Correlation effects in ionic crystals: I. The cohesive energy of MgO

High-level quantum-chemical calculations, using the coupled-cluster approach and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m clusters embedded in a Madelung potential. The results of these calculations are used for setting up an incremental expansion for the correlation energy of bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal is recovered. It is shown that only 60% of the correlation contribution to the cohesive energy is of intra-ionic origin, the remaining part being caused by van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure

Theoretical studies of inorganic and organometallic reaction mechanisms 13: Methane, ethylene, and acetylene activation at a Cationic Iridium Center

The oxidative-addition/reductive-elimination (OA/RE) reactions of methane, ethylene and acetylene with the CpIr(PH3)(CH3)+ complex are investigated by ab initio methods and density functional theory (DFT). The calculated results shows that the OA reaction from CpIr(PH3)(CH3)(agostic-alkane)+ to CpIr(PH3)(CH3)(H)(alkyl)+ is endothermic by 4.4 and 0.8 kcal/mol with a low barrier of 11.5 and 10.0 kcal/mol at the DFT-B3LYP and coupled cluster with singles and doubles (CCSD) levels of theory, respectively. The RE reaction from CpIr(PH3)(CH3)(H)(alkyl)+ to a β-agostic complex, CpIr(PH3)(alkyl)+, is exothermic with a low barrier of 7.1 and 9.2 kcal/mol. A strong stabilizing interaction between either ethylene or acetylene and CpIr(PH3)(CH3)+ leads to a high activation barrier (24-36 kcal/mol) for the OA processes of either one. Compared to ethylene, the OA/RE reaction of acetylene with CpIr(PH3)(CH3)+ complex is more favorable. Thus, the dimerization of terminal alkynes catalyzed by cationic iridium complexes is plausible