Dynamic polarization of protons in methanol-ethyleneglycol mixtures

Results of a measurement of the polarization of protons in mixtures of ethyleneglycol and methanol at 4.2 K and 1.3 K in a field of 9 kG are presented. Higher polarizations were obtained than in pure ethyleneglycol and also a slightly better free-to-bound proton ratio.     

Dynamic polarization of protons in irradiated polymers

Protons were polarized in different types of irradiated polymers by means of the solid effect method. The polarization was measured in fields of 3150 Oe and 19200 Oe in a range of temperatures from 1·65 to 4·2°K. The highest polarization was reached on protons in polycaprolactam of type II in a field of 19200 Oe ( _e =53·5 GHz) at a temperature of 2·1°K. Further, the dependence of the polarization on the microwave power and nuclear relaxation times were measured. The temperature dependence of the distance between positive and negative polarization maxima was observed.     

Electron spin polarization in a rotating triplet

The triplet model of electron spin polarization in fluid media is evaluated. The model consists of an initial singlet molecule rotating in a static, externally applied magnetic field. Intersystem crossing into different zero-field states is represented by a rate matrix diagonal in the molecular frame, and this matrix is expressed as an effective spin operator. The triplet rotates, and the motion affects the polarization in the laboratory frame, and also causes spin relaxation in the triplet manifold. The triplet is chemically quenched, and the polarization appears in the doublet fragments. The model is treated in a density matrix formalism and on the basis of anisotropic rotational diffusion of the triplet molecule. Explicit expressions are obtained in terms of the molecular parameters, the various rate constants, and the rotational correlation time.     

Vacuum polarization effects in elastic scattering of protons by nuclei

Sensitivity of spin observables in elastic proton scattering by nuclei to the vacuum polarization correction δϱ_VAC is explored. The Dirac optical potential is calculated from a complete set of Lorentz invariant amplitudes, Dirac-Hartree densities and the existing estimate of δϱ_VAC based on quantum hadrodynamics.     

Spin-lattice relaxation and dynamic polarization of protons in ethyleneglycol-methanol samples

We report measurements of nuclear spin-lattice relaxation and dynamic polarization of protons in ethyleneglycol-alcohol mixtures at 9 kG and 4·2 K and 1·3 K. Comparison of experimental results with a proposed model of spin-lattice relaxation indicates that the role of dipolar and exchange reservoirs of paramagnetic ions in nuclear relaxation is not necessarily negligible, particularly in the presence of high concentration of paramagnetic impurities. The influence of methanol is to enhance the dynamic polarization of protons compared to pure ethyleneglycol, in the most favourable cases by almost 50% in our experimental conditions.     

Photoinduced polarization inversion in a polymeric molecule

The polymeric molecule can exhibit a new photoinduced phenomenon where the electric dipole of the molecule with a bipolaron is reversed by absorbing one photon. This photoinduced polarization inversion occurred in a single molecule is an ultrafast process with a relaxation time of 200 fs.     

Longitudinal nuclear spin relaxation in solids in a triply rotating coordinate system: Selective observation of the multispin dipole contribution

The longitudinal nuclear spin relaxation in an effective magnetic field H _e3 acting in a triply rotating coordinate system is recorded. Rotating and doubly rotating coordinate systems are employed for strong suppression of the secular nuclear dipole interactions in the first two orders and for separation of higher-order interactions (four-and five-spin). Experiments on protons in polycrystalline benzene showed that the contribution of such multispin dipole interactions to this relaxation can be observed selectively as a pronounced local minimum in the temperature dependence of the relaxation time. This contribution correponds to ultraslow molecular motions with rates ≃ γH _e3≃2π(10¹−10³) s⁻¹ and can be employed to study such motions in detail, including for purposes of identification of the form of the motion. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 335–340 (10 September 1996)     

Dynamic H nuclear magnetic resonance of rotating solids

The sensitivity of one-dimensional dynamic magic-angle spinning (MAS) and off-MAS ²H nuclear magnetic resonance spectra to changes in the parameters of jump-type molecular motions is studied. The Floquet theory approach is used to simulate spectra of spins with I = 1, which are involved in exchange processes in rotating solids. The solution of the Bloch-McConnell equations for rotating samples are derived and some simulated frequency spectra are shown. The dependence of the lineshapes of the center and sidebands of the MAS and off-MAS spectra on the exchange parameters are discussed. Experimental results of ²H spectra of perdeuterated dimethyl sulfone, obtained in the temperature range 20–55 °C, are demonstrated. The methyl groups in this molecule undergo π flips at rates that can be detected by MAS and off-MAS NMR. The shapes of the experimental sidebands are compared with simulated results.     

Effect of polarization nonreciprocity in rotating fiber ring interferometers

The effect of polarization nonreciprocity in sensors of angular velocity of rotation—fiber ring interferometers (FRIs)—is considered for arbitrary polarization of eigenmodes of the single-mode optical fiber of an FRI loop and arbitrary polarization state of radiation at the FRI input. A new method for detecting the polarization nonreciprocity in an FRI is proposed. Numerical estimations are made.     

Time inversion symmetry and transport properties of solids

Identification of the time inversion operator θ with the magnetic moment inversion operator A in finding the symmetry restrictions on the form of transport tensors in solids is shown to be incorrect.     

