Temperature Dependence Of T1 Research Articles

An Evaluation of the Overall Rotational Correlation Times of Tetraalkylammonium Ions in Solution. 13C NMR Relaxation Study

Abstract The 13C NMR spin-lattice and spin–spin relaxation times (T1 and T2), and the nuclear Overhauser effect (NOE) were measured for tetraalkylammonium ions (R4N+, R = C2H5 (Et), n-C3H7 (n-Pr), n-C4H9 (n-Bu), and n-C5H11 (n-Pen)) in the concentrated aqueous solutions of their bromides or chlorides. A minimum was observed in the temperature dependence of T1 for the α-carbon in each solution. The value was 1.2—1.4 times higher than that predicted assuming only the isotropic overall rotation of the R4N+ ion. This increase in the T1 minimum values was ascribed to the rotations around the N–α-C bonds with an restriction in the azimuthal angle for each bond. The ranges where the N–α-C bonds could rotate with the time scales much less than those for the overall rotations were determined by the values of the T1 minima for the corresponding R4N+ ions as ±20°, ±17°, ±20°, and ±20° centering the gauche conformations, respectively, for Et4N+, n-Pr4N+, n-Bu4N+, and n-Pen4N+. These values well reproduced the T2 and NOE values observed at various temperatures in the concentrated aqueous solutions of the R4N+ halides. By using the thus obtained ranges of the azimuthal angles, the overall rotational correlation times in the dilute aqueous solutions (0.01 mol kg−1) were also determined. The obtained rotational correlation times were compared with those calculated using the Stokes–Einstein–Debye equation and the effect of the hydrophobic hydration was discussed.

Radical Anions of Mono- and Bis-fulleropyrrolidines: An EPR Study

The radical anions of three C60 derivatives (N-methyl-3,4-fulleropyrrolidine, N-methyl-2-(3,6,9-trioxadecyl)3,4-fulleropyrrolidine, and bis-N-methyl-3,4-fulleropyrrolidine are respectively MFP - , tegMFP - , and bisMFP - ) have been investigated by continuous wave (cw) and pulsed EPR in methyltetrahydrofuran and tetrahydrofuran solutions. The EPR spectra for MFP - and tegMFP - show a 1:1:1 triplet, due to the hyperfine coupling with the pyrrolidine nitrogen. The EPR spectrum of bisMFP - shows a superimposition of two signals, each due to a 1:2:3:2:1 quintet. Each quintet is due to the hyperfine coupling with the two pyrrolidine nitrogen atoms. The two signals are attributed to two different conformers of the radical. A direct transfer of spin density from the C60 sphere to the nitrogen atoms is discussed as arising from the bent conformation of the pyrrolidine rings. All the spectra show several weaker satellite lines, due to the radical anions containing a 13 C atom. From their analysis a C 2V symmetry for the spin distribution is obtained in the case of MFP - and tegMFP - . The EPR line widths of these latter radicals at room temperature are larger than those expected on the basis of spin relaxation due to rotational diffusion in solution, and the line widths decrease upon decreasing the temperature. Pulsed EPR measurements of T1 in liquid and frozen solutions show that the EPR line widths are determined by the anomalously short lifetimes of the spin states. The temperature dependence of T1 obeys to the Arrhenius law, giving an activation energy of 7 kJ/mol. The latter results are compared with those obtained in the past for the lone C60 . They are attributed to the closeness of the electronic levels, due to the weak perturbation of the pyrrolidine ring on the symmetrical electronic distribution of C 60. On the other hand, the spin relaxation properties of bisMFP - are shown those expected for a nonsymmetrical radical. The comparison of cw- and pulsed-EPR spectra of MFP - in frozen solutions of MeTHF and THF indicates the presence of aggregates of interacting radicals in THF.

