Dipolar Spin-lattice Relaxation Time Research Articles

Proton Dipolar Spin-Lattice Relaxation in Nano-channels of Natrolite.

The 1H nuclear magnetic resonance (NMR) spectra and the dipolar spin–lattice relaxation time T1D for 1H in the natural natrolite (Na2Al2Si3O10·2H2O) have been measured in the temperature range of 190–390 K. From the temperature transformations of 1H NMR spectra, it follows that at T > 300 K, the diffusion of water molecules along the nano-channels is observed. From experimental T1D data, it follows that the 180° flip motion of the water molecules takes place in natrolite. At low temperature (T < 250 K), the dipolar interaction with paramagnetic impurities as a relaxation mechanism of 1H nuclei becomes significant.

NMR probe of band tail states in a-Si:H

We measured the dipolar spin–lattice relaxation time ( T 1d) of the bonded hydrogen in hydrogenated amorphous silicon at low temperature. At 15 K, after the sample is irradiated by λ=6500 A ̊ light for 2 s, T 1d is reduced by about one order of magnitude. The reduction of T 1d is attributed to the cross-relaxation between the hydrogen and localized paramagnetic band tail states that are induced by the light. Within experimental error, T 1d is proportional to the inverse of the square of the density of the paramagnetic states, consistent with a previously proposed model.

Hydrogen motion in hydrogenated amorphous silicon (a-Si:H)

Dipolar spin lattice relaxation time, T 1D, measurements of 1H in a-Si:H have been used to probe the local motion of hydrogen in a-Si:H. Variations of T 1D with doping level reveal a trend similar to the changes in hydrogen diffusion constants measured in similar samples using secondary ion mass spectroscopy (SIMS). The temperature variations of T 1D indicate that, if the local motion is thermally activated, the activation energies are at least an order of magnitude smaller than those measured using SIMS. The ‘diffusion constants’ for local hydrogen motion inferred from T 1D measurements are many orders of magnitude faster than the diffusion constants obtained from SIMS. The T 1D measurements are made on the timescale of milliseconds while the SIMS measurements are performed on a scale of several hours to days. A phenomenological model provides a plausible connection between these two measurements, which are made on very different timescales.

Local and Long-Range Hydrogen Motion in a-Si:H

ABSTRACTNuclear magnetic resonance (NMR) measurements of 1H dipolar echoes in a-Si:H indicate that the dipolar spin-lattice relaxation time, T1D, can be used as a measure of local hydrogen motion. In general, the microscopic hydrogen motion is orders of magnitude faster, and the microscopic activation energies are much smaller, than those measured macroscopically using secondary ion mass spectroscopy (SIMS). Although no detailed model exists to explain these apparently divergent results, these discrepancies can be understood phenomenologically by assuming a very broad distribution of rates. In this case the measured activation energies can be shown to scale roughly as the logarithm of the time over which the measurement is made.

Molecular dynamics of hydantoins and barbiturates assessed by1H,13C and15N relaxation data

The molecular dynamics of hydantoin,5,5-dimethylhydantoin,5,5-diphenylhydantoin, barbituric acid and 5-ethyl-5-phenylbarbituric acid in solution are determined to investigate the molecular basis for the activity of these neuroactive drugs. From dipolar 13C and 15N spin-lattice relaxation times the correlation times for the rotational dynamics of specific bond vectors are evaluated. Conclusions can be drawn concerning the anisotropy of the overall molecular rotational motion and the internal rotations of molecular segments. The translational molecular diffusion coefficients are obtained from self diffusion measurements by the pulsed gradients method. The observed differences in the dynamics as well as structural differences may account for the variation in the neuroactivity of the studied compounds.

