The ultrafast dynamics of hydrogen-bonded liquids: molecular structure-dependent occurrence of normal arrhenius or fractional Stokes−Einstein−Debye rotational diffusive relaxation

Hunt, N. T., Turner, A. R., Tanaka, H. and Wynne, K. (2007) The ultrafast dynamics of hydrogen-bonded liquids: molecular structure-dependent occurrence of normal arrhenius or fractional Stokes−Einstein−Debye rotational diffusive relaxation. Journal of Physical Chemistry B, 111(32), pp. 9634-9643. (doi:10.1021/jp072409h)

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Publisher's URL: http://dx.doi.org/10.1021/jp072409h

Abstract

The ultrafast rotational-diffusive dynamics of the peptide linkage model compounds N-methylacetamide (NMA), acetamide (Ac), and N,N-dimethylacetamide (DMA) have been studied as a function of temperature using optically heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy. Both NMA and Ac exhibit a non-Arrhenius temperature dependence of the rotational diffusive relaxation time. By contrast, the non-hydrogen-bonding DMA exhibits normal hydrodynamic behavior. The unusual dynamics of NMA and Ac are attributed to the decoupling of single-molecule rotational diffusive relaxation from the shear viscosity via a transition between stick and slip boundary conditions, which arises from local heterogeneity in the liquid due to the formation of hydrogen-bonded chains or clusters. This provides new insight into the structure and dynamics of an important peptide model compound and the first instance of such a phenomenon in a room-temperature liquid. The OHD-OKE responses of carboxylic acids acetic acid (AcOH) and dichloroacetic acid (DCA) are also reported. These, along with the terahertz Raman spectra, show no evidence of the effects observed in amide systems, but display trends consistent with the presence of an equilibrium between the linear and cyclic dimer structures at all temperatures and moderate-to-high mole fractions in aqueous solution. This equilibrium manifests itself as hydrodynamic behavior in the liquid phase.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Wynne, Professor Klaas
Authors: Hunt, N. T., Turner, A. R., Tanaka, H., and Wynne, K.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Journal of Physical Chemistry B
Publisher:American Chemical Society
ISSN:1520-6106
ISSN (Online):1520-5207

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