Electronic and relaxation contribution to linear molecular polarizability. An analysis of the experimental values

This paper reports and comments on the experimental values of <(alpha)over bar>(e) and <(alpha)over bar>(r) of more than 650 molecules, where <(alpha)over bar>(e) and <(alpha)over bar>(r) are the mean values of the electronic polarizability tensor alpha(e) and of the relaxation polarizability tenser alpha(r), respectively. It is assumed that alpha(e) and alpha(r) sum up to alpha, the static polarizability of the molecule that can be directly measured through the dielectric constant. Thus, only values of alpha(e) independent of the frequency are reported. The principal targets of this paper are: (1) to collect from literature the largest amount of experimental data of <(alpha)over bar>(e) and <(alpha)over bar>(r) and try to group the data for classification and characterization of the electrical behaviour of molecules; (2) to find out which classes of molecules have unusual <(alpha)over bar>(e) and/or <(alpha)over bar>(r); (3) to interpret the data in terms of charge distribution; and (4) to provide a basis for comparing with experiment the values <(alpha)over bar>(e) and <(alpha)over bar>(r) calculated by quantum chemical methods. The main conclusions are: <(alpha)over bar>(e) increases with the number Z of electrons in the molecule so that the polarizability per electron (<(alpha)over bar>(e)/Z) is reasonably constant except for conjugated molecules (where it is slightly larger) and for halogenated molecules (where it is much smaller); <(alpha)over bar>(r) ranges from very low to rather large values, apparently without any correlation nor with the corresponding <(alpha)over bar>(e) or the number Z of electrons or the number nu of oscillators in the molecule; the largest values of <(alpha)over bar>(r) are reached for molecules having spherical shape and the largest ratio <(alpha)over bar>(r)/<(alpha)over bar>(e) is attained by molecules containing several halogen atoms. These data seem to suggest an additive scheme for the mean value of the electronic polarizability. On the contrary, <(alpha)over bar>(r) does not obey any additive scheme; large <(alpha)over bar>(r) and large <(alpha)over bar>(r)/<(alpha)over bar>(e) Seem to be connected to large charge fluxes associated to vibrations and therefore to large electron-phonon coupling. All the above considerations hold for the mean values of the polarizability tensors. A different behaviour of the single components of alpha(e) and alpha(r) is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.