The dynamics of liquid formamide, N-methylformamide, N,N-dimethylformamide, and N,N-dimethylacetamide. A dielectric relaxation study

The results of dielectric relaxation experiments on formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA), performed between 238.15 K and 338.15 K in the frequency range 0.2 less than or equal to v/GHz less than or equal to 89 are presented. Whereas the FA spectra can be formally fitted with a Davidson-Cole relaxation-time distribution, two relaxation processes can be resolved for the other liquids. For DMF and DMA the long relaxation time, tau(1)(T), follows the Vogel-Fulcher-Tammann equation. For these liquids tau(1)(T) probes the essentially isotropic rotational diffusion of the molecular dipole vector. For the hydrogen-bonding liquids FA and NMF a modified Eyring equation is more appropriate to describe tau(1)(T). From the parameters the average number of monomers in the chains of H-bonded NMF molecules is deduced, as well as the number of hydrogen bonds which must be broken before relaxation in FA can occur. The data suggest that the observed high-frequency contribution in the spectra mainly reflects the onset of inertial contributions. (C) 2002 Elsevier Science B.V. All rights reserved.