Spin-lattice Relaxation Mechanism Research Articles

A Heteronuclear NMR Study of Aqueous Lithium Salt Solutions of l-Alanine: Revealing Solute Hydrophobic Association through the NMR B' Coefficient.

In the present study, a set of heteronuclear NMR approaches has been adopted to investigate the solution behavior of a small hydrophobic solute l-alanine in the presence of lithium (Li) salts. The presence of salts plays a major role in determining the structure and solvation of biomolecules. It therefore evokes interest to understand the effect of Li salts on amino acids (alanine), the building block of biomolecules. The ionic solute dynamics in the present case has been probed using 1H, 7Li, and 13C nuclei available in the aqueous Li salt solution of l-alanine. Nuclear longitudinal spin relaxation of alanine protons was examined at a variable concentration range of three lithium salts, i.e., LiCl, Li2SO4 and LiClO4, to introduce the NMR B' coefficient for each salt defining ionic solute/solvent interaction in the solution. Analysis of the active relaxation mechanism of 7Li spin-lattice relaxation further revealed the presence of alanine in the solvation shell of Li ion depending on the anionic counterpart.

EPR study of Mn site substituted Pr based Doped Rare Earth Manganites

In this paper, we present the investigations of Electron Paramagnetic Resonance (EPR) on Mn site substituted Pr based Doped rare Earth Manganites i.e. Pr0.60Ca0.40MnO3 and Pr0.60Ca0.40Mn0.85Zn0.15O3. Changes in physical properties as lattice parameters, average valence of Mn site was observable of those manganites. X-ray diffraction pattern shows that both Pr0.60Ca0.40MnO3 and Pr0.60Ca0.40Mn0.85Zn0.15O3 have single phase and without the other secondary or impurity phase and indexed supported the Pbnm space group. The value of x in Pr0.60Ca0.40Mn1-xZnxO3 increases, the average valence V was increased except for a fixed composition, i.e. x remains unchanged, the average valence V was decreased as we go from less valency to high valency (i.e., from divalent to trivalent and from trivalent to tetravalent. The EDXS analysis of those materials shows good homogeneity, but there are experimental errors in composition. It is seen from the SEM images that is formed in different shape grains. The average grain sizes of the samples are different for Pr0.60Ca0.40MnO3 and Pr0.06Ca0.40Mn0.85Zn0.15O3 The paramagnetic resonance spectra parameters (effective g-factor, peak-to-peak line width) of Pr0.60Ca0.40MnO3 and Pr0.60Ca0.40Mn0.85Zn0.15O3.have been used to study the paramagnetic spin correlations and spin dynamics. As for Pr0.60Ca0.40MnO3 the line width becomes wider because of the contribution of small polaron jumping within the PM mechanism. However, as for Pr0.60Ca0.40Mn0.85Zn0.15O3 the broadening of EPR line-width is understood with the spin-lattice relaxation mechanism, g value decreased from 1.99 to 1.79. Therefore, the Zn dopant not solely changes the parent spin correlation in the PM regime however additionally suppresses the development of orbital ordering.

Study of the structure and dynamics at various parts of the antibacterial drug molecule cefpodoxime proxetil

The structure and dynamics of cefpodoxime proxetil are elucidated by measuring chemical shift anisotropy (CSA) tensor, spin-lattice relaxation time, and local correlation time at twenty-one crystallographically different 13C nuclei sites. The principal components of CSA tensor of cefpodoxime proxetil are extracted by the two-dimensional phase adjusted sinning sideband (2DPASS) cross-polarization magic angle spinning (CP-MAS) solid-state NMR experiment, and the spin-lattice relaxation time is measured by the method outlined by Torchia(T1CP). The local correlation time is calculated by bearing in mind that the spin-lattice relaxation mechanism of 13C nuclei is mainly governed by the CSA interaction and the heteronuclear dipole-dipole interaction. The aminothiazole ring, β-lactam ring, and dihydrothiazine ring provide stability to the drug molecule and increase the affinity of the drug to penicillin-binding proteins (PBPs) receptors. The principal components of CSA parameters, spin-lattice relaxation time, and local correlation time vary substantially for carbon nuclei residing on these three rings. These signify that not only the electronic environment, but the molecular conformation, and the local dynamics are also altered within the ring. The substitution of the acyl side chain, oxime group, and the aminothiazole ring at the C7 position of the β-lactam ring enhances the antibacterial activity and the binding affinity of the drug. A huge variation of the spin-lattice relaxation time and local correlation time is observed in those regions. The change in the electron charge distribution and nuclear spin dynamics at different parts of the drug molecule is clear by CSA and spin-lattice relaxation measurements, which will enrich the field “NMR crystallography”.

