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-dichloroallyl and dichloromethyl systems show that the dimeric structures are favoured for both species, but to a significantly lesser extent than in the analogous difluoro systems previously studied. The two dichloromethyl systems show geometric and energetic features very close to those found in the larger systems, whose vinylic part is not directly involved in the dimerization process. In both cases about the same dimerization free energies are computed for the lithium and sodium systems (ca. -24 kcal mol(-1)). Dichloromethyl is then used as a model for dichloroallyl re investigate, for two different dimerization equations, the structural and energetic effects of the interaction with the counterion of a discrete number of water molecules (used as a model for ether). The sign of the energy differences obtained by taking into account this interaction is reversed, and, in the case of lithium, the equilibrium is now clearly in favour of the monomeric species (by ca. 16 or 20 kcal mol(-1), depending on the equation considered). The inclination to dimerize of the sodium systems is less clearly set, because smaller approximate Delta G values are estimated (close to zero or ca. 7 kcal 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.