MOLECULAR-DYNAMICS WITH A QUANTUM-CHEMICAL POTENTIAL - SOLVENT EFFECTS ON AN S(N)2 REACTION AT NITROGEN

Solvent effects on an S(N)2 reaction at nitrogen (C1(-) + NH2Cl --> Cl NH2 + Cl-) in dimethyl ether solution were studied by means of molecul ar dynamics simulation with a combined quantum-chemical and molecular- mechanical potential. The energetics and geometrical parameters of the reaction in the gas phase, calculated by means of the semiempirical m odel PM3 (the quantum chemical part of the combined potential), were c ompared with ab initio calculations up to the 6-311 + G*/MP2 and 6-31 1 + G(2d,p)/MP 2 levels of theory. Compared with the gas phase potenti al energy surface, the free energy profile of the reaction in dimethyl ether solution shows that the solvent makes the ion-dipole complex we ll shallower by approximately 6.4 kcalmol(-1) and raises the height of the effective barrier from the complex to the transition state by abo ut 2.2 kcalmol(-1). The overall transition barrier between the separat ed reactants and the products is raised from 6.4 kcalmol(-1) to 15.0 k calmol(-1) upon solvation. The radial distribution functions between s olvent-solute atom pairs at different stages of the reaction course we re compared. Results show that better solvation of the charge localise d separated reactants is responsible for the increase in the barrier h eight. Polarisation of the solute by its surroundings is also discusse d.