Strongly Entangled Triplet Acyl-Alkyl Radical Pairs in Crystals of Photostable Diphenylmethyl Adamantyl Ketones.

Radical pairs generated in crystalline solids by bond cleavage reactions of triplet ketones offer the unique opportunity to explore a frontier of spin dynamics where rigid radicals are highly entangled as the result of short inter-radical distances, large singlet-triplet energy gaps (ΔEST), and limited spin-lattice relaxation mechanisms. Here we report the pulsed laser generation and detection of strongly entangled triplet acyl-alkyl radical pairs generated in nanocrystalline suspensions of 1,1-diphenylmethyl 2-ketones with various 3-admantyl substituents. The sought-after triplet acyl-alkyl radical pairs could be studied for the first time in the solid state by taking advantage of the efficient triplet excited state α-cleavage reactions of 1,1-diphenylmethyl ketones and the slow rate of CO loss from the acyl radicals, which would have to generate highly unstable phenyl and primary alkyl radicals or relatively unstable secondary and tertiary alkyl radicals. With the loss of CO prevented, the lifetime of the triplet acyl-alkyl radical pair intermediates is determined by intersystem crossing to the singlet radical pair state, which is followed by immediate bond formation to the ground state starting ketone. Experimental results revealed biexponential kinetics with long-lived components that account for ca. 87-92% of the transient population and lifetimes that extend to the range of 53-63 μs, the longest reported so far for this type of radical pair. Structural information inferred from the starting ketone will make it possible to analyze the affects of proximity and orientation of the singly occupied orbitals and potentially help set a path for the use of triplet radical pairs as qubits in quantum information technologies.

An atomic resolution description of folic acid using solid state NMR measurements.

The chemical shift anisotropy tensor and site-specific spin-lattice relaxation time of folic acid were determined by a 13C 2DPASS CP-MAS NMR experiment and Torchia CP experiment respectively. The molecular correlation time at various carbon nuclei sites of folic acid was evaluated by assuming that the 13C spin-lattice relaxation mechanism is mainly governed by chemical shift anisotropy interaction and hetero-nuclear dipole–dipole coupling. CSA parameters are larger for the carbon nuclei residing at the heteroaromatic ring and aromatic ring, and those attached to double-bonded electronegative oxygen atoms. It is comparatively low for C9, C19, C21, and C22. The molecular correlation time is of the order of 10−4/10−5 s for C9, C19, C21 and C22 carbon nuclei, whereas it is of the order of 10−3 s for the rest of the carbon nuclei sites. Spin lattice relaxation time varies from 416 s to 816 s. For C23 and C14, the value is 816 s, and it is 416 s for C7 nuclei. The correlation between structure and dynamics on an atomic scale of such an important drug as folic acid can be visualized by these types of extensive spectroscopic measurements, which will help to develop an advanced drug for DNA replication.

