Spin-locking Conditions Research Articles

133Cs Nuclear Magnetic Resonance Relaxometry as a Probe of the Mobility of Cesium Cations Confined within Dense Clay Sediments

133Cs nuclear magnetic resonance is used to quantify the mobility of cesium counterions confined within the interlamellar space of synthetic Hectorite clay under controlled hydration conditions. The degree of lamellae ordering within the macroscopic clay sediment is determined by analyzing the variation of the 133Cs NMR spectra as a function of the sample orientation in the static magnetic field. Furthermore, a detailed analysis of the 133Cs NMR spectra allows one to determine the order of magnitude of the quadrupolar and heteronuclear dipolar couplings monitoring the NMR relaxation of the cesium cations confined within clay sediments. Finally, a lower limit to the average residence time of the cesium cations within the clay interlamellar space is determined by using 133Cs NMR relaxation measurements under spin-locking conditions.

Multiscale Water Dynamics within Dense Clay Sediments Probed by 2H Multiquantum NMR Relaxometry and Two-Time Stimulated Echo NMR Spectroscopy

2H NMR spectroscopy, relaxometry, and two-time correlation measurements are used to investigate the structural and dynamical properties of water molecules confined within the multiscale porous network of dense clay sediment. The residual quadrupolar splitting detected by 2H NMR spectroscopy is the fingerprint of the specific orientation of the water molecules pertaining to the first hydration shell of clay lamellae. Multiquantum 2H NMR relaxation measurements are used to quantify the distribution of the clay platelet orientations within the dense sediment. The average residence time of the water molecules confined within the clay interlamellar space is determined by exploiting 2H multiquantum NMR relaxation measurements under spin-locking conditions. Finally, long-time scale diffusion of the water molecules within the multiscale porous network of the clay sediment is quantified by measuring the attenuation of the 2H NMR two-time stimulated echo.

Density matrix tomography of singlet states

First direct and quantitative study of singlet states using density matrix tomography is reported. A robust scheme for the tomography of a general density matrix of two spin 1/2 nuclei is introduced for this purpose. The study is carried out at different spin-lock conditions and the results are compared.

Long-Time Scale Ionic Dynamics in Dense Clay Sediments Measured by the Frequency Variation of the7Li Multiple-Quantum NMR Relaxation Rates in Relation with a Multiscale Modeling

7Li NMR relaxation measurements under spin-locking conditions are used to probe the dynamical properties of the lithium counterions within dense dispersions of charged anisotropic nanoplatelets. By simultaneously measuring the T1ρ and T2ρ relaxation times in addition to triple-quantum filtered relaxation times under the same spin-locking conditions, it is possible to separately quantify the contributions from the quadrupolar and the heterogeneous dipolar relaxation mechanisms. Thanks to the contribution from the residual quadrupolar coupling felt by the condensed lithium counterions, that procedure allows a broad dynamical range to be probed by performing spin-locking relaxation measurements using a limited number of irradiation powers. As illustrated by a multiscale modeling of the lithium diffusion and relaxation within such heterogeneous system, the frequency variation of the spectral densities characterizing the decorrelation of the quadrupolar coupling is a sensitive probe of the ionic mobility and the structure of the colloidal dispersion.

Nuclear magnetic resonance noise spectroscopy using two-photon excitation

Two-photon excitation is employed in conjunction with stochastic nuclear magnetic resonance (NMR) at high magnetic field to acquire pulsed NMR spectra with high sensitivity in the absence of an on-resonance radio-frequency excitation. These conditions allow the NMR receiver to be active during excitation and therefore eliminate the receiver dead-time. Examples are presented demonstrating the direct observation of nuclear magnetization under nutation and spin-locking conditions. Applications of the techniques are discussed.

Thermal relaxation and coherence dynamics of spin 3/2. II. Strong radio‐frequency field

AbstractThe relaxation dynamics of the spin‐3/2 density operator under strong, on‐resonance radio‐frequency (RF) irradiation and in the presence of fluctuating and static quadrupolar interactions is reviewed. The evolution under the lock is analyzed for any value of the static quadrupolar interaction, ranging from isotropic systems to systems exhibiting large splitting far exceeding the line widths. For isotropic systems, relaxation under off‐resonance spin‐locking conditions and in the multiple‐pulse quadrupolar echo (QE) experiment is reviewed also. Spin‐lock pulse sequences optimized for the selective detection of nuclei involved in slow molecular motion and/or in anisotropic, liquid crystalline environment are discussed. In Part I, the classical relaxation dynamics of spin 3/2 was reviewed. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part A 19A: 117–133, 2003.

