NMR Spin-lattice Relaxation Times Research Articles

A 1H NMR study of molecular motions in bromo- and iodopentamethylbenzene

The molecular motions and the phase transitions in crystalline bromopentamethylbenzene (BPMP) and iodopentamethylbenzene (IPMB) have been studied 1H NMR spin–lattice relaxation time T 1 measurements and differential scanning calorimetry. A solid–solid phase transition was observed at 399 K for BPMB and three solid–solid phase transitions were 237, 277, and 316 K for IPMB. T 1 was calculated theoretically with the stochastic process for the reorientations in the six potential wells of five different depths depending on the halogen position. The C 3 reorientation of CH 3 groups was observed as a gradual decrease of T 1 in both BPMB and IPMB at temperatures lower than 145 and 130 K, respectively. It has been clarified that all the in-plane reorientations activated simultaneously result in a deep T 1 minimum of 43 ms for BPMB. For IPMB, the calculation implies that the reorientations among the six potential wells are activated separately in a wide range of temperature; those from moderately deep two potential wells contribute mainly to the deepest T 1 minimum of 135 ms and those from the deepest two potential wells are not yet activated in the measured temperature region. IPMB melts at 412 K, where the latter start to contribute to the magnetic relaxation, while BPMB melts at 436 K, where all the reorientations have been already activated much.

1HNMR Study on the Motion of NH4 + in Ferroelectric NH4H(ClH2CCOO)2 and Mixed (NH4)1-xRbxH(ClH2CCOO)2 (x = 0.15)

Temperature-dependent proton NMR relaxation time measurements have been performed at 60 MHz in order to study the NH4 + dynamics in ferroelectric NH4H(ClH2CCOO)2 and mixed Rbx(NH4)1-x(ClH2CCOO)2, where x = 0.15. The data indicate that the dominant relaxation mechanism for the NMR spin-lattice relaxation time T 1 in both crystals involves simultaneous NH4 group reorientation about their C2 and C3 symmetry axis in the paraelectric phase. Details of the NH4 +reorientation have been inferred from analysis of temperature dependence of T1 assuming the Watton model. The activation parameters of the motionshave been determined.It has been found that the substitution of Rb does not change the activation parameters of the NH4 group dynamics.

1H Spin-Lattice Relaxation in a NH4HSO4Single Crystal

The proton NMR line width and spin-lattice relaxation times for the NH 4 HSO 4 single crystal were studied over a wide temperature range. The proton spin-lattice relaxation time for NH 4 HSO 4 has ...

PRELIMINARY NMR AND ESR INVESTIGATION OF LOCAL DYNAMICS IN SOME POLYISOPRENE-CCl4 SOLUTIONS

The temperature dependence of the NMR spin-lattice relaxation time is analyzed for preliminary evaluation of the correlation time of the local dynamics of the polymeric chain, for the molten polyisoprene and some polyisoprene- CCl 4 solutions. Complementary information concerning this dynamics were obtained from the analyze of the ESR spectra recorded from the nitroxide molecules (2, 2, 6, 6, tetramethyl piperidin-1-yloxyl) dissolved in the polyisoprene- CCl 4 solutions.

SPINLESS IMPURITIES IN CUPRATES: LOCAL MAGNETISM AND KONDO EFFECT IN THE NORMAL AND SUPERCONDUCTING STATES

Spinless point defects in the CuO 2 planes of the cuprates have been shown for long to induce a magnetic response on the neighboring copper sites which behaves as a nearly free local moment in the under-doped case.1,2 In the case of Li + substitution on the Cu site, accurate 7 Li NMR shift data allow to evidence that the local moment susceptibility displays a ( T + T K ) -1 dependence, with a Kondo like temperature T K which increases abruptly near optimal doping.3 While a Curie paramagnetic response can be understood on general theoretical grounds in the case of undoped quantum spin systems, the Kondo screening of the moment might directly reflect the influence of charge carriers. This Kondo like behavior is also corroborated by measurements of the fluctuation time of the local moment deduced from 7 Li NMR spin lattice relaxation time.4 The local moment is found to survive in the superconducting state although a marked reduction of the screening is observed when T K > T c (see Ref. [5]), as might be expected from theoretical considerations. Correlatively it will be shown that the scattering of charge carriers by spinless defects can be studied accurately in single crystals in which such defects are created by electron irradiation. The low T upturns of the resistivity can be associated with a Kondo-like T dependence of the scattering in the under-doped case. In the over-doped case, T K being very high, the scattering becomes T-independent and 2D weak localization effects dominate.6

