Direct Measurement of the Angular Pair Correlation Coefficients in Molecular Liquids Using NMR. Benchmarking Force Fields for Atomistic Simulations.

High-field deuterium NMR spectroscopy is used to characterize a number of molecular liquids and their mixtures in order to probe the directional part of the intermolecular interactions through the orientational ordering induced in the isotropic liquid phase by the spectrometer magnetic field. The systems studied include benzene, chloroform, hexafluorobenzene, and thiophene at various concentrations and in mixtures. Dilution with the magnetically isotropic tetramethylsilane provides quantification of ordering at "infinite magnetic dilution", that is, in the absence of magnetic intermolecular correlations, and thereby allows identification of the contribution of these correlations to the orientational ordering in neat phases and at various degrees of magnetic dilution. Such contributions are conveyed by angular pair correlation coefficients, which, in addition to being accessible to direct NMR measurement, are also possible to evaluate directly from molecular dynamics simulations. By using various force fields, simulations provide benchmark quantities for testing and possibly further improving the force field performance, particularly with respect to the directional components of the intermolecular interactions. The latter are critical for the simulation of self-assembly generally and particularly in biological systems.