Microwave-Induced Quantum Beats in Micellized Radical Pairs under Spin-Locking Conditions

The effect of short microwave pulse irradiation on the spin correlated radical pairs generated by the hydrogen abstraction reaction of triplet excited xanthone with tert-butyl-substituted phenols in a micellar sodium dodecyl sulfate solution is investigated by a time-resolved transient optical absorption technique. An increase in the microwave pulse duration under the electron spin resonant conditions leads to the alternation of the radical pair concentration. It is explained by the periodical change of the radical pair spin state population with an angular frequency of ω1 = γeB1 due to the pumping with resonant microwave field B1. The double frequency (2ω1) quantum beats are observed under spin-locking conditions. The behavior of quantum beats in this case is rationalized by the effect of microwave pulse on the electron spins of both radicals in the radical pair and reflects the spin dynamics in the triplet manifold of radical pairs that is isolated from that of the singlet during the microwave pulse by ...

Dynamics of spin I=3/2 under spin-locking conditions in an ordered environment.

We have derived approximate analytic solutions to the master equation describing the evolution of the spin I=3/2 density operator in the presence of a radio-frequency (RF) field and both static and fluctuating quadrupolar interactions. Spectra resulting from Fourier transformation of the evolutions of the on-resonance spin-locked magnetization into the various coherences display two satellite pairs and, in some cases, a central line. The central line is generally trimodal, consisting of a narrow component related to a slowly relaxing mode and two broad components pertaining to two faster relaxing modes. The rates of the fast modes are sensitive to slow molecular motion. Neither the amplitude nor the width of the narrow component is affected by the magnitude of the static coupling, whereas the corresponding features of the broad components depend in a rather complicated manner on the spin-lock field strength and static quadrupolar interaction. Under certain experimental conditions, the dependencies of the amplitudes on the dynamics are seen to vanish and the relaxation rates reduce to relatively simple expressions. One of the promising emerging features is the fact that the evolutions into the selectively detected quadrupolar spin polarization order and the rank-two double-quantum coherence do not exhibit a slowly relaxing mode and are particularly sensitive to slow molecular motion. Furthermore, these coherences can only be excited in the presence of a static coupling and this makes it possible to discern nuclei in anisotropic from those in isotropic environment. The feasibility of the spin-lock pulse sequences with limited RF power and a nonvanishing average electric field gradient has been demonstrated through experiments on sodium in a dense lyotropic DNA liquid crystal.

NMR Relaxation in Multipolar AX Systems under Spin Locking Conditions

A relaxation matrix has been calculated for a multipolar AX spin system under the on-resonance spin-locking condition. Auto- and cross-correlation terms between dipolar, quadrupolar, and CSA interactions are considered. It is shown that the spin-lock leads to many relaxation pathways being blocked, resulting in a considerably simplified relaxation network. The presence of spectral densities at zero frequency, associated with locked nuclei, allows efficient relaxation also in the absence of fast molecular motions.

Illustrating NMR cross-polarization (CP) with a microcomputer

As a continuation of previous works, Quick Basic computer programs have been developed for the solution of the equations describing the transfer of polarization in a system of coupled spins, when spin-lock conditions are applied. The transfer is considered to take place through a simple model of dipolar spin-spin interactions. The presence of a randomly changing surrounding is modeled by the random number generator of a microcomputer. The discussed examples allow one to understand the effects of cross-polarization as a function of the correlation time τ0, and to introduce the concept of spin temperature. ©1998 John Wiley & Sons, Inc. Concepts Magn Reson 10: 343–354, 1998

Illustrating rotating‐frame (T1) NMR relaxation with a microcomputer

This paper is a continuation of previous works, in which QuickBasic computer programs are used to solve equations that describe the loss of quantum coherence in a two-state spin system, in the present case spin-locking conditions are applied. The coherence is considered to be lost by interaction with a randomly changing surrounding, modeled by the random-number generator of a microcomputer. The examples illustrate the behavior of rotating-frame relaxation time (T1ρ) as a function of correlation time (τ0). ©1998 John Wiley & Sons, Inc. Concepts Magn Reson 10: 157–166, 1998

11B{ 27Al} and Al{ 11B} double resonance experiments on a glassy sodium aluminoborate

