Adsorption of (CO)-C-12-O-16-(CO)-C-13-O-18 isotopic mixtures was used to establish polycarbonyl structures formed with Cu+ ions in Cu-ZSM-5. The principal Cu+((CO)-C-12-O-16)(2) dicarbonyls are characterized by nu(s) at 2178 cm(-1) and nu(as) at 2151 cm(-1). In excellent agreement with theory. Cu+((CO)-C-12-O-16)((CO)-C-13-O-18) mixed-ligand species were found to absorb at 2168 and 2065 cm(-1) Another kind of dicarbonyls, characterized by nu(s) at 2183 cm(-1), was also unambiguously detected. Tricarbonyls are characterized by two IR modes, at 2191 and 2166 cm(-1). Different bands of Cu+((CO)-C-12-O-16)(x)((CO)-C-13-O-18)(y) (x + y = 3) species were established. The use of isotopic mixtures also helped in the assignment of a shoulder at 2194 cm(-1) to Cu2+-CO adducts. (C) 2011 Elsevier B.V. All rights reserved.