Spin-lattice relaxation in liquid entrapped in a nanocavity

We consider the spin lattice relaxation in bulk liquid and liquid entrapped in a nanocavity. The kinetic equation which describes the spin lattice relaxation is obtained by using the theory of the nonequilibrium state operator. A solution of the kinetic equation gives the quadrature expression for the relaxation time, T1. The calculated relaxation time agrees well with the experimental data.The spin-lattice relaxation time is calculated for nanocavities with a characteristic size much less than 700 nm, with the assumption that the spin-lattice relaxation mechanism is determined by nanocavity fluctuations. The resulting expression shows an explicit dependence of the relaxation time T1 on the volume, density of nuclear spins, and parameters of the cavity (shape and orientation relatively to the applied field). To compare with the experiment on the detection of the anisotropy of the relaxation time, we average the expression that describes the relaxation time over the orientation of the nanocavities relative to the applied magnetic field. The good agreement with the experimental data for fibril tissues was achieved by adjustment of few fitting parameters - the standard deviation, averaged fiber direction, and weight factors - which characterize the ordering of fibrils.

Investigation of the Detailed Internal Structure and Dynamics of Itraconazole by Solid-State NMR Measurements.

The structure and dynamics of itraconazole were investigated by 13C 2DPASS MAS SSNMR and spin-lattice relaxation time measurement to get an insight into its multiple biological activities, e.g., antifungal, antiviral, anticancer activities, etc. The molecular correlation time at chemically different sites of carbon nuclei was calculated by considering that the spin-lattice relaxation mechanism is mainly dominated by chemical shift anisotropy interaction and heteronuclear dipole–dipole interaction. The spin-lattice relaxation time is long for C35, C6, C5, and C34 carbon nuclei that participated in the 1, 2, 4-triazole ring. On the contrary, it is comparatively shorter for C1, C2, C3, and C4 carbon nuclei associated with the sec-butyl group in the triazolane side-chain region. Chemical shift anisotropy (CSA) parameters of C5, C6, C34, and C35 nuclei are much higher than those of C1, C2, C3, C4 nuclei, indicating that the relaxation mechanism at a high value of magnetic field is predominated by chemical shift anisotropy interaction. The molecular correlation time of carbon nuclei residing at the side-chain region is 2–3 orders of magnitude lesser than that of those participated in the 1,2,4-triazole ring. The spin-lattice relaxation time is very long for carbon nuclei C28 and C30 bonded with chlorine. Asymmetry and anisotropy parameters are also very high for the spinning CSA sideband pattern corresponding to the C28 and C30 nuclei. The molecular correlation time is on the order of 10–3 s for C28 and 10–4 s for C30, whereas for side-chain carbon nuclei, it is on the order of 10–6 s. This suggests that the effective magnetic field experienced by C28 and C30 nuclei is affected by the polarization of the chemical bond. A huge variation in molecular correlation time is observed for chemically different sites of carbon nuclei of the itraconazole molecule. These investigations vividly portrayed how the structure is correlated with the dynamics of a valuable drug, itraconazole, with multiple biological activities. This study will enlighten the way of inventing advance medicine for multiple biological activities in the pharmaceutical industry.

Spin current polarization and electrical conductivity in metal helimagnets

The peculiarities of spin and charge kinetics in helical magnetic metals that are due to forces acting on the magnetic moment of conduction electrons in inhomogeneous magnetic field have been considered. Analyzing the equations of motion for non-equilibrium spin density shows that an electric field directed along the axis of the helix produces conduction-electron spin polarization along this direction. Under the same conditions, the directions of polarization of the spin current and polarization of the locally equilibrium spin density are collinear. The specific structure of the effective exchange field acting on the conduction electrons in helical magnets causes two additional spin relaxation mechanisms to emerge. In addition to the mechanism of spin-lattice relaxation, “diffusion” and “precession” mechanisms of spin relaxation exist in conductive helical magnets. The diffusion mechanism is analogous to the Dyakonov-Perel spin relaxation mechanism. The key point is that the inhomogeneities of a magnetic field initiate a coupling between the spin and charge systems. It has been shown that, in a helical metal, the electrical conductivity decreases due to the action of the non-uniform exchange magnetic field.

