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

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.

Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

“…All reasonable relaxation models up to n = 4 were tested, including fits with inertia-corrected (superscript i ) variants of the HN, CD and D equations. 37 The fits were assessed by their values of the reduced error function, w r 2 , and the smooth concentration dependence of the obtained parameters.…”

Universal nonexponential relaxation: Complex dynamics in simple liquids

“…For this, the empirical function 1 − exp͑−k RT t͒ has been used. 5,12 However, in analyzing these data, we find that a steeper rise is necessary to be consistent with the oscillatory behavior at high frequency through which the anisotropy arises and here a Gaussian rise function is employed. As is seen in Fig.…”

“…Ultrafast optical Kerreffect ͑OKE͒ spectroscopy probes many-body interactions and is established as a powerful probe of structural relaxation in both complex and simple liquids. [5][6][7][8][9][10][11][12] However, no detailed OKE study of the noble-gas liquids exists. OKE spectroscopy ͑the time-domain variant of Raman spectroscopy͒ measures the time-derivative of the two-point time-correlation of the anisotropic part of the many-body polarizability tensor.…”

“…12 The OKE response of these liquids is relatively weak, so sample purity is critical and as it was found that atmospheric gases, including nitrogen, are readily dissolved by the samples, especially argon, the apparatus was made gas-tight. A gradient-seal quartz cuvette ͑Starna Optiglass͒ was extended by fusing an additional 30 cm glass tube to the opening.…”

Local Orientational Order in Liquids Revealed by Resonant Vibrational Energy Transfer

“…To quantitatively investigate the relation between liquid structure and anisotropy decay, we performed molecular dynamics (MD) simulations of two hydrogen-bonded liquids for which evidence exists that the angles between nearest neighbors are very different: ethanol (CH 3 -CH 2 -OH, EtOH) and liquid N-methylacetamide (CH 3 -NH-CO-CH 3 , NMA). Simulations and experiments have shown evidence that in NMA the molecules form linear chains connected by NH Á Á Á OC hydrogen bonds, such that neighboring NH groups are approximately parallel [27][28][29], whereas in ethanol hydrogen bonding occurs between the OH group of one molecule and a lone-electron pair on the O atom of another, such that the angle between two hydrogen-bonded OH groups is approximately 180°− 109.5°≈ 70° [30,31]. This difference in local ordering is also observed in our MD simulations [32].…”