Solvent effect on the isomerization rate of calix[4]arene studied by molecular dynamics simulations

The isomerization rates of a calix[4]arene in benzene and in chloroform hav e been calculated by using molecular dynamics simulations. The reaction coo rdinate that is employed is based on the unstable normal mode at the saddle point of the potential energy surface. The free energy as a function of th is coordinate has been calculated by means of umbrella sampling. Comparison of the free energies in the solvents with those in vacuum reveals that bot h solvents destabilize the pace conformation and stabilize the transition s tate region. In chloroform the calix[4]arene shows a stronger preference fo r the cone conformation than in benzene or in vacuum. The isomerization rat e has been determined by the reactive flux method. In benzene the transmiss ion coefficient is about a third higher than in chloroform. The calculated rates are in perfect agreement with experimental data.