Temperature dependence of the proton exchange time in pure water by NMR

The temperature dependence of the proton spin-spin relaxation rate 1/T₃ on 180° pulse spacing (Meiboom dispersion) was measured for pure water enriched at 4% ¹⁷O to obtain the proton exchange time. At 58°C, the dispersion of the proton spin-lattice relaxation in the rotating frame (T_1ρ) was shown to be explained by a comparable proton exchange time.     

35Cl quadrupole resonance study of the thermally activated motion of the nitro group in chlorinated nitrobenzenes

The results of NQR studies of the thermoactivated motion of NO₂ groups in chlorine-containing nitrobenzenes are presented. A procedure for analyzing the experimental data is suggested. It uses the characteristic temperature of the minimum of the modulation contribution to the temperature dependence of the spin-lattice relaxation time T ₁ of the resonant probe nuclei (³⁵Cl) bonded with the moving group by nonvalent interactions. The activation energy of the thermoactivated motion was correlated with the temperature of the minimum. The possibility of a temperature dependence of the activation energy of the NO₂ group was examined.     

NMR study of hindered rotation in solid (ND4)2GeF6

Nuclear spin-lattice relaxation times T₁ for deuterons and ¹⁹F nuclei in polycrystalline (ND₄)₂GeF₆ were measured by the pulse method at 8 MHz between 40 K and 300 K and between 4 K and 400 K, respectively. Correlation times and activation energies for the reorientational motions of ND₄⁺ and GeF₆^2? ions were calculated from the measured T₁ values.     

Phase transitions and molecular motions in [Cd(H2O)6](BF4)2 studied by DSC, H and F NMR and FT-MIR

Two solid phase transitions of [Cd(H₂O)₆](BF₄)₂ occurring on heating at T_C2=183.3 K and T_C1=325.3 K, with 2 K and 5 K hysteresis, respectively, were detected by differential scanning calorimetry (DSC). High value of entropy changes indicated large orientational disorder of the high temperature and intermediate phase. Nuclear magnetic resonance (¹H NMR and ¹⁹F NMR) relaxation measurements revealed that the phase transitions at T_C1 and T_C2 were associated with a drastic and small change, respectively, of the both spin-lattice relaxation times: T₁(¹H) and T₁(¹⁹F). These relaxation processes were connected with the “tumbling” motions of the [Cd(H₂O)₆]²⁺, reorientational motions of the H₂O ligands, and with the iso- and anisotropic reorientation of the BF₄⁻ anions. The cross-relaxation effect was observed in phase III. The line width and the second moment of the ¹H and ¹⁹F NMR line measurements revealed that the H₂O reorientate in all three phases of the title compound. On heating the onset of the reorientation of 3 H₂O in the [Cd(H₂O)₆]⁺², around the three-fold symmetry axis of these octahedron, causes the isotropic reorientation of the whole cation. The BF₄⁻ reorientate isotropically in the phases I and II, but in the phase III they perform slow reorientation only about three- or two-fold axes. A small distortion in the structure of BF₄⁻ as well as of [Cd(H₂O)₆]²⁺ is postulated. The temperature dependence of the bandwidth of the O-H stretching mode measured by Fourier transform middle infrared spectroscopy (FT-MIR) indicated that the activation energy for the reorientation of the H₂O did not change much at the T_C2 phase transition.     

High-resolution solid-state 13C NMR study of ethylcellulose films

Ethylcellulose films cast from concentrated solutions of chloroform, benzene, and carbon tetrachloride were subjected to the NMR relaxation measurements including ¹H spin-lattice relaxation time (T_1H), rotating-frame ¹H spin-lattice relaxation time (T_1ρH), and ¹³C spin-lattice relaxation time (T_1C). The values of T_1ρH for carbons in the glucose units of ethyl-cellulose were of the same order of magnitude as those reported for the crystalline and noncrystalline regions of ramie cellulose. The values of T_1C for unsubstituted C2, C3 carbons were smaller than those for the corresponding carbons in the noncrystalline region of native celluloses. The T_1C values for unsubstituted C2, C3, and substituted C6 carbons showed a small but definite dependence on the solvent from which the films were cast. © 1993 John Wiley & Sons, Inc.     

