Quest for the origin of basicity: Initial vs final state effect in neutral nitrogen bases

The problem of the origin of the intrinsic basicity of neutral nitrogen bases, as reflected in their gas phase proton affinities, is addressed and a simple solution is found. It is rooted in an intuitively appealing picture involving ionization of the base in question by pruning an electron, subsequent creation of the hydrogen atom with the incoming proton, and the formation of the homolytic chemical bond between a radical cation and the hydrogen. The role of the initial state (base) is mirrored by the ionization potential of the pruned electron given by Koopmans' approximation, whereas the contribution of the final state (conjugate acid) encompasses the electron affinity of the proton, the relaxation energy of the produced radical cation, and finally the homolytic bond association energy of the newly formed N-H bond. This dissection of the protonation process into three sequential steps has a high cognitive value, enabling classification of bases into three categories at the same time. The first is given by compounds such as ammonia and its alkylated derivatives, the basicity of which is dictated by the initial state effect. The second grouping is formed by those molecules in which the final-state effects decisively influence their basicity values such as, e.g., in methyleneimine and its amino derivatives, whereas the last category encompasses systems exhibiting basicities governed by an interplay between the initial and final-state properties. Phosphazenes belong to the latter set of compounds. Finally, the solvent effect in acetonitrile is considered and briefly discussed within the context of the isodensity polarized continuum model (IPCM). It is shown that a correct hierarchy of basicity in the NH3-n(Me)(n) series requires explicit account of the solvent effect. Although the present analysis is quite general, it should be particularly useful in discussing trends of changes in basicities of intimately related molecules.