Development of an All-Atom Force Field for Heterocycles. Properties of Liquid Pyrrole, Furan, Diazoles, and Oxazoles

The approach to the extension of the optimized potentials for liquid simulations all-atom (OPLS-AA) force field, which was tested for pyridine and the diazenes in the first paper of this series, has been extended to pyrrole, pyrazole, imidazole, furan, isoxazole, and oxazole. Standard OPLS Lennard-Jones parameters are used for the nonbonded interactions in conjunction with partial charges obtained from fitting to RHF/6-31G* electrostatic potential surfaces. The harmonic bond stretching and angle bending terms are adopted mostly from the AMBER force field, although the addition of two new atom types was required. The resultant force field was tested by computing the gas-phase structures of the heterocycles, heterocycle−water interaction energies, properties of the pure liquids, and the relative free energies of hydration for pyrrole and imidazole.