Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland.
Phys Chem Chem Phys. 2011 Mar 7;13(9):3911-6. doi: 10.1039/c0cp01900c. Epub 2011 Jan 5.
The impedance spectroscopy studies performed for two strongly hydrogen-bonded liquid amides: N-methylpropionamide (NMP, CH(3)·NH·CO·C(2)H(5)) and N-ethylacetamide (NEA, C(2)H(5)·NH·CO·CH(3)) have shown that the two centers of the peptide linkage, -NH·CO-, active in the C=OH-N hydrogen bonds formation, exhibit quite different sensibilities to the steric screening effects. In contrast to the oxygen atom, a relatively small change (CH(3)- to C(2)H(5)-) 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(DC) vs. logτ(D) dependence is nonlinear and can be described with the fractional Stokes-Einstein-Debye relation, σ(DC)τ ≅ const, with the exponent s varying from about -0.8 to about -0.6 in the temperature range from 5 °C to 110 °C.
N-甲基丙酰胺（NMP，CH(3)·NH·CO·C(2)H(5))和 N-乙基乙酰胺（NEA，C(2)H(5)·NH·CO·CH(3))进行的阻抗谱研究表明，肽键的两个中心，-NH·CO-，在 C=OH-N 氢键形成中活跃，对空间屏蔽效应表现出相当不同的敏感性。与氧原子相比，氢原子屏蔽的相对较小变化（CH(3)- 到 C(2)H(5)-）导致酰胺自缔合程度的显著降低。结果，NEA 的静态介电常数和取向熵增量都明显低于 NMP。然而，发现研究的动态过程仅受到弱影响（对于直流电导率，σ(DC)）或完全不受影响（介电弛豫时间，τ(D)) 通过 NMP 和 NEA 自缔合程度的不同。实验表明，对于两种酰胺，logσ(DC) vs. logτ(D) 关系是非线性的，可以用分数斯托克斯-爱因斯坦-德拜关系描述，σ(DC)τ ≅ const，指数 s 在 5°C 至 110°C 的温度范围内从约-0.8 变化到约-0.6。
