AB-INITIO THEORETICAL-STUDY OF THE MONOMER-DIMER EQUILIBRIUM IN LITHIUM AND SODIUM GEM-DICHLORO ALLYL AND METHYL SYSTEMS

Non-empirical free energy calculations on lithium and sodium gem-dichl oroallyl and dichloromethyl systems show that the dimeric structures a re favoured for both species, but to a significantly lesser extent tha n in the analogous difluoro systems previously studied. The two dichlo romethyl systems show geometric and energetic features very close to t hose found in the larger systems, whose vinylic part is not directly i nvolved in the dimerization process. In both cases about the same dime rization free energies are computed for the lithium and sodium systems (ca. -24 kcal mol(-1)). Dichloromethyl is then used as a model for di chloroallyl re investigate, for two different dimerization equations, the structural and energetic effects of the interaction with the count erion of a discrete number of water molecules (used as a model for eth er). The sign of the energy differences obtained by taking into accoun t this interaction is reversed, and, in the case of lithium, the equil ibrium is now clearly in favour of the monomeric species (by ca. 16 or 20 kcal mol(-1), depending on the equation considered). The inclinati on to dimerize of the sodium systems is less clearly set, because smal ler approximate Delta G values are estimated (close to zero or ca. 7 k cal mol(-1), depending again on the equation considered). This picture contrasts the results previously published for the analogous difluoro compounds, for which approximate Delta G values ranging from -13 to - 18 kcal mol(-1) had been obtained.