Coexistence of spin-lattice and spin-spin relaxation mechanism in perovskite manganite (La0.5Pr0.5)0.67Ca0.33MnO3

Electron paramagnetic resonance (EPR) was applied extensively to probe the phase separation and spin relaxation mechanism in the perovskite manganites. In this study, we conducted the measurement of EPR for the perovskite manganites (La0.5Pr0.5)0.67Ca0.33MnO3. We found that both paramagnetic and ferromagnetic resonance peaks simultaneously emerged on resonance spectrum near below TC, exhibiting a typical characteristic of phase separation. The linewidth of EPR spectrum progressively enlarged upon the rising of temperature in the temperature range of T>TC and a quasilinear variation of peak-peak width was observed at TC<T<500 K. In contrast to the usual beliefs that the variation of peak-peak linewidth can be understood either by spin-lattice relaxation or by spin-spin relaxation, however, the linewidth behavior observed here shows a coexistence of two spin relaxation mechanisms. We proposed that the inhomogeneous electronic phases due to the intrinsic self-organizing growth were responsible for the appearance of multiple spin exchange interactions and variational relaxation mechanism. This finding would be possible to pave a new way for further investigating magnetic correlations and spin dynamics in the paramagnetic hole-doped manganites.

Layered metal(IV) phosphonate materials: Solid-state 1 H, 13 C, 31 P NMR spectra and NMR relaxation.

Multinuclear solid-state NMR and powder X-ray diffraction data collected for phosphonate materials Zr(O3 PC6 H4 PO3 ) · 3.6H2 O and Sn(O3 PC6 H4 PO3 )0.85 (O3 POH)0.30 ·3.09H2 O have resulted in the layered structure, where the phosphonic acids cross-link the layers. The main structural motif (the 111 connectivity in the PO3 group) has been established by determination of chemical shift anisotropy parameters for phosphorus nuclei in the phosphonate groups. An analysis of the variable-temperature 31 PT1 measurements and the shapes of the phosphorus resonances in the 31 P static NMR spectra have resulted in the dipolar mechanism of the phosphorus spin-lattice relaxation, where the rotating phenylene rings reorient dipolar vectors P… H as a driving force of the relaxation process. It has been found that water protons do not affect the 31 PT1 times. The activation energy of the phenylene rotation in both compounds has been determined as low as 12.5kJ/mol. The interpretation of the phosphorus relaxation data has been independently confirmed by the measurements of 1 H T1 times for protons of the phenylene rings.

Detailed mechanisms of 1H spin-lattice relaxation in ammonium dihydrogen phosphate confirmed by magic angle spinning

Mechanisms of the 1H spin-lattice relaxation in NH4H2PO4 were studied in detail by use of the effect of magic angle spinning on the relaxation. The acid and the ammonium protons have different relaxation times at the spinning rates higher than 10 kHz due to suppression of spin diffusion between the two kinds of protons. The intrinsic relaxation times not affected by the spin diffusion and the spin-diffusion assisted relaxation times were evaluated separately, taking into consideration temperature dependence. Both mechanisms contribute to the 1H relaxation of the acid protons comparatively. The spin-diffusion assisted relaxation mechanism was suppressed to the level lower than the experimental errors at the spinning rate of 30 kHz.

Locking of electron spin coherence above 20 ms in natural silicon carbide

We demonstrate that silicon carbide (SiC) with natural isotope abundance can preserve a coherent spin superposition in silicon vacancies over unexpectedly long time approaching 0.1 seconds. The spin-locked subspace with drastically reduced decoherence rate is attained through the suppression of heteronuclear spin cross-talking by applying a moderate magnetic field in combination with dynamic decoupling from the nuclear spin baths. We identify several phonon-assisted mechanisms of spin-lattice relaxation, ultimately limiting quantum coherence, and find that it can be extremely long at cryogenic temperature, equal or even longer than 8 seconds. Our approach may be extended to other polyatomic compounds and open a path towards improved qubit memory for wafer-scale quantum techmologies.