Modified Jeener Solid-Echo Pulse Sequences for the Measurement of the Proton Dipolar Spin-Lattice Relaxation-Time ( T1D) of Tissue Solid-like Macromolecular Components

Modified Jeener solid-echo pulse sequences are proposed for the measurement of the proton dipolar spin-lattice relaxation time, T 1D, of motionally restricted (solid-like) components in the presence of mobile molecular species, such as encountered in biological tissue. A phase-cycled composite-pulse sequence was used for detection of the dipolar signal and cancellation of the Zeeman signal. A homospoil gradient pulse was added to the Jeener echo pulse sequence to enhance dephasing of the transverse magnetization components of mobile species, thereby aiding in elimination of the Zeeman signal during dipolar signal acquisition. A modified Jeener echo sequence incorporating water suppression is also proposed as a means to further depress the Zeeman signal arising from mobile components. The modified Jeener echo sequences were successfully used for the measurement of proton T 1D values of solid 2,6-dimethylphenol and Sephadex gels of differing degrees of cross linking and hydration.

Effect of Light Soaking on the Local Motion of Hydrogen in Hydrogenated Amorphous Silicon

ABSTRACTPrevious measurements of local hydrogen motion in intrinsic, doped, and compensated hydrogenated Amorphous silicon (a-Si:H) using the 1H nuclear magnetic resonance (NMR) dipolar echo method have shown that the local hydrogen motion is much faster than the macroscopic diffusion would indicate but that the local motion follows the same trends with doping and defect density as the macroscopic diffusion. We report the effect of light soaking on the local motion of hydrogen in hydrogenated Amorphous silicon. Measurements are presented on 10−3 P-doped a-Si:H at 297 K. After light soaking with infrared-filtered, white light of intensity -400 MW/cm2 for 75 hours, the electron spin resonance (ESR) spin density increases to -101 spins/cm After light soaking 1H NMR dipolar echo measurements on this sample show that the dipolar spin-lattice relaxation time, T1D, is ∼4 Ms. After thermal annealing at 190 C for two hours the value of T1Dreturns to its pre-irradiation value of ∼ 11 Ms. The local rate of motion, which scales with TID-1 thus increases with the paramagnetic defect density. The general implications of this result for descriptions of both microscopic and macroscopic Motion of hydrogen in a-Si:H are discussed.

Microscopic motion of hydrogen in the dilute and clustered phases of hydrogenated amorphous silicon

1H NMR dipolar echo measurements have been performed on a series of B-doped, P-doped, compensated and intrinsic a-Si:H samples. At room temperature the dipolar spin lattice relaxation time, T 1D, correlates with the macroscopic diffusion constants as determined by SIMS experiments. The local jump frequencies determined from these values of T 1D are orders of magnitude faster than those inferred from the macroscopic diffusion results and the associated activation energies are much less (∼ 0.2 eV). At 150 K and 100 K the dipolar echo in 10 −3 P-doped a-Si:H exhibits both a narrow line and a broad line. The full widths at half maximum of the narrow line and the broad line are the same as those of the narrow and broad components of the free induction decay, respectively. Because the narrow line exhibits a T 1D with a slightly different temperature dependence from the T 1D for the broad line the local motion is slightly different for hydrogen atoms in the clustered and dilute phases.

Doping Dependence of Local Hydrogen Motion in Hydrogenated Amorphous Silicon

1H NMR dipolar echo measurements have been performed on a series of samples of phosphorus- and boron-doped a-Si:H. The dipolar echo sequence consists of three rf pulses followed by an echo in the form: (π/2)x - τ1 - (π /4)y - π2- (π /4)y - echo. The echo height is plotted against π2 and the slope yields the dipolar spin-lattice relaxation time (T1D). T1D is a measure of fluctuations in the local dipolar field surrounding each hydrogen atom in a-Si:H, and measurement of this quantity can be employed as a probe of hydrogen motion on a microscopic scale. The T1D measurements of 10-5 B-doped, 10-3 P-doped and undoped a-Si:H are compared to the previously measured T1D of 10-4 B-doped a-Si:H. The T1D values for 10-4B-doped, 10-4 B-doped and undoped a-Si:H are, respectively, 1.7 ms, 11 ms and 22 ms at 300 K. The T1D for 10-3 P-doped is found to be the same as for 10-5 B-doped within experimental error. These trends are similar to the variation of the macroscopic diffusion of hydrogen with respect to various doping levels, but the details of the local motion are very different from those of the macroscopic diffusion.

