A vibrational analysis with Fermi resonances for methoxy adsorption on Cu(111) using ab initio cluster calculations

Abstract

An analysis of the vibrational frequencies observed with high-resolution electron energy loss spectroscopy (HREELS) and reflection adsorption infrared spectroscopy (RAIRS) for methoxy adsorbed at the threefold sites on the Cu(111) surface has been performed using ab initio cluster calculations. The OC stretch and the CH3 bending and rock modes can readily be assigned with the computed harmonic vibrational frequencies, but the three-peak structure experimentally observed in the CH stretching region is more challenging. The frequencies of the first overtone for the CH3 bending modes are found to be very close to the fundamental CH stretching frequencies suggesting that a Fermi resonance between the two types of modes might take place. We verify this origin for the three-peak structure in the CH stretching region by using a simple numerical differentiation procedure to estimate the energy third derivatives, which should be the principal cause of the coupling between the CH stretching fundamentals and the bending overtones. Using this procedure we compute three dipole active modes in the CH stretching region with a relative intensity pattern very similar to that observed in the HREELS and RAIRS. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 350–357, 2000