Structural relaxation in the hydrogen-bonding liquids N-methylacetamide and water studied by optical Kerr-effect spectroscopy

TL;DR

This study investigates the structural relaxation dynamics of N-methylacetamide and water using ultrafast optical Kerr-effect spectroscopy, revealing the presence of alpha and beta relaxations with Arrhenius behavior and proposing modifications to traditional relaxation models.

Contribution

It provides the first detailed comparison of relaxation processes in N-methylacetamide and water, introducing modified Cole-Cole functions to better model physical relaxation behavior.

Findings

Beta relaxation is present in both liquids and fits the Cole-Cole function.

Alpha and beta relaxations in NMA follow Arrhenius temperature dependence.

Proposed modifications improve the physical realism of relaxation models.

Abstract

Structural relaxation in the peptide model N-methylacetamide (NMA) is studied experimentally by ultrafast optical Kerr-effect spectroscopy over the normal-liquid temperature range and compared to the relaxation measured in water at room temperature. It is seen that in both hydrogen-bonding liquids, beta relaxation is present and in each case it is found that this can be described by the Cole-Cole function. For NMA in this temperature range, the alpha and beta relaxations are each found to have an Arrhenius temperature dependence with indistinguishable activation energies. It is known that the variations on the Debye function, including the Cole-Cole function, are unphysical, and we introduce two general modifications: one allows for the initial rise of the function, determined by the librational frequencies, and the second allows the function to be terminated in the alpha relaxation.