Mobility of Silver Ions in Silver Ion Conductor Ag7NbS6 Studied by Ag and Nb NMR

Temperature dependences of the 109Ag chemical shift and the spin−lattice relaxation time, T1, of 109Ag, 107Ag, and 93Nb NMR have been measured in the room-temperature phase of Ag7NbS6 and related compounds. The 109Ag chemical shift is about 1300 ppm and is much larger than those of other silver compounds. The 109Ag and 107Ag line widths are about 200 Hz, and these indicate that the motional narrowing of silver ions has been completed in this phase. T1 of all nuclei increases with increasing temperature. The relaxationl process is explainable by the fluctuations of scalar coupling between silver nuclei, because the T1 of 109Ag does not show any magnetic field dependence. The temperature dependences of T1 of the mobile 109Ag and 107Ag are weaker than that of T1 of the immobile 93Nb as well found in the case of Ag9GaSe6. Though Ag7NbS6 and related compounds show a high ionic conductivity of 0.1−1 S m-1, high affinity between the silver and chalcogenides and correlation among silver ions are suggested by the ...

Evidence for a gap in the excitation spectrum of CrO2

Magnetization and resistivity of CrO2 powders and oriented CrO2 films on different TiO2 single-crystal substrates are reported, and the low-temperature specific heat is measured on powders. The M(T) data follow a Bloch T3/2 law, with no sign of a spin-wave gap. The residual resistivity depends strongly on film texture, but a T2 temperature dependence of the resistivity is found in all samples above a cut-off temperature of the order of 70 K. The behavior is attributed to electron-magnon spin-flip scattering which is suppressed at low temperature in a half-metallic ferromagnet.

NMR Studies of Molecular Motion and Compatibility in Polymer Blends of Poly(epichlorohydrin)/Poly(methyl methacrylate)

Molecular motion of polymer chains depends on blending and differs in bulk and in polymer blend. In this research the molecular motion of poly(epichlorohydrin) (PECH) was studied by 13C NMR and showed different behavior in bulk from polymer blend. Two types of blend films, PECH/poly(methyl methacrylate) (PMMA)=(60/40) (wt%), were prepared from toluene and tetrahydrofuran (THF) solution; the former is transparent and the latter is opaque film. The line shape and temperature dependence of T1 and T2 in PECH indicate that the molecular motion of PECH is inhibited by surrounding PMMA chains and that the decrease in molecular motion depends on miscibility. The slow motion described by T2 is more influenced by blending than the fast one described by T1. Distribution curves of T2 were obtained for bulk and three types of blend films prepared from toluene, THF, and THF/H2O solution. The bulk and blend films cast from toluene show sharp distributions are homogeneous in molecular motions. The blend films cast from THF and THF/H2O have wide distribution in molecular motion, which indicates the existence of the gradient in local concentration. T1 in PMMA agrees with T1 and T2 in PECH.

CW and Pulsed EPR Study of Silver Nanoparticles in a SiO2 Matrix

Metallic silver nanoparticles were prepared by the sol-gel method in an SiO2 matrix. The process includes complexation of silicon alkoxides with metal salts, hydrolysis, polycondensation, formation of powder, and subsequent thermal treatment first in oxidizing and second in reducing atmospheres. The sizes of metallic particles were determined both by X-ray diffraction and transmission electron microscopy. These measurements revealed sizes of metallic particles between 1 and 20 nm, depending upon processing conditions. The magnetic properties were investigated using electron paramagnetic resonance spectroscopy in the temperature range between 4 and 300 K. The spin-lattice relaxation time T1 was measured by pulsed EPR. The temperature dependence of T1 is described by the relation 1/T1 ∝ Tn, where 0.4 < n < 1. This behavior is unusual and different from any well-known relaxation processes such as Raman, Direct or the Orbach-Aminov.

35Cl NQR Studies of Bismuth Trichloride

Abstract The temperature dependence of the 35Cl nuclear quadrupole resonance (NQR) frequency and spin lattice relaxation time (T1) are studied in crystalline BiCl3 , in the range 40-300 K. The positive temperature coefficient observed for one of the 35Cl resonances is explained in terms of the strong intermolecular interactions that exist in this compound. Variation of with temperature is found to be similar at the chemically inequivalent halogen sites. Semiclassical descriptions based on torsional oscillator dynamics are found to be inadequate for explaining the spin lattice relaxation. T1 (T) data follow an AT2 + BT3 type behaviour, indicating that the anharmonic nature of the lattice vibrations plays a significant role in determining the temperature dependence of T1 at high temperatures.

