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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.
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PMID:The ultrafast dynamics of hydrogen-bonded liquids: molecular structure-dependent occurrence of normal arrhenius or fractional Stokes-Einstein-Debye rotational diffusive relaxation. 1764 27

In a previous study of room temperature ionic liquid/water mixtures, the first clearly observed biexponential decays in optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments on a liquid were reported, (Sturlaugson, A. L.; Fruchey, K. S.; Fayer, M. D. J. Phys. Chem. B 2012, 116, 1777), and it was suggested that the biexponential behavior is indicative of the approach to gelation. Here, new OHD-OKE experiments on mixtures of the room temperature ionic liquid 1-methyl-3-octylimidazolium chloride (OmimCl) with water are presented. The OmimCl/water system is shown to gel over the water mole fraction range of 0.69-0.81. In the OHD-OKE decays, the biexponential behavior becomes more distinct as the gelling concentration range is approached from either high or low water concentrations. The biexponential decays are analyzed in terms of the wobbling-in-a-cone model, and the resulting diffusion constants and "relative" order parameters and cone angles are reported. Comparison of the OmimCl/water data with the previously reported room temperature ionic liquid/water OHD-OKE decays supports the previous hypothesis that the biexponential dynamics are due to the approach to the liquid-gel transition and suggests that the order of the concentration-dependent phase transition can be tuned by the choice of anion.
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PMID:Orientational dynamics in a lyotropic room temperature ionic liquid. 2417 52