Proton spin-lattice relaxation in MBDBP

We have measured the proton spin-lattice relaxation rate R in 4,4′-methylenebis(2,6-di-t-butylphenol) (MBDBP) and MBDBP-OD (deuteriated hydroxyl proton) between 88 K and 380 K. The experimental results are compared with those from a previous study in the closely related but simpler molecule 4-methyl-2,6-di-t-butylphenol (MDBP). The present experiment is analysed in terms of relaxation resulting from intramolecular reorientation modulation of the proton spin-spin interaction. This intramolecular reorientation involves different combinations of superpositions of methyl, t-butyl, aromatic ring and hydroxyl group reorientation and a discussion is presented concerning the effects on the relaxation of superimposing these motions. The MBDBP-OD data is best fitted with the assumption that only methyl and aromatic ring reorientations occur. In MBDBP it is found that, in addition, an OH flip-flop motion is superimposed on the aromatic ring reorientation.     

Proton spin-lattice relaxation in iron fluosilicate

Proton T₁ of FeSiF₆·6H₂O is proportional to exp (Δ/kT)at liquid helium temperatures, with Δ ≈ D - 3E. We find the crystal field splitting of the Fe²⁺ ion in this salt to be D = (12.2 ± 1.0) cm^-1 using E = 0.54 cm^-1.     

Proton dipolar spin-lattice relaxation in hexagonal ice

The ultraslow motion of defects in high purity hexagonal H₂O ice has been studied by proton dipolarT _1D measurements in the strong collision limit, using the Jeener technique. The obtained NMR correlation times agree rather well with both the Schottky H₂O diffusion timest _s=r ²/6D and the deuteron correlation times in D₂O ice, suggesting that Schottky rather than interstitial diffusion dominates spin-lattice relaxation in both H₂O and D₂O ice.On leave of absence from University of Ljubljana, Institute J. Stefan.     

Proton spin-lattice relaxation in aqueous ionic solutions

The proton spin-lattice relaxation time, T ₁, has been measured for aqueous solutions of sodium, potassium, rubidium and magnesium chlorides up to a concentration of about 4N at 25°c. The sodium and the magnesium chloride solutions were also measured at 60°c and 100°c. The variation of 1/T ₁ is not linear with concentration, at least above about ¼N, in contrast with previous reports [1, 2]. The behaviour depends markedly on the nature of the salt and on the temperature. It is shown that almost the whole of the variation of T ₁ as compared with that of water can be directly and simply related to the corresponding changes in the shear viscosity of the solutions. It is noted that the viscosity correction is better the higher the temperature of the solution.     

Proton spin-lattice relaxation times in ytterbium hydrides

The measurements of the proton (NMR) spinlattice relaxation times have been made in a series of ytterbium hydrides, YbH _x . Results are reported forx=1.80, 1.95, 2.00 and 2.62 and temperatures 4.2T297K. In the orthorhombic phase (1.80x2.00), the spin-lattice relaxation times are dominated by the hyperfine interaction of protons with conduction electrons and the spin diffusion mechanism. In the cubic phase (x=2.62), the relaxation times are five orders of magnitude shorter than in the orthorhombic one. This is interpreted in terms of the proton coupling with the Yb³⁺ ion spin fluctuations.     

Proton spin-lattice relaxation and charge transfer in quinolinium-tetracyanoquinodimethane

The proton spin-lattice relaxation of selectively deuterated quinolinium-tetracyanoquinodimethan O(D₈) (TCNQ)₂ is reported. Its temperature dependence indicates a metallic state down to at least ～ 130°K. The magnitude of the relaxation rate, when compared to the rate of the non-deuterated compound indicates nearly complete charge transfer.     

Proton spin-lattice relaxation time of hydrated gelatin

In this work we obtain a model to describe the relaxativity of water molecules, adsorbed on macromolecules, as a function of the concentration. Excellent agreement with experimental data was obtained. The model allows us to estimate the adsorption energy of water molecules on different sites of the macromolecules.     

