Computation of adsorbate vibrational frequencies: methyl, methylidyne and methoxy adsorbed on the Al(111) surface

Abstract

A general strategy for computing adsorbate vibrational frequencies using cluster calculations is illustrated for methyl, methylidyne and methoxy adsorbed on an Al(111) surface. The method uses energy second derivatives rather than the numerical fitting procedures often used in cluster calculations and avoids having to make guesses at the form of the normal modes for the vibrating polyatomic adsorbate. Using energy derivatives also facilitates performing a convergence check on the adsorbate frequency values. The methyl and methylidyne computed frequencies are in good agreement with vibrational frequencies for the CH3I dissociation product observed by high resolution electron energy loss spectra (HREELS). The computed methoxy adsorbate frequencies are also consistent with HREELS experiments, but the optimized methoxy structure, while agreeing with chemical intuition, differs from the methoxy adsorption at the hcp threefold site recently determined by normal incidence standing X-ray wavefield absorption (NISXW) experiments.