Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Two-dimensional inverse Laplace transform NMR: altered relaxation times allow detection of exchange correlation

Abstract: A pulse sequence for a two-dimensional inverse Laplace transform NMR experiment is proposed and demonstrated. The experiment is analogous to the two-dimensional Fourier transform protocol called EXSY, but detects exchange by monitoring alterations in the transverse relaxation time rather than the NMR frequency. The sequence may be useful for measurement of exchange and diffusion of water in vivo and for detecting slow exchange phenomena in glassy polymers. The application of the Fourier transform (FT) to nuclear magnetic resonance spectroscopy' and its extension to multiple dimensions2 has radically altered modern chemistry and medical science; NMR imaging3 and the determination of the structure of proteins in solution4 are two important advances that would be either impossible or orders-of-magnitude more difficult without the FT. The crucial feature of the transformation is the increase in availablespectroscopic resolution. If two signals havedifferent precession frequencies VI and vs as a result of some coherent interaction, the time evolution of the total transverse nuclear magnetization M(r) will be the sum of the time evolution of these two species (M1(t) + Ms(t)), and at every point in time it will be dependent on VI and us. However, the FT will still show resolved signals in frequency space, thus discriminating the contributions of the two species.

In-plane phonon transport in thin films

Abstract: The in-plane phonon thermal conductivities of argon and silicon thin films are predicted from the Boltzmann transport equation under the relaxation time approximation. We model the thin films using bulk phonon properties obtained from harmonic and anharmonic lattice dynamics calculations. The input required for the lattice dynamics calculations is obtained from interatomic potentials: Lennard-Jones for argon and Stillinger–Weber for silicon. The effect of the boundaries is included by considering only phonons with wavelengths that fit within the film and adjusting the relaxation times to account for mode-dependent, diffuse boundary scattering. Our model does not rely on the isotropic approximation or any fitting parameters. For argon films thicker than 4.3 nm and silicon films thicker than 17.4 nm, the use of bulk phonon properties is found to be appropriate and the predicted reduction in the in-plane thermal conductivity is in good agreement with results obtained from molecular dynamics simulation and ex...

Solvent relaxation dynamics and electron transfer

Abstract: In this paper we explore the effects of medium dynamics on activationless and inverted-region electron transfer (ET), when medium-induced dynamics is slow on the time scale of the electronic processes. ET dynamics, with electron-nuclear coupling to the medium modes, was characterized in terms of incoherent population decay of vibronic states in the initial donor-acceptor manifold, which is characterized by nonadiabatic, energy ( E )-dependent, microscopic ET rates, k ( E ). These k ( E )'s are determined by average Franck-Condon densities (AFDs), which were evaluated by quantum and classical formalisms, with model calculations being performed for multimode harmonic systems with displaced potential surfaces. In spite of the intrinsic limitations of the classical AFDs, which do not account for mode specificity and nuclear tunneling effects, the classical Franck-Condon factors provide a good description of the E dependence of the microscopic ET rates. For activationless ET we show that k ( E )∝( E + n ϵ) − 1 2 , where n ϵ is the zero point energy, implying a weak energy dependence of k ( E ). Accordingly, the averaged experimental activationless ET rates exhibit a weak variation between the limits of slow medium-induced relaxation and that of fast medium-induced dynamics. Subsequently, the theory of k ( E ) was extended to include the effects of ET-induced excitations of high-frequency intramolecular vibrational modes, providing a unified description of the weak E dependence of k ( E ) in the activationless and inverted regions. We predict that for activationless and inverted-region ET the experimental ET rates are only weakly dependent on the characteristics of medium relaxation dynamics, and can be appreciably higher than the solvent-controlled values (i.e., the reciprocal values of the medium relaxation time induced by a constant charge distribution). Our analysis provides an adequate explanation for recent experimental observations of ultrafast ( k =(1 ps) −1 –(100 fs) −1 ) activationless and inverted-region ET, which apparently violate the predictions of solvent-controlled ET theory.

Kinetic properties and the electric field effect of the helix-coil transition of poly(gamma-benzyl L-glutamate) determined from dielectric relaxation measurements.

Abstract: Dielectric relaxation of poly(γ-benzyl L-glutamate) in solution has been studied in the 5 kcps-10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*max = 5 × 10−7 sec. The elementary process of helical growth turns out to be practically diffusion-controlled (with a rate constant of hydrogen bond formation of 1.3 × 1010 sec−1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action of an electric field.