Cross-polarization/magic-angle spinning (CP/MAS) and rotational echo double resonance (REDOR) experiments involving two half-integer quadrupolar nuclei, 11B and 27A1, are reported, to demonstrate boron-aluminum connectivities in a model aluminoborate glass. A detailed study of the spin-lock behavior of 11B and 27A1 proves to be a prerequisite for successful CP/MAS experiments. Under MAS conditions, two distinct boron sites are observed, corresponding to tetrahedral BO − 4 2 sites (nuclear electric quadrupole coupling constant near 0.3 MHz) and trigonal BO 3 2 sites (nuclear electric quadrupole coupling constant near 2.7 MHz). The BO − 4 2 sites are most successfully spin-locked in the adiabatic regime at high radio frequency (RF) field strengths, whereas for the BO 3 2 sites optimum spin-lock conditions are achieved in the sudden regime (low RF field strengths). These differences can be exploited for spectral editing purposes in REDOR experiments. Using corresponding T 1 π filters, it becomes possible to measure individual REDOR dephasing curves for both types of boron sites. The results illustrate the possible utility of heteronuclear X-Y double resonance techniques in unravelling the intermediate range order in amorphous systems containing quadrupolar nuclei.

Polymeric coordination complexes based on cobalt, nickel, and ruthenium that exhibit synergistic thermal properties

d-Block transition-metal-containing polymer blends which form coordination complexes are the focus of this research. The model compounds are Co chloride hexahydrate, Ni acetate tetrahydrate, and the dimer of dichlorotricarbonylruthenium (II). The ligand is poly (4-vinylpyridine), P4VP, or copolymers that contain 4-vinylpyridine repeat units. Differential scanning calorimetry suggests that the glass transition temperature (gtt) of the polymeric ligand(s) is enhanced by these d-block metal salts in binary and ternary blends. Co and Ni salts function as transition-metal compatibilizers for two immiscible copolymers of styrene with 4-vinylpyridine and butyl methacrylate with 4-vinylpyridine. At the molecular level, FTIR of P4VP-Ru precipitates reveals that the pyridine nitrogen lone pair coordinates to the metal center and strengthens Ru-carbonyl bonds in the polymeric complex. IR absorption frequencies of the CO ligands are consistent with {pi}-backbonding between the t{sub 2g} molecular orbitals of the metal in the octahedral point group and the {pi}* antibonding orbitals of CO. High-resolution C-13 solid-state NMR spectroscopy identifies at least two, and possibly three, carbonyl signals in the undiluted pseudooctahedral Ru dimer via the Bloch-decay pulse sequence. In the polymeric complex, carbonyl C-13 magnetization unique to the Ru salt is generated via intermolecular polarization transfer from the proton spin manifold of poly(4-vinylpyridine)more » using a cross-polarization thermal mixing time of 2 ms. Since there are no protons in the Ru dimer, the observation of energy-conserving H-1/C-13 spin diffusion between dissimilar molecules under matched Hartmann-Hahn spin-lock conditions argues convincingly that heteronuclear dipolar distances are small enough for the proposed polymeric complex to form. Thermodynamic interpretation of ligand field stabilization energies appropriate to tetrahedral Co complexes is employed to estimate the synergistic enhancement of the gtt.« less

Dynamic nuclear polarization enhanced nuclear magnetic resonance of polymer-blend interfaces

Heterogeneous blends of [3,3′- 13C 2]polycarbonate and [uniform-ring- 12C 6]polystyrene were formed by serial film casting. The polystyrene phase of each blend was homogeneously doped with 2% by weight of a bis (diphenylene) phenylalyl free-radical complex with benzene. Proton polarization enhanced by dynamic nuclear polarization was generated in the polycarbonate phase by dipolar coupling to electrons in the polystyrene phase under 39 GHz microwave irradiation at the difference of the electron and proton Larmor frequencies. Proton magnetization was then transferred to carbons under matched spin-lock conditions for detection with chemical-shift selectivity by magic-angle spinning 13C nuclear magnetic resonance. The 13C signal from polycarbonate arises exclusively from chains which are at the polycarbonate-polystyrene interface. Signals from bulk polycarbonate were suppressed by differencing techniques. The dominant mechanism of polarization transfer from the electrons in the polystyrene phase to the protons in the polycarbonate phase is by direct polarization transfer. The interface signal arises from a 60 Å region which is 2% of the film thickness. As monitored by dynamic nuclear polarization selected dipolar rotational spin-echo 13C nuclear magnetic resonance, polycarbonate chains have less motion at the interface than in the bulk.

