Correlation of bond orders and softnesses

An analytically established approximate linear relationship between covalen t bond orders (atomic charge-charge correlations) and canonical ensemble so ftness kernels (atomic charge how polarizabilities) has been studied in a s eries of Y-conjugated molecules and ions (BX3, CX2Y, NX2Y, SO3, PO3, X, Y = F, Cl, O). The proportionality coefficients, identified as mean excitation energies, depend on the nature of the atom or bond. The present calculatio ns seem to confirm the predicted relationship and reflect the plausible fac t that electrons flow most easily between covalently bonded atoms. Quite la rge softness matrix elements and covalent bond orders of about 0.2 were obs erved between nonbonded pairs of easily polarizable ligand atoms. (C) 2000 Elsevier Science B.V. All rights reserved.