Fractional Stokes–Einstein–Debye relation and orientational entropy effects in strongly hydrogen-bonded liquid amides

Abstract

The impedance spectroscopy studies performed for two strongly hydrogen-bonded liquid amides: N-methylpropionamide (NMP, CH₃·NH·CO·C₂H₅) and N-ethylacetamide (NEA, C₂H₅·NH·CO·CH₃) have shown that the two centers of the peptide linkage, –NH·CO–, active in the CO⋯H–N hydrogen bonds formation, exhibit quite different sensibilities to the steric screening effects. In contrast to the oxygen atom, a relatively small change (CH₃– to C₂H₅–) in the screening of the hydrogen atom leads to an essential decrease of the degree of the amide self-association. As a consequence, both the static dielectric permittivity and the orientational entropy increment of NEA are essentially lower than those of NMP. However, it was found that the dynamic processes studied are only weakly influenced (in the case of dc conductivity, σ_DC) or totally not influenced (the dielectric relaxation time, τ_D) by the different degrees of NMP and NEA self-association. The experiment shows that for both the amides, the logσ_DCvs. logτ_D dependence is nonlinear and can be described with the fractional Stokes–Einstein–Debye relation, σ_DCτ^s_D ≅ const, with the exponent s varying from about −0.8 to about −0.6 in the temperature range from 5 °C to 110 °C.