Studies on the phase structure of ethylene‐vinyl acetate copolymers by solid‐state 1H and 13C NMR

AbstractThe phase structure of a series of ethylene‐vinyl acetate copolymers has been investigated by solid‐state wide‐line 1H NMR and solid‐state high‐resolution 13C NMR spectroscopy. Not only the degree of crystallinity but the relative contents of the monoclinic and orthorhombic crystals within the crystalline region varied with the vinyl acetate (VA) content. Biexponential 13C NMR spin–lattice relaxation behavior was observed for the crystalline region of all samples. The component with longer 13C NMR spin–lattice relaxation time (T1) was attributed to the internal part of the crystalline region, whereas the component with shorter 13C NMR T1 to the mobile crystalline component was located between the noncrystalline region and the internal part of the crystalline region. The content of the mobile crystalline component relative to the internal part of the crystalline region increased with the VA content, showing that the 13C NMR spin–lattice relaxation behavior is closely related to the crystalline structure of the copolymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2199–2207, 2002

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.

85,87Rb, 14N NMR Studies of Successive Phase Transitions and Incommensurate Phase in R2Pb[Cu(NO2)6] (R = NH4, Rb)

85,87Rb and 14N NMR spectra and spin-lattice relaxation time (T1) were measured for R2Pb[Cu(NO2)6] (R = Rb, NH4). The quadrupole coupling constant (e2Qq/h), asymmetry parameter (η), and the effective transverce relaxation time (T2) were estimated from the simulation of NMR spectra. The NMR spectra in commensurate phase III can be explained by the superposition of two components corresponding to two inequivalent sites of the R+ ion. In the incommensurate phase II, e2Qq/h and T2 decreased with increasing temperature, while was almost temperature independent. T1 in phase II is found to be determined by the contribution of acoustic phason with multi-soliton limits

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

Molecular Motions in Halogen-Bridged One-Dimensional Pt Complexes, [PtII(en)2][PtIVX2(en)2](ClO4)4 (X = Cl, Br) Studied by 2H and 1H NMR

2H and 1H NMR measurements were performed on crystalline [Pt(en)2][PtX2(en)2](ClO4)4 (X = Cl, Br), where the protonated and partially deuterated ethylenediamines (en’s), NH2(CH2)2NH2, NH2(CD2)2NH2 and ND2(CH2)2ND2 were used as ligands. Measurements of 2H and 1H NMR spin-lattice relaxation times showed the presence of motions of en chelate rings at the temperatures near the phase transitions, whereas broad 2H NMR spectra and the reported X-ray diffraction data showed no marked motions. These results were consistently explained by introducing the en puckering motion between highly asymmetric potential wells with an energy difference of 10 - 13 kJ mol-1. This difference was shown to be much larger than 2 - 5 kJ mol-1, reported for the iodo-complex, [Pt(en)2][PtI2(en)2](ClO4)4

Phase transition kinetics and reorientational dynamics of the plastic crystal meta-carborane studied by deuteron NMR

Meta-carborane shows an orientationally disordered, orthorhombic phase which transforms into an ordered monoclinic phase near 170 K and into a quasi-isotropic cubic phase near 280 K. From deuteron magnetization measurements both transitions are found to exhibit a thermal hysteresis and slow transformation kinetics. Furthermore, solid-echo NMR spectra and spin-lattice relaxation times are reported. These results indicate that in the orthorhombic phase the meta-carborane molecules perform a strongly anisotropic motion which is characterized by a temperature dependent amplitude.

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.