Nuclear quadrupole and magnetic resonance studies of structure and the molecular motion in SYMC6Cl3F3

³⁵Cl and ¹⁹F relaxation time measurements were carried out on symC₆Cl₃F₃. The analyses of the ¹⁹F high-resolution NMR spectra and the ³⁵Cl spin-lattice relaxation time showed that the crystal belongs to a trigonal or a hexagonal crystal system in which the molecules undergo three-fold reorientation about the molecular figure axis with the correlation time τ_c/s=3.42·10^?7 exp(18.2 kJ mol^?1/RT). Temperature dependences of the ³⁵Cl NQR frequency and ¹⁹F spin-lattice relaxation times are such that only a minor structural change is associated with the phase transition at 296 K.     

Pulsed NMR studies of reorientation and phase transitions in solid NaSbF6 and KSbF6

Nuclear spin-lattice relaxation times T₁ for ¹⁹F nuclei in polycrystalline NaSbF₆ and KSbF₆ were measured between 250 K and 550 K and between 150 K and 450 K, respectively. Correlation times and activation energies for the reorientation of SbF₆^? ions were calculated from the measured T₁ data. Phase transitions not reported previously were found in NaSbF₆ and KSbF₆.     

The effect of silica nanoparticles on the morphology, mechanical properties and thermal degradation kinetics of PMMA

Silica-PMMA nanocomposites with different silica quantities were prepared by a melt compounding method. The effect of silica amount, in the range 1-5 wt.%, on the morphology, mechanical properties and thermal degradation kinetics of PMMA was investigated by means of transmission electron microscopy (TEM), X-ray diffractometry (XRD), dynamic mechanical analysis (DMA), thermogravimetric analyses (TGA), Fourier-transform infrared spectroscopy (FTIR), ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (¹³C{¹H} CP-MAS NMR) and measures of proton spin-lattice relaxation time in the rotating frame (T_1ρ(H)), in the laboratory frame (T₁(H)) and cross-polarization times (T_CH). Results showed that silica nanoparticles are well dispersed in the polymeric matrix whose structure remains amorphous. The degradation of the polymer occurs at higher temperature in the presence of silica because of the interaction between the two components.     

CP/MAS Carbon-13 NMR Study of Spin Relaxation Phenomena of Cellulose Containing Crystalline and Noncrystalline Components

Abstract

Cross-polarization, ¹³C rotating frame spin-lattice relaxation and C laboratory frame spin-lattice relaxation processes have been studied for different cellulose samples by CP/MAS ¹³C NMR spectroscopy. It was found that the CP process can be described by a simple thermodynamic model and relative intensities of the respective resonance lines are consistent with the atomic ratios for the spectra obtained at a contact time of about 1 ms. The observed rotating frame spin-lattice relaxation times T^C ₁₀ were dominantly dependent on the time constant T^D _CH by which ¹³C nuclei were coupled to the ¹H dipolar spin system. It was, therefore, impossible to obtain information about molecular     

NMR study of molecular reorientations in NH4H2AsO4

Zeeman (T_1Z) and dipolar (T_1D) spin-lattice relaxation times of protons in NH₄H₂AsO₄ were measured as a function of temperature. The existence of a slow motion (τ ≈ 10^?3 see) is established, which is most probably a low frequency hindered reorientation of H₂AsO₄ groups. This motion is slowed down below the Curie point T_c. A sharp increase of the dipolar relaxation rate above T = 314°K indicates the possibility of a new high temperature phase transition in this compound.     

Interactions and mobility of copper(II)-imidazole-containing copolymers

Poly(1-vinylimidazole-co-methyl methacrylate) copolymers (PVM) were obtained from copolymerization of 1-vinylimidazole and methyl methacrylate with 2,2^′-azobisisobutyronitrile as an initiator. The formation of random copolymers was substantiated by the glass transition temperature (T_g) and the proton spin-lattice relaxation time in the rotating frame (T^H_1ρ). Cu(II)-PVM complexes were prepared by mixing tetrahydrofuran solution of PVM and copper sulfate solution. The formation of coordination bond between PVM and Cu²⁺ ions was studied using differential scanning calorimetry, infrared and ¹³C solid-stated nuclear magnetic resonance spectroscopy. A single composition dependent T_g was obtained for the PVM copolymers, and that increased with increasing VI content. The T_g value of the Cu(II)-PVM complex was much higher than that of the PVM copolymer with the same composition. The T^H_1ρ of the VI units and MMA units in the copolymers and complexes had one value, and that in the complexes was much lower than that in the copolymers. The dramatic decrease in T^H_1ρ for the Cu(II)-PVM complexes was due to Cu(II) complexation and electron-nuclear dipolar interactions.     

