Anisotropic Molecular Rotation in Liquid N,N-Dimethylformamide by Nuclear Magnetic Resonance

Abstract

The anisotropy in the molecular rotation of N,N-dimethylformamide (DMF) in the liquid is investigated by considering the NMR relaxation times of the 14N, 17O, and formyl deuteron nuclei in DMF, DMF17O, and DMF-d1, respectively. Assuming that the molecular rotation is diffusional and that one principal diffusion axis (the z axis) is perpendicular to the molecular plane, a range of possible orientations for the diffusion axes in the plane and a corresponding range of values for each of the three diffusion constants are calculated. The range of possible orientations of the x diffusion axis, which varies little with temperature, extends over about 26°, from the direction of the x principal inertial axis toward and past the direction of the molecular dipole moment. At 280°K the diffusion constants are Dx = (2.0 ± 0.4) × 1011sec−1, Dy = (6 ± 3) × 109sec−1, and Dz = (2.9 ± 0.7) × 1010sec−1. At 390°K, Dx = (4.0 ± 0.7) × 1011sec−1, Dy = (10 ± 8) × 109sec−1 and Dz = (2.1 ± 0.7) × 1011sec−1. The ratio of the rotation rate about the ith axis in the free gas to Di is a direct measure of the extent to which rotation about the ith axis is hindered in the liquid by intermolecular interactions. Defining this ratio as χi (where larger values of χi indicate a more hindered rotation), it is found that at low temperatures χy, χz ≫ χx and at high temperatures χy ≫ χz, χx. Once the rotation of the molecular framework is known, the relaxation times of the methyl deuterons in DMF-d7 can be used to calculate the internal rotation rates of the deuterated methyl groups. These rotation rates are shown to be comparable to the rate of the molecular rotation.