NMR multiple-quantum processes in a doubly rotating frame in solids

Multiple-quantum transitions between nuclear spin Zeeman levels of fluorine nuclei in a CaF₂ single crystal are studied by a rotary-saturation method in NMR. The theoretical studies are performed for the system consisting of spins interacting with each other through dipole-dipole coupling. The analysis of the behavior of nuclear magnetization, the second quantization of a perturbing field, and the calculation of the nth-order timedependent perturbation are all performed in the reference frame of a doubly rotating system. We found evidence that (1) one type of single spin-n quantum transitions is taking place, but another type of two spin-n quantum transitions is not confirmed yet in this crystal; (2) the linewidths of the multiple-quantum absorptions are extremely narrow at the magic angle but do not completely vanish because of the presence of a perturbing field; (3) the effects of the energy level mixing through the dipole-dipole interactions on the multiple-quantum transitions are negligible over a wide range around the magic angle; and (4) the inhomogeneity of the RF field used to quantize the spin system in the rotating frame affects considerably the results of the rotary-saturation experiments.     

High-frequency dynamic nuclear polarization in the nuclear rotating frame

A proton dynamic nuclear polarization (DNP) NMR signal enhancement (epsilon) close to thermal equilibrium, epsilon = 0.89, has been obtained at high field (B(0) = 5 T, nu(epr) = 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spin-lock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T(1rho)), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T(1n)). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T(1rho) and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T(1n)). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time(1/2) of epsilon(t) = 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for high-field DNP/NMR studies of biological systems.     

Selective inversion in inhomogeneously broadened powder spectra

It is shown how amplitude-modulated radio-frequency pulses optimized for selective inversion of magnetization can be applied to static powder samples with inhomogeneously broadened spectra. Selective one-dimensional methods can in principle provide information similar to two-dimensional exchange spectroscopy, and they are often more efficient for determining the rates of dynamic processes. However, in systems where transverse magnetization is subject to significant homogeneous decay during the rf pulses, selective one-dimensional methods do not necessarily offer the expected advantages over two-dimensional spectroscopy.     

Amplitude-modulated decoupling in rotating solids: a bimodal Floquet approach

This paper centers on a theoretical study of amplitude-modulated heteronuclear decoupling in solid-state NMR under magic-angle spinning (MAS). A spin system with a single isolated rare spin coupled to a large number of abundant spins is used in the analysis. The phase-alternating decoupling scheme (XiX decoupling) is analyzed using bimodal Floquet theory and the operator-based perturbation method developed by van Vleck. An effective Hamiltonian correct to second order is calculated for the spin system under XiX decoupling. The results of these calculations indicate that under XiX decoupling the main contribution to the residual line width comes from a cross-term between the heteronuclear and the homonuclear dipolar couplings. This is in contrast to continuous-wave decoupling, where the residual line width is dominated by the cross-term between the heteronuclear dipolar coupling and the chemical-shielding tensor of the irradiated spin. For high-power decoupling the method results in very good decoupling provided that certain unfavorable recoupling conditions, imposed by specific ratios of the amplitude modulation frequency and the MAS frequency, are avoided. For low-power decoupling, the method leads to acceptable decoupling when the pulse length corresponds to an integer multiple of a 2pi rotation and the rf-field amplitude is less than a quarter of the MAS frequency. The performance of the XiX scheme is analyzed over a range of values of the rf power, and numerical results that agree well with the most recent experimental observations are presented.     

Dynamical polarization of protons in some polymers using the solid effect method

Experimental results on the dynamical polarization of protons in various polymers irradiated by neutrons are recapitulated. The measurements were performed at liquid nitrogen temperatures. The dependence was investigated of the degree of polarization on the microwave power, on the irradiation dose and on the change in frequency of the nuclear magnetic resonance. The maximum enhancement of polarization was obtained for polyethylene of a density 0·95 g/cm³ and was equal to 30.

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SOLID EFFECT

. . , . , 30, 0,95 g/cm³.

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In conclusion we thank R. Tichý for assistance in the measurements and S. Roda for help in the construction of the apparatus.     

Indirect detection of nitrogen-14 in solids via protons by nuclear magnetic resonance spectroscopy

This Communication describes the indirect detection of 14N nuclei (spin I=1) in solids by nuclear magnetic resonance (NMR) spectroscopy. The two-dimensional correlation method used here is closely related to the heteronuclear multiple quantum correlation (HMQC) experiment introduced in 1979 to study molecules in liquids, which has recently been used to study solids spinning at the magic angle. The difference is that the coherence transfer from neighboring 1H nuclei to 14N is achieved via a combination of J couplings and residual dipolar splittings (RDS). Projections of the two-dimensional correlation spectra onto the 14N dimension yield powder patterns which reflect the 14N quadrupolar interaction. In contrast to the indirect detection of 14N via 13C nuclei that was recently demonstrated [Gan, J. Am. Chem. Soc. 128 (2006) 6040; Cavadini et. al., J. Am. Chem. Soc., 128 (2006) 7706], this approach may benefit from enhanced sensitivity, and does not require isotopic enrichment in 13C, although the 1H line-widths may have to be reduced upon selective deuteration.