Acetone dimer, molecular mechanics

Molecular mechanics methods are also omitted from the present discussion for similar reasons. Although very sophisticated force fields are available for water (including polarizable models), most force fields for weakly bound clusters are essentially 2-body (dimer) potentials that have been adjusted empirically to reproduce bulk-phase properties. This procedure leads to very reliable descriptions of liquid water, but diminishes the quality of results for small clusters. Although force fields that include 3-body interactions are beginning to appear, ° the effects of higher order interactions (4-body, 5-body, etc.) are still untested. Furthermore, the composition of a weakly bound cluster, not just its size, is a major concern with molecular mechanics force fields. The highly refined potentials that have been developed for water are not necessarily transferable to other weak noncovalent systems (methanol, acetone, etc.). 

For the first time we report the synthesis of macrocycles like calixpyrrole, cyclotriveratrylene (CTV), cyclotetraveratrylene (CTTV), porphyrine ete over molecular sieve as a catalyst. Calixpyrroles are synthesized from pyrrole and a ketone like acetone over MCM-41 under reflux conditions using suitable solvent. In case of MCM-41 cyclic calixpyrroles were obtained. On the other hand due to shape selectivity in case of Y zeolite linear di-, tri- and tetra- polypyrroles were obtained and cyclic tetramers were not observed. The mechanism of the synthesis of calixpyrrole is either by the dimerization of dimer with simultaneous cyclization to cyclic tetramer or cyclization of linear tetramer via recoil phenomenon. 

The product of the reduction of [Co(bpy)3] by Cr, upon aerial oxidation, is a red dimeric species, postulated to have the structure [(H20)4Cr(/x-OH)2Cr(OH2)2]. This product and the stoichiometry of the reaction suggests a two-electron process, with the bpy ligand serving as a temporary bridging radical. An investigation of the Cr(II) reduction of [Co(pd)3] (pd = pentane-2,4-dione) in water/acetone mixtures reveals outer-sphere, and mono- and di-bridged ([H ] dependent) pathways.The effect of the cosolvent on the activation parameters is observed at an acetone mole fraction of 0.06, at which point its solvation of the activated complex becomes important. The reduction of [Co(en)2(dppd)] (dppd = l,3-diphenylpropane-l,3-dione) by Cr occurs by a multistep mechanism in which the first step is the formation of the [Co(en)2(dppd )] radical, which catalyzes the inner-sphere Co(III)/Cr(II) electron transfer process. " A molecular orbital study indicates that the [Co(en)2(dppd)] reduction likely involves attack of Cr " at the methine carbon of dppd, in contrast to the attack on an oxygen in the [Co(en)(pd)2] reduction. 

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