a
[
v
-“
S~dEKXXW~3id-
submitted for publication to T’he Journal of Physical Chemistry B
Transferable Potentials for Phase Equilibria.
4. United-Atom Description of Linear and
Branched
Alkenes and Alkylbenzenes
Collin D. Wick,l Marcus G. Martin,l’2 and J. Hja Siepmann*’l
1Departments of Chemistry and of Chemical Engineering and Materials Science,
University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455-0431
and
2Computational Biology and Materials Technology,
Sandia National Laboratory, Albuquerque, NM 87185-1111
4ECEIVED
AM 172000
(“3~ ~ j
* Corresponding author: siepmann@chem.umn. edu
1
t 1
Abstract
The Transferable Potentials for Phase Equilibria-United .+tom (Tra.PPE-LJ.A) force ficki
for hydrocarbons is extended to alkenes and alkylbenzenes by introducing the followins
pseudo-atoms: CHZ (sp2), CH(sp2), C(sp2), CH(aro), R–C(aro) for the link to aliphatic side
chains, and C (are) for the link of two benzene rings. In this united-atom force field. the
nonbonded interactions of the hydrocarbon pseudo-atoms are solely governed by Lemwd-
Jones 12–6 potentials, and the Lennard-Jones well depth and size parameters for the new
pseudo-atoms were determined by fitting to the single-component vapor-liquid phase equi-
libria of a few selected model compounds. Configurational-bias Monte Carlo simulations in
the .N’VT~’ersicn of the Gibbs ensemble were carried out to calculate the sillgle-co~llpol]e~lt
vapor-liquid coexistence curves for ethene, propene, l-butene, tnzns- and cis-2-butene. 2-
methylpropene, 1..5-hexadiene, l-octene, benzene, toluene, ethylbenzene, propylbenzene,
isopropylbenzene. o-, m-, and p-xylene, and naphthalene. The phase diagrams for the
binary mixtures of (supercritical) ethene/n-heptane and benzene/n-pentane were deter-
mined from simulations in the NpT Gibbs ensemble. .Although the TraPPE-U.\ force field
is rather simple and makes use of relatively few different pseudo-atoms, its performance.
2
%sjudged by comparisons to other popular force fields and available experimental data. k
very satisfactory.
DISCLAIMER
This report was,.prepared as an account of work sponsored
by an agency of the United States Government. Neither
the United States Government nor any agency thereof, nor
any of their employees, make any warranty, express or
implied, or assumes any legal Iiabiiity or responsibility for
the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disciosed, or
represents that its use wouid not infringe privateiy owned
rights.
Reference herein to any specific commercial
product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute
or impiy its endorsement, recommendation, or favoring by
the United States Government or any agency thereof. The
views and opinions of authors expressed herein do not
necessarily state or refiect those of the United States
Government or any agency thereof.
DISCLAIMER
Portions of this document may be illegible
in electronic image products. Images are
produced from the best available original
document.
1. Introduction
Unsaturated hydrocarbons arc ubiquitous as reacrants. iutermcdiates. or products
in man!’ prowsscs ill t-he (]][:t,r(~)cllcl]lical industries: and the rigidity and distinct, con-
formation of unsaturated carbon-cwbon bonds is exploited by many biological systems.
However, despite the importance of unsaturated hydrocarbons little attention has been
devoted to the development, of appropriate molecular-mechanics-type force fields for these
sj-stems that are transferable over a wide range of temperature, pressure! and composition.
‘The popular OPLS unitecl-atom force field developed by Jorgensen et al. 1 contains
the Lennard-.Jones (LJ ) parameters (and also the torsional parameters) for some of the
pseudo-atoms required for modeling of unsaturated hydrocarbons. In particular, these
OPLS-UA force field parameters were fitted to liquid densities: heats of vaporization, and
liquid structures of l-butene, cis-z-butene, 2-methylpropene, and benzene (at 7’ = 298.15 K
and p = 101.3 kPa) determined from isobaric-isothermal ensemble Monte Carlo simula-
tions. Using a similar procedure, ,Jorgensen et al.23 also determined force field parameters
(LJ ~~elldepths and LJ diameters PIUS partial charges) for an all-atom description of the
unsaturated hydrocarbons. However. over the past few years it has become clear that
simulations over a wider range of physical conditions (temperature and pressure) are re-
cluired to yield force fields for a given system that strike a good balance between enthalpic
and entropic contributions to the free energy and that are thus transferable to different
phjsical conclitions.4.3
Recently, Spyriouni et al.~ proposed a united-atom model for medium-sized o-olefines
(denoted here as the SET force field) that yields very good predictions for many thermo-
physical properties (critical constants, ~’apor pressures, heats of vaporization, and second
I
virial coefficients) over the entire fluid phase regime.
To further test their wolefine force
field, Spyriouni et al.6 carried out calculations for binary mixtures of I-butene (or l-hexene)
3