Spin-lattice Relaxation Time T1 Research Articles

Spin Structure and Dynamics in a Spin 1/2 One Dimensional Antiferromagnet, [NiBr(chxn)2]Br2 (chxn: 1R,2R-cyclohexanediamine)

Abstract The 1H NMR spin-lattice relaxation time T1 in a spin 1/2 1-D antiferromagnetic complex [NiBr(chxn)2]Br2 was observed down to 1.8 K. The observed T1 temperature dependence was well reproduced by the Sachdev’s treatment giving an exchange interaction energy (J) of ca. 2500 K. 1-D spin migration assignable to paramagnetic electrons in impurity amounts of NiII sites was observed at low temperatures.

Study of inclusion complexes of acridine with β‐ and (2,6‐di‐O‐methyl)‐β‐cyclodextrin by use of solubility diagrams and NMR spectroscopy

AbstractPrevious molecular modeling studies, in our laboratory, have shown that some esters of type RCOO(CH2) nC5H5N+Cl− are potentially active against Alzheimer's disease. We have also demonstrated that acridine, which has strong anticholinesterase activity appears to be a suitable R substituent. The main obstacle to the possible pharmaceutical application of these compounds is their limited solubility in water, which is due to the poor aqueous solubility of acridine itself (0.26 mM). Inclusion complexation with cyclodextrins may overcome this problem. Solubility diagrams and NMR spectroscopy were used to study the inclusion of acridine (Acr) within β‐cyclodextrin (βCD) and heptakis(2,6‐di‐O‐methyl)cyclomaltoheptose (DMβCD). A 1:1 complex was formed for the Acr–βCD system and both 1:1 and 2:1 complexes for the Acr–DMβCD system (apparent Ka 215 ± 20 and 1150 ± 100 M −1, respectively). Data from 1H NMR studies corrected the assignment of the acridine H1, H8 and H4, H5 protons in D2O, which have been erroneously assigned in previous publications. 1H spin–lattice relaxation times T1 measured with selective, non‐selective and null inversion characterize the dynamics of the inclusion complexes. Geometric features of the host–guest inclusion complexes were inferred from intermolecular dipolar interactions obtained by 2D adiabatic off‐resonance ROESY experiments. A more detailed picture of the structure of the inclusion complexes was obtained by combining NMR structural data and molecular modeling (docking, dynamics under NMR constraints and energy calculations). Copyright © 2002 John Wiley & Sons, Ltd.

133Cs NMR Spin-Lattice Relaxation Time and Chemical Shift Studies on Cs2MX4 Crystals with Sr2GeS4 and β-K2SO4 Structures Performing no Low Temperature Phase Transition

133Cs NMR spectra and spin-lattice relaxation times T1 in crystalline Cs2ZnCl4 and Cs2ZnBr4 with -K2SO4 structure, and Cs2CdI4 and Cs2HgI4 with Sr2GeS4 structure, which have no phase transition in the lower temperature regions, weremeasured to clarify the relation between the interionic covalency and crystal structures. Two central lines corresponding to two crystallographically inequivalent Cs sites were observed in all compounds. One of the two peaks in β-K2SO4-type compounds appears below 40 ppm, but another peak in those compounts and the both lines in Sr2GeS4-type compounds show larger shifts, 130 - 200 ppm. The temperature dependences of T1 observed in Sr2GeS4-type compounds were close to the theoretical behaviour, calculated by considering contributions from normal lattice vibrations. Deviations from the calculation obtained for β-K2SO4-type systems are attributable to the difference in interionic interactions, i. e., partial covalency, in the crystals

Incommensurability and Domain Structure of K2SbF5

Phase transitions and incommensurability in K2SbF5 have been studied by means of 123Sb NQR spectra and spin-lattice relaxation measurements. The phase transitions occur at 117, 135 and 260 K. The line shape and temperature dependence of the spin-lattice relaxation time T1 at 135 to 260 K are characteristic for an incommensurate state with a plane wave modulation regime. At 117 to 135 K a distinct fine structure of the NQR spectra has been observed. The X-ray diffraction pattern of this phase is interpreted as a coexistence of two modulation waves along the a and b axis with wave vectors (a*/6 + b*/6) and (a*/2 + b*/2), respectively. The best interpretation that fits our NQR data is a coexistence of two domains, the structures of which are modulated with different periods in such a manner that each domain exhibits only one of the aforementioned modulation waves. Redistribution of line intensities with the variation of temperature shows that one of the domains becomes energetically preferable on cooling and is transformed into the low temperature phase at 117 K. The 123SbNQR measurements in K2SbF5 show unusually short values of T1, which become close to the spin-spin relaxation time T2 with increasing temperature. - Pacs: 61.44.Fw, 64.60, 64.70, 64.70.Rh, 76.60

