Dipole relaxation time, study molecular dynamics

The molecular dynamics, such as flexibility and inter-intramolecular interactions, and the electrical properties of polar molecules like the poly(alkylene oxide)s can be investigated by measurement of simple and complex dielectric phenomena. From dipole relaxation times, the time and temperature dependence of polymer flexibility and mobility or viscoelasticity in bulk and in solution, which is important to flow characteristics and utility, can be analyzed and studied. It is well known that complex mechanical moduli are analogous to dielectric phenomena. 

N and 13C labeling of peptides facilitates the study of their molecular dynamics in solution by measurements of relaxation parameters (42,43). Heteronuclear relaxation times and heteronuclear NOEs are predominantly affected by the dipole-dipole interaction of the heteronucleus with the directly attached proton. Since the intemuclear (i.e., chemical bonding) distances are known from the molecular geometry, correlation times for overall and internal motions can be determined. 

When the H- H dipole-dipole interaction can be measured for a specific pair of H nuclei, studies of the temperature dependence of both the H NMR line-shape and the H NMR relaxation provide a powerful way of probing the molecular dynamics, even in very low temperature regimes at which the dynamics often exhibit quantum tunnelling behaviour. In such cases, H NMR can be superior to quasielastic neutron scattering experiments in terms of both practicality and resolution. The experimental analysis can be made even more informative by carrying out H NMR measurements on single crystal samples. In principle, studies of both the H NMR lineshape and relaxation properties can be used to derive correlation times (rc) for the motion in practice, however, spin-lattice relaxation time (T measurements are more often used to measure rc as they are sensitive to the effects of motion over considerably wider temperature ranges. 

In early years of NMR, extensive studies of molecular dynamics were carried out using relaxation time measurements for spin 1/2 nuclei (mainly for 1H, 13C and 31P). However, difficulties associated with assignment of dipolar mechanisms and proper analysis of many-body dipole-dipole interactions for spin 1/2 nuclei have restricted their widespread application. Relaxation behaviour in the case of nuclei with spin greater than 1/2 on the other hand is mainly determined by the quadrupolar interaction and since the quadrupolar interaction is effectively a single nucleus property, few structural assumptions are required to analyse the relaxation behaviour. 

NMR relaxation and its field dependence are a very important source of experimental information on dynamics of molecular motions. This information is conveyed through spectral density functions, which in turn are related to time-correlation functions (TCFs), fundamental quantities in the theory of liquid state. In most cases, characterizing the molecular dynamics through NMR relaxation studies requires the identification of the relaxation mechanism (for example the dipole-dipole interaction between a pair of spins) and models for the spectral densities/correlation functions." During the period covered by this review, such model development was concerned with both small molecules and large molecules of biological interest, mainly proteins. 

The time taken for reorganization of the solvent molecules around an instantly created dipole is termed as solvation time or solvent relaxation time (r, ) [72]. As the ILs are polar, time-resolved fluorescence studies on dipolar fluorescent molecules provide valuable information on the timescales of reorganization of the constituents of the ILs around a photoexcited molecule. The timescale of solvation depends on the viscosity, temperature, and molecular structure of the surrounding solvent [72]. As ILs are highly viscous, the solvation in ILs is a much slow process compared with that in less viscous conventional solvents. The dynamics of solvation is commonly studied by monitoring the time-dependent fluorescence Stokes shift of a dipolar molecule following its excitation by a short pulse (Scheme 7.2). This phenomenon is called dynamic fluorescence Stokes shift [73], and the solvation dynamics in several ILs has been studied by this method using various fluorescent probes. 

The main applications covered in this study are the accurate determination of rotation and rotation-vibration molecular energies the determination of the molecular geometry of simple molecules the evaluation of force field and of the vibration- and rotation-vibration interactions the measurement of pressure broadening and pressure shift of the spectral lines the determination of electric dipole moments via laser-Stark spectroscopy the studies of intramolecular dynamics the calculation of rate constants, equilibrium constants and other thermodynamic data the evaluation of relaxation times. 

Dielectric relaxation study is a powerful technique for obtaining molecular dipolar relaxation as a function of temperature and frequency. By studying the relaxation spectra, the intermolecular cooperative motion and hindered dipolar rotation can be deduced. Due to the presence of an electric field, the composites undergo ionic, interfacial, and dipole polarization, and this polarization mechanism largely depends on the time scales and length scales. As a result, this technique allowed us to shed light on the dynamics of the macromolecular chains of the rubber matrix. The temperature as well as the frequency window can also be varied over a wide... 

An alternative approach to DS study is to examine the dynamic molecular properties of a substance directly in the time domain. In the linear response approximation, the fluctuations of polarization caused by thermal motion are the same as for the macroscopic rearrangements induced by the electric field [27,28], Thus, one can equate the relaxation function < )(t) and the macroscopic dipole correlation function (DCF) V(t) as follows ... 

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