Multiband electronic characterization of the complex intermetallic cage systemY1−xGdxCo2Zn20

A detailed microscopic and quantitative description of the electronic and magnetic properties of Gd$^{3+}$-doped YCo$_{2}$Zn$_{20}$ single crystals (Y$_{1-x}$Gd$_{x}$Co$_{2}$Zn$_{20}$: (0.002 $\lesssim x \leq $ 1.00) is reported through a combination of temperature-dependent electron spin resonance (ESR), heat capacity and $dc$ magnetic susceptibility experiments, plus first-principles density functional theory (DFT) calculations. The ESR results indicate that this system features an \emph{exchange bottleneck} scenario wherein various channels for the spin-lattice relaxation mechanism of the Gd$^{3+}$ ions can be identified via exchange interactions with different types of conduction electrons at the Fermi level. Quantitative support from the other techniques allow to extract the exchange interaction parameters between the localized magnetic moments of the Gd$^{3+}$ ions and the different types of conduction electrons present at the Fermi level ($J_{fs}$, $J_{fp}$ and $J_{fd}$). Despite the complexity of the crystal structure, our combination of experimental and electronic structure data establish GdCo$_{2}$Zn$_{20}$ as a model RKKY system by predicting a Curie-Weiss temperature $\theta_{C} = -1.2(2)$~K directly from microscopic parameters, in very good agreement with the bulk value from magnetization data. The successful microscopic understanding of the electronic structure and behavior for the two end compounds YCo$_{2}$Zn$_{20}$ and GdCo$_{2}$Zn$_{20}$ means they can be used as references to help describe the more complex electronic properties of related materials.

77Se Nuclear Spin–Lattice Relaxation in Binary Ge–Se Glasses: Insights into Floppy Versus Rigid Behavior of Structural Units

The mechanism of (77)Se nuclear spin-lattice relaxation is investigated in binary Ge-Se glasses. The (77)Se nuclides in Se-Se-Se chain sites relax faster via dipolar coupling fluctuation compared to those in Ge-Se-Ge sites shared by GeSe4 tetrahedra that relax slower via the fluctuation of the chemical shift anisotropy. The relaxation rate for the Se-Se-Se sites decreases markedly with increasing magnetic field, whereas that for the Ge-Se-Ge sites displays no appreciable dependence on the magnetic field such that the extent of differential relaxation between the two Se environments becomes small at high fields on the order of 19.6 T. The corresponding dynamical correlation time is three orders of magnitude shorter (∼10(-9) s) for the Se-Se-Se sites, compared to that for the Ge-Se-Ge sites (∼10(-6) s). The large decoupling in the time scale between these Se environments provides direct experimental support to the commonly made assumption that the selenium chains are mechanically floppy, and the interconnected GeSe4 tetrahedra form the rigid elements in the selenide glass structure.

Electron spin-lattice relaxation mechanisms of nitroxyl radicals in ionic liquids and conventional organic liquids: temperature dependence of a thermally activated process.

During the past two decades, several studies have established a significant role played by a thermally activated process in the electron spin relaxation of nitroxyl free radicals in liquid solutions. Its role has been used to explain the spin relaxation behavior of these radicals in a wide range of viscosities and microwave frequencies. However, no temperature dependence of this process has been reported. In this work, our main aim was to investigate the temperature dependence of this process in neat solvents. Electron spin-lattice relaxation times of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (TEMPOL), in X-band microwave frequency, were measured by the pulse saturation recovery technique in three room-temperature ionic liquids ([bmim][BF4], [emim][BF4], and [bmim][PF6]), di-isononyl phthalate, and sec-butyl benzene. The ionic liquids provided a wide range of viscosity in a modest range of temperature. An auxiliary aim was to examine whether the dynamics of a probe molecule dissolved in ionic liquids was different from that in conventional molecular liquids, as claimed in several reports on fluorescence dynamics in ionic liquids. This was the reason for the inclusion of di-isononyl phthalate, whose viscosities are similar to that of the ionic liquids in similar temperatures, and sec-butyl benzene. Rotational correlation times of the nitroxyl radicals were determined from the hyperfine dependence of the electron paramagnetic resonance (EPR) line widths. Observation of highly well-resolved proton hyperfine lines, riding over the nitrogen hyperfine lines, in the low viscosity regime in all the solvents, gave more accurate values of the rotational correlation times than the values generally measured in the absence of these hyperfine lines and reported in the literature. The measured rotational correlation times obeyed a modified Stokes-Einstein-Debye relation of temperature dependence in all solvents. By separating the contributions of g-anisotropy, A-anisotropy and spin-rotation interactions from the observed electron spin-lattice relaxation rates, the contribution of the thermally activated process was obtained and compared with its expression for the temperature dependence. Consistent values of various fitted parameters, used in the expression of the thermal process, have been found, and the applicability of the expression of the thermally activated process to describe the temperature dependence in liquid solutions has been vindicated. Moderate solvent dependence of the thermally activated process has also been observed. The rotational correlation times and the spin-lattice relaxation processes of nitroxyls in ionic liquids and in conventional organic liquids are shown to be explicable on a similar footing, requiring no special treatment for ionic liquids.