Molecular Relaxations of a Branched Poly(oxyethylene) Network Polymer

Dielectric relaxation measurements and 13C NMR measurements were carried out on an amorphous network polymer derived from poly[2-(2-methoxyethoxy)ethyl glycidyl ether]triol with 2,4-toluenediisocyanate. The main relaxation and the higher frequency relaxation were observed in the dielectric spectra. The main relaxation was found to be symmetrical and broad with a half width of 5 decades in a frequency domain. The higher frequency process is considered to be the γ process of ethylene oxide chains due to the local twisting mode. The temperature dependence of T1 and T1ρ of the ethylene oxide carbon showed minima and it is clearly shown that the relaxation times obtained from the dielectric measurements correspond to the relaxation time obtained from the T1ρ measurements of the ethylene oxide carbon. The temperature dependence of T1 was well described assuming the Cole-Cole spectral density function with the shape factor β obtained for the dielectric main relaxation. The 13C magnetization decay of the ethylene oxide carbon in the rotating frame was found to be given by two components (slow and fast components) below the relaxation temperature. The broad dielectric relaxation and the decay profiles of the ethylene oxide carbon are discussed in terms of the branched structure of the network.

Recent progress in surface NMR-electrochemistry

NMR spectroscopy is one of the newer spectroscopic techniques for investigating the static, dynamic and electronic structures of molecules adsorbed onto metal catalyst surfaces. We review recent progress in the application of solid-state NMR methods to the investigation of molecules adsorbed onto metal surfaces in an electrochemical environment: in the presence of electrolyte, and at an electrified interface under external potentiostatic control. While at a very early stage of development, the NMR-electrochemistry approach has considerable potential for investigating otherwise inaccessible aspects of electrode and adsorbate structure, and should enable a comparison of results obtained from different spectroscopies, in particular from IR spectroscopy. We present a brief review of the development of the subject, followed by details of the instrumentation necessary for NMR-electrochemistry studies. We show how spin–spin relaxation can give information on surface structure and surface diffusion, how spin–lattice relaxation can give information on the presence of conduction electron spillover onto the adsorbate, and how the NMR of surface species responds to an externally applied electric field. The 13 C-NMR of CO on Pt in an electrochemical environment is compared with the 13 C-NMR of CO on Pt catalysts in vacuum, which are well characterized. In the case of CN on Pt, we show large spectral shifts of the resonance as the electrode potential is varied, providing an independent measurement of the effects of the electrified interface on the chemisorption bond. Spectral sensitivity is also now adequate to observe nuclei which produce even weaker signals than 13 C, such as 15 N. The NMR-electrochemistry method thus opens up a broad new array of possibilities for probing static structures (from T 2 ), surface diffusion (from the temperature dependence of T 2 ) as well as electronic properties of the chemisorption bond (from T l , and from electrode potential effects) at electrochemical interfaces, and for studying reactive intermediates and poisons on high-surface-area catalysts, such as those utilized in hydrogen and organic fuel cells.

Temperature and concentration dependences of the spin-lattice relaxation rate in four borate glasses doped with Fe2O3

Spin-lattice relaxation time (T 1) of Fe3+ ions has been measured in four borate glasses doped with 0.25, 0.5, 1 and 3 wt.% Fe2O3 by the modulation method over the temperature range 5–250 K. In the three less concentrated samples, it was observed thatT 1 −1. αT at intermediate temperatures. This is explained in terms of a relaxation mechanism involving TLS (Two Level Systems). At higher temperatures, the temperature dependence ofT 1 −1 is slowed down by cross-relaxation. In the most concentrated sample, the exchange interaction plays a dominant role leading to a very different relaxation-time behaviour, described well by the Bloembergen and Wang three-reservoir model.

