SIMULATION OF LIQUID AMIDES USING A POLARIZABLE INTERMOLECULAR POTENTIAL FUNCTION

We have developed a polarizable intermolecular potential function (PIP F) for simulation of liquid amides. The PIPF potential includes a pair wise additive component, consisting of the familiar Lennard-Jones and Coulomb form, and a nonadditive polarization term. The empirical param eters were optimized through a series of statistical mechanical Monte Carlo simulations of liquid formamide, N-methylacetamide (NMA), N-meth ylformamide (NMF), and N,N-dimethylformamide (DMF). In deriving the em pirical potential functions, bimolecular complexes of the amides dimer s were studied by nb initio molecular orbital calculations using the 6 -31G(d) basis set, and the results were compared with the PIPF predict ions. The computed heats of vaporization and densities for the liquids using the final parameters are within 2% and 3% of experimental value s, respectively. The polarization effects are found to be significant in all liquids, ranging from 6% for DMF to 14% for formamide of the to tal liquid energy. Electrostatic and polarization components dominate in primary and secondary amides, while the van der Waals contribution is greater than electrostatic terms for the tertiary amide DMF. In the present parameter optimization, polarization energies and induced dip ole moments in the liquids are compared with results obtained from sep arate Monte Carlo simulations employing a combined quantum mechanical and molecular mechanical (QM/MM) approach. In the latter calculation, one amide monomer is treated quantum mechanically by the semiempirical AM1 theory, which is embedded in the liquid of the same amide represe nted by the empirical OPLS potential. In addition, structural features including hydrogen-bonding interactions and radial distribution funct ions are examined and found to be in good agreement with the previous computational results.