Quadrupole Spin-lattice Relaxation Research Articles

Dynamic crossover in metallic glass melt detected by NMR

The structure and dynamics of melts have critical influence on the glass formation and solidification of crystals, which attracts broad interest to clarify the underlying mechanisms. In this paper, the quadrupolar spin-lattice relaxation rates of 63Cu and 65Cu nuclei in a Au50Cu25.5Ag7.5Si17 metallic glass melt are studied by using a home-designed high-precision high-temperature NMR equipment. The relaxation rates follow the Arrhenius behavior at high temperatures above 870 K, but become non-Arrhenius at low temperatures, which indicates a non-Arrhenius dynamical crossover. The crossover temperature TA is about 2.3 times of the glass-transition temperature (Tg=378 K). Based on the analysis of Knight shift and magnetic relaxation rates, Cu and Si atoms are found to tend to form stronger covalent-like bonds below TA. Such an increase in bonding strength may promote the connectivity of energetically preferred short-range clusters, which is responsible for the observed sluggish dynamics.

1H-Detected quadrupolar spin-lattice relaxation measurements under magic-angle spinning solid-state NMR.

Proton detection and phase-modulated pulse saturation enable the measurement of spin-lattice relaxation times of "invisible" quadrupolar nuclei with extensively large quadrupolar couplings. For nitrogen-14, efficient cross-polarization is obtained with a long-duration preparation pulse. The experiment paves the way to the characterization of a large variety of materials containing halogens, metals and more.

Неравноямные реориентации в кристалле Cl3P=NCCl(CF3)2 по данным ядерного квадрупольного резонанса хлора-35

The crystal Cl 3 P=NCCl(CF 3 ) 2 , which discovered autonomous reorientations of the asymmetric fragment CCl(CF 3 ) 2 between unequal potential wells, is a unique object that allowed to observe, using the NQR method, the effect of unequal-wells reorientations on quadrupole nuclei, both participating in the hindered motion and fixed quadrupole nuclei of neighboring atoms, which are connected with the moving fragment by nonvalent interactions. This study repeats the quantitative analysis of the temperature dependences of the spin-lattice relaxation time T 1 measured in this compound earlier by the NQR method for both types of quadrupole nuclei. An experimental proof is obtained that the dependence T 1 ( T ) of quadrupole nuclei directly involved in the unequal-wells reorientations provides information about the potential barrier of the transition from the ground state to the metastable, which is the opposite of the previously accepted point of view. This fact eliminates contradictions in the earlier interpretation of the NQR data for the fixed quadrupole nuclei. New analysis gives a self-consistent adequate description of the experimental results obtained by the NQR method and clarifies the real parameters of all molecular movements affecting the relaxation behavior of quadrupole nuclei in different molecular fragments. On the other hand, the dependence T 1 ( T ) of quadrupole nuclei involved in unequal-wells reorientation directly allowed to establish the principal feature of such reorientation, which consists in reducing the efficiency of the reorientation mechanism and manifests itself in shifting the dependence T 1 ( T ) towards higher temperatures. Quantitatively, we can observe such a shift as a decrease in the pre-exponential factor in the Arrhenius equation, which describes the reorientation contribution to the dependence T 1 ( T ) of moving nuclei. This decrease can reach several orders in comparison with the usual value of the factor (10 12 –10 13 s –1 ) and makes it possible to evaluate the difference D between the energy levels of the main and metastable orientations. On the basis of the obtained data, it is assumed that the general form of the expression for the quadrupole spin-lattice relaxation rate of moving nuclei should contain a factor depending on the energy difference D of the orientational positions, and a factor depending on the angle between the direction of the electric field gradient on the quadrupole nucleus and the axis of the reorientational motion.

Nuclear quadrupolar spin-lattice relaxation with realistic phonon density of states in alkali halides

The nuclear quadrupolar spin-lattice relaxation times T1 have been calculated in alkali halides, whereby the phonon densities of states have been extracted from the shell model lattice dynamics. The calculations at room temperature were performed on 79Br in KBr, NaBr, and RbBr; 23Na in NaI, NaCl, and NaBr; 35Cl in NaCl and KCl; and 127I in NaI and KI. The obtained values of relaxation time T1 are an order of magnitude smaller than those calculated with the Debye-model phonon density of states and agree well with experiments. The temperature dependence of acoustic and optical phonon contributions to the nuclear relaxation rate (first-order Raman phonon process) are carried out in the temperature range 20 to 300 K for 23Na and 127I in NaI. The role of optical phonons becomes important from 77 K on; the temperature dependence of T1 agrees with experimental data throughout most of the temperature range studied (77–300 K), while differing quantitatively from the predictions obtained on the basis of the Debye model.

