SEMIEMPIRICAL MOLECULAR ORBITAL THEORY AND MOLECULAR GEOMETRY .2. ANALYTIC PROCEDURES FOR CHARGE REDISTRIBUTION METHODS

Analysis has been carried out to obtain closed-form energy and electron distribution expressions for the first-row atom dihydrides as represented by extended-Hückel theory modified by self-consistent charge redistribution procedures, and by the similar but theoretically more soundly grounded SCF CND02 method. Previous examination of this group of molecules revealed deficiencies in the ability of two important variations of zeroth-order Hückel theory to represent energy upon geometry dependences. From the results obtained here, the specific way in which the charge imbalances established in the zeroth-order results are modified by the variations in electronegativities of the atoms so charged may be followed. Good approximations to the self-consistent energies and charge densities are obtained in both the Hückel and CNDO methods by familiar closed form summation procedures. Exactly how the original and the redistribution parameters interact to change the energy upon geometry dependence are revealed and the reasons why the Hückel redistribution procedure is inherently inferior to the SCF redistribution of the CND02 method are examined. The approximate cancellation of hydrogen-hydrogen electron and nuclear repulsions are recognized in the CNDO angle deformation problem and the influence of this cancellation on making the CNDO and extended Huckel procedures appear similar is discussed. © 1969, American Chemical Society. All rights reserved.