Temperature dependence of the spin-lattice relaxation time for quadrupole nuclei under conditions of NMR line saturation

The contributions of different mechanisms of nuclear spin-lattice relaxation are experimentally separated for 69Ga and 71Ga nuclei in GaAs crystals (nominally pure and doped with copper and chromium), 23Na nuclei in a nominally pure NaCl crystal, and 27Al nuclei in nominally pure and lightly chromium-doped Al2O3 crystals in the temperature range 80–300 K. The contribution of impurities to spin-lattice relaxation is separated under the condition of additional stationary saturation of the nuclear magnetic resonance (NMR) line in magnetic and electric resonance fields. It is demonstrated that, upon suppression of the impurity mechanism of spin-lattice relaxation, the temperature dependence of the spin-lattice relaxation time T 1 for GaAs and NaCl crystals is described within the model of two-phonon Raman processes in the Debye approximation, whereas the temperature dependence of T 1 for corundum crystals deviates from the theoretical curve for relaxation due to the spin-phonon interaction.

Spin−Lattice Relaxation and 13C NMR Line Shape at Multiaxial Reorientation of Molecules in Fullerite C60

Molecular dynamics in the fcc-sc phases of fullerite C60 (13C NMR line shape and spin-lattice relaxation time T1) is described in terms of discrete orientational states of molecules. Molecular reorientations in the low temperature orientationally ordered sc phase are assumed to take place about molecular symmetry axes. Transition into the high-temperature disordered fcc phase is modeled by additional molecular reorientations about C4 symmetry axes of the cubic unit cell. At low temperatures, the NMR line shape functions are demonstrated to depend significantly on the model of molecular reorientations in contrast with T1. In the vicinity of the phase transition at 260 K, T1 changes dramatically for all models, unlike the NMR line shape. The model of relatively fast molecular jumps about a single C3 symmetry axis along with slow reorientations about the other axes is found to adequately reproduce the T1 experiment and qualitatively reflects the temperature changes in NMR line shape.

6Li and 7Li MAS NMR Studies on Fast Ionic Conducting Spinel-Type Li2MgCl4, Li2–xCuxMgCl4, Li2–xNaxMgCl4, and Li2ZnCl4

6Li and 7Li MAS NMR spectra including 1D-EXSY (exchange spectroscopy) and inversion recovery experiments of fast ionic conducting Li2MgCl4, Li2–xCuxMgCl4, Li2–xNaxMgCl4, and Li2ZnCl4 have been recorded and discussed with respect to the dynamics and local structure of the lithium ions. The chemical shifts, intensities, and half-widths of the Li MAS NMR signals of the inverse spinel-type solid solutions Li2–xMIxMgCl4 (MI=Cu, Na) with the copper ions solely at tetrahedral sites and sodium ions at octahedral sites and the normal spinel-type zinc compound, respectively, confirm the assignment of the low-field signal to Litet of inverse spinel-type Li2MgCl4 and the high-field signal to Lioct as proposed by Nagel et al. (2000). In contrast to spinel-type Li2–2xMg1+xCl4 solid solutions with clustering of the vacancies and Mg2+ ions, the Cu+ and Na+ ions are randomly distributed on the tetrahedral and octahedral sites, respectively. The activation energies due to the various dynamic processes of the lithium ions in inverse spinel-type chlorides obtained by the NMR experiments are Ea=6.6–6.9 and ΔG*>79 KJ mol−1 (in addition to 23, 29, and 75 kJmol–1 obtained by other techniques), respectively. The largest activation energy of >79 KJ mol−1 corresponds to hopping exchange processes of Li ions between the tetrahedral 8a sites and the octahedral 16d sites. The smallest value of 6.6–6.9 KJ mol−1, which was derived from the temperature dependence of both the spin-lattice relaxation times T1 and the correlation times τC of Litet, reveals a dynamic process for the Litet ions inside the tetrahedral voids of the structure, probably between fourfold 32e split sites around the tetrahedral 8a site.

