Theoretical models for the description of the IR frequency shifts of carbon monoxide interacting with silanol groups

Several theoretical models were tried to reproduce the nearly equal values of blue and red shifts of the CO vibrational frequency of surface species formed by the CO adsorption on silica and which were ascribed to SiOH⋯CO and SiOH⋯OC complexes. The adequacy of the theoretical methods applied was assessed on frequency shifts for the analogous complexes of CO with water and methanol. B3LYP has the advantage over MP2 in the reproduction of these shifts due to a better description of the CO dipole moment. However, B3LYP predictions for CO complexes with silanols R3SiOH (R = H,F,OH) give the 2:1 ratio of shifts, in contrast to the 1:1 ratio observed for surface species. The high red shift observed for surface species with SiOH⋯OC bonding may be reproduced only in the cases when the interaction of CO with atoms imitating lattice oxygens is present.