Dielectric Studies. XXVI. Relaxation Data and Apparent Dipole Moments of N-Methylaniline, N,N-Dimethylaniline, and N,N-Diethylaniline

Abstract

Dielectric studies have been carried out at microwave frequencies on dilute solutions of N-methylaniline, N,N-dimethylaniline, and N,N-diethylaniline, and the static dielectric constant has been measured at 2 MHz. N-Methylaniline has been studied at 25 and 50°C in cyclohexane, while N,N-dimethylaniline and N,N-diethylaniline have also been examined in this solvent at 15, 25, 35, and 50°C. N-Methylaniline and N,N-dimethylaniline have been studied at 25°C in 1,4-dioxan solution. Both the mean relaxation time and dipole moment data indicate intermolecular hydrogen bonding between N-methylaniline and 1,4-dioxan of the type NH···O. The dielectric data have been analyzed into contributions from two relaxation times, the longer of which has been identified with reorientation of the molecule. N,N-Dimethylaniline and N,N-diethylaniline yield dipole moments of the same order as that obtained by the non-dielectric absorption methods, whereas N-methylaniline has an appreciably lower moment and thus behaves similarly to aniline and o-, m-, and p-toluidine. The difference in behavior is mainly to be attributed to the presence of intermolecular hydrogen bonding of the type NH···N of the –NH2 and –NHCH3 groups which is not feasible for the –N(CH3)2 and –N(C2H5)2 groups. This leads to an additional higher-frequency dispersion exhibited by primary and secondary aromatic amines which seems (at least in part) to be related to the ability of the molecule to self-associate through forming intermolecular hydrogen bonds.

Charge Asymmetries in Hydration of Polar Solutes

“…In real solute pairs, such as these, there is not perfect charge mirroring. Thus, the dipole moment of N , N -dimethylaniline in cyclohexane is 1.5 D and that of nitrobenzene is 4.0 D …”

“…In real solute pairs, such as these, there is not perfect charge mirroring. Thus, the dipole moment of N,N-dimethylaniline in cyclohexane is 1.5 D 30 and that of nitrobenzene is 4.0 D. 31 Experimentally, nitrobenzene is more strongly hydrated, with a hydration free energy of −4.12 kcal/mol; N,N-dimethylaniline has a hydration free energy of −3.45 kcal/mol, a difference of 0.67 kcal/mol. 32 Because of the large difference in dipole moments, the Poisson-Boltzmann equation predicts a difference of 4.6 kcal/mol between the two hydration free energies.…”

Dielectric absorption of a few aromatic amines in a polystyrene matrix

“…Numerous dielectric studies have been made in the microwave region on the intramolecular motion associated with aromatic amines in the pure liquid and solution (1)(2)(3)(4)(5). Internal rotation about the C-N bond of the non planar CarOmatic-NH, grouping or nitrogen inversion or both may be considered as the possible relaxation process(es) giving rise to dielectric absorption at higher frequencies in addition to the lower-frequency one for molecular rotation (6).…”