Calculation of the nmr relaxation time of dilute 129 Xe gas

The spin-lattice relaxation time T₁ of ¹²⁹ Xe gas is calculated with the kinetic theory due to Chem and Snider. A Lennard-Jones (12,6) potential functions is employed as a model for the spherical potential while the transient spin-rotation interaction is assumed to be responsible for the relaxation of the nuclei. Cross sections for spin transitions on collisions are calculated either quantum mechanically or semiclassically depending on the relative energy. The temperature dependence of T₁ is determined in the range 200–450 K. The calculated value of T₁ at 298 K and 1 amagat is 2.8 x 0⁵ s while the value measured by Hund and Carr is (2.0 ± 0.2) x 10⁵s.     

Micropore size analysis in hydrated cement paste by NMR

We present a time evolution of ¹H spin-lattice relaxation rates in the laboratory (1/T₁) and in the rotating (1/T_1ρ) frame of a synthetic cement paste. The typical results found for both rates allow us to follow the main hydration stages of the cement paste and the refinement of its microporosity. In particular the texturation of the porosity and the structuration of the surface of the material are evidenced on two model cement pastes. An interpretation in terms of fractal size distribution is considered as well as the effect of the curing temperature.     

Spin-lattice relaxations study of water mobility in natural natrolite

The mobility of water molecules in natural natrolite (Na₂Al₂Si₃O₁₀?2H₂O) is investigated by the ¹H NMR method. The spin-lattice relaxation times in the laboratory and rotating frames (T₁ and T_1ρ) are measured as a function of the temperature for a polycrystalline sample. From experimental T₁ data it follows that at T > 286 K the diffusion of water molecules along channels parallel to the c axis is observed. From experimental T_1ρ data it follows that at T > 250 K the diffusion of water molecules in transversal channels of natrolite is also observed. At a low temperature (T < 250 K) the dipolar interaction with paramagnetic impurities (presumably Fe³⁺ ions) becomes significant as a relaxation mechanism of ¹H nuclei.     

Physical properties of the nonlinear optical material Li2B4O7 studied by static NMR and MAS NMR

The mechanisms behind the nonlinear optical (NLO) properties of Li₂B₄O₇ are characterized by ⁷Li static nuclear magnetic resonance (NMR) and magic-angle spinning (MAS) NMR. Furthermore, the structural nature of 3-coordinate BO₃ and 4-coordinate BO₄ groups is also characterized by the same method. For ⁷Li and ¹¹B, the spin-lattice relaxation time T₁ in laboratory frame gradually decreases with increasing temperature, whereas the spin-lattice relaxation time T_1ρ in rotating frame, which differs from T₁, is nearly constant. In addition, the activation energies of ⁷Li and ¹¹B, which are obtained via the values of T₁ and T_1ρ, are also compared.     

Investigating the New Interaction Model between Cu(II) and PVA by the Proton Relaxation Method

An aqueous PVA-Cu²⁺ solution at pH = 3 is in the hydrated form and becomes green at pH ? 6 with a decrease in viscosity. The structure of the copper ion is suggested to be that of a polynuclear complex at pH > 6. For the green solution the polynuclear chains of the copper complex are believed to be surrounded by the PVA chains with the hydrophobic backbones facing toward the inside and the hydrophilic OH groups oriented toward the outside facing the bulk water. The proton spin-lattice relaxation rate 1/T_1p and the spin-spin relaxation rate 1/T₂ of CH and CH₂ in PVA and H₂O for aqueous PVA-Cu²⁺ solutions at pH = 3, can be explained by the two site exchange model in the region of the fast exchange limit. The dipolar correlation time τ_c is dominated by the reorientational process with a dipolar correlation time of 2.11 × 10^?11 s. When the pH rises from pH=3 to pH=12.5, the variation of 1/T₁p and 1/T₂p of CH and CH₂ in PVA with Cu²⁺ ion concentration in aqueous PVA-Cu²⁺ solution at pH=12.5 can be explained in terms of the relaxation by an inclusive model of the polynuclear copper complex and PVA. Furthermore, the frequency (or field) dependence of 1/T₁p, 1/T₂p of CH in PVA for aqueous PVA-Cu²⁺ solution at pH = 12.5 suggests that the dipolar relaxation is dominated by the electron-spin relaxation with the electron spin relaxation time T_1e = 1 ? 2 × 10^?10 s. The invariance of 1/T₁p and 1/T₂p of H₂O with the variation of the Cu²⁺ ion concentration in aqueous PVA-Cu²⁺ solution at pH = 12.5 supports the hypothesis that the water is not directly bound to the Cu²⁺ ion.     