Pulsed ESR Study of the Conduction Electron Spin Center in μc-Si:H

AbstractThe spin-lattice relaxation times (T1) of conduction electron (CE) and dangling bond (DB) centers in μc-Si:H have been directly measured using the 3-pulse inversion recovery method. For both CE and DB, the inversion recovery curve follows a stretched exponential form. T1 of DB is about twice the T1 of CE, however the temperature dependence of T1 seems to be the same for both CE and DB and can be approximated by T−4 While the DB echo decay is modulated by both 29Si and 1H nuclei, we found no modulation of the CE echo decay by the H nucleus, indicating that CE centers are located in H-depleted phases in μc-Si:H. The modulation results are direct evidence that CE centers are located in the crystalline grains and DB centers in the amorphous phases.

Influence of γ‐irradiation on proton spin–lattice relaxation of SBR used as antirads and its ESR investigation

AbstractProton spin–lattice relaxation time t1 was measured on SBR samples with carbon black or kaolin filler using modified linseed oil. The NMR pulse technique at 90MHz was used in the temperature range from 180 to 400 K. The temperature dependence of t1 indicates that samples filled with carbon black have similar molecular dynamics to the standard unfilled SBR samples. The activation energy for the motion of the main chain for these samples amounts to 16.4kJ/mol. Samples containing linseed oil modified with para‐toluidine showed an activation energy of about 14.6kJ/mol and were not affected by γ‐irradiation. Values of the minimum relaxation time tmin1 were increased by γ‐irradiation in comparison with a standard SBR sample. ESR measurements carried out at room temperature by means of an X‐band spectrometer indicated that unidentified radicals within the rubber were formed during its mastication with vulcanizing additives. The ESR spectra did not change during the vulcanization process. Samples filled with carbon black showed a broadening of the ESR line; this is consistent with the increase in the electrical conductivity.

Investigation of molecular motions of crosslinked polyepichlorohydrin by nuclear magnetic resonance spectroscopy

AbstractThe molecular motion of crosslinked polyepichlorohydrin (PECH) is studied qualitatively by NMR techniques. The results of temperature dependence of 1H T2 and T1 indicate that the crosslinking (crosslink density &lt; 3%) restricts molecular motions of the polymer even far above its Tg. The 1H T1 minimum, corresponding to the large‐scale chain‐motion of crosslinked PECH, shifts to higher temperatures with increasing crosslink density. 1H T2 data also show that the crosslinking hinders free chain motions of the polymer above its Tg. The 13C T1 relaxation time is sensitive to such motional changes as well. 13C linewidths of crosslinked PECHs vary with the crosslink density in both the swollen state and the solid state. The mechanism of 13C linewidth broadening of crosslinked polymers is discussed in detail. In the case of PECH, the linewidth broadening is caused by changing molecular environment due to crosslinking (such as presence of various chemical shift structures and freezing effects in conformational environment as chain mobility decreases), rather than increasing correlation times, which shorten the relaxation time (T2) of polymer chains. © 1994 John Wiley &amp; Sons, Inc.

Neutron diffraction and 35Cl-NQR-T 1 measurements of N-[2,6-dichlorophenyl]-2,2,2-trichloroacetamide, 2,6-Cl2C6H3NHCOCCl3 1

Abstract N-[2,6-dichlorophenyl]-2,2,2-trichloroacetamide (2,6-Cl2C6H3NHCOCCl3) shows a bleaching out of certain 35Cl-NQR frequencies at a temperature of 159 K. The 35Cl-NQR spin lattice relaxation time (T 1) was determined as function of temperature. The data of the trichloromethyl resonances lead to an activation energy of 23.7 kJ/mole for the bleaching out process. The chlorine atoms bonded to the phenyl ring show a normal temperature dependence of T 1. In addition crystal structure determinations were made at three temperatures using neutron diffraction (117 K, 195 K, 373 K). The space group of the compound is P21/c = C[unk]h with four molecules per unit cell. The cell parameters are a = 1007(1) pm, b = 1168(1) pm, c = 1002(1) pm, β = 101.79(7)° (117 K), a = 1017(3) pm, b = 1194(3) pm, c = 1007(3) pm, β = 101.9(2)° (295 K), a = 1018.5(9) pm, b = 1208(1) pm, c = 1009(1) pm, β = 101.63(8)° (373 K). The molecules form intermolecular hydrogen bonds of the N–H…O–C type along [001]. An anharmonic behaviour of two of the trichloromethyl chlorine atoms was observed and studied as a function of temperature. The intramolecular distances and angles show no significant temperature variations.

