Relaxation time, in water

The ratio of the values for the NMR relaxation times in water and at the silica surface are in good agreement with that observed experimentally. Effects due to ionic strength are minor. For kaolinite there is no published experimental data for comparison due to problems with uneven coagulation. However, predicted values for kaolinite are of the same order as for silica. 

This prediction is confirmed by observation of a very rapid relaxation of the spin equilibrium in Co(terpy)22+ in solution. A relaxation time of less than 15 nsec was observed in a Raman laser temperature-jump experiment (14). This is consistent with the absence of any relaxation of the small excess sound absorption found in ultrasonic experiments. An upper limit of 0.2 nsec for the relaxation time in water at 298 K can be calculated from the magnitude of the excess absorption, which is... 

The change in the emission spectrum with time after pulsed excitation (TRES) is a method for assessing the overall response of the solvent to a change in solute geometry or polarity [22]. The precise values of the relaxation times depend upon the method of measurement. At room temperature the TRES solvent correlation times are subnanosecond and, in some cases subpicosecond. The Debye relaxation time in water is 8 ps, while the TRES correlation time is shorter [22]. Although there is not, in general, a... 

A stig — dielectric increment per gm. protein per liter /r = dipole moment in debye units t H O is the relaxation time in water at 25° (correcting for the relative viscosity of water and the solvent actually employed) To = relaxation time of a sphere, of volume equal to that of the protein, in water at 25° ajb = ratio of major to minor axis, calculated from r and observed relaxation times, by the equations of Perrin (92) [Cohn and Edsall (Jd)], neglecting hydration. 

Much work has been carried out on pressure-dependent studies of properties (including diffusion measurements and spin-lattice relaxation times) in water and aqueous solutions. There has been particular interest in supercooled metastable states. Heterogeneous nucleation is dependent on the sample volume. Consequently, a small volume capillary cell is required for high-pressure NMR studies on aqueous solutions under these conditions. This was discussed in Section 5.1.2.3. In order to stabilize aqueous solutions under... 

J. C. Hindman, A. Svirmickas, M. Wood 1973, (Relaxation processes in water. A study of the proton spin-lattice relaxation time),/. Chem. Phys. 59 (3), 1517— 1522. 

In solution, [Co(terpy)2]2+ is also in a high-spin/low-spin equilibrium. Ultrasonic absorption measurements determined the spin equilibrium relaxation time in both water and MeOH solution to be less than 2 ns.249 Electron-donating functional groups such as methoxyl appended to the terpy ring result in a shift towards the high-spin form of the complex,250 as does replacement of one pyridyl ring with a pyrazole.251... 

Migus et al. s (1987) delineation of the formation of a primary species absorbing in the IR, which develops in -110 fs and which transforms to the well-known spectrum of the hydrated electron in -240 fs, which is consistent with the longitudinal dielectric relaxation time of water (Mozumder, 1969a, b). 

Reasoning on the basis of the short relaxation time of water molecules compared to the time of hydrophobic interaction, there may perhaps be small differences between the volume changes involved in equilibrium and kinetic processes. However, so far no realistic comparison has been made. 

In order to obtain information on the electron relaxation time in copper aqua ion, measurements should not be performed in water solution, because the correlation time for proton relaxation is in that case the reorientational time, which is much smaller than T g. NMRD profiles should be actually acquired in ethylene glycol solution and at temperatures lower that room temperature, so that the reorientational time increases one two orders of magnitude (see Section II.B). In this way T e of the order of 10 s can be estimated. 

In the previous discussion, the electron-nucleus spin system was assumed to be rigidly held within a molecule isotropically rotating in solution. If the molecule cannot be treated as a rigid sphere, its motion is in general anisotropic, and three or five different reorientational correlation times have to be considered 79). Furthermore, it was calculated that free rotation of water protons about the metal ion-oxygen bond decreases the proton relaxation time in aqua ions of about 20% 79). A general treatment for considering the presence of internal motions faster than the reorientational correlation time of the whole molecule is the Lipari Szabo model free treatment 80). Relaxation is calculated as the sum of two terms 8J), of the type... 

Fig. 1. Magnetic field dependences of the proton spin-lattice relaxation time of water in Bioran B30 and Vycor glasses at temperatures above 27°C and below the temperature where the non-surface water freezes ( —25°C and —35°C). The solid lines represent the power law in the Larmor frequency with an exponent of 0.67 (34).

Table 4 Estimated average NMR relaxation times of water in various environments...

Regardless of the truth of these stories, it is undeniable that microwave radiation can be hazardous to life. One damage mechanism is merely intense heating of the water bound up in all living organisms. It is clear, therefore, that the potential hazard of radiation of a given frequency depends on the optical constants, particularly c", of water at that frequency. For example, at room temperature the maximum value of c" occurs at about 20 GHz (see Fig. 9.15), that is, at the relaxation frequency 1/2 ttt, where r is the relaxation time in (9.41). 

Figure 4. Temperature dependence of the longitudinal relaxation time of water in the water-NaLS system.

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