Temperature‐dependent effects in base oils: Carbon‐13 NMR spin‐lattice relaxation time and viscometry studies

AbstractCarbon‐13 NMR spin‐lattice relaxation time (T1) data have been used to study the molecular dynamic aspects of base oils with different physical properties. Relaxation time measurements have been carried out on a few model compounds and a number of mineral base oils at various temperatures (273–373 K). Effective correlation time (°C) and rotational mobility data obtained for the model compounds and base oils have provided evidence of relationships between molecular flexibility and the temperature dependence of viscosity. It is possible to determine the average carbon alkyl chain lengths and the molecular weights of the base oils from the ratio of the T1 values of Cβ and Cint carbons and the optimised value of the microviscosity factor (fr), respectively. A qualitative correlation between Arrhenius energies (Ea) for microscopic motion and macroscopic bulk properties such as viscosity and viscosity‐temperature characteristics has been observed. Base fluids having better viscosity‐temperature characteristics were also associated with lower values of Ea for micro‐ as well as macroscopic processes.

Dynamic Properties of the Guest Molecules in the Pyrazine/α-Zirconium Phosphate Intercalation Compound: a Multinuclear Solid-State NMR Study

Dynamic properties of perdeuterated pyrazine (pyrazine-d(4)) guest molecules in the layered a-zirconium phosphate host structure have been established from solid-state H-2 NMR lineshape analysis and H-2 NMR spin-lattice relaxation time measurements. Both techniques indicate that the intercalation compound contains two different types of pyrazine-d(4) guest molecules, which undergo different dynamic processes. The relative proportions of the two types of pyrazine-d(4) guest molecules are approximately 30% (component A) and 70% (component B). The pyrazine-d(4) molecules representing component B undergo two-site 180degrees jumps about the N...N axis of the molecule, whereas the pyrazine-d(4) molecules representing component A undergo an effectively isotropic motion. The activation energy for the two-site 180degrees motion of component B is estimated to be 39 kJ mol(-1) from the H-2 NMR spin-lattice relaxation time measurements and 49 kJ mol(-1) from the H-2 NMR line-shape analysis. Structurally, it may be inferred that component B comprises guest molecules for which the N...N axis has a restricted orientation in space, presumably because of direct interactions with the alpha-zirconium phosphate host structure, whereas component A comprises guest molecules that are held rather weakly in the host structure (or on its external surfaces) such that their reorientation is relatively unestricted. Thermogravimetric analysis provides direct evidence for the existence of two different types of pyrazine guest molecules in the alpha-zirconium phosphate host structure, with different strengths of binding and with populations in the approximate ratio 30:70. High-resolution solid-state N-15 NMR and C-13 NMR spectra provide further support for the existence of two different types of pyrazine guest species within the alpha-zirconium phosphate host structure and provide direct evidence that the pyrazine guest molecules of component B are N-protonated (from the host structure).

Quantification and visualization of oxygen partial pressure in vivo by 19F NMR imaging of perfluorocarbons

Due to the high solubility of molecular oxygen in perfluorocabons (PFC), this class of fluorinated compounds has gained wide-spread interest for its biomedical application as temporary blood substitutes and as radiosensitizers. Since the observation that the NMR spin-lattice relaxation times (T1) of some 19F PFC resonances are sensitive to oxygen tension (pO2), this paramagnetic effect has been used to non-invasively probe pO2 in vivo. In this study, combined 19F/1H NMR image data of Copenhagen rats after PFC application were evaluated with the software package MATLAB. The analysis of the 19F NMR data resulted in image matrices with calculated T1 values in each pixel. By using a calibration curve, the corresponding pO2 values were computed. Color overlays of pO2 contour lines on T1-weighted 1H images show a good anatomical-functional correspondence.

