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 <200cm−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 110cm−1 mainly originates from the libration mode of amide molecules in the chain-like hydrogen-bonded structure and (2) the higher frequency band (approximately 200cm−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.