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Water
Published in Thomas M. Nordlund, Peter M. Hoffmann, Quantitative Understanding of Biosystems, 2019
Thomas M. Nordlund, Peter M. Hoffmann
Computer simulations of the molecular motion of water molecules in the condensed, liquid state show rotations on a picosecond (10–12 s) time scale. The rotational correlation time determined from dielectric and NMR (nuclear magnetic resonance) measurements is about 8 ps and measures approximately how fast the molecular motions are that govern the viscosity and rotational adjustment to perturbations in the liquid.11 We will later treat translational motion of water molecules in water and rotations of H2O in the gaseous state, but it is clear that water rotates on a very short time scale compared to most cellular processes, even when we take into account that, on average, a water molecule is H-bonded to two others at any given time. Some measurements suggest discreet trimeric (3-water), tetrameric (4-water), pentameric (5-water) and hexameric (6-water) H-bonded complexes in liquid water, though rotational vibrational spectroscopy of a hexameric form shows “geared” motions of H2O pairs, facilitated by quantum mechanical tunneling, within the hexamer on times scales of about 10−10 s.12
Fluorescence Spectroscopy and Its Implementation
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
Here, r is the measured anisotropy; r0 is the fundamental anisotropy (i.e., in the absence of any rotational diffusion); τ is the fluorescence lifetime (i.e., the average time a fluorophore stays in the excited S1 state before emitting a photon); and θ is the so-called rotational correlation time. For a brief summary of fluorescence anisotropy, polarization, and rotational correlation time, see, e.g., the appendix in Owicki (2000). Finally, it is worth noting that the said rotational correlation time θ exhibits the proportionality θ ∝ (ηV/RT), where η is the viscosity of the sample environment in which the fluorophore or the fluorophore-labeled molecule is imbedded; V is the volume of the rotating unit; and R and T are the gas constant and sample temperature, respectively.
Nanoviscosity effect on the spin chemistry of an electron donor/Pt-complex /electron acceptor triad - classical and quantum kinetics interpretation
Published in Molecular Physics, 2019
Stefan Riese, Lena Mungenast, Alexander Schmiedel, Marco Holzapfel, Nikita N. Lukzen, Ulrich E. Steiner, Christoph Lambert
In an attempt to test our classical model we sought for an experimentally easy to vary parameter with an extreme effect on the relaxation-related second step of the double Lorentzian function. Such a parameter is solvent viscosity, because the electron spin relaxation time is critically dependent on the rotational correlation time of a radical. To achieve such a high viscosity variation we selected the solvent pair THF/poly-THF (pTHF). Whereas the polarity of these two solvents is rather similar, the viscosity is about three orders of magnitude higher in the polymer. That would lead to a drastic prolongation of the rotational correlation time and, hence, to an extreme acceleration of spin relaxation at low fields. The solvent viscosity effect might even lead to a situation, where the fast motional Redfield limit is no longer valid and where a specific slow motional treatment would have to be applied. In the extreme, one could approach the situation of a rigid medium where anisotropic interactions like the ahfc become time-independent, such that both isotropic and anisotropic interactions lead to coherent spin mixing, albeit depending on the molecular orientation.