Dipolar Measurements of Hydrogen in Amorphous Silicon

ABSTRACTThe dipolar interaction of hydrogen (∼ 10 at. %) in boron-doped amorphous silicon has been studied using the Jeener-Broekaert pulse sequence. The sample was prepared on an Al foil substrate at a temperature of 230°C using a standard glow discharge system (operating at 1 W rf power). The diborane/silane ration was 10-4. The same sample was removed from the Al substrate using dilute hydrochloric acid. The Jeener-Broekaert pulse sequence consists of three pulses: π/2|x′ – τ1 – π/4|y′ – τ2 – π/4|y′ – echo. We measured T1D, the dipolar spin lattice relaxation time, for τ1 = 82 μs, τ1 = 100 μs and τ1 = 50 μs at 299°K. The value of τ1D was found to be independent of τ1. At 335°K we found τ1D to be much longer than at room temperature. The values of τ1D at 299 K, 314 K and 335 K are, respectively, 0.7 ms, 1.2 ms and 1.8 ms. From the data we estimate an activation energy for microscopic motion to be ∼ 0.2 eV.

Motional dynamics in liquid 1,2,3,4-tetrahydro-5,6-dimethyl-1,4-methanonaphthalene

The rotational motion of molecules about the three space axes in the liquid state can be described by the rotational diffusion tensor. Until recently most investigations concerning rotational motions of molecules in liquids employing dipolar spin-lattice relaxation data have proceeded on the assumption that the orientation of the rotational diffusion tensor principal axis system coincides with that of the intertial tensor. But this is not necessarily the case for liquids which consist of molecules rotating as asymmetrictop rotors with less than two mutually perpendicular mirror planes. Usually it is assumed that, in hydrocarbons, a rotation of the two principal axis systems with respect to each other does not occur because of their weak intermolecular interactions. In order to examine this statement the rotational behaviour of the molecules of the hydrocarbon 1,2,3,4-tetrahydro-5,6-dimethyl-1,4-methanonaphthalene (1) was investigated using 13C dipolar spin-lattice relaxation time measurements. The orienta...

Glassy crystals. V : structural and dynamic studies of large amplitude molecular motions

This paper on glassy crystals contains new data obtained by thermally stimulated current (TSC) measurements in cycloalkanols and 1,2 -difluorotetrachloroethane and NMR measurements of spin-spin, spin-lattice and dipolar spin-lattice relaxation times in cyclohexanol and cyclooctanol. TSC results as a function of thermal treatment show that the hypothesis of structural heterogeneity is not necessary. NMR data show the existence of two different large amplitude motions in cyclo-hexanol and —octanol, but the conventional BPP model does not adequately describe our results.Nevertheless, using literature data, it is possible to describe the quenching mechanism. For cyclohexanol and 1-cyanoadamatane, the structural models of the plastic phases are also good space and time averaged descriptions of the resulting glassy crystals. The latter contains the same molecular orientations and essentially the same short-range disorder, in spite of a change in molecular orientational correlations. From the dynamic point of view, glass transition, for the 4 studied compounds, seems closely related to the freezing of endospherical reorientational motions ; some anisotropic large amplitude motions can continue within the glassy crystalline phase and can eventually lead to secondary glass transitions, as in cyclohexanol, when their characteristic times reach 103 —104 s.

Anisotropic rotational motion and internal rotation in liquid methylbenzenes

The usual semiquantitative evaluations of temperature-dependent dipolar 13C spin-lattice relaxation times of liquid methylbenzenes (toluene, xylenes, tri- and tetramethylbenzenes), which assume isotropic molecular motion, do not allow any reliable statements to be made about the various hindered rotations of the methyl groups, and about preferred molecular rotation axes. The quantitative evaluation of the experimental data using a new formalism (complete anisotropic motion with arbitrary position of the internally rotating methyl groups) gave the following: physically meaningless results were obtained in all cases for the optimizations for one of the three possible axially symmetrical ellipsoidal motions, and in most cases for complete anisotropic motion. A quantitative evaluation of dipolar relaxation times which takes complete anisotropic motion into account is—at least for the methylbenzenes—only promising if the present 10 per cent level of experimental error in the dipolar relaxation times can be red...