NMR relaxation study of crosslinked cis-1,4-polybutadiene

Proton relaxation measurements have been used to investigate the effects of crosslinking on the segmental motion in cis-1,4-polybutadiene samples. The temperature dependence of proton spin–lattice relaxation time T₁ and spin–spin relaxation time T₂ at 60 and 24.3 MHz are reported in cis-1,4-polybutadiene (PB) samples with different crosslink density including uncrosslinked PB and samples with 140, 40, and 14 repeat units between crosslinks. In addition, spin-lattice relaxation times in rotating coordinate frame, T_1p, have also been determined. The relaxation data are interpreted in terms of the effects of crosslinks on segmental chain motions. Because of their sensitivity to low-frequency motion, T₂ data are of major interest. At temperatures well above the T₁ minimum the small T₂ temperature dependence resembles solidlike behavior reflecting the nonzero averaging of dipolar interactions due to anisotropic motion of the chain segments between crosslinks. The magnitude of T₂ at 60°C is found to be proportional to the average mass between crosslinks.     

EPR of Mn2+ in [Co(H2O)6] PtCl6 single crystals: Estimation of Co2+ spin-lattice relaxation time

EPR of Mn²⁺ in single crystals of [Co(H₂O)₆]PtCl₆ has been studied from room temperature to 77 K at ∼ 9.35 GHz. Mn²⁺ has been found to substitute for Co²⁺ exhibiting a unique magnetic complex. The observation of resolved Mn²⁺ spectra has been interpreted in terms of a random modulation of the interaction between the Mn²⁺ and Co²⁺ ions by the rapid spin-lattice relaxation of Co²⁺ ions. It has been found that the effective spin-relaxation time T₁αT⁻ⁿ where n = 1.8 for 105 < T< 293 K.     

A method for obtaining potential parameters for helium from T1 measurements

The temperature dependence of T₁ for ³He gas in the range 0–4°K is calculated for a Lennard-Jones (12,6) potential. The relaxation of the nuclear spins is assumed to be due to a dipolar interaction between the nuclei. A minimum value in the relaxation time, T_1,min, is found to occur at a temperature denoted by T_min. By repeating the calculation for different pairs of values of the potential parameters ? and σ, we have found that for a density of 10^?2 g/cm³,σ²T_min = 13.0?1.12 × 10³ ?σ², T_1,min/σ²(T_min)^{$\text{1}\text{2}$} = 17.4?6.56 × 10² ?σ², with ?, σ, T_min and T_1,min in eV, Å, °K and minutes, respectively. From measurements of T_min and T_1,min, ? and σ can be determined.     

Molecular mobility of counterion functional groups in ionic liquid 1-ethyl-3-methylimidazolium acetate according to 1H and 13C NMR relaxation data

1-Ethyl-3-methylimidazolium acetate was studied by NMR relaxation. The temperature dependences of the spin-lattice relaxation rates (1/T ₁) for ¹H and ¹³C were obtained. The curves with maxima were observed for the majority of the temperature dependences 1/T ₁, which provided a reliable temperature dependence of the correlation times (τ_c). In the low-temperature range, the proton relaxation rates tend to an asymptotic value, which is related, most likely, to spin diffusion manifested in the studied samples. The values of correlation times τ_c calculated for ¹H and ¹³C of the same functional group almost coincide at high temperatures, which confirms that the used approach is adequate for the determination of characteristic times of rotational reorientation of counterions in the studied ionic liquid.