Thermoactivated molecular motions in solids and determination of the energy of their activation by NQR spectroscopy methods

The activation energy of thermoactivated molecular motions in solids is determined by examining the influence of these motions on the temperature dependence of the nuclear quadrupole spin-lattice relaxation rate and quadrupole resonance (NQR) signal intensity for chlorine-35. In the latter case, the heating of the crystals is accompanied by the fading of resonance signals, which is analyzed using a linear correlation between the activation energy of this motion and the fade-out temperature. The correlation parameters are demonstrated to be dependant on the type of molecular motion. The relaxation method is shown to be more effective in studying molecular motion and evaluating its activation energy as compared to the NQR signal fading technique.

Nuclear quadrupole spin-lattice relaxation in Bi4Ge3O12 single crystals doped with atoms of d or f elements. Crystal field effects in compounds exhibiting anomalous magnetic properties

The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi4Ge3O12 doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole spin-spin relaxation time T2* occurred upon doping the pure Bi4Ge3O12 sample. Unlike T2*, the effective spin-lattice relaxation time T1* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects.

Parameters of thermally activated motions in crystals according to the chlorine-35 NQR spectroscopy data

The data on a wide range of molecular crystals with internal thermally activated mobility in the form of reorientations of atomic groups and whole molecules and intramolecular position exchange of atoms were used to analyze the interrelations of activation parameters present in the temperature dependence of the rate of quadrupole spin-lattice relaxation of chlorine-35 nuclei. The parameters were found to be linearly correlated. The ratio between them was analyzed to draw the conclusion that the activation energy of these motions played a determining role. The data used to construct correlations included dynamic processes with activation energy of from 10 to 100 kJ/mol without temperature effects on this energy in molecular solids considered.

NMR study of liquid to solid transition in a glass forming metallic system

Quadrupolar spin-lattice relaxation effect was used to study the temperature dependence of the correlation of electric field gradient (EFG) observed by (63)Cu and (65)Cu NMR in the liquid and supercooled liquid states of Pd(43)Cu(27)Ni(10)P(20) metallic glass forming system. The correlation time of EFG was shown to have a dramatic temperature dependence that cannot be accounted for by available theory. Analyzed in the context of mode coupling theory (MCT), it is shown that the correlation time of EFG follows the scaling equation of MCT and reveals a T(c), the critical temperature of MCT, at 700 K. Other NMR techniques such as chemical exchange line narrowing and stimulated echo pulse sequences were used to study motion of (31)P at lower temperatures. Combined together, these techniques cover the whole range of liquid to solid transition. By comparing the NMR results with data obtained by other techniques, a decoupling of motion for different types of atoms is revealed starting from T(c) and below. This essentially demonstrates a transition from liquidlike to solidlike motion at T(c) as predicted by MCT.

35Cl NQR and pseudorotation in crystalline molecular complexes of antimony trichloride with aromatic hydrocarbons

The pseudorotation of chlorine atoms in molecular complexes of SbCl3 with benzene and its substituted derivatives was used as an example to consider the effect of the thermally activated intramolecular mobility on the quadrupole spin-lattice relaxation of moving nuclei. The data obtained by nuclear quadrupole resonance were analyzed using the model that implies the existence of a temperature dependence of the activation energy for pseudorotation. This model accounts for the great preexponential factors in the expression for the thermal activation component of the relaxation rate of chlorine nuclei found for some complexes and provides the way for rapid estimation of the pseudorotation activation energy in the characteristic temperature points using both relaxation and spectral NQR parameters.

Tacticity effects on the barriers to rotation of the ester methyl group in poly (methyl methacrylate): a deuteron magnetic resonance study.

