Relaxation Of Quadrupolar Nuclei Research Articles

NMR Relaxation Rates of Quadrupolar Aqueous Ions from Classical Molecular Dynamics Using Force-Field Specific Sternheimer Factors.

The nuclear magnetic resonance (NMR) relaxation of quadrupolar nuclei is governed by the electric field gradient (EFG) fluctuations at their position. In classical molecular dynamics (MD), the electron cloud contribution to the EFG can be included via the Sternheimer approximation, in which the full EFG at the nucleus that can be computed using quantum density functional theory (DFT) is considered to be proportional to that arising from the external, classical charge distribution. In this work, we systematically assess the quality of the Sternheimer approximation as well as the impact of the classical force field (FF) on the NMR relaxation rates of aqueous quadrupolar ions at infinite dilution. In particular, we compare the rates obtained using an ab initio parametrized polarizable FF, a recently developed empirical FF with scaled ionic charges and a simple empirical nonpolarizable FF with formal ionic charges. Surprisingly, all three FFs considered yield good values for the rates of smaller and less polarizable solutes (Li+, Na+, K+, Cl-), provided that a model-specific Sternheimer parametrization is employed. Yet, the polarizable and scaled charge FFs yield better estimates for divalent and more polarizable species (Mg2+, Ca2+, Cs+). We find that a linear relationship between the quantum and classical EFGs holds well in all of the cases considered; however, such an approximation often leads to quite large errors in the resulting EFG variance, which is directly proportional to the computed rate. We attempted to reduce the errors by including first order nonlinear corrections to the EFG, yet no clear improvement for the resulting variance has been found. The latter result indicates that more refined methods for determining the EFG at the ion position, in particular those that take into account the instantaneous atomic environment around an ion, might be necessary to systematically improve the NMR relaxation rate estimates in classical MD.

Dynamic isotope effects on relaxation of quadrupolar nuclei in 12 simple organic molecules

Dynamic isotope effects on relaxation rate of quadrupolar nuclei are preliminarily reported. The relaxation rates of 17O and 14N in 12 simple organic molecules and their 18 corresponding deuterated species have been systematically measured. The principal components of the molecular inertia tensors have been calculated. The results show that there is an intrinsic correlation between the dynamic isotope effects of the relaxation rate and the static isotope effects of the molecular inertia. The concepts of molecular collision frequency and translation-rotation coupling have been introduced into the NMR relaxation theory. Therefore, a reasonable explanation of the experimental results has been given.

Some Recent Developments in NMR Approaches for Studying Liquid Molecular Dynamics and Their Biological Applications

AbstractTwo aspects of NMR relaxation are reviewed and their use in studying molecular dynamics in the liquid state and their application to biological systems are discussed. The first aspect concerns the use of ‘null‐point spectra’ for observing the effects of higher rank multipoles (tensors) in the relaxation of quadrupolar nuclei. The second aspect concerns the effects of cross‐correlation in homo‐ and heteronuclear spin systems. The application of null point spectral analysis to study cross‐correlation in homonuclear spin systems is also discussed. Analysis of the effects of higher rank multipoles and of cross‐correlation effects are shown to provide substantially more structural and motional information about spin systems than that available from traditional relaxation measurements. While similar in information content to multiple quantum techniques, the null point technique is experimentally less demanding. Over the past several years, work in this laboratory has been heavily involved in the theoretical formulation and the application of these techniques to biological systems. Illustrations of the use of these techniques to biological systems are drawn from our recent work.

Micelle size and order in lyotropic nematic phases from nuclear spin relaxation

Nuclear magnetic resonance (NMR) relaxation of quadrupolar nuclei is introduced as a new method for determining micelle size and nematic order in lyotropic nematic mesophases from the dependence of the spin relaxation rates on molecular diffusion over the curved micelle surface. The approach is illustrated by an experimental study of two uniaxial nematic phases (the calamitic NC and discotic ND phases of the sodium dodecyl sulphate/decanol/water system) using two nuclei: 2H in the α-deuterated surfactant and 23Na in the counterions. The two nuclei yield similar results: an apparently temperature independent axial ratio of 3–4 in both phases and a nematic order parameter which decreases from ca. 0.9 (0.75) at the lowest temperature to ca. 0.6 (0.5) at the highest temperature in the NC (ND) phase. As compared to the predictions of the Maier–Saupe theory, the nematic order parameter in the NC phase is considerably larger and decreases more strongly as the nematic–isotropic transition is approached.

NMR Relaxation of quadrupolar Nuclei as a Probe of the Motion of Guest Species in Sodalite Cavities

AbstractUseful information on interactions and motion of guest molecules in sodalite cavities is obtained by estimating the spin-lattice and spin-echo decay rates of encapsulated Br− ions. An extremely fast spin-lattice relaxation rate is observed for 81Br nuclei, adjacent to Br−-empty cages which contain rapidly tumbling water molecules. We detect a further significant different T1 of the 81Br in Na8Br2- sodalite and AgsBr2-sodalite, respectively, which probably indicates a less restricted motion within the cavities of the Ag+-exchanged material. Spin-echo decay experiments additionally indicate a difference between the dynamic properties of the Br− anion in the encapsulated Na4Br and Ag4Br clusters, respectively.

Accurate measurement of NMR amplitudes using a computer-controlled wide line cw nuclear double resonance spectrometer

A computer-controlled cw nuclear magnetic double-resonance spectrometer, designed for studying the spin-lattice relaxation of quadrupolar nuclei in single crystals, is described. Resonances are detected by a Robinson spectrometer using a technique involving sinusoidal field modulation in conjunction with first and second harmonic phase-sensitive detection. The output of the first harmonic detector provides an error signal, which is used to ''lock'' the spectrometer to the center of the resonance. The output of the second harmonic detector is used as the measure of NMR amplitude. Comprehensive tests have shown that the amplitudes of weak NMR signals can be compared with an accuracy of better than +/-0.1%. Numerous precautions to obtain such accuracy are discussed.