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 frequ encies were performed for small Cu-containing complexes in an effort t o assess models of exchanged Cu ion sites in zeolites and to help inte rpret 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 fre quencies for Cu-bound mono- and dicarbonyl and mono- and dinitrosyl sp ecies lie in ranges consistent with experimentally observed frequencie s and confirm earlier assignments. Our results show a clear linear cor relation between bond length and frequency for both carbonyl and nitro syl 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 ex plicit countercharges producing secondary effects.