Spin-lattice relaxation in solid 3He in weak magnetic fields

Spin-lattice relaxation times T1 were measured for solid 3He at temperatures of 0.22 to 0.73 K in a 44-Oe magnetic field. An increase in T1 at temperatures higher than approximately 0.4 K was related to switching on the vacancy mechanism of atomic mobility in the crystal. At a melting curve minimum, in the region of predominance of exchange motions of atoms in the crystal, measurements of T1 were performed in magnetic fields of 2 to 71 Oe. The data obtained in fields higher than 5 Oe were in agreement with the Cowan-Fardis theory.

The pressure dependence of self-diffusion and spin–lattice relaxation in cold and supercooled H2O and D2O

Measurements of the self-diffusion coefficients D and the oxygen-17 and deuterium spin–lattice relaxation times T1 in light and heavy water have been extended into the supercooled region to a maximum pressure of 400 MPa and to a minimum temperature of 240 K and partly to even lower temperatures. With these measurements the maxima of the T1 isotherms could be established for the first time. For the deuterium and oxygen-17 T1 of heavy water as well as for the oxygen-17 T1 of light water the maxima occur around 250 MPa, their position being independent of temperature. Previously established maxima in the D-isotherms for both liquids are found at around 150 MPa. The heights of the maxima increase strongly with decreasing temperature. The increase in mobility with pressure is much more pronounced for the rotational mobility as given by T1 than for the translational mobility characterised by the self-diffusion coefficient. This anomaly, which appears to have been observed experimentally in cold water only, is discussed qualitatively.

35Cl NQR studies of 1‐chloro‐2,4‐dinitrobenzene and 1,2‐dichloro‐3‐nitrobenzene as a function of pressure and temperature

AbstractThe temperature and pressure dependences of 35Cl nuclear quadrupole resonance (NQR) frequency and spin–lattice relaxation time (T1) were investigated for 1‐chloro‐2,4‐dinitrobenzene and 1,2‐dichloro‐3‐nitrobenzene. T1 was measured in the temperature range 77–300 K. Furthermore, the NQR frequency (ν) and T1 for these compounds were measured as a function of pressure up to 5.1 kbar at 300 K. Relaxation was found to be due to the torsional motion of the molecule and the reorientation motion of the nitro group. By analysing the temperature dependence of T1, the activation energy for the reorientation motion of the nitro group was obtained. The temperature dependence of the average torsional lifetimes of the molecules and the transition probabilities W1 and W2 for the Δm = ±1 and Δm = ±2 transitions, were also obtained. Both compounds showed a non‐linear variation of NQR frequency with pressure. The pressure coefficients were observed to be positive. A thermodynamic analysis of the data was carried out to determine the constant‐volume temperature coefficients of the NQR frequency. The spin–lattice relaxation time T1 for both the compounds was found to be weakly dependent on pressure, showing that the relaxation is mainly due to the torsional motions. Copyright © 2002 John Wiley & Sons, Ltd.

Dynamics of Water Molecules in Micropores of AlPO4-5 and SAPO-5 Studied by 1H NMR

Abstract Dynamics of water molecules in one-dimensional micropores AlPO4-5 and SAPO-5 frameworks was studied by measuring 1H NMR spectra, second moment M2 of line-width and spin-lattice relaxation time T1. Although the presence of two kinds of water molecules free and bound on the capillary wall were reported in AlPO4-5, our NMR T1 results imply the absence of two kinds of H2O motions, but the motional rate distributes from slow to fast jumps in both AlPO4-5 and SAPO-5.

