  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.
