Bond dissociation energies of OV+-O, V+-N, and VN+-NH3 measured by photodissociation.

A newly constructed cryogenically cooled ion photodissociation spectrometer has been used to measure the bond dissociation energies (BDEs) of D0(OV+-O) = 4.562(4) eV, D0(V+-N) = 4.688(12) eV, and D0(VN+-NH3) = 2.249(40) eV, where the estimated error limit is given in parentheses in units of the last quoted digit. The instrument employs a laser-ablation, supersonic expansion source to generate jet-cooled metal-containing ions that are mass selected and collected in a cryogenically cooled linear quadrupole ion trap. The cold cations are then irradiated with a pulse from a tunable optical parametric oscillator laser, given time to dissociate, and released from the trap. The mass-selected photofragment ion signal as a function of laser wavelength provides the photodissociation action spectrum. With the laser intensity reduced to eliminate two-photon processes, the measured predissociation threshold provides the BDE of the selected ion species, which is compared to the literature and to quantum chemical calculations conducted at the coupled cluster with single, double, and perturbative triple excitations [CCSD(T)], using a complete basis set extrapolation from aug-cc-pVXZ (X = T and Q) basis sets. The measured BDEs were combined in thermochemical cycles to derive the ionization energy IE(VO2) = 8.40(21) eV and additional BDEs of D0(V+-O2) = 5.490(4) eV and D0(N-V+NH3) = 4.97(13) eV. For VN, the BDEs of VN and VN+ combined with the ionization energies of V and VN satisfy a thermochemical cycle to high precision (within 12 meV), demonstrating the validity of predissociation thresholds as a means of measuring BDEs.