Predissociation measurements of bond dissociation energies: VC, VN, and VS

The abrupt onset of predissociation in the congested electronic spectra of jet-cooled VC, VN, and VS has been observed using resonant two-photon ionization spectroscopy. It is argued that because of the high density of electronic states in these molecules, the predissociation threshold occurs at the thermochemical threshold for the production of separated atoms in their ground electronic states. As a result, the measured threshold represents the bond dissociation energy. Using this method, bond dissociation energies of D0(V C) = 4.1086(25) eV, D0(V N) = 4.9968(20) eV, and D0(V S) = 4.5353(25) eV are obtained. From these values, enthalpies of formation are derived as Δf,0KH°(V C(g)) = 827.0 ± 8 kJ mol(-1), Δf,0KH°(V N(g)) = 500.9 ± 8 kJ mol(-1), and Δf,0KH°(V S(g)) = 349.3 ± 8 kJ mol(-1). Using a thermochemical cycle and the well-known ionization energies of V, VC, and VN, our results also provide D0(V(+)-C) = 3.7242(25) eV and D0(V(+)-N) = 4.6871(20) eV. These values are compared to previous measurements and to computational results. The precision of these bond dissociation energies makes them good candidates for testing computational chemistry methods, particularly those that employ density functional theory.