Molecular conformations and the liquid structure in bis(methylthio)methane and diethyl sulfide: diffraction experiments vs molecular dynamics simulations

Abstract

Molecular dynamics computer simulations with the OPLS-AA and EncadS all-atom force fields have been performed for liquid bis(methylthio)methane and diethyl sulfide. For bis(methylthio)methane, additional simulations using gas electron diffraction and ab initio bond length and bond angle parameters were also performed. Although there were small differences between the structures provided by the different molecular dynamics simulations of bis(methylthio)methane, the main conformer was found to be the AG one (54%), followed by the G(+)G(+) one (31%) in all the calculations. This is in contrast with gas phase electron diffraction and ab initio calculations, where the main conformer was G(+)G(+) (70%; beside that, 30% AG was assumed). For diethyl sulfide, no experimental data was present for the gas phase and the different interatomic potential sets produced different conformer ratios: applying OPLS-AA resulted in 51% AG and 40% AA, while from EncadS the main conformer was G(+)G(+), with an occurrence of 69.5% (AG was present with 29.6%). Comparing the MD calculated structure factors to results of X-ray diffraction experiments, some differences could be found at Q < 5 Å(-1) in all cases. For bis(methylthio)methane, the EncadS potential produced the X-ray structure factor with the smallest difference to the experimental one, whereas for diethyl sulfide the OPLS-AA parameter set proved to be the more successful. The EncadS force field provided a better approximation to the experimental value of the enthalpy of vaporization for bis(methylthio)methane; for diethyl sulfide, the two different force fields gave similar results.