THERMOCHEMISTRY AND STRUCTURE OF THE CLUSTER IONS CH3-PHASE((CO)(N) AND SIH3+(CO)(N) IN THE GAS)

The gas-phase clustering reactions of CH3+ and SiH3+ with CO were stud ied using a high-pressure mass spectrometer. For the duster CH3+(CO)(n ), the acylium ion CH3CO+ is generated with n = 1, and it forms the co re in larger clusters. Theoretical calculations predict that CH3CO+ is 58.6 kcal/mol more stable than the isomeric CH3O+ ion. The interactio n between CH3CO+ and CO is mainly electrostatic and the CH3CO+ ion is solvated preferentially by three CO ligands, giving the shell structur e, CH3CO+(CO)(3)(CO)(n-4). The bond dissociation energy of SiH3+... CO is 34 kcal/mol. The cluster ion SiH3+(CO)(n) was found to have the sh ell structure SiH3+(CO)(2)(CO)(6)(CO)(n-8). Theoretical binding energi es for both CH3+(CO)(n) and SiH3+(CO)(n), differ from the observed one s by less than 0.7 kcal/mol. The n = 3 and 4 geometries have a stagger ed conformation of CO coordinations relative to C-M and Si-H bonds.