Hydrogen bonding structure and dynamics of cis- and trans- conformers of N-methylformamide in water, DMSO and water-DMSO mixtures at varying compositions

Abstract

The local hydrogen bonding structure and dynamics of cis- and trans- N-methylformamide (NMF) in water, DMSO and in water–DMSO mixture is investigated by classical molecular dynamics simulations. We have considered five different concentrations of NMF in water as well as in DMSO at 298 K. In the case of NMF (XNMF = 0.20) in water–DMSO mixtures, we have considered six different concentrations, varying from NMF in pure water to DMSO. It is observed that the donating ability of amide–hydrogen of NMF to the oxygen of DMSO is higher compared to the oxygen of water. The variation of DMSO shows negligible effects on the HNMF…ODMSO radial distribution function (RDF), whereas the HNMF…OWAT decreases with the addition of water to the solution. In the case of NMF in water–DMSO mixtures, the addition of DMSO strengthens the HNMF…ODMSO as well as HNMF…OWAT correlation, but it differently affects for cis- and trans- NMF in the solution. DMSO prefers cis- conformer, whereas water prefers trans- NMF. In NMF–water–DMSO mixtures, the addition of DMSO strengthens HWAT…ONMF up to intermediate DMSO concentration, but at very high DMSO concentration, it decreases due to preferable HNMF…ODMSO interaction. The probability of hydrogen bonding between NMF–NMF is higher in NMF–DMSO solution compared to NMF–water. The dynamical slowdown is observed for all the species in water–NMF mixtures at intermediate concentration, whereas in the case of NMF–DMSO solution, faster dynamics are observed with the addition of DMSO. For NMF in water–DMSO mixtures, the dynamical slowdown is observed as DMSO is added to the NMF–water solution. Our calculated hydrogen-bond lifetime suggested that the DMSO forms a stronger hydrogen bond with cis-NMF whereas water generally prefers trans-NMF.