Spin-lattice relaxation, aqueous system

Chief among the interfacial properties of aqueous systems that suggest the occurrence of thermal anomalies are the following index of refraction, density, activation energy for ionic conductance, rates of surface reactions, surface tension, surface potentials, membrane potentials, heats of immersion, zeta potentials, rate of nucleation, viscous flow, ion activities, proton spin lattice relaxation times, optical rotation, ultrasonic velocity and absorption, sedimentation rates, coagulation rates, and dielectric properties. 

C Spin lattice relaxation times, Tj, spin-spin relaxation times, T2, and nuclear Overhauser enhancements, NOE, for the a-carbons of PBLG of various molecular weights have been used to study transitions from rigid to flexible forms of this polymer (Allerhand and Oldfield, 1973). Effective rotational correlation times, reff, calculated from 7 - and NOE-values, for the a-carbons were 24-32 nanoseconds for the helical form and approximately 0-8 nanoseconds for the random coil (Allerhand and Oldfield, 1973). The transition from the a-helix to the random-coil of PLM causes the resonances of the a- and carbonyl carbons to move upfield 2-3 and 3-4 ppm respectively (Tadokoro et al., 1973), which is consistent with results obtained for PBLG and PCBO. Further work is required before the reasons for the chemical shift differences between the corresponding carbons in the helical and random-coil forms in deuterochloroform-TFA systems can be elucidated. Plots of chemical shifts and relaxation times vs. pH have been used to study the helix-coil transition of poly-L-lysine hydrochloride in aqueous solution (Saito and Smith,... 

A chromia-alumina catalyst may contain protons both as surface hydroxyl groups and as adsorbed water, and therefore it exhibits a proton NMR spectrum. Alumina itself has been extensively studied by proton NMR, but this experimental technique has not been widely applied to the chromia-alumina system. Until recently, the nearest approach to this application was by Selwood and his co-workers 165, 166) who found that the spin-lattice relaxation time of the protons in water was modified by contacting the water with a chromia-alumina catalyst. By comparing this effect with that of Cr + ions in aqueous solution, the authors obtained a rough index of the accessibility of the ohromia, which in effect could be regarded as an approximate measure of the number of surface chromium ions. Ibers et al. 167) studied the proton NMR of HCr02. 

Table 5.56 lists the main features of NMRI. As NMRI is non-invasive, multiple measurements can be made on the same sample under different conditions. The main problem with NMRI is the long data collection time, mainly due to the long spin-lattice relaxation time Ti ( 0.5 s for aqueous systems). The high cost of imaging facilities is a hindrance to exploitation of NMRI in polymer science. However,... 

Spin-echo techniques have a unique position in NMR applications. The main problem with NMR imaging is the long data collection time, due mainly to the spin-lattice relaxation time Each measurement necessitates a time period of the order of T, (which is 0.5 s for aqueous systems) for the system to return to... 

Furthermore, the short scan times of EPR (usually 500 ms or less) and the ability to measure species in diamagnetic matrices, for example, aqueous solutions, enable the time-resolved monitoring of chemical reactions involving radical reactants or intermediates. In this way, kinetics of such reactions can be studied even if multiple magnetic species are involved, as their characteristic signals typically differ sufficiently to deconvolute the resulting EPR spectra. Commercial pulsed EPR spectrometers are also available, enabling the study of spin dynamics, that is, the relaxation of the excited system via spin-spin and spin-lattice mechanisms. 

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