Pressure Dependence of the Chlorine NQR in Three Solid Chloro Anisoles

Abstract The 35C1 Nuclear Quadrupole Resonance (NQR) frequency (VQ) and spin lattice relaxation time (T1) in the three anisoles 2,3,4-trichloroanisole, 2,3,6-trichloroanisole and 3,5-dichloroanisole have been measured as a function of pressure upto 5.1 kbar at 300 K, and the data have been analysed to estimate the temperature coefficients of the NQR frequency at constant volume. All the three com­ pounds show a non linear variation of the NQR frequency with pressure, the rate of which is positive and decreases with increasing pressure. In case of 3,5-dichloroanisole the value becomes negative in the higher range of pressure studied. The spin lattice relaxation time T1 in all the three compounds shows a weak dependence on pressure, indicating that the relaxation is mainly due to the torsional motions.

Raman spin-lattice relaxation, Debye temperature and disorder effectsstudied with electron spin echo of Cu2+ in Tutton saltcrystals

Spin-lattice relaxation time T1 was determined by the electron spin echo(ESE) method in the temperature range 4-60 K in a series of Tutton saltcrystals MI2MII(SO4)2·6X2O (MI = NH4, K; MII = Zn, Mg; X = H, D) weaklydoped (⩽1018 ions cm-3) with the 63Cu2+isotope. The ESE signal was undetectable at higher temperatures. Therelaxation rate increases over the six decades in the studied temperaturerange with T1 equal to 1 s at 4 K and 0.5 µs at 50 K. Variouspossible relaxation mechanisms are discussed with the conclusion that therelaxation is governed by two-phonon Raman processes without a noticeablecontribution from the reorientations of Cu(H2O)6octahedra between Jahn-Teller distorted configurations. Deuteration of thecrystal has no effect in spin-lattice relaxation. For a few crystals, havingthe largest Cu2+ concentration among the studied crystals, a strongand linear in temperature contribution to the relaxation rate was found below15 K. Possible explanations are discussed with the final conclusion that thiseffect is due to a non-uniform Cu2+ distribution in the hostlattice producing effective relaxation via pairs and triads of the Cu2+ ions. From the T1(T) dependence the Debye temperature ΘDwas determined for the all crystals studied. This varies fromΘD = 166 K for K2Zn(SO4)2·6H2O to ΘD = 238 K for (NH4)2Mg(SO4)2·6H2O. The ΘD values are discussed and usedfor calculation of the sound velocity which was found to be similar in allcrystals and equal to ν = 4150(±150) m s-1.

Continuous monitoring of the zinc-phosphate acid-base cement setting reaction by proton nuclear magnetic relaxation

Proton nuclear magnetic relaxation is a well-established technique for continuous and non destructive monitoring of hydration of conventional Portland building cements. Here, we demonstrate the feasibility of nuclear magnetic resonance (NMR) monitoring of the setting reaction of zinc-phosphate acid-base dental cements, which harden in minutes as compared to days, as in the case of Portland cements. We compare the setting of cement powder (mainly, zinc oxide) prepared with clinically used aluminum-modified orthophosphoric acid solution with the setting of a model system where cement powder is mixed with pure orthophosphoric acid solution. In contrast to previously published NMR studies of setting Portland cements, where a decrease of spin-lattice relaxation time is attributed to enhanced relaxation at the growing internal surface, spin-lattice relaxation time T1 increases during the set of clinically used zinc-phosphate cement. Comparison of these results with a detailed study of diffusion, viscosity, and magnetic-field dispersion of T1 in pure and aluminum-modified orthophosphoric acid demonstrates that the increase of T1 in the setting cement is connected with the increase of molecular mobility in the residual phosphoric acid solution. Although not taken into account so far, such effects may also significantly influence the relaxation times in setting Portland cements, particularly when admixtures with an effect on water viscosity are used.

Fourier-transform EPR at high-field/high-frequency (3.4 T/95 GHz) using broadband stochastic microwave excitation.

Stochastic excitation with a full-width-half-maximum bandwidth of 250 MHz was used to perform Fourier-transform (FT) high-field/high-frequency electron paramagnetic resonance (EPR) at 3.4T/95 GHz (W-band). Thereby, the required microwave peak power is reduced by a factor of tau(p)/T1 as compared to equivalent pulsed FT EPR in which the spin system with spin-lattice relaxation time T1 is excited by a single microwave pulse of length tau(p). Stochastic EPR is particularly interesting under high-field/high-frequency conditions, because the limited output power of mm microwave sources, amplifiers, and mixers makes pulse FT EPR in that frequency domain impossible, at least for the near future. On the other hand, FT spectroscopy offers several advantages compared to field-swept magnetic resonance methods, as is demonstrated by its success in NMR and X-band EPR. In this paper we describe a novel stochastic W-band microwave bridge including a bimodal induction mode transmission resonator that serves for decoupling the microwave excitation and signal detection. We report first EPR measurements and discuss experimental difficulties as well as achieved sensitivity. Moreover, we discuss future improvements and the possibility for an application of stochastic W-band FT EPR to transient signals such as those of photoexcited radical pairs in photosynthetic reaction centers.

