A finite expansion method for the calculation and interpretation of molecular electrostatic potentials

Because it is useful to have the molecular electrostatic potential as an element in a complex scheme to assess the toxicity of large molecules, efficient and reliable methods are needed for the calculation and characterization of these potentials. A multicenter multipole expansion of the molecular electron charge density calculated with a limited Gaussian basis set is shown here to have only a finite number of nonzero terms from which the molecular electrostatic potential can be calculated. The discrete contributions to the electrostatic potentials from the terms of this expansion provide a physically meaningful decomposition of the potential and a means for its characterization. With pyrrole as an example, the electrostatic potential calculated from this finite expansion of the electron density is compared to that obtained from exact calculations from the same wave function. Good agreement is obtained at distances greater than 1.5 A from any atom in the molecule. In contrast, rearrangement of the terms into an expansion corresponding only to Mulliken atomic charges and dipoles yields a decomposition that produces electrostatic potentials which agree less well with the exact potential. This discrepancy is attributable to the neglect of terms due to higher moments.     

PHOTOCATALYSIS, PHOTOREDUCTION, PHOTOOXIDATION AND PHOTOREDOX HYDROGEN TRANSFER REACTIONS OF METALLOPORPHYRINS

Abstract— Hydrogen transfer from tertiary amines to nitroaromatics is photocatalysed by Sn^IV and Ge^IV complexes of tetraphenylporphyrin. There are two photochemical steps, one involving hydrogen transfer to the catalyst, and the second being a hydrogen donation from the intermediate formed. The photoredox properties of para-substituted tin tetraphenyl porphyrin dichlorides can be correlated with the one electron oxidation-reduction potentials of the porphyrins and of the reduced intermediates. The steady-state concentration of intermediate and rate of product formation depend also on the electron donor and acceptor properties of, respectively, the amine and nitro-compound. Fluorescence results indicate that in both photochemical steps excited singlet states of porphyrins are involved and there is evidence for exciplex formation with ground state amine. Acetic anhydride has a cocatalytic effect but does not influence the quenching of porphyrin fluorescence by amine or the quertchirjg of the fluorescence of the intermediate by nitrobenzene.