Study of the orientation of thiourea adsorbed on aluminum oxide by tunneling spectroscopy. II. Comparison of experimental and calculated intensities

Experimental and calculated tunneling intensities were compared for several vibrational modes of thiourea [CS(${\mathrm{NH}}_2$ ${)}_2$] adsorbed on aluminum oxide. The partial charge model of Kirtley, Scalapino, and Hansma, as extended by the use of floating valence charges, was used to compute the theoretical intensities. The required partial charges on each atom were determined for each vibrational mode using a method developed by Momany and described in the preceding paper. The Coulomb potential resulting from point charges located at atom sites was fitted to the quantum-mechanical electrostatic potential of a molecule calculated from Hartree-Fock theory. Two charges were associated with each atomic site since an investigation of the effects of a vibrational mode pattern on the electrostatic potential of a molecule had shown that the potential could not be acceptably modeled with a single point charge located on each atom. One charge was used to represent the core charge of each atom and a second charge was used to represent the valence-charge cloud. In fitting the potential, the valence charge was allowed to move independently of the core charge during a molecular vibration. The motions of the two charges were found to be very different for hydrogen atoms. This ‘‘floating-valence-charge’’ model gave very reasonable agreement between the theoretical and observed relative intensities for the in-plane vibrational modes of thiourea. An acceptable set of out-of-plane force constants could not be found, which caused problems in the interpretation of the out-of-plane relative intensities.