Proton spin-lattice relaxation in binary liquid mixtures: determination of the mixed translational contributions

Proton spin-lattice relaxation measurements in some binary liquid mixtures suggest a method for determining, with the BPP translational model, the relaxation contributions related to the coupling of protons belonging to molecules of different types. These mixed translational contributions can be evaluated when the experimental values of the apparent relaxation time and those corresponding to ideal behaviour are not very different. When the translational relaxation times corresponding to the mixed contributions are shorter than the translational relaxation times related to molecules of the same type, a decrease in the reciprocal mobility occurs which involves interaction phenomena such as molecular associations and structure-breaking processes. The opposite case refers to the existence of rather stable structures.     

Proton spin-lattice relaxation in deuterated diluted copper tutton salts

The spin-lattice relaxation time, T_1I, of the protons in (Cu,Zn)Cs₂(SO₄)₂·6(H,D)₂O was measured. The sample was 94% deuterated and contained 0.5% Cu²⁺ ions. The T_1I measurements were carried out at liquid helium temperatures and at fields in the region from 5–15 kOe. Under these conditions, T_1I shows a strong dependence on the direction of H₀ with respect to the crystal axes, in the magnetic K₁–K₃ plane. This angular dependence has been accurately determined close to the K₁ and K₃ axes, at 1.4K. Between the angular dependent behaviour of the proton relaxation time and the EPR spectrum of the Cu²⁺ ions exists a striking correlation: T_1I exhibits sharp minima at angles where the frequency distance between the EPR lines is about equal to the proton resonance frequency.The observed temperature dependence in the liquid helium region is weak and can very well be described with a factor (1−P²₀)⁻¹, where P₀ denotes the electron polarization. The field dependence of T_1I appears to be between H^1.0₀ and H^1.5₀.     

Proton spin-lattice relaxation by critical polarization fluctuations in KH2PO4

The observed anomalous decrease in the proton spin-lattice relaxation timeT ₁ on approaching the Curie point in a rather pure KH₂PO₄ single crystal is explained by magnetic dipolar coupling to the ferroelectric mode. The isolated “non-interacting” O?H...O proton flipping time is estimated from theT ₁ data as τ=0.66·10^?12 sec for the paraelectric phase and τ=2.24·10^?12 sec for the ferroelectric phase, in good agreement with the results obtained from other methods.     

Proton spin-lattice relaxation time in ordering VHx alloys

Proton spin-lattice relaxation time is determined in VH_0.68 and VH_0.73 alloys in the temperature range of β₁, β₂ and δ ordered phases (400-100 K). Experimental results are described in terms of dipole-dipole interaction mechanism. On comparing calculated and experimental data in the complete temperature range the values of temperature dependent activation energy E_a and time constant τ₀ are estimated.     

Proton spin-lattice relaxation time in the α-VHx

The ¹H spin-lattice relaxation time T₁ has been measured in the α-VH_x. It is found that T₁ in the α-phase is essential ly determined by contact hyperfine interactions, and its variation with hydrogen concentration is in accord with a rigid-band model.     

Nonexponential nuclear spin-lattice relaxation in the isotropic phase of thermotropic liquid crystals

In the isotropic phase of nematic and smectic liquid crystals T₁ of CH3 and chain protons is larger than that of ring and ring-neighboured protons being caused by fast CH₃ reorientation and internal motions in chains, respectively.     