Proton Dipolar Spin-lattice Relaxation in the Smectic Phases of TBBA

Abstract The proton dipolar spin-lattice relaxation time T 1D was measured at several Larmor frequencies in the various smectic phases of the liquid crystal terephthal-bis-butylaniline (TBBA) as a function of temperature and orientation of the sample in an external magnetic field. The angular dependent T 1D measurements are used to determine the importance of orientational order director fluctuations (ODF) in the dipolar field of the smectic phases of TBBA. In particular, the T 1 D −1 angular dependence in the SA phase is different from that in the SC and SG phases. This contrasts sharply with the T 1 −1 angular dependence in these phases which are all similar.

Proton Zeeman and dipolar relaxation study of the ferroelectric liquid crystal DOBAMBC

The proton Zeeman spin-lattice relaxation time T1 and the proton dipolar spin-lattice relaxation time T1D of a chiral compound p-decyloxybenzylidene p′-amino-2-methyl butyl cinnamate are studied as a function of temperature and sample rotation in a magnetic field at 30 MHz. In addition, proton T1 is measured in the smetic A, ferroelectric smectic C* and H* phase at 15.1 and 45.5 MHz. The spin relaxation mechanisms for the proton T1 and T1D in the smectic A and C* phase are ‘‘diffusion-induced’’ orientational order director fluctuations and molecular self-diffusion. In contrast with the achiral compound TBBA, the T1 angular dependence in the smectic C* and C phase are different, while the T1D angular dependence appears to be similar in these smectic phases.

NMR study of a smectic A-smectic B transition in a liquid crystal

Proton Zeeman spin-lattice relaxation time and dipolar spin-lattice relaxation time measurements are used to study the smectic A-smectic B transition in a pure liquid-crystal 4- n-butoxybenzylidene-4′-aminopropiophenone. Both the orientational order director fluctuations and the molecular self-diffusion are responsible for the Zeeman spin-lattice relaxation, while the latter is responsible for the dipolar spin-lattice relaxation.

On the dynamics of the pseudo one-dimensional ferroelectric ordering in CsH2PO4and CsD2PO4

Abstract Measurements of the temperature dependence of the phosphorus Zeeman and dipolar spin-lattice relaxation times in CsH2PO4 show in the temperature range dominated by one-dimensional correlations an extremely strong dynamic central peak which decreases by four orders of magnitude on going to the range dominated by three dimensional correlations. The order fluctuation spectrum is qualitatively different in the case of CsD2PO4.

NMR investigation of the layered transition metal phosphorus trichalcogenides and the intercalation compounds Li xNiPS 3

The transition metal phosphorus trichalcogenides MPX 3 are layered semiconductors which can be intercalated either chemically or electrochemically by lithium atoms. We present a NMR study of the magnetic properties of the MPX 3 compounds (M = Ni, Mn, Fe; X = S, Se) and their modification as a function of intercalation in the compounds Li xNiPS 3 (x = 0 − 1.29). In the concentration range x = 0.02 − 0.5, the relative shift of the P 31 resonance line K iso = -0.052 % and the Neel temperature T N = 155 K are constant and barely different from that measured in pure NiPS 3 ( K iso = -0.057 %, T N = 165 K). For x > 0.5 a new P 31 resonance line is observed, which grows with x at the expense of the previous one. It corresponds to non magnetic layers S - Ni 2 3 (P 2) 1 3 - S , which progressively replace the paramagnetic ones up to the limit of intercalation x = 1.29. Investigation of the lithium ions mobility through Zeeman and dipolar spin lattice relaxation times T 1∼ and T 1D measurements indicates a very low self-diffusion coefficient - D = 10 -13 − 10 -14 cm s -1 -, that is four orders of magnitude smaller than those measured in Li xTiS 2 compounds. This strongly contrasts with the good electrochemical activity of the Li xNiPS 3 system.

Proton NMR study of molecular motion in solid SiH 4

Proton Zeeman- and dipolar spin—lattice relaxation times and second moments have been measured in polycrystalline SiH 4 at 24 MHz, between 90 and 3 K. Down to 25 K the classical model for molecular applies; below 2S K, quantum effects have to be taken into account.

The determination of the spin diffusion rate from the dipolar relaxation of a rotating sample

It is shown that the observed dipolar spin-lattice relaxation time is shortened by sample rotation. This can be used to determine experimentally the spin diffusion rate.