Single water molecule reorientational

Buchner and Hefter 2009). The former relaxation time can be attributed to a concerted reorientation of several dipoles in a water cluster, due to the co-operative nature of the hydrogen bonding, and the latter, shorter, re-orientation time to the movement of a single water molecule, but its relaxation time was later revised on measurements from410GHz to 18 THz frequencies to = 0.42 ps (Fukasawa et al. 

The high quantum yield of 18% for the flip of a single water molecule becomes obvious by a comparison of the number of absorbed IR photons (typically lO s ) with the total number of water molecules (about 10 ) in the sample volume. Since a strong induced spectral diffusion is observed within several minutes, a local reorientation process with a high quantum yield must be involved. 

The dielectric response of water can be described by a Debye model including two relaxation times, a slow (x ) and a fast (x ), which are related to the collective reorientation of the hydrogen bonded liquid and the fast reorientation of a single water molecule, respectively [139,140]. According to this treatment, the solvent is modeled as a structureless fluid with a frequency-dependent dielectric constant e(m). In the case of water, e(m) is generally expressed in the Debye form ... 

The emersed electrode, in principle, may be treated as the experimental realization of a single electrode. However, it is doubtful whether its liquid layer has the same bulk properties. This is probably the main reason for the different results of E°H(abs) found for emersed electrodes, e.g., -4.85 V.83 Samec et al. have found that emersion of electrodes in a nitrogen atmosphere decreases the Volta potential and therefore the absolute electrode potential by ca. 0.32 V relative to the value in solution. They have attributed this mainly to the reorientation of the water molecules at the free surface. 

An important signature of the dynamics of water molecules is the reorientation of its dipole vector that can be probed by dielectric and NMR measurements. We have calculated the single molecule dipole-dipole time correlation function (TCF), defined as,... 

Employing the additivity approximation, we find dielectric response of a reorienting single dipole (of a water molecule) in an intermolecular potential well. The corresponding complex permittivity jip is found in terms of the hybrid model described in Section IV. The ionic complex permittivity A on is calculated for the above-mentioned types of one-dimensional and spatial motions of the charged particles. The effect of ions is found for low concentrated NaCl and KC1 aqueous solutions in terms of the resulting complex permittivity e p + Ae on. The calculations are made for long (Tjon x) and rather short (xion = x) ionic lifetimes. 

Investigation of the microscopic origin of these TLSs has demonstrated the feasibility of modulating resonance shifts in a single molecule by interrogation of neighboring solvent molecules coupled to the system.1158 In poly(methyl methacrylate) doped with free base phthalocyanine and small amounts of water, it has been shown that reorientation of nearby water molecules is the source of spectral diffusion observed in the phthalocyanine. 

The hrst molecular dynamics computation of single molecule orientational correlation functions at liquid interfaces was reported by Benjamin. In bulk water, the water dipole correlation time (4 0.2 ps) and the water HH vector correlation time (1.5 0.1 ps, which can be approximately deduced from the NMR line shape) are in reasonable agreement with experiments. The reorientation was found to be faster at the water liquid/vapor interface. The reorientation dynamics of water molecules at the water/l,2-dichloroethane interface is, in contrast, slightly slower (to 6 0.3 and 2.3 0.2 ps for the dipole and the HH vectors, respectively).Similar results were found in a recent study by Chowdhary and Ladanyi of water reorientation near hydrocarbon liquids having different structure (different branching). The slower reorientation was limited to water molecules immediately next to the organic phase. Slower dynamics were observed when the reorientation was calculated in the intrinsic frame (thus eliminating the effect of capillary fluctuations). 

Even if we consider a single solvent, e g., water, at a single temperature, say 298K, depends on the solute and in fact on the coordinate of the solute which is under consideration, and we cannot take xF as a constant. Nevertheless, in the absence of a molecular dynamics simulation for the solute motion of interest, XF for polar solvents like water is often approximated by the Debye model. In this model, the dielectric polarization of the solvent relaxes as a single exponential with a relaxation time equal to the rotational (i.e., reorientational) relaxation time of a single molecule, which is called Tp) or the Debye time [32, 347], The Debye time may be associated with the relaxation of the transverse component of the polarization field. However the solvent fluctuations and frictional relaxation occur on a faster scale given by [348,349]... 

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