VIBRATIONAL-SPECTRA, STRUCTURES, AND NORMAL-COORDINATE ANALYSIS OF AL-CO2 COMPLEXES ISOLATED IN SOLID ARGON

The codeposition of atomic Al and carbon dioxide molecules in argon matrices led to the formation of AlCO2 molecules, which are found to reversibly interconvert between two geometrical isomers. The low-temperature form presents a C(s) symmetry, with a large inequivalence of the two CO bonds. The higher temperature form has a ring structure in which the metal interacts symmetrically with the two oxygen atoms. Normal-coordinate analysis based on four isotopic precursors ((CO2)-C-12-O-16, (CO2)-C-13-O-16, (CO2)-C-12-O-18, and (COO)-C-12-O-16-O-18) and a harmonic model enable a determination of some molecular constants. The C(s) symmetry structure has CO bonds with force constants almost corresponding to those of double and single bonds (F(C = O) = 13.1 and F(C-O) = 6.75 mdyn angstrom-1) and a relatively strong Al-O bond (F(Al-O) = 2.2 mdyn angstrom-1), while the C2-nu symmetry structure has equally perturbed CO bonds comparable in stiffness to carbonate species and also two looser Al-O interactions (F(Al-O) = 1 mdyn angstrom-1). The OCO valence bond angles are estimated in the 120 +/- 5-degrees and 115 +/- 5-degrees ranges, respectively. Temperature studies of the relative population yield an enthalpy difference of 1.55 +/- 0.4 kJ/mol between the two isomeric forms. For larger Al clusters, reductive elimination is evidenced upon warming the sample above 30 K, yielding Al2O and, presumably, CO.