HEXAMETHYLBENZENE-TETRACYANOETHYLENE ELECTRON-TRANSFER - ACCEPTOR ANDDONOR ENERGETICS AND ELECTRONIC-STRUCTURE

We report results of ab initio calculations of the energies, geometrie s and molecular electronic structures of hexamethylbenzene (HMB) and t etracyanoethene (TCNE). These are the species that form the donor-acce ptor pair in on-going electron transfer experiments. Vertical and adia batic ionization potentials and vertical and adiabatic electron affini ties are reported for the relevant species: HMB, TCNE. HMB(+), and TCN E(-). The computations for HMB(+) are performed by taking into conside ration the Jahn-Teller effect. Comparisons of the energies and geometr ies with experimental data are made, and our computed energetic quanti ties for TCNE are in much better agreement with experimental data than previously computed quantities found in the literature. The charge di stribution analysis for the neutral and ionic species is carried out a ccording to several methods: orbital (Mulliken and Lowdin), spatial (a toms in molecules), and distributed multipoles. We find that the 10(-3 ) au electron density contours of the species HMB(+) and TCNE(-) defin e molecular volumes that differ very little from the corresponding vol umes of the neutral species. The ab initio molecular electrostatic pot entials in relevant molecular neighborhoods are computed for the neutr al and ionic species and they are compared with the electrostatic pote ntials derived from the charge distributions computed according to the four methods referred to above. Although the implications of the nono bservable character of atomic charges in molecules have been considera bly analyzed, our results are particularly well suited to illustrate t he consequences of different choices of atomic charges and their effec ts on the molecular electrostatic potentials, and we consequently offe r a brief discussion of such representations of the molecular electric charge distribution.