MECHANISM OF THE GRIGNARD REAGENT FORMATION-PART 2-AN AB INITIO STUDY OF THE RMgX FORMATION FROM THE RMgn(1) CLUSTERS

摘要

The results of density functional calculations arereported for CH3Mg2, CH3Mg4T and for CH3Mg5TB Cl modelclusters, with T= tetrahedral and TB= trigonal bipyramid. Thesecalculations aim at a simulation of the migration of a methylgroup in the proposed intermediates RMg(1)n (n=2 and 4) and ofthe succession of steps from the substrate to RMgX. The mono-coordination of the methyl group in the clusters CH3Mgn (n=2 or4) represents the most stable structure. The energy barrier to passfrom poly-coordination structure to mono-coordination structureis low (1.213 eV=27.9 kca/mol). It is sufficient, however, toprevent the methyl migration from a magnesium atom to anotherone but is probably frozen at lower temperatures. We also presenta Grignard reagent formation proposal different from theclassical linear mechanism generally proposed. This mechanismimplies a neutral species (RMg(1I)X Mgn-1 RMg(1I)X linked to themetallic surface) as intermediate. The reaction would evolve thentoward the Grignard reagent RMgX and a magnesium clusterwith n-1 magnesium atoms in an irreversible pathway.