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FTIR / Raman Study of the Concentration Dependence of the nu(C=O) Band of N, N-Dimethylformamide

Amides are subject of permanent interest in view of their importance as common organic solvents. In that respect, one of the most interesting points to resolve were the formation of aggregates and their structural and thermodynamical consequences in liquid systems. N, N-dimethylformamide (DMF) is the simplest amide which contains no N-H group responsible for hydrogen-bonded aggregates of N-H containing amide molecules. So, the question arises of the ability of DMF molecules to build structurally well defined aggregates, i.e. about the nature of the intermolecular interactions between DMF molecules in liquid state. From the longstanding discussion of the solvation process of DMF in aprotic solvents, a discrepancy in the interpretation of the spectral data is evident. First interpretation is that DMF molecules build structurally well defined aggregates in liquid state. Much of the infrared, NMR and, more recently, Raman spectroscopic works were supported by this interpretation. Asymmetric shape of the isotropic Raman carbonyl stretching band was considered in terms of well defined DMF-DMF aggregate formation. However, second interpretation is based on assumption that in the liquid state one can not consider any intermolecular interactions between DMF molecules which give rise to well structured complexes. From that point of view, asymmetric shape of isotropic Raman carbonyl stretching band was interpreted in terms of concentration fluctuations within the interaction volume of DMF molecule5 and in terms of the hydrodynamic and electronic dispersion force dependence of the Raman band shape. In the present study we attempt to resolve the above described discrepancy. The main motivation for carrying out this study is to characterise intermolecular interactions between DMF molecules in a liquid state. We report results obtained from experimental infrared and Raman spectra of the DMF solutions in full concentration range, from very dilute solutions to pure liquid DMF. Combination of the obtained data enables us to monitor dynamic and interaction processes in a liquid state in terms of orientational dipole-dipole interactions. It is observed that in DMF/CCl4 system resonant transfer of vibrational energy (RET) is predominant factor of band broadening, while effect of local field (LFE) is of negligible importance. In DMF/CH2Cl2 system is, however, observed that importance of LFE to band broadening increases, while RET contribution rapidly decreases. Single DMF molecule infrared nu(C=O) band is also broader in CH2Cl2 than in CCl4, which is another reflection of the relative importance of solvation to band width. In both systems, infrared nu(C=O) band is symmetric in the whole concentration range, being predominantly of Lorentzian shape. Raman isotropic nu(C=O) band is symmetric for dissolved solutions, but, increasing DMF concentration, its asymmetry also increases, being steeper on its lower wavenumber side. Anisotropic band is symmetric in the whole concentration range for both systems. Noncoincidence splitting increases with DMF concentration, being in accordance with Logan’ s model for DMF/CCl4 system, and with McHale-Mirone’ s model for DMF/CH2Cl2 system.

N; N-dimethylformamide; FTIR; Raman; liquids; dipole; intermolecular interactions

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