Thermochemistry and structure of the cluster ions CH 3+(CO) n and SiH 3+(CO) n in the gas phase

Abstract

The gas-phase clustering reactions of CH 3 + and SiH 3 + with CO were studied using a high-pressure mass spectrometer. For the cluster CH 3 +(CO) n , the acylium ion CH 3CO + is generated with n = 1, and it forms the core in larger clusters. Theoretical calculations predict that CH 3CO + is 58.6 kcal/mol more stable than the isomeric CH 3OC + ion. The interaction between CH 3CO + and CO is mainly electrostatic and the CH 3CO + ion is solvated preferentially by three CO ligands, giving the shell structure, CH 3CO +(CO) 3(CO) n−4 . The bond dissociation energy of SiH 3 + … CO is 34 kcal/mol. The cluster ion SiH 3 +(CO) n was found to have the shell structure SiH 3 +(CO) 2(CO) 6(CO) n−8 . Theoretical binding energies for both CH 3 +(CO) n and SiH 3 +(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.