Radio-frequency-driven nuclear spin diffusion in solids

The rate of spin diffusion between natural abundance 13C spins in rigid solids is enhanced by more than two orders of magnitude by performing the experiment under spin-locking conditions. While the application of a continuous wave spin lock suffers from a strong offset dependence, spin locking by a WALTZ-17 sequence easily covers the entire 13C spectrum using radio-frequency fields of less than 50 kHz. Simultaneous continuous wave proton decoupling is required. The principal features of the experiment are demonstrated on adamantane. In 1,5-dimethylnaphthalene spin diffusion between all the carbons was observed in contrast to the conventional proton-driven experiment.

Dynamic 2H NMR by quadrupole echo train in the fast-exchange limit. Ring inversion of cyclohexane- d12 in liquid crystalline solvents

A NMR method is proposed to determine the kinetic and magnetic parameters for spin I = 1 nuclei with quadrupole interactions, undergoing exchange in the fast dynamic limit. The method consists of measuring the quadrupole echo decay rate under the effect of a sequence of π 2 pulses and is the quadrupole echo analog of the Can-Purcell-Meiboom-Gill experiment on spin I = 1 2 systems. The echo decay is exponential, and by use of the average Hamiltonian formalism, a general expression is derived for its relaxation rate as function of the pulse repetition rate and the π 2 pulse width. Expressions are also given for the limiting cases of infinitely narrow pulses at one end and spin-lock conditions which correspond to T 1 ϱ , on the other. The method is applied to a liquid crystalline solution of cyclohexane- d 12 undergoing rapid ring inversion. The kinetic and magnetic parameters derived by the present method are in perfect agreement with those obtained earlier by complete lineshape analysis of the deuteron NMR spectrum of this system.

Proton NMR studies of a metallothionein from Neurospora crassa: sequence-specific assignments by NOE measurements in the rotating frame

Sequential 1H NMR assignments of a metallothionein from Neurospora crassa have been accomplished by the combined use of COSY, 2QF-COSY, HOHAHA, and rotating-frame NOE experiments. All potentially observable resonances were assigned except for the epsilon-NH3 group of the C-terminal lysine. 1H NOEs, when observed in the laboratory frame and at 500-MHz spectrometer frequency, were negligible in this protein due to the inherent rotational correlation time of the molecule. This difficulty was circumvented by measuring transverse NOEs in the rotating frame under spin-locking conditions. The observed pattern of NOEs reveals a marked absence of "regular" secondary structures in the protein. Thus, the stability of this metallothionein's tertiary structure must arise primarily from its metal ligation. This appears to be a general feature of MTs since a general lack of extensive secondary structural elements was also observed in other metallothioneins.

A method for T1 rho imaging.

The spin lattice relaxation time (T1) is dependent on the strength of the polarizing magnetic field. The relaxation at low field strengths provides information from the processes at macromolecular level. However, the decrease of the polarizing magnetic field decreases the signal-to-noise ratio that determines the resolution of magnetic resonance images. In this report we describe a method for T1 rho imaging. The method possesses the relaxation time contrast of low field strengths with signal-to-noise ratio provided by the higher polarizing field. The relaxation time T1 rho is obtained under spin lock conditions. The spin system relaxes toward thermal equilibrium along the locking field. This process is analogous to the spin lattice relaxation at low field strength and characterized by the time constant T1 rho. T1 rho and T1 rho-dispersion may provide new imaging parameters for noninvasive tissue characterization.

On the convergence of the magnus expansion for spin systems in periodic magnetic fields

An analytically solvable single-spin problem is employed to investigate the convergence of the Magnus expansion for spin systems in periodic magnetic fields. The slow decay of quasi-stationary magnetization under multiple-pulse spin-locking conditions is explained within the framework of a theory based on a time-independent effective hamiltonian.

Averaging of transverse relaxation times of scalar coupled nuclei under spin lock conditions

The response of a scalar coupled spin system to a train of closely spaced 180° pulses is analyzed by means of an effective Liouville operator from which terms involving chemical shifts are omitted. It is shown that the resulting equations of motion for the spin density matrix are most easily solved in a basis for which the spin coupling part of the Hamiltonian is diagonal. Transverse relaxation behavior for each line in the spectrum may then be calculated by transforming to the basis in which the entire time independent Hamiltonian is diagonal. The analysis shows that under spin lock conditions produced by rapid nonselective pulses, the existence of scalar coupling causes all transitions of the same symmetry to exhibit identical transverse relaxation behavior. For two coupled nuclei of spin 1/2 all four transitions are shown to decay with the same single exponential, and the relaxation time is the average of relaxation times expected for each spin in the absence of scalar coupling. Experimental results demonstrating this averaging effect are reported for the protons in neat 2,3-dibromothiophene.