Theoretical study of CO and NO vibrational frequencies in Cu-water clusters and implications for Cu-exchanged zeolites

Local spin density functional theory calculations of vibrational frequencies were performed for small Cu-containing complexes in an effort to assess models of exchanged Cu ion sites in zeolites and to help interpret infrared spectroscopy results. Model complexes consisted of Cuⁿ⁺ (n = 0-2) ions with varying coordination to water ligands and to more realistic fragments of zeolites. Calculated CO and NO vibrational frequencies for Cu-bound mono- and dicarbonyl and mono- and dinitrosyl species lie in ranges consistent with experimentally observed frequencies and confirm earlier assignments. Our results show a clear linear correlation between bond length and frequency for both carbonyl and nitrosyl complexes. The (nominal) oxidation state of Cu in these complexes is the most important factor in determining CO and NO frequencies and bond lengths, with the local coordination of Cu and the presence of explicit countercharges producing secondary effects.