Thermochemistry and structure of the cluster ions CH3+(CO)n and SiH3+(CO)n in the gas phase

The gas-phase clustering reactions of CH3+ and SiH3+ with CO were studied using a high-pressure mass spectrometer. For the cluster CH3+(CO)n, the acylium ion CH3CO+ is generated with n = 1, and it forms the core in larger clusters. Theoretical calculations predict that CH3CO+ is 58.6 kcal/mol more stable than the isomeric CH3OC+ ion. The interaction between CH3CO+ and CO is mainly electrostatic and the CH3CO+ ion is solvated preferentially by three CO ligands, giving the shell structure, 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 shell structure SiH3+(CO)2(CO)6(CO)n−8. Theoretical binding energies for both CH3+(CO)n and SiH3+(CO)n differ from the observed ones by less than 0.7 kcal/mol. The n = 3 and 4 geometries have a staggered conformation of CO coordinations relative to CH and SiH bonds.