Structure and discrimination in chiral fluids: A molecular dynamics and integral equation study

An analysis of structure and discrimination in simple chiral fluids is pres ented. The chiral molecules consist of a central carbon bonded to four dist inct groups. Molecular-dynamics simulations have been performed on a one-co mponent chiral fluid and on two racemic mixtures. For the racemates, discri mination, as measured by differences in pair distribution functions, is pre sent but found to be small. Intermolecular pair interaction energies are fo und to be good predictors of the magnitude and the sign (mirror-image pairs favored) of the differences observed in site-site distribution functions. For the one-component fluid, the quality of structural predictions from the reference-interaction-site method and Chandler-Silbey-Ladanyi (CSL) integr al equation theories, with the hypernetted chain (HNC) and Percus-Yevick cl osures, has been examined. These theories generally provide a qualitatively correct description of the site-site distributions. Extensions beyond the HNC level have been explored: Two-field-point bridge diagrams have been exp licitly evaluated and included in the CSL theory. The inclusion of these di agrams significantly improves the quality of the integral equation theories . Since the CSL theory has not been used extensively, and bridge diagrams h ave been evaluated in only a few instances, a detailed analysis of their im pact is presented. For racemic mixtures, diagram evaluation is shown to be crucial. Specifically, the differences in site-site distributions for sites on identical and mirror-image molecules are found to originate from bridge diagrams which involve interactions between four-site, or larger, clusters . Discrimination cannot be predicted from an integral equation theory which neglects these diagrams. (C) 2000 American Institute of Physics. [S0021- 9 606(00)50630-8].