Nitranions and Their Precursors: Charge Density Rearrangements and 15N NMR Chemical Shift Changes

RPA LORG (random phase approximation, localized orbitals-local origin) calculations of the changes in the I5N nuclear magnetic shielding tensor occurring upon deprotonation of pyridinium cation, and of a number of nitranion precursors, shed light on the corresponding changes in the experimental isotropic I5N chemical shifts. The downfield shift of deprotonated nitrogens results from the dominance of the decrease in shielding in the molecular plane over the concurrent increase (or near constancy) of the out-of-plane shielding. Conversely, the upfield shift of pyridyl nitrogens, with respect to nitranion precursors, is due to the dominance of the increase in the molecular-plane shielding over the decrease in the out-of-plane shielding. The charge density rearrangements occurring upon deprotonation are rationalized in terms of the relevant RHF/6-3 1G+ charge density topologies, within the framework of the quantum theory of atoms in molecules. Deprotonation of a planar tricoordinated nitrogen increases its u population and allows an efficient release of its r charge to the ring r system. The u density made available upon deprotonation at nitrogen is to a great extent transferred to the whole molecular u framework. The removal of r charge allows the amino nitrogen to effectively conjugate to the ring ?r system or, in the case of pyrrole anion and pyridine, to realize r conjugation throughout the heterocyclic ring. These mechanisms are evidenced by changes in the Laplacian of the charge density, and in the preferred direction of charge accumulation along the N-C bond, following deprotonation. The correlation between the charge density redistribution mechanisms and the changes in shielding tensor components are highlighted. Empirical relationships between the electron populations and the I5N NMR shifts are investigated for both virial partitioning (VP) and Mulliken partitioning (MP) charges. I5N NMR shifts correlate with both u and r VP charges, but not with their sum. Correlations between anisotropies in shielding and anisotropies in charge