On the alkali-induced vibrational frequency shift of CO chemisorbed on transition metal surfaces

LCGTO-LDF model cluster calculations have been performed to evaluate various explanations for the reduction of the stretching frequency of CO chemisorbed at alkali-promoted transition metal surfaces. Potassium ions and point charges were used to describe electrostatic interactions and to probe direct interaction occurring at promoted surfaces. It is concluded that the alkali-induced reduction of the CO stretching frequency is caused mainly by electrostatic interaction and, to a smaller degree, by increased backdonation. No direct orbital interaction is found between coadsorbed species. An all-electron calculation of the cluster Ni₈COK₂ yields a CO frequency shift of 590 cm^-1 below the gas phase value, in very good agreement with experimental results.