In isotactic poly(methyl methacrylate) (PMMA), we investigate the dynamics of the ester methyl groups by means of deuteron magnetic resonance (DMR) in a deuterated sample. We find that the motion of the CD(3)-group affects the deuteron spin-lattice relaxation as well as the DMR line shape in a characteristic way. Quadrupolar order spin lattice relaxation measurements between T=291 K and T=70 K reveal a broad temperature dependent probability distribution of autocorrelation times tau(c) for the 2pi/3 reorientation. This broad distribution corresponds to a temperature independent Gaussian distribution of activation energies rho(E(a)) with variance sigma(E(a) )=13.8+/-0.5 meV (1.33 kJ/mol). The line shape transition between T=70 K and T=23 K is explained with the freezing in of the methyl group reorientation. By comparing our results in an 88% isotactic sample with results obtained from a 50% syndiotactic, 30% atactic, and 20% isotactic sample of a previous investigation, we demonstrate the higher local order of the 88% isotactic sample, which corresponds to a ratio of 1.6 in the relative width sigma(E(a) )/E(a) of the E(a) distribution. We show that different stereospecific forms of PMMA can be easily distinguished by the characteristics of their line shape transition between T=70 K and T=23 K.

Dynamics of a Liquid Crystal in its Smectic A Phase from Angle Dependent Deuterium Spin Relaxation Measurements

The Zeeman and quadrupolar spin-lattice relaxation times of the aromatic deuterons of OAB-d 12 were measured throughout the smectic A phase at two different frequencies (15 and 46.04 MHz). At 15 MHz measurements were performed as a function of the angle between the phase director and the external magnetic field. Eight spectral densities were consequently determined, and were analyzed by means of a global target fitting procedure using diffusional models for the overall molecular reorientations and the internal motions. Diffusional coefficients of about 10 9 s −1 were found for molecular spinning and phenyl ring rotations, three orders of magnitude higher than for molecular tumbling.

NQR data on the activation energy of orientational defects in crystal lattice

The nuclear quadrupole spin-lattice relaxation related to the formation of orientational defects in crystals has been analyzed. The defects arise because of thermally activated reorientational motion of molecules and their fragments between the nonequivalent potential wells. These molecular reorientations are considered for double-and triple-well potentials, both periodic and bounded by external side walls. It was shown that, for such potentials, the temperature dependence of the relaxation time provides the data on the activation energy for transitions of molecules in the crystal lattice from the equilibrium ground-state positions to those corresponding to metastable states. We systematized the known NQR data on the activation energies for orientational defects arising in the intracrystal rotations of molecules and atomic groups possessing the imperfect two-, three-, and sixfold axial symmetries.

14N spin relaxation study in the hexagonal phase of a lyotropic liquid crystal

14N quadrupolar splitting and spin-lattice relaxation times T IZ and T IQ were measured in the hexagonal phase of a binary mixture of dodecyltrimethylammonium chloride and water. The derived spectral densities of motion at seven temperatures together with their corresponding quadrupolar splittings were analysed based on a simple model of ‘classical’ aggregates. Both fast local motion and slower surface self-diffusion about the cylindrical aggregate axis were required to account for the 14N spin relaxation in this phase. The azimuthal correlation time was found to be tens of nanoseconds.

14N spin relaxation study in the hexagonal phase of a lyotropic liquid crystal

14N quadrupolar splitting and spin-lattice relaxation times T IZ and T IQ were measured in the hexagonal phase of a binary mixture of dodecyltrimethylammonium chloride and water. The derived spectral densities of motion at seven temperatures together with their corresponding quadrupolar splittings were analysed based on a simple model of ‘classical’ aggregates. Both fast local motion and slower surface self-diffusion about the cylindrical aggregate axis were required to account for the 14N spin relaxation in this phase. The azimuthal correlation time was found to be tens of nanoseconds.

Atomic Detail Peptide-Membrane Interactions: Molecular Dynamics Simulation of Gramicidin S in a DMPC Bilayer

Molecular dynamics simulations have been performed of the sequence-symmetric cyclic decapeptide antibiotic gramicidin S (GS), in interaction with a hydrated dimyristoylphosphatidylcholine (DMPC) bilayer, and the results compared with a “control” simulation of the system in the absence of GS. Following experimental evidence, the GS was initially set in a single antiparallel β-sheet conformation with two Type II′ β-turns in an amphiphilic interaction with the membrane. This conformation and position remained in the 6.5 ns simulation. Main-chain dihedrals are on average ∼26° from those determined by NMR experiment on GS in dimethylsulfoxide (DMSO) solution. Sequence-symmetric main-chain and side-chain dihedral angle pairs converge to within ∼5° and ∼10°, respectively. The area per lipid, lipid tail order parameters, and quadrupole spin-lattice relaxation times of the control simulation are mostly in good agreement with corresponding experiments. The GS has little effect on the membrane dipole potential or water permeability. However, it is found to have a disordering effect (in agreement with experiment) and a fluidifying effect on lipids directly interacting with it, and an ordering effect on those not directly interacting.

