DEPENDENCE OF THE HARDNESS OF ATOMS IN MOLECULES ON THE LOCAL ENVIRONMENT - AN AB-INITIO STUDY

The effect of the deformation of the electron cloud on the chemical ha rdness of atoms in molecules was studied by ab initio calculations at the CCSD(T)/6-31++G(d,p) level. The influence of the molecular environ ment generally results in an increased hardness of the atoms. The exte nt of this effect depends on the kind of coordination as well as on th e bond distances. Higher coordination results in higher atomic hardnes ses; a shortening of the bond distances results in an exponential incr ease of this parameter. The hardening coefficient of an atom in a mole cule as a function of the distance seems to obey an exponentional deca ying regime, of which the falloff parameter correlates with the atomic polarizability. The present approach allows to improve existing elect ronegativity equalization schemes by explicitly taking into account th e dependence of the hardness parameter on the molecular geometry.