The electrostatic V+(OCO) complex has a vibrationally resolved photodissociation spectrum in the visible. Photodissociation produces V++CO2 (nonreactive pathway) and VO++CO (reactive pathway). Production of VO+ is energetically favored, but spin forbidden. One-photon dissociation studies confirm mode selectivity observed by Lessen [J. Chem. Phys. 95, 1414 (1991)]: excitation of one quantum of rocking motion enhances VO+ production by >30%. Branching ratio measurements in one-photon dissociation are extended to higher energy. The effect of OCO antisymmetric stretch vibrations on reactivity is investigated using vibrationally mediated photodissociation, in which the OCO antisymmetric stretch is excited at 2390.9 cm(-1). Vibrationally excited molecules are then dissociated in the visible. Seven vibronic bands are investigated, involving the antisymmetric stretch alone and in combination with the CO2 bend, the V+(OCO) stretch and rock. Exciting the antisymmetric stretch leads to a similar to 15% increase in the reactive VO+ channel, compared to other states at similar energy. Combination bands involving the antisymmetric stretch all show slightly higher reactivity. Electronic structure calculations were performed to characterize the dissociation pathways and excited electronic states of V+(OCO). The geometries of reactants, products, and transition states and relative energies of quintet and triplet states were determined using hybrid density functional theory; energies were also calculated using the coupled cluster with single, double and perturbative triple excitations method. In addition, time-dependent density functional theory calculations were performed to predict the excited electronic states of quintet and triplet V+(OCO). Spin-orbit coupling of quintet states to triplet states was calculated and used to compute intersystem crossing rates, which reproduce many of the observed mode selective trends. The V+-OCO stretch and OCO antisymmetric stretch appear to enhance reactivity by increasing the intersystem crossing rate. (c) 2008 American Institute of Physics.