posted on 2013-06-11, 00:00authored byAlauddin Ahmed, Stanley I. Sandler
Nitroaromatic
compounds (NACs) are used as energetic materials,
reagents, and pesticides; however, they are potentially hazardous
for the environment and human health. To predict the environmental
distribution of these compounds, the vapor pressure, aqueous solubility,
and Henry’s law constant are important properties, as is the
solvation free energy in water from which the latter two can be computed.
Here, we have calculated the hydration free energies for a set of
nine nitroaromatic compounds containing one, two, and three nitro
groups using the expanded ensemble molecular dynamics simulation method
with TIP3P water and the GAFF, CGenFF, OPLS-AA, and TraPPE force field
parameters and the RESP (gas phase), CHELPG (gas phase), and CM4 (aqueous
phase) partial atomic charges calculated here. Also, we have computed
hydration free energies using the reported default partial atomic
charges of the OPLS-AA force field and using the semiempirical AM1-BCC
charges with GAFF parameters. The effect of water model flexibility
on the computation of hydration free energy is examined with CGenFF/(CHELPG+SPC-Fw)
model. All the force fields studied generally led to less accurate
predictions with increasing numbers of nitro groups. The average unsigned
errors (AUE) show that 6 of 16 force-field/(charge+water) models used
perform approximately equally well in predicting measured hydration
free energies: these are CGenFF/(CHELPG+TIP3P), CGenFF/(CM4+TIP3P),
OPLS-AA/(CHELPG+TIP3P), OPLS-AA/(CM4+TIP3P), TraPPE-UA/(CHELPG+TIP3P),
and TraPPE-UA/(CM4+TIP3P). When using the default atomic charges,
the OPLS-AA force field was the most accurate, though using CHELPG
and CM4 charges led to better predictions. Our analyses indicate that
not only the charges but also the van der Waals interaction parameters
for the nitro-group nitrogen and oxygen atoms in the force fields
are partly responsible for the performance variations in predicting
solvation free energies. We also compared the force field-based simulation
results with the predictions from the SM6 solvation model and Abraham
linear solvation energy relationship (LSER) method. With an appropriate
choice of theory and basis set both for geometry optimization and
computation, which unfortunately is not known a priori, the SM6 model hydration free energy predictions for the NACs are
comparable to the simulation results here. The Abraham LSER predictions
with descriptors obtained from the Platts method are of reasonable
accuracy. A useful addition to this paper is the Supporting Information that contains a compiled and evaluated
list of the hydration free energies of the NACs studied here assembled
from the literature.