates: a theoretical study

The present investigation deals with the study of the N-H bond dissociation enthalpies (BDEs) of the Y-substituted (NH2-C(=X)Y-R) and N-substituted ((R)(H)NC(=X)YH) carbamates (X, Y = O, S, Se; R = H, CH3, F, Cl, NH2), which have been evaluated using ab initio and density functional methods. The variations in N-H BDEs of these Y-substituted and N-substituted carbamates as the effect of substituent have been understood in terms of molecule stabilization energy (ME) and radical stabilization energy (RE), which have been calculated using the isodesmic reactions. The natural bond orbital analysis indicated that the electrode-localization of the lone pairs of heteroatoms in the molecules and radicals affect the ME and RE values depending upon the type and site of substitution (whether N- or Y-). The variations in N-H BDEs depend upon the combined effect of molecule stabilization and radical stabilization by the various substituents.