Rotational dynamics of a chiral mesogen by 2H NMR study: can it be anomalous?

Deuterium NMR spectroscopy is used to study a ring-deuteriated chiral liquid crystal 4-(2-methylbutyl)oxycarbonylphenyl 4-(10-undecenyloxy)benzoate. The quadrupolar and proton-deuteron dipolar splittings, and deuteron quadrupolar and Zeeman spin-lattice relaxation times were measured as a function of temperature in the smectic A phase at two different Larmor frequencies. The derived spectral densities of motion at different temperatures were analysed simultaneously using a rotational diffusion model which also includes internal ring rotations. Motional parameters (D ⊥, D ∥, D R) and order parameter tensors (Szz , Sxx -Syy ) were obtained. Although the present data seem insufficient to draw a definitive conclusion, we believe that it is possible for this particular chiral molecule to have D⊥ >D∥ , which is different from non-chiral rod-like liquid crystals.

Rotational dynamics of a discotic liquid crystal studied by deuteron spin relaxation

We report on measurements of deuteron quadrupolar splitting and spin-lattice relaxation times T1Q and T1Z in the columnar phase of a ring-deuteriated hexaoctyloxyrufigallol at 46 MHz as a function of temperature. To describe small-step rotations of these molecules within each column in the columnar phase, a space-fixed frame is used to diagonalize the molecular diffusion tensor. The principal diffusion constants in this so called 'anisotropic viscosity' model Dalpha and Dbeta are for rotations of a molecule around and perpendicular to the columnar axis, respectively. A global target analysis of the spectral densities at seven temperatures in a minimization procedure was carried out. We found that Dalpha > Dbeta, which is consistent with the picture that the motion towards or away from the local director tends to disrupt the packing of molecules within the column.

Molecular dynamics and quadrupole relaxation in solid trimethylamine– and trimethylphosphine–boron trichloride complexes

The temperature dependence of quadrupole spin–lattice relaxation time of 35Cl nuclei has been measured in solid trimethylamine– and trimethylphosphine–boron trichloride complexes. The nature of reorientation processes occurring in the complexes has been established and the activation parameters determined. The obtained results indicate that in BCl 3–trimethylamine and –trimethylphosphine the hindered rotation of the whole complex occurs rather than uncorrelated motion of the two moieties. The motions are discussed in relation to the molecular structure of the complexes. The temperature dependence of the 35Cl NQR frequency of trimethylphosphine–boron trichloride is also reported.

Conformational dynamics of a metallomesogen studied by 2H-NMR spectroscopy

In this work we present a quantitative analysis of both quadrupolar splittings and deuterium Zeeman and quadrupolar spin-lattice relaxation times reported in the literature for two isotopomers of Azpac, an acetylacetonate derivative of the cyclopalladated 4, 4'-bis(hexyloxy) azoxybenzene. Azpac-d(4) is deuterated at the aromatic rings and Azpac-d(26) is deuterated on the alkoxy chains. The additive potential method is used to model the splittings, while the derived spectral densities are interpreted using the decoupled model in conjunction with the Nordio model. The two side chains are assumed to be noninteracting and identical in their conformations in order to limit the size of the transition rate matrix needed to describe correlated internal bond rotations in the chains. Rotational diffusion constants and internal jump rate constants are derived for this metallomesogen.

35Cl NQR and structural-dynamic properties of crystalline CCl3C(O)OC6H2Cl3-2,4,6

The temperature dependences of the resonance frequency and quadrupole spin-lattice relaxation time of the chlorine-35 nuclei in crystalline CCl3C(O)OC6H2Cl3-2,4,6 were studied. Owing to the presence of resonant nuclei in various fragments of the molecules, the character of dynamics of these molecular fragments and the entire molecule was established. It is shown that thermal librations of molecules are quasiharmonic over the whole temperature range from 77 K to the melting point of the crystal. The reorientational motion of the CCl3 group bonded to the three-coordinated carbon atom was revealed. This motion causes an exponential increase in the spin-lattice relaxation time of the chlorine nuclei of this group and subsequent damping of NQR signals (chlorine-35 resonance signals of the aryl radical were observed before melting of the sample). The activation energy of the reorientational motion is found; its value, which is 27.3 kJ/mole, is considered in comparison with the activation energies obtained by the Cl NQR method in solids for CCl3 reorientations in similar molecular structures.