Dipolar nuclear polarization via the spin diffusion of a dipole order

We propose transfer of the paramagnetic impurity (PI) polarization to nuclei in bulk, outside the diffusion barrier, by using dipolar system of the nuclear spins. The transfer can overcome influence of the diffusion barrier and is proposed to be implemented in four stages. At the first stage, transition of the Zeeman PI order to the Zeeman order of nuclear spins inside the spin-diffusion barrier is occurred. During the second stage the Zeeman order of both the nuclear spins inside the barrier and the nuclear spins in bulk is transferred into the nuclear dipolar spin order. As a result, the nuclear dipolar spin reservoir inside the barrier acquires a lower spin temperature, and thus a gradient of the spin temperature of the nuclear dipolar spin system is created. Since the external magnetic field and the magnetic field created by PIs do not effect on the dipole-dipole interaction between the nuclear spins, the dipolar reservoir is common for all nuclear spins, both inside and outside the diffusion barrier. Restriction of the diffusion barrier is removed and the spin diffusion of the dipole energy and transfer of the spin dipolar order to bulk spins occurs without obstacles (the third stage). At the last stage, to register an NMR signal, the dipolar order of the bulk spins is transferred into the Zeeman order of these spins. Estimations show that enhancement of the polarization can reaches in the case of a ¹H nuclear spin, ~220, for ¹³C ~850, and for ¹⁵N ~2130.

MQ NMR dynamics in dipolar ordered state at negative temperature

We investigate theoretically the Multiple Quantum (MQ) NMR dynamics at negative absolute temperatures in systems of nuclear spins 1/2 coupled by the dipole-dipole interactions and with the initial conditions determined by the dipolar ordered state. Two different methods of MQ NMR are used. One of them is based on the measurement of the dipolar energy. The other method uses an additional resonance (π/4)y-pulse after the preparation period of the standard MQ NMR experiment. It is shown that at negative temperatures many-spin clusters and spin correlations are created faster, and the intensities of MQ coherences are higher than in the usual MQ NMR experiments. So, the eighth-order MQ coherence in 10-spin system of the cyclopentane molecule appears to be 1.5 times faster and its intensity is four orders higher than at positive temperatures. The proposed MQ NMR methods at negative absolute temperatures can be used for the investigation of many-spin dynamics of nuclear spins in solids.

Low temperature MQ NMR dynamics in dipolar ordered state

We investigate analytically and numerically the Multiple Quantum (MQ) NMR dynamics in dipolar ordered spin systems of nuclear spins 1/2 at low temperatures. We consider two different methods of MQ NMR. One of them is based on the measurement of the dipolar energy. The other method uses an additional resonance (π/4)y-pulse after the preparation period of the standard MQ NMR experiment in solids and allows one to measure the Zeeman energy. Both considered methods are sensitive to the contribution of remote spins in the interaction and to the spin system structure. The QS method is sensitive to the spin number in the molecule while the PS method gives very similar time dependencies of the intensities of MQ coherences for different spin numbers. It is shown that the use of the dipolar ordered initial state has the advantage of exciting the highest order MQ coherences in clusters of 4m identical spins, where m=1,2,3,…, that is impossible to do with the standard MQ method. MQ NMR methods based on the dipolar ordered initial states at low temperatures complement the standard NMR spectroscopy for better studying structures and dynamic processes in solids.

Spin diffusion in multiple pulse spin-locking in solids containing paramagnetic impurities

Spin diffusion and spin-lattice relaxation in solids containing paramagnetic impurities under influence of a multiple-pulse spin-locking radio-frequency sequence are studied theoretically and experimentally. The diffusion equation obtained provides a clue for determination of the time dependent magnetization. The spin-lattice relaxation time is calculated as a function of the correlation time and multiple-pulse field parameters. From the experimental data the spin diffusion coefficient, the radius of the spin diffusion barrier, and the correlation time for very slow molecular motion in polycrystalline (C(2)F)(n) system are estimated and found to be D∼7.1×10(-12)cm(2)/s, r(c)∼4.8×10(-10)m, and τ(c)∼10.2μs, respectively.

