NATURAL RESONANCE THEORY - II - NATURAL BOND ORDER AND VALENCY

Resonance weights derived from the Natural Resonance Theory (NRT), int roduced in the preceding paper are used to calculate ''natural bond or der,'' ''natural atomic valency,'' and other atomic and bond indices r eflecting the resonance composition of the wave function. These indice s are found to give significantly better agreement with observed prope rties (empirical valency, bond lengths) than do corresponding MO-based indices. A characteristic feature of the NRT treatment is the descrip tion of bond polarity by a ''bond ionicity'' index (resonance-averaged NBO polarization ratio), which replaces the ''covalent-ionic resonanc e'' of Pauling-Wheland theory and explicity exhibits the complementary relationship of covalency and electrovalency that underlies empirical assignments of atomic valency. We present nb initio NRT applications to prototype saturated and unsaturated molecules (methylamine, butadie ne), polar compounds (fluoromethanes), and open-shell species: (hydrox ymethyl radical) to demonstrate the numerical stability, convergence, and chemical reasonableness of the NRT bond indices in comparison to o ther measures of valency and bond order in current usage. (C) 1998 Joh n Wiley & Sons, Inc.