Structure and infrared spectroscopy of group 6 transition-metal carbonyls in the gas phase: DFT studies on M(CO)n (M = Cr, Mo, and W; n = 6, 5, 4, and 3)

B3LYP-based density functional theory (DFT) calculations with effective core potentials (ECPs) (LANL2DZ) on M and 6-311+G(2d) all-electron basis function sets on C and O are used to interpret the symmetry characteristic vibrational absorption patterns of CO ligands in the "naked" coordinatively unsaturated transition-metal carbonyls M(CO)n-1 (M = Cr, Mo, and W; n = 4-6) observed by a time-resolved infrared absorption spectroscopy after the UV pulse laser photolysis of M(CO)6 in the gas phase. The UV photolysis results can be reasonably explained by the trends in the calculated bond dissociation enthalpies of M(CO)n-1-CO for group 6 metal carbonyls. M(CO)n-1 produced through one CO elimination from M(CO)n is found out to keep its parent skeleton, resulting in the structure with symmetry of C4v for M(CO)5, C2v for M(CO)4, and C3v for M(CO)3.