Hydrogen bonding in protic and aprotic amide mixtures: Low-frequency Raman spectroscopy, small-angle neutron scattering, and molecular dynamics simulations

Abstract The hydrogen-bonding interactions in protic and aprotic amide solvent mixtures, i.e., formamide (FA) and N , N -dimethylformamide (DMF), were investigated via low-frequency Raman spectroscopy, small-angle neutron scattering (SANS) experiments, and molecular dynamics (MD) simulations. In a neat amide system, the low-frequency Raman spectra R ( ν )s were well reproduced by the corresponding S ( ν ) spectra derived from the MD simulations. The observed peaks in R ( ν )s at around − 1 were assigned to the intermolecular interactions, particularly in terms of the hydrogen-bonding network formation and its dimensionality in the liquid state. The SANS experiments for the FA–DMF mixtures demonstrated that the FA molecules forming an extended three–dimensional hydrogen-bonding structure in the neat system interacted with DMF molecules through the hydrogen bonds in the mixtures over the whole range of solvent compositions, resulting in a homogeneous mixing state. Additionally, the R ( ν ) spectra for the mixtures were represented by the corresponding S ( ν ) spectra. From the R ( ν ) and S ( ν ) spectra of the FA–DMF mixtures, we found that (1) the Raman band at around 110 cm − 1 mainly originates from the libration mode of amide molecules in the chain-like hydrogen-bonded structure and (2) the higher frequency band (approximately 200 cm − 1 ) was attributed to the libration of the FA molecule restricted by the three-dimensional hydrogen-bonded network, which remained even in the DMF-rich compositions.