Intermolecular Interactions in N, N-Dimethylacetamide without and with LiCl Studied by Infrared Spectroscopy and Quantum Chemical Model Calculations

Abstract

The mixture of LiCl and N,N-dimethylacetamide (DMAc) is an important laboratory-scale solvent for cellulose. However, the mechanism of cellulose dissolution in DMAc/LiCl could not be fully established due to the limited knowledge about the interactions between DMAc and LiCl. To address this issue, we studied neat DMAc and DMAc/LiCl mixtures by ATR FTIR spectroscopy and quantum chemical model calculations. On the basis of the calculations, we newly assigned the bands at 1660 and 1642 cm-1 in the ν(C=O) region of the spectra to DMAc monomeric and dimeric structures. The latter are presumably stabilized by the C-H···O=C weak hydrogen bonds that prevail in both neat DMAc and DMAc/LiCl mixtures. The analysis of the concentrated (7.9 wt % of LiCl) DMAc/LiCl mixture revealed that only about half of DMAc molecules interact directly with LiCl. The resulting average stoichiometry of about 2.8:1 (DMAc:LiCl), indicating the predominance of [(DMAc)2-LiCl] and [(DMAc)3-LiCl] complexes, was found to be temperature independent. Conversely, the stoichiometry was considerably temperature sensitive for the diluted DMAc/LiCl mixture (2.6 wt % of LiCl), indicating that further DMAc molecules can be incorporated into the primary solvation shell of LiCl at higher temperatures. These results highlight the dynamic character of the DMAc/LiCl system that needs to be considered when studying the cellulose dissolution mechanism.