Site of ionization of polyfunctional bases and acids .2. Ab initio electric field gradients at nitrogen, oxygen, phosphorus, and sulfur in neutral and ionized forms

The electric field gradients (efg) at nitrogen, oxygen, phosphorus, and sulfur have been investigated with ab initio theoretical methods for a series of monofunctional and polyfunctional bases and acids and their ionized forms. Absolute efg values of neutral species are compared with theoretical results from the literature and used to give estimates of NMR linewidths insolution. Efg changes occurring upon the acid-base process, expressed as effective nuclear quadrupolar coupling constants, are used to predict changes in the NMR relaxation rates of the pertinent quadrupolar nuclei (14N, 17O, 33S) upon ionization. This information, joined to relative proton affinities, allows one to predict the preferred ionization site in solution of a given polyfunctional acid or base and to establish criteria whereby such equilibria can be investigated by heteronuclear NMR.