Nuclear magnetic spin-lattice relaxation and molecular motion in solid white phosphorus and in liquid phosphorus

The ³¹P spin-lattice relaxation rates have been measured in solid white phosphorus and in liquid phosphorus over the temperature range 110 K to 400 K and at Larmor frequencies of 10 MHz and 30 MHz. The contributions to the measured relaxation rate from the different interactions have been separated. In the low-temperature, crystalline phase there are important contributions to the relaxation rate from the anisotropic chemical shielding and the intramolecular dipole-dipole interactions which are modulated by the reorientational motion of the molecule. Interference effects between these two interactions, which are important in liquids, are demonstrated to be quenched by the strong dipolar interactions in the solid. The reorientational correlation time is given by

and the chemical shielding anisotropy by

In the high-temperature, plastic-crystalline phase the reorientational correlation time is

as obtained from the anisotropic chemical shielding relaxation rate which is separated from the other contributions by its quadratic dependence on the Larmor frequency. Using this τ _R the intramolecular dipole-dipole relaxation rate is calculated. The contribution from the translational diffusion modulated intermolecular dipole-dipole interaction is calculated from the self-diffusion coefficient. When these contributions are subtracted from the observed relaxation rate, there remains a frequency-independent relaxation rate, proportional to 1/δ _R , which is attributed to the spin-rotational interaction. The latter is shown to be quantitatively consistent with large-angle reorientational jumps of the P₄ molecules by 120° about their C _3v axes. The relaxation in the liquid phase is dominated by the spin-rotational interaction and the expression representing the spin-rotational relaxation rate is the same as the one derived in the plastic-crystalline phase. The mechanism of molecular reorientation in the liquid is therefore the same as in the plastic-crystalline phase.     

Proton NMR relaxation of adsorbed water in gelatin and collagen

The dependence of the water self-diffusion coefficients as well as of the proton spin-lattice and spin-spin relaxation rates on the concentration have been studied in the gelatin-water system and in hydrated native collagen. The bound and free water fractions and the corresponding spin-spin and spin-lattice relaxation rates have been determined within the multi-phase water proton exchange model. Various theoretical models for the water proton cross-relaxation to the biopolymer have been studied and the results compared with the observed Larmor frequency dependence of the water proton spin-lattice relaxation rate.     

Deuteron spin-lattice relaxation in ammonium hexachlorometallates

Deuteron spin-lattice relaxation via the motion-dependent part of the electric quadrupole interaction is discussed in partly and fully deuterated ammonium ions of ammonium hexachlorometallates. The dominant motion at temperatures T>50K is normally 120 degrees reorientations of the ammonium ions. In some hexachlorometallates the instantaneous equilibrium directions of the nitrogen-hydrogen vectors make a certain angle Delta with the metal-nitrogen vectors and they appear in groups of six near each metal-hydrogen vector. Each N-D vector jumps between the six directions of one group and this motion (called limited jumps) dominates the deuteron relaxation at lower temperatures. In some samples one direction of each group seems to become more populated than the others when the deuteration degree exceeds a certain value and the ammonium ions become ordered. A model is derived for the relaxation rate in the absence of tunnelling splittings, which includes the effects of reorientations and limited jumps also in the ordered structure, where the limited-jump rate of a N-D vector to the preferred direction, r(p), differs from that to the nonpreferred direction, r(n). The obtained relaxation rate depends, in addition to the angle Delta, also on the ratio d=r(n)/r(p). The effect of d is discussed and estimates for it are presented on the basis of earlier experiments. The recent model for the deuteron relaxation in NH(3)D(+) ions, including the effect of proton tunnelling, is shortly reviewed. At lowest temperatures the motional rates can be dominated by corresponding incoherent tunnelling and the rate of the incoherent tunnelling contributing to limited jumps is argued to be clearly larger than that of the incoherent tunnelling contributing to approximately 120 degrees rotations.     

Calculation of spin-lattice relaxation times

A general expression is found for the probability for a direct (single-phonon) relaxation transition for arbitrary symmetry of the surroundings of a paramagnetic ion having an effective spin of S = 1/2. The expression is compared with experiment for the case of tetragonal symmetry. Since certain approximations frequently used in the calculations are not used here, a markedly improved agreement is found with the experimental relaxation times.Translated from Izvestiya VUZ. Fizika, No. 2, pp. 36–39, February, 1970.