Niobium oxochlorides in the gas phase: Quantum chemical calculations of the structure and relative stability of isomers

The gas-phase niobium oxochloride anions that result by the interaction between the finely dispersed stereoselective acetylene cyclotrimerization catalyst NbCl2(C (n) H (n) ) (n = 10-12) and atmospheric oxygen and moisture have been characterized by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. From the relative intensities of mass spectrometric lines, it has been deduced that, among the various niobium oxochloride species passing into the gas phase under the action of laser radiation, the most abundant monomer ion is NbO2Cl (2) (-) , the most abundant dimers are Nb2O4Cl (3) (-) and Nb2O3Cl (5) (-) , the most abundant trimer is Nb3O6Cl (5) (-) , and the most abundant tetramer is Nb4O8Cl (5) (-) . The gas phase also contains low concentrations of fragments corresponding to the pentanuclear anion Nb3O11Cl (4) (-) and the hexanuclear anion Nb6O15Cl (2) (-) . The geometric parameters and total energy of the stable isomers of the dinuclear and polynuclear niobium oxochloride anions existing in the gas phase has been calculated by quantum chemical methods, and their relative thermodynamic stabilities have been determined for different metal core configurations and different arrangements of oxygen and chlorine ions. The stereochemistry of the niobium oxochlorides is discussed.