We have developed a polarizable intermolecular potential function
(PIPF) for simulation of liquid amides.
The PIPF potential includes a pairwise additive component,
consisting of the familiar Lennard-Jones and
Coulomb form, and a nonadditive polarization term. The empirical
parameters were optimized through a
series of statistical mechanical Monte Carlo simulations of liquid
formamide, N-methylacetamide (NMA),
N-methylformamide (NMF), and
N,N-dimethylformamide (DMF). In deriving the
empirical potential functions,
bimolecular complexes of the amides dimers were studied by ab
initio molecular orbital calculations using
the 6-31G(d) basis set, and the results were compared with the
PIPF predictions. The computed heats of
vaporization and densities for the liquids using the final parameters
are within 2% and 3% of experimental
values, respectively. The polarization effects are found to be
significant in all liquids, ranging from 6% for
DMF to 14% for formamide of the total 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 dipole
moments in the liquids are compared with results obtained from separate
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 represented by the empirical OPLS
potential. In addition, structural features
including hydrogen-bonding interactions and radial distribution
functions are examined and found to be in
good agreement with the previous computational results.
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