Anomalous temperature dependence of T2 for dilute solutions of 3He in solid 4He

Upon warming it is observed that the spin-spin relaxation time for dilute solutions of 3He in solid 4He decreases. This is contrary to common expectation, where the effect of increasing motion is to lengthen the relaxation time. Anomalous behaviour of spin diffusion measurements in this system have been explained by the quasi one dimensional motion of the 3He atoms along dislocation lines in the crystal. We show that this will also account for the unusual behaviour of the spin relaxation time.

Inequivalent Reorientation of the Trichloromethyl Groups in 1,4-Bis(TrichloromethyI) Benzene as Studied by Pulsed 35Cl NQR

Abstract The temperature dependence of T1 of 35Cl NQR of the title com pound evidenced that the three crystallographically inequivalent CCl3 groups have different potential barriers against reorientation (about 30, 36, and 42 kJ/mol). An intramolecular interaction between π-electrons on the benzene ring and the C -Cl bond is proposed to interpret the characteristic frequency splitting common to the 35Cl NQR spectrum of each of the three inequivalent CCl3 groups. The magnitude of T2 of each Cl atom is interpreted by intra-and intermolecular H ··· Cl interactions. It was found that only part of the temperature dependence of the NQR frequency can be interpreted by reorientation.

The electronic quantum size effect observed by195Pt NMR in the metal cluster compound Pt309Phen 36 * O30

195Pt NMR on the organic ligand stabilized metal cluster compound Pt309Phen 36 * O30 reveals two separate peaks in the lineshape. Ligand-bonded platinum atoms at the surface of the core are thought to be responsible for the peak that does not show any Knight shift. The corresponding spin-lattice relaxation timeT1 is of the order of seconds. The second peak is Knight shifted and is attributed to the other Pt atoms, for which metallic behavior is inferred from the temperature dependence ofT1. The Korringa relation holds down to 65 K. Below 65 K the relaxation of the magnetization becomes increasingly non-exponential with decreasing temperature. The relaxation process can be successfully modelled under the assumption of a Poisson distribution of the energy levels around the Fermi energy (the electronic quantum size effect).

Influence of electron-electron scattering on the electrical conductivity in organic conductors

We have measured electrical resistance of the organic conductors with different anisotropy in the wide temperature range : quasi one-dimensional (TMTSF)2X (X=C104, PF6 and NO3) and two-dimensional β-(BEDT-TTF)2AuI2. We have looked for the T2 temperature dependence of the resistivity which is expected for the electron-electron scattering in the standard transport theory for isotropic systems. The agreements are, for all studied materials, satisfactory in the wide temperature region. We discuss and commpare electrical resistivity curves of this materials.

The methyl torsional levels of solid acetonitrile (CH3CN)—A neutron scattering study

Neutron powder diffraction has been used to obtain the thermal parameters for the deuterium atoms and to confirm the crystal structure of acetonitrile-d3 at 4 K. Inelastic neutron scattering from both isotopic species is used to determine the energies of the first and second rotational levels of the methyl group. These four levels are reasonably reproduced by a threefold potential with V3=125 meV. The activation energy derived from this potential is in agreement with that previously obtained from the temperature dependence of T1 in proton magnetic resonance measurements.

Faraday communications. Nuclear spin relaxation of 7Li in aqueous solutions of LiCl

Spin–lattice relaxation times, T1, of 7Li NMR have been measured for 12.3 mol kg–1 LiCl–D2O and 13.8 mol kg–1 LiCl–H2O samples at temperatures of 80 K to 313 K. The temperature dependence of T1 for both samples was very similar over the whole temperature range. The minimum of T1 was found to be 27 ± 1 ms for both samples. From the experimental results, it is concluded that the quadrupole interaction is responsible for the relaxation of 7Li NMR.