Bond dissociation energies of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi.

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi. It is argued that because of the high density of electronic states at the ground separated atom limit in these molecules, the predissociation threshold in each case corresponds to the thermochemical bond dissociation energy. The resulting bond dissociation energies are D(TiSi) = 2.201(3) eV, D(ZrSi) = 2.950(3) eV, D(HfSi) = 2.871(3) eV, D(VSi) = 2.234(3) eV, D(NbSi) = 3.080(3) eV, and D(TaSi) = 2.999(3) eV. The enthalpies of formation were also calculated as ΔH°(TiSi(g)) = 705(19) kJ mol, ΔH°(ZrSi(g)) = 770(12) kJ mol, ΔH°(HfSi(g)) = 787(10) kJ mol, ΔH°(VSi(g)) = 743(11) kJ mol, ΔH°(NbSi(g)) = 879(11) kJ mol, and ΔH°(TaSi(g)) = 938(8) kJ mol. Using thermochemical cycles, ionization energies of IE(TiSi) = 6.49(17) eV and IE(VSi) = 6.61(15) eV and bond dissociation energies of the ZrSi and NbSi anions, D(Zr-Si) ≤ 3.149(15) eV, D(Zr-Si) ≤ 4.108(20) eV, D(Nb-Si) ≤ 3.525(31) eV, and D(Nb-Si) ≤ 4.017(39) eV, have also been obtained. Calculations on the possible low-lying electronic states of each species are also reported.