RELAXATION OF ELECTRON CHARGE-DISTRIBUTIONS AND FORCE-CONSTANTS IN POLYATOMIC-MOLECULES

Using the example of the compounds H2O, NH3, CH4, CH2, PH3 and SH2 the characteristic features of the electron charge distribution relaxatio n (CDR) induced by nuclear displacements and its relation to the magni tude of the diagonal and off-diagonal force constants in polyatomic mo lecules have been qualitatively examined using HF-LCAO wavefunctions. For the molecules with second-row atoms this relaxation is localized i n the region of the varying bond and this region seems to make the mai n contribution to the magnitudes of both diagonal and off-diagonal for ce constants. In contrast to generally made assumptions, the charge de nsity remains unaltered along the lines of fixed bonds and changes on the periphery of these bonds. In the molecules with third-row atoms th e surrounding atoms exert a much more significant influence upon the c haracter of the relaxation. Relaxation maps show the increasing role i n the relaxation process of the inner-core electron density polarizati on on heavy atoms and the increasing flexibility of the valence charge density. With the variation of the valence angle alpha = angle HAH, t he CDR shows common behaviour for all types of molecules, radicals and ions considered. The relaxation contribution to the angular force con stant is defined mostly by that relaxation in the inner region of the valence angle alpha that is consistent with simple qualitative assumpt ions.