Anti-parallel dimer and tetramer formation of cyclic and open structure tertiary amides, N-methyl-2-pyrrolidone and N,N-dimethylacetamide, in solution of a non-polar solvent, benzene.

We examined the dielectric (DE) and nuclear magnetic resonance (NMR) spectroscopic behaviour of N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc), which are tertiary amide compounds that bear five-membered cyclic and open structures, respectively. They were examined to investigate the formation of anti-parallel dimers and tetramers in a dipole configuration in the pure liquid state and in benzene solution over a wide concentration range at 25 °C. Because the Kirkwood correlation factors of electric dipoles considerably decreased with increasing concentrations in both the NMP and DMAc systems and because the second DE relaxation modes were clearly observed to exhibit relaxation times that are longer than those of the first modes assigned to the rotational relaxation of monomeric molecules, anti-parallel dimers are formed due to dipole-dipole interactions with increasing concentrations.