Structural study of supercritical water. III. Rotational dynamics

The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O–D axis of a single water molecule. It is then found that while τ2R decreases rapidly with the temperature on the liquid branch of the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied up to twice the critical at a fixed supercritical temperature of 400 °C. The comparison of τ2R with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with molecular dynamics simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.

Peculiarities in the Water Adsorption and Morphology of Carbon Chars Assessed by NMR

Proton spin–lattice relaxation and 1H NMR spectroscopic data are used for studies of the water adsorption and morphology of several newly synthesized carbon-based chars. Two NMR lines have been observed in accordance with a difference between macropore and micropore water in a system with heterogeneous pore structure. The macropores occupy about the half of the char particle volume. In aqueous suspension fast molecular exchange between water in macropores and bulk water leads to an intense NMR line with a single spin–lattice relaxation time T1, which exhibits a strong particle size effect. This effect has been used for the estimation of T1 of water molecules in the vicinity of paramagnetic centers at the surface of chars, as well as for macropore size estimation. The kinetics of water vapor adsorption in chars as seen by 1H NMR indicates that this process begins in micropores, which serve as primary adsorption centers. Water molecules in micropores exhibit specific properties which are characteristic of liquids in a small, restricted space, and they give rise to the observed accumulation as well as saturation effects.

Measurement of spin-lattice relaxation times with longitudinal detection

New experimental schemes to measure spin-lattice relaxation times T1 on the basis of inversion-recovery and saturation-recovery experiments with longitudinal detection are introduced. With this approach, paramagnetic species with T1 values as short as 20 ns can be measured. Possibilities to reduce unwanted signals and instrumental artifacts are analyzed. An experiment where the signal is induced directly by the time-dependent M2 magnetization is also proposed. Experimental results for organic radicals and defect centers are presented and compared with data obtained with conventional techniques, and a metal complex at 250 K is analyzed where it is very difficult to get information about relaxation times with established methods because of fast spin-spin relaxation.

Nuclear quadrupole resonance study of phase transitions andincommensurability in K2SbF5

Phase transitions and incommensurability in K2SbF5 has been studied bymeans of 123Sb NQR spectra and spin-lattice relaxation measurements. Thephase transitions have been obtained at 117, 135 and 260 K. Line shape andtemperature dependence of the spin-lattice relaxation time T1 in thetemperature range from 135 to 260 K are characteristic for the incommensuratestate with the plane wave modulation regime. At 117 to 135 K, a distinct finestructure of NQR spectra has been observed. Redistribution of line intensitieswith the variation of temperature allows us to suggest that this phaseexhibits domain structure, and that one of the domains becomes energeticallybeneficial on cooling and transforms into the low temperature phase at 117 K.The 123Sb NQR measurements in K2SbF5 show unusually short values ofT1, which become close to the spin-spin relaxation time T2 withincreasing temperature.

Molecular dynamics in supercooled m-fluoroaniline

Molecular dynamics of meta-fluoroaniline (mFA) — the system which makes a simple glass forming liquid — was examined by 13C NMR and dielectric relaxation measurements. Spin–lattice relaxation times T1, nuclear Overhauser (NOE) factors of chemically non-equivalent carbons and dielectric relaxation in mFA were measured as a function of temperature. The analysis showed that the correlation function describing molecular dynamics in mFA could be well described in terms of an asymmetric distribution of correlation times predicted by the Davidson–Cole model. The overall correlation times of mFA, derived from the simultaneous fitting of our T1, NOE, and dielectric data (as well as of the dielectric data obtained by Roessler's group) to the Davidson–Cole model, follow the non-Arrhenius temperature type of the behaviour in accordance with the Vogel–Fulcher equation.