ABINITIO AND SEMIEMPIRICAL MOLECULAR-ORBITAL STUDIES OF SURFACE ENHANCED AND BULK HYPER-RAMAN SCATTERING FROM PYRIDINE

Ab initio and semiempirical molecular orbital methods have been used to calculate the bulk and surface Raman and hyperRaman spectra of pyridine. Similar calculations for benzene are also reported. For both molecules, we compare the results with available bulk and surface experiments. In all calculations, the polarizability and hyperpolarizability derivatives are evaluated using normal coordinates that are obtained from Hartree-Fock (HF) calculations. The Raman results are somewhat sensitive to the basis set used in determining the normal coordinates, and the hyper-Raman spectra are strongly sensitive. The Raman and hyper-Raman spectra have been calculated at zero frequency using HF methods (GAUSSIAN 90), and finite frequency using INDO (intermediate neglect of differential overlap) methods (ZINDO). The frequency dependence of the results is found to be minor. Comparisons with experiment indicate that the inclusion of diffuse basis functions is crucial to the accuracy of the HF calculations. For such basis sets, the benzene Raman and hyper-Raman spectra provided by HF are in excellent agreement with experiment. Raman spectra from HF for pyridine also agree well, but hyper-Raman spectra agree only in part. Changes in Raman and hyper-Raman spectra calculated in going from bulk to surface generally agree with observations. The corresponding bulk and surface spectra from ZINDO are usually less accurate, especially the hyper-Raman spectra, and especially the out of plane modes. Estimates of the surface hyper-Raman enhancement factor are in the range 10(11)-10(12) for pyridine on silver electrodes.