Water rotational dynamics

We discuss the rotational dynamics of water molecules in terms of the time correlation functions, Ciit) = (P [cos 0 (it)]) (/ = 1, 2), where Pi is the /th Legendre polynomial, cos 0 (it) = U (0) U (it), u [, Is a unit vector along the water dipole (HOH bisector), and U2 is a unit vector along an OH bond. Infrared spectroscopy probes Ci(it), and deuterium NMR probes According to the Debye model (Brownian rotational motion), both... 

In many products, the spin-relaxation properties of the components can be different due to molecular sizes, local viscosity and interaction with other molecules. Macromolecules often exhibit rapid FID decay and short T2 relaxation time due to its large molecular weight and reduced rotational dynamics [18]. Mobile water protons, on the other hand, are often found to have long relaxation times due to their small molecular weight and rapid diffusion. As a result, relaxation properties, such as T2, have been used extensively to quantify water/moisture content, fat contents, etc. [20]. For example, oil content in seeds is determined via the spin-echo technique as described according to international standards [64]. 

A = solv, pol. The striking similarity in the spectra and their components is evident, justifying the use of OKE data to model SD in this liquid. It should be noted that the two experiments have very (tifierent dynamical origins in some liquids such as, for example, water, where SD is strongly dominated by rotational dynamics, " whereas OKE probes mainly translational motions due to the very small molecular polarizability anisotropy. ... 

In these equations, the symbols have their customary meanings (see Toth et al. in this volume for an excellent review of the topic), and the correlation times given in Eq. (3) have the following typical values at 50 MHz in water Tle (electron spin-lattice relaxation time) =10 ns, T2e (electron spin-spin relaxation time) = 1 ns, rm (inner sphere water exchange correlation time) = 130 ns [3], and rR = 60 ps. These values, in the context of Eq. (1 - 3), show why rotational dynamics control relaxivity for such chelates. 

Now we shall generalize previous consideration (1) by taking into account also the effect of the bending force constant Ka on rotational dynamics of an H-bonded molecule, (2) by rigorous solution of the ID equation of motion, and (3) by application theory also for heavy water. 

Very recently, detailed further analyses of the translation and reorientation dynamics of SCW have been reported showing consistency with our results in the limit of very low-density supercritical states for water [47], The dynamical behavior of SCW in this study is also compared to that of supercritical benzene showing that the density dependence of the self-diffusion coefficient and rotational dynamics of SCW is smaller than that of supercritical benzene because SCW is capable of maintaining stronger degrees of structural correlations and orientational anisotropy than benzene, which tends to lose intermolecular correlations at a much faster rate upon decreasing density [47],... 

In-plane and out-of-plane rotational dynamics of CigRB at the toluene/water interface was evaluated using time-resolved TIR fluorescence spectroscopy [27]. The known transition dipole moment for the absorption of rhodamine B(RB) at about 530 nm (So Si) is almost parallel to that for the emission at about 570 nm (Si -> So) [28]. Time-resolved in-plane fluorescence anisotropy (r[Pg.213]

Faraone A, Liu L, Mou CY, ShuhPC, Copley JRD, Chen SH. (2003) Translational and rotational dynamics of water in mesoporous silica materials MCM-41-S and MCM-48-S. J Chem Phys 119 3963. 

Zawodzinski et al. [58] have reported NMR relaxation measurements on water in Nafion membranes. In contrast with proton NMR relaxation studies, which are difficult to interpret because of various inseparable contributions to the observed relaxation rates, a direct relationship often exists between the observed relaxation rate and rotational dynamics of the deuteron-bearing species. The time scale probed by such measurements is in the pico- to nanosecond range, and thus very short-range motions are probed. In a membrane equilibrated with saturated water vapor, a Ti on the order of 0.2s was observed. This relaxation rate for D2O in the membrane is only higher by a factor of two than that in liquid D2O, indicating a bulk water-like mobility within the pore at high membrane hydration levels. The relaxation rate increases (i.e., local water motion in the membrane becomes slower) as the water to ion-exchange site ratio decreases. 

MD simulations of [C6mim][PF6] and water mixtures were carried out [98], The authors found water to be closely associated with the anions and that its presence enhances both the translational and rotational dynamics of the IL. The mean square displacement and rotational correlation functions reflected that the diffusive regime was achieved faster when water was present in the IL or the observed decay of correlations was faster when water was present, respectively [98], From this the authors deduced that the experimentally observed decrease in viscosity is a consequence of the faster translational and rotational dynamics caused by the presence of water [98], In agreement with experiments, the authors found that the fluorescence spectra of Coumarin-153 is red-shifted because of the presence of water [98],... 

For the mixture of [bmim]Br and D20, anomalous dynamics were found utilizing diffuson NMR as well as lH and 81Br NMR relaxation measurements [41]. The addition of water here also was found to lead to a network of hydrogen bonding and very dilute samples behaved in a special manner (whatever that may mean). The strength of solvation for D20 and C6D6 mixtures with [bmim]Cl and -PF6 was investigated via the rotational correlation time (cf. Sect. 2.2) [42]. Water rotated two times slower in the Cl-IL as compared with the PF6-IL while no difference in... 

Water hydration and counter-ion solvation of biological macro molecules considerably alter the translational and rotational dynamics ofbiological macromolecules. The effect can be correlated with a number of different solution properties of a given biological macromolecule such as the hydrated volume Vh- The term Vh is given by... 

To be historically fair, other people did observe the existence of jump motions in the rotation of water molecules in the liquid state but detailed analysis of the dynamics of an individual event was not carried out before. Given that perspective, the Laage-Hynes mechanism of water rotation by large-amplitude jumps is indeed a departure from conventional and prevailing wisdom that water rotation is Brownian that is, it occurs differently in water from in other liquids where motion by small steps dominates. Experimental verification of the jump diffusion model came from a beautiful study of the temperature-dependent rate of water rotation. However, both the experiments and the interpretation of results are quite involved. We shall discuss the results as simply as possible. 

The absence of slow dynamics in this system can be attributed to the fact that the penta-alanine peptide does not have any polar side-chain atom which can form a strong HB with water. With a higher level of hydration, the rotational dynamics of water approached that of bulk water, again as expected. A QENS study of protein dynamics was also carried out on the picosecond timescale of a protein, lysozyme solvated in glycerol at different water contents, h (g water/g lysozyme). For all h, a well-visible low-frequeney vibrational bump was observed. The quasi-elastic scattering can be decomposed into two Lorentzian components, corresponding to motions with charaeteristic time constants of 15 ps and 0.8 ps. The 15 ps component is the slow component, which is in the same range observed in many other experimental studies. 

The rotational dynamics on the protein surface is basically shaped by electrostatic interactions alone and the HBs formed by water with the protein surface break the quasi-isotropic nature of the dipolar rotation that is found in the bulk. Also, for the fiilly thermalized protein, a ratio between the characteristic times of the first and the second dipole-dipole correlation function, yff = /r, of about 5 is at variance... 

Translational and rotational dynamics of water molecules in the grooves 153... 

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