Dynamics of negatively charged divacancies in neutron irradiated type Ib diamond

Thermal neutron irradiation of synthetic type Ib diamond, followed by annealing, results in a number of paramagnetic point defect centers. One of these is the composite defect designated W29. Previous EPR measurements have shown that the W29 center (S=3/2) is a charged divacancy [VV]− with spin Hamiltonian parameters similar to those of the R4/W6 neutral divacancy center [VV]0 (S=1) found in irradiated high purity type IIa diamond. The present EPR linewidth and spin-lattice relaxation rate measurements, made as a function of temperature on the W29 center, show that an Orbach two-phonon process, with a gap of ~20meV, plays a dominant role in determining the relaxation behavior above 20K. Below 20K a direct single phonon process provides the spin-lattice relaxation mechanism and the relaxation times become long, of the order of milliseconds. The linewidth behavior with temperature is accounted for in terms of changes in the spin dynamics.

Electron spin–lattice relaxation mechanisms of rapidly-tumbling nitroxide radicals

Electron spin relaxation times at 295K were measured at frequencies between 250MHz and 34GHz for perdeuterated 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (PDT) in five solvents with viscosities that result in tumbling correlation times, τR, between 4 and 50ps and for three 14N/15N pairs of nitroxides in water with τR between 9 and 19ps. To test the impact of structure on relaxation three additional nitroxides with τR between 10 and 26ps were studied. In this fast tumbling regime T2-1∼T1-1 at frequencies up to about 9GHz. At 34GHz T2-1>T1-1 due to increased contributions to T2-1 from incomplete motional averaging of g-anisotropy, and T2-1-T1-1 is proportional to τR. The contribution to T1-1 from spin rotation is independent of frequency and decreases as τR increases. Spin rotation dominates T1-1 at 34GHz for all τR studied, and at all frequencies studied for τR=4ps. The contribution to T1-1 from modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as τR increases; it dominates at low frequencies for τR>∼15ps. The contribution from modulation of g anisotropy is significant only at 34GHz. Inclusion of a thermally-activated process was required to account for the observation that for most of the radicals, T1-1 was smaller at 250MHz than at 1–2GHz. The significant 15N/14N isotope effect, the small H/D isotope effect, and the viscosity dependence of the magnitude of the contribution from the thermally-activated process suggest that it arises from intramolecular motions of the nitroxide ring that modulate the isotropic A values.

Electron paramagnetic resonance studies on manganite Pr0.5Sr0.5Mn1−x Ga x O3 (x=0 and 0.05)

In this paper, we present the investigations of electron paramagnetic resonance on perovskite manganite Pr0.5Sr0.5MnO3 and Ga-doped Pr0.5Sr0.5Mn0.95Ga0.05O3. The temperature dependent paramagnetic resonance spectra parameters (effective g-factor, peak-to-peak linewidth ΔH pp and double integrated intensities) have been used to study the paramagnetic spin correlations and spin dynamics. The gradual increase of effective g-factor is attributed to the presence of orbital ordering above T C. The model fittings of temperature dependent double integrated intensities reveal Arrhenius law is appropriate for describing Pr0.5Sr0.5Mn0.95Ga0.05O3 instead of Pr0.5Sr0.5MnO3 system. As for Pr0.5Sr0.5MnO3, the broadening of linewidth with the temperature increase origins from the contribution of small polaron hopping in the PM regime. However, as for Pr0.5Sr0.5Mn0.95Ga0.05O3, the broadening of EPR linewidth can be understood with the spin-lattice relaxation mechanism.

