Study of the orientation of thiourea adsorbed on aluminum oxide by tunneling spectroscopy. I. Determination of partial charges from molecular electrostatic potential calculations for thiourea and urea

Abstract

Ab initio electrostatic molecular potentials for urea [CO(${\mathrm{NH}}_2$ ${)}_2$] and thiourea [CS(${\mathrm{NH}}_2$ ${)}_2$] have been calculated with use of restricted Hartree-Fock theory. A nonlinear least-squares regression package was used to fit atomic partial charges to the potentials. Calculations were made with use of several different basis sets with and without polarization functions. Basis-set scaling factors calculated for each atom showed that the potential-derived (PD) charges were much more reliable than the Mullikan values. The effects of a vibrational mode on the calculated partial charges were also evaluated. Dipole moments were calculated with use of various basis sets and compared with observed values. The potential was also fitted by splitting the static charge determined for each atomic site into two portions. The core-charge portion was forced to follow the nuclear motion, whereas the valence-charge portion was allowed to float in a direction such that the best fit to the potential was obtained. This latter procedure provided reproductions of the calculated quantum-mechanical electrostatic potentials which were determined to be sufficiently accurate for electron-tunneling-intensity calculations. Comparison of experimental and calculated tunneling intensities for thiourea are presented in the following paper.