Theoretical studies of CO/Ni(100): Geometry, vibrational frequencies and ionization potentials for the on-top site

The chemisorption of CO on the (100) surface of Ni has been studied using an Ni₁₄ cluster and generalized valence bond (GVB) methods. CO is found to bond perpendicular to the Ni surface with optimized NiC and CO bond distances of 1.94 and 1.15 Å, respectively. The calculated NiCO bond strength is 29.7 kcal (experimental values 30-32 kcal). Vibrational frequencies are calculated to be 401 cm^-1 for NiC stretch, 327 cm^-1 for NiCO bend, and 2129 cm^-1 for CO stretch. This decrease of the CO frequency by 71 cm^-1 from the free molecule value is consistent with experiment based on self-consistent calculations of the positive ion states. We propose a new explanation for the loss of one PES peak upon chemisorption.