Radical ions with nearly degenerate ground state: Correlation between the rate of spin-lattice relaxation and the structure of adiabatic potential energy surface

Paramagnetic spin-lattice relaxation (SLR) in radical cations (RCs) of the cycloalkane series in liquid solution was studied and analyzed from the point of view of the correlation between the relaxation rate and the structure of the adiabatic potential energy surface (PES) of the RCs. SLR rates in the RCs formed in x-ray irradiated n-hexane solutions of the cycloalkanes studied were measured with the method of time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs. Temperature and, for some cycloalkanes, magnetic field dependences of the relaxation rate were determined. It was found that the conventional Redfield theory of the paramagnetic relaxation as applied to the results on cyclohexane RC, gave a value of about 0.2 ps for the correlation time of the perturbation together with an unrealistically high value of 0.1 T in field units for the matrix element of the relaxation transition. The PES structure was obtained with the DFT quantum-chemical calculations. It was found that for all of the cycloalkanes RCs considered, including low symmetric alkyl-substituted ones, the adiabatic PESes were surfaces of pseudorotation due to avoided crossing. In the RCs studied, a correlation between the SLR rate and the calculated barrier height to the pseudorotation was revealed. For RCs with a higher relaxation rate, the apparent activation energies for the SLR were similar to the calculated heights of the barrier. To rationalize the data obtained it was assumed that the vibronic states degeneracy, which is specific for Jahn-Teller active cyclohexane RC, was approximately kept in the RCs of substituted cycloalkanes for the vibronic states with the energies above and close to the barrier height to the pseudorotation. It was proposed that the effective spin-lattice relaxation in a radical with nearly degenerate low-lying vibronic states originated from stochastic crossings of the vibronic levels that occur due to fluctuations of the interaction between the radical and the solvent. The magnitude of these fluctuations, ~100 cm(-1), determines the upper scale of the unperturbed splitting between the vibronic states, for which the manifestation of this paramagnetic relaxation mechanism could be expected. Our estimate for the relaxation rate derived using standard Landau-Zener model of nonadiabatic transitions at the level crossing agrees with the experimental data. This paramagnetic relaxation mechanism can also be operative in paramagnetic species of other types such as linear radicals, radicals with threefold degeneracy, paramagnetic centers in crystals, etc. It looks likely that the proposed SLR mechanism can be quenched by a fast vibrational relaxation in radicals.

Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms

Dynamic nuclear polarization (DNP) is used to enhance signals in NMR and MRI experiments. During these experiments microwave (MW) irradiation mediates transfer of spin polarization from unpaired electrons to their neighboring nuclei. Solid state DNP is typically applied to samples containing high concentrations (i.e. 10–40 mM) of stable radicals that are dissolved in glass forming solvents together with molecules of interest. Three DNP mechanisms can be responsible for enhancing the NMR signals: the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Recently, numerical simulations were performed to describe the SE and CE mechanisms in model systems composed of several nuclei and one or two electrons. It was shown that the presence of core nuclei, close to DNP active electrons, can result in a decrease of the nuclear polarization, due to broadening of the double quantum (DQ) and zero quantum (ZQ) spectra. In this publication we consider samples with high radical concentrations, exhibiting broad inhomogeneous EPR line-shapes and slow electron cross-relaxation rates, where the TM mechanism is not the main source for the signal enhancements. In this case most of the electrons in the sample are not affected by the MW field applied at a discrete frequency. Numerical simulations are performed on spin systems composed of several electrons and nuclei in an effort to examine the role of the DNP inactive electrons. Here we show that these electrons also broaden the DQ and ZQ spectra, but that they hardly cause any loss to the DNP enhanced nuclear polarization due to their spin-lattice relaxation mechanism. Their presence can also prevent some of the polarization losses due to the core nuclei.