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, an dsulfur have been investigated with ab initio theoretical methods for a series of monofunctional and polyfunctional bases and acids and thei r ionized forms. Absolute efg values or neutral species are compared w ith theoretical results from the literature and used to give estimates of NMR line widths in solution. Efg changes occurring upon the acid-b ase process, expressed as effective nuclear quadrupolar coupling const ants, are used to predict changes in the NMR relaxation rates of the p ertinent quadrupolar nuclei (N-14, O-17, S-33) upon ionization. This i nformation, joined to relative proton affinities, allows one to predic t 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.