Infrared Photodissociation Spectroscopy of Benzene-V⁺(CO)_n "Piano Stool" Cations

Benzene-vanadium-carbonyl cations of the form bz-V⁺(CO)_n are produced in a pulsed nozzle laser vaporization source and studied by infrared photodissociation spectroscopy and density functional theory. Photofragmentation patterns indicate a stable tricarbonyl core ion. However, infrared spectroscopy and comparison to the predictions of density functional theory computations indicate that the coordination behavior is more nuanced. The small clusters contain a 16-electron tricarbonyl core ion, consistent with the fragmentation patterns. However, larger clusters with more CO ligands begin to form the tetracarbonyl ion with 18 valence electrons. The tricarbonyl and tetracarbonyl species have distinctly different infrared spectra. The asymmetric carbonyl stretch for both complexes is red-shifted from that of gas phase vanadium carbonyl, but the red shift is greater for the tetracarbonyl species.