Multiple quantum NMR dynamics in pseudopure states

We investigate numerically the multiple quantum (MQ) NMR dynamics in systems of nuclear spins 1/2 coupled by dipole-dipole interactions in the case of the pseudopure initial state. Simulations of the MQ NMR with real molecular structures such as six dipolar-coupled proton spins of benzene, hydroxyl proton chains in calcium hydroxyapatite, and fluorine chains in calcium fluorapatite open the way to experimental NMR testing of the obtained results. It was found that multiple-spin correlations are created faster in such experiments than in the usual MQ NMR experiments and can be used for the investigation of many-spin dynamics of nuclear spins in solids.

Dynamics of multiple quantum coherences in dipole-coupling spins in solid

A perturbation method deals with dipolar coupling spins in solids is presented. As example of application the method, the multiple-quantum coherence dynamics in clusters of a linear chain of four nuclear spins and a ring of six spins coupled by dipole-dipole interaction are considered. The calculated 0Q and 2Q intensities in a linear chain of four nuclear spins and 6Q intensity in a ring of six spins vs. the duration of the preparation period agree well with the exact solutions (for linear chain of four nuclear spins) and simulation data (for linear chain of four nuclear spins and a ring of six spin).

Spin diffusion of dipolar energy in NQR

The theory of spin diffusion was extended to the case of nuclear dipolar order in solids containing paramagnetic impurities and nuclei with spin I > 1/2 having nuclear quadrupole moment. We show that spin diffusion process of dipolar order takes place in solids containing paramagnetic impurities. At the start of relaxation process, the direct relaxation regime is realized with non-exponential time dependence. Then the relaxation regime will be changed to diffusion-limited one. Using obtained expressions for the spin lattice relaxation times for these two relaxation regimes, the diffusion coefficient of the dipolar order in nuclear quadrupole resonance can be estimated from experimental data.

Spin diffusion and nuclear spin-lattice relaxation in irradiated solids: a multiple-pulse NQR study

We present a detailed theoretical and experimental NQR multiple-pulse spin-locking study of spin-lattice relaxation and spin diffusion processes in the presence of paramagnetic impurities in solids. The relaxation function of the nuclear spin system at the beginning of the relaxation process is given by exp (-t/T1rho)alpha, where T1rho is spin-lattice relaxation time in rotating frame and alpha = d/6, d is the sample dimensionality. Then the relaxation proceeds asymptotically to an exponential function of time, which was attributed to the spin-diffusion regime. Using the experimental data obtained from the analysis of those two relaxation regimes in gamma-irradiated powdered NaClO3, spin diffusion coefficient has been determined and the radius of the diffusion barrier has been estimated.

Spin-locking in one pulse NMR experiment

The response of a spin system to a long (in comparison to spin-spin relaxation time T2) radiofrequency pulse has been studied. We observed that the magnetization after the long pulse does not fall to zero at time t >> T2 for both on-resonance and off-resonance conditions. The dependencies of the magnetization on frequency offset, linewidth and radiofrequency power are investigated, both theoretically and experimentally. The question of the effective field direction is also discussed.

Fractional power time dependence of the nuclear magnetization in the presence of paramagnetic impurities

We extend the theory of growth of the nuclear magnetization in the presence of paramagnetic impurities and the absence of spin diffusion to the case of multi-paramagnetic centers. We show that for short times after saturation pulses, the rate of growth of the magnetization is proportional to t alpha where t is the time and alpha = 1/3, 1/2 and 2/3 for one-, two- and three-dimensional systems, respectively. We also present experimental data for which the total time-dependent magnetization is proportional to exp[-(t/T1) alpha], which reduces to the above time dependence for short times.