NMR Study of the Polymer Electrolyte Formed by Ethylene Glycol, Citric Acid and LiClO 4

In this work we report a proton ( 1 H) and lithium ( 7 Li) NMR lineshape and spin-lattice relaxation time investigation of this polymer electrolyte obtained from the polymerization of ethylene glycol and citric acid with lithium salt. The 7 Li data indicates that the lineshape and spin-lattice relaxation is affected by the coupling of its quadrupolar moment to the surrounding electric field gradients. The activation energy for the lithium motion extracted from the 7 Li relaxation data is 0.23 eV. The dynamics parameters obtained from the NMR data indicate that the lithium mobility in the EG:CA:LiClO 4 is comparable to those found in polymer gel electrolytes and in plasticized polymer electrolytes.

NMR Study on (CH 3 ) 2 NH 2 H 2 AsO 4

Proton NMR line width and spin-lattice relaxation time T 1 for the ferroelectric (CH 3 ) 2 NH 2 H 2 AsO 4 have been measured at the temperature range between 77 K and 300 K. The temperature dependence of T 1 is well described by the expressions based on rotation of the methyl group using the value of activation energy E a = 99 meV and elementary correlation time 0 = 1.5 2 10 m 13 sec. Moreover, a sharp dip of T 1 near ferrroelectric phase transition temperature ( T c = 274.5 K) results from the ordering of protons. In addition, the temperature dependence of T 1 deviates from the expression for molecular rotation above around 200 K. It is deduced that the deviation is caused by the suppression of some kind of proton motion.

Polymer Relaxational Dynamics Associated With Ionic Conduction in Confined Geometry

ABSTRACTResults of a quasi-elastic incoherent neutron scattering study of the influence of confinement on polyethylene oxide (PEO) and (PEO)8Li+[(CF3SO2)2N]- (or (POE)8LiTFSI) dynamics are presented. The confining media is Vycor, a silica based hydrophilic porous glass. We observe a strong slowing down of the bulk polymer dynamics under presence of Li salt. The confinement also affects dramatically the apparent mean-square displacement of the polymer. As supported by DSC measurements, the PEO melting transition at 335 K is strongly attenuated under confinement, suggesting that confinement modifies the global structure of the system, increasing the fraction of amorphous PEO by respect to crystalline phase. Local relaxational PEO dynamics is successfully described by the DLM (Dejean-Laupretre-Monnerie) model usually used to interpret NMR spin-lattice relaxation time data. The scattering vector dependence of the correlation times deduced from inelastic neutron scattering data is found to obey a power-law dependence. DSC and preliminary ionic conduction measurements are also presented.

Dynamics of the Hydration of Halogenoalcohols in Aqueous Solution

Abstract The 17O NMR spin-lattice relaxation times (T1), of the solvent water in aqueous solutions of halogenoalcohols were determined as a function of the concentration at 298 K. The values of the dynamic hydration number, nDHN ≡ nh(τch/τc0 -1), were determined from the concentration dependence of T1. In order to investigate the effect of the halogen atom on hydrophobic hydration, the ratios (τch/τc0), of the rotational correlation times,(τch), of water molecules around the solute to those of pure water,(τc0), were estimated from the nDHN and the coordination numbers (nh), which were calculated from the water-accessible surface area (ASA) of the solute molecule. (i) The τch/τc0 value decreased in the order EtOH > IEtOH ≥ BrEtOH ≥ ClEtOH > FEtOH for monohalogenoethanols, EtOH ≥ Cl3EtOH > Cl2EtOH ≥ ClEtOH and EtOH ≥ F3EtOH > FEtOH for polyhalogenoethanols, and PrOH > ClPrOH and i-PrOH > (CF3)2CHOH for halogenopropanols. (ii) The ASA dependence of the τch/τc0 value for chloroethanols was smaller than that for fluoroethanols. These results are explained by the disturbance of hydrophobic hydration by the halogen atom and the size effect.

Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal

The proton NMR line width and spin-lattice relaxation times for LiNH4SO4 single crystal were studied at low temperature range of 6 and 280K. The changes in the proton relaxation behavior near the phase transition temperature indicates a change in the state of internal motion at the transition. The molecular motions obtained by the spin-lattice relaxation processes were found to be determined by molecular reorientation of the NH4 ions in phases III, IV, and V. We also confirmed that the phase transitions occur at 26 and 133K.