Weak Interactions in Dimethyl Sulfoxide (DMSO) -- Tertiary Amide Solutions: the Versatility of DMSO as a Solvent

TL;DR

This study uses neutron diffraction and EPSR to reveal detailed molecular interactions in DMSO solutions with amides, highlighting the solvent's versatility and the subtlety of weak intermolecular forces.

Contribution

It provides a comprehensive structural analysis of amide-DMSO solutions, demonstrating the complex hierarchy of weak interactions and the effectiveness of neutron diffraction in such studies.

Findings

Amide-DMSO solutions have a coordination shell of 13-14 neighbors.

Weak C-H...O hydrogen bonds dominate the local interactions.

DMSO's structural role is versatile and well-resolved by neutron diffraction.

Abstract

The structures of equimolar mixtures of the commonly used polar aprotic solvents dimethylformamide (DMF) and dimethylacetamide (DMAc) in dimethylsulfoxide (DMSO) have been investigated via neutron diffraction augmented by extensive hydrogen/deuterium isotopic substitution. Detailed 3-dimensional structural models of these solutions have been derived from the neutron data via Empirical Potential Structure Refinement (EPSR).The intermolecular Centre-of-Mass (CoM) distributions show that the first coordination shell of the amides comprises 13-14 neighbours, of which approximately half are DMSO. In spite of this near ideal coordination shell mixing, the changes to the amide-amide structure are found to be relatively subtle when compared to the pure liquids. Analysis of specific intermolecular atom-atom correlations allows quantitative interpretation of the competition between weak interactions in solution. We find a hierarchy of formic and methyl C-H to O hydrogen bonds form the dominant local motifs, with peak positions in the range 2.5-3.0 {\AA}. We also observe a rich variety of steric and dispersion interactions, including those involving the amide pi-backbones. This detailed insight into the structural landscape of these important liquids demonstrates the versatility of DMSO as a solvent and the unparalleled resolution of neutron diffraction, which is critical for understanding weak intermolecular interactions at the nanoscale and thereby tailoring solvent properties to specific applications.