DETERMINATION OF THE SIZE OF MAGNESIUM AG GREGATES OBTAINED BY VAPORIZATION - QUANTUM EFFECT AND NATURE OF THE ACTIVE-SITES IN THE MAGNESIUM SAMPLES USED FOR THE PREPARATION OF GRIGNARD-REAGENTS

The mechanism of formation of Grignard reagents has been studied using clusters of magnesium obtained by metal vapor synthesis. These aggreg ates, although being pyrophoric, have been observed by electronic micr oscopy; their average size is 32A; this size is far smaller than other activated forms oi magnesium. Density function type calculations allo ws a simulation of the properties of small clusters of magnesium both in terms of stability and reducing power (ionisation potential). These calculations completed by a Marcus approah of reaction rate constants suggest that the rate of electron transfer may vary by more than six orders of magnitude depending upon the size and structure of the clust er. These results suggest that the active sites of a magnesium surface are constitued by sets of clusters of highly variable reactivity rath er than by a unique ''entity'' called metallic magnesium. This proposi tion provides an alternative to the macroscopic vision of surface stat es.