Different weak C-H••O contacts in N-methylacetamide-water system:: Molecular dynamics simulations and NMR experimental study

An all-atom N-methylacetamide (NMA) model and a refined SPC water model have been adopted for molecular dynamics simulations. H-1 NMR chemical shifts of water, as well as the relative chemical shifts of the NMA methyl group and amide hydrogen atom in the NMA-water system, are measured and calculated over an entire composition range at different temperatures: 308, 323, 338, and 353 K. NMA molecules can act as a hydrogen bonding acceptor or a donor competing with water molecules. Interestingly, the two methyl groups in the NMA molecule are found to have different capabilities in forming weak C-(HO)-O-... contacts in the mixtures from the radial distribution functions (RDFs). Also, temperature-dependent NMR results of the different methyl groups show excellent agreements with the MD simulations.