Reactivity and Spectroscopic Studies of Selected Transition Metal Clusters.

Reactions of the group 5 transition-metal clusters (V_{rm n}, Nb _{rm n}, and Ta_ {rm n}) have been investigated in the gas-phase by using a fast-flow chemical reactor. Dissociative chemisorption of D_ 2 and N _ 2 is found to be size selective for these bare metal clusters, as well as for the cluster monoxides V_{rm n}O and Ta _{rm n}O and monocarbides V_{rm n}C. The effect of a single atomic impurity (C or O) decreases for clusters larger than 10 atoms and becomes negligible for clusters larger than 15-20 atoms. This loss of sensitivity to impurities is consistent with the loss of cluster size specificity, which occurs in the same size range. Evidence of structural isomerism is found for Nb_ 9, Nb_{11}, Nb _{12}, and Ta_ {12} which react with N_ 2 with biexponential reaction kinetics. The rate of chemisorption with ethane (C_ 2H _ 6) displays a steady increase with cluster size for V_{rm n} and Nb_{rm n}, but Ta_{rm n} clusters are only slightly reactive. Finally, the rate of addition of a second ligand to a metal cluster is considered, and examples are analyzed for the Nb_{ rm n}N_ 2 + N _ 2 case. Resonant two photon ionization spectroscopy has been used to investigate the optical spectra of jet-cooled MoO. By coupling the technique to a time-of-flight mass spectrometer, spectra of all isotopic species of MoO have been separately recorded. The observed spectra establish the ground state of MoO as ^ 5Pi_ {rm r}, deriving from the 2 delta^{2}12sigma^{1 }6pi^ 1 molecular configuration. Transitions from this ground state to the excited 2delta ^{1}12sigma^{1}6 pi^ 2, A^' ^ 5Delta_{rm r}; 2delta^{2}6 pi^{1}13sigma^ 1; B ^ 5Pi_{rm r} ; and 2delta^{1}12 sigma^{1}6pi^{1 }13sigma^ 1, B^ ' ^ 5Pi_{ rm r} states have been located, rotationally resolved, and anlayzed. Numerous other weak transitions have been observed in the visible, many of which must be due to S = 1 >=ts X^ 5Pi_{rm r} intercombinations. Several strong transitions are observed in the near infrared where the 2delta^{2}12sigma ^{1}13sigma^ 1, ^ 5Sigma^ - >=ts X ^ 5Pi and 2 delta^{2}6pi^ 2, A ^ 5Sigma^ + >=ts X ^ 5Pi band systems are expected. The general electronic structure of MoO is shown to be quite analogous to that of its congener, CrO, and the pattern of molecular orbitial energies is also quite comparable to that found for NbO.