Modeling Aromatic Liquids:  Toluene, Phenol, and Pyridine

Abstract

Aromatic groups are now acknowledged to play an important role in many systems of interest. However, existing molecular mechanics methods provide a poor representation of these groups. In a previous paper, we have shown that the molecular mechanics treatment of benzene can be improved by the incorporation of an explicit representation of the aromatic π electrons. Here, we develop this concept further, developing charge-separation models for toluene, phenol, and pyridine. Monte Carlo simulations are used to parametrize the models, via the reproduction of experimental thermodynamic data, and our models are shown to outperform an existing atom-centered model. The models are then used to make predictions about the structures of the liquids at the molecular level and are tested further through their application to the modeling of gas-phase dimers and cation-π interactions.

The structures of liquid pyridine and naphthalene: the effects of heteroatoms and core size on aromatic interactions

“…This relatively simplistic atom centred potential provides a much better description of the orientational structure than the charge separated (OPLS-CS) model that explicitly described the p-electron density, which gives a considerably stronger preference for perpendicular nearest neighbours. 36 For naphthalene the OPLS-AA force-field clearly does not replicate the experimental g(r,y), showing no clear preference for parallel nearest neighbours. The OPLS force-field treats the ''condensed'' carbon as having the same Lennard-Jones parameters as a standard benzene carbon.…”

“…These intermolecular force-fields, such as OPLS-AA, 37 are parameterized for a range aromatic molecules, and are able to closely match experimental values for enthalpy of vaporisation, density and heat capacity. 36,37 Jorgensen and McDonald 37 calculated the N-N radial distribution function for liquid pyridine using OPLS-AA, which showed two broad peaks at B4.8 and 6.0 Å. These features were attributed to head-to-head and head-to-tail antiparallel stacked structures ( Fig.…”

“…1), similar to those seen in ab initio calculation of dimers. However a more detailed analysis by Baker and Grant, 36 again using the OPLS-AA force-field, showed that perpendicular arrangements were preferred over parallel ones in the liquid state. This study went further, by proposing a new force-field, OPLS-CS, which more realistically represents the 3-dimensional charge distribution in aromatic molecules, by the use of virtual sites to represent charge in the aromatic ring.…”

“…The result is however similar to calculations from classical simulation studies of bulk phase liquid pyridine. 36 For a simulation run using the EPSR Monte Carlo simulation, but without refinement to the data, there is indeed a stronger preference for parallel molecular nearest neighbours (see SI-S2, ESI †). The refinement to the data reduces this parallel peak in the angular radial distribution function.…”

“…To date, the main tools applied to tackle this problem have been ab initio calculations [18][19][20][21][22][23][24][25][26][27][28][29][30] and gas phase spectroscopy 31,32 of dimers or small clusters, spectroscopy of model systems such as molecular balances, 33 solid and liquid state diffraction, 34,35 and classical molecular modelling. [36][37][38][39][40] As the simplest aromatic molecule and, therefore, benchmark, benzene has received significant attention in this context. Extensive ab initio and experimental studies of the benzene dimer have in general shown a similar energy minimum for parallel-displaced, tilted T-shaped, and perpendicular T-shaped geometries (Fig.…”