Spin-lattice Relaxation Mechanism Research Articles

Proton spin-spin and spin-lattice relaxation in CaSO4 · xH 2O below 1 K

Proton NMR has been studied between 0.01 and 1 K in various hydrates of CaSO 4 as part of a program to investigate spin-lattice relaxation mechanisms at low temperatures. All T 1 values were found to be independent of temperature, consistent with the dominance of surface relaxation through interaction with 3He spins in the heat transfer bath. The recovery of the signal after inversion and after hole burning show different time scales for approach to internal Zeeman equilibrium and for relaxation of Zeeman energy to the lattice.

Mechanism of 3He-mediated nuclear spin-lattice relaxation at surfaces

We measured the nuclear spin-lattice relaxation time T 1, of several surface-bound nuclei, 1H, 19F, 11B, 13C, 29Si, and 2H, immersed in liquid 3He over the temperature range 0.01 K ⩽ T < 1 K. The Larmor frequencies of these nuclei in a 3.39 T field extended from 22 to 144 MHz. All T 1 values were temperature-independent and ranged from a few seconds to several hours, depending on the particular nucleus and the surface geometry of the sample. The results indicate that the coupled relaxation of surface spins is a phenomenon occurring in all solids immersed in 3He and thus provides a general mechanism for obtaining high nuclear polarization in solids, that the relaxation is controlled by direct dipole-dipole interactions between the surface spins and 3He in the first surface layer, that the 3He motion dynamics do not change appreciably from one surface to another, and that measurements of T 1 may thus be useful for determining the structure of surfaces.

Nuclear orientation and nuclear spin-lattice relaxation in insulating magnetically ordered crystals

The mechanism of nuclear spin-lattice relaxation in insulating magnetically ordered crystals at low temperatures is discussed. In 3-dimensional systems the relaxation time T1 is long, but in the lower dimensional systems it can be quite short making them possible candidates for hosts in on-line experiments. An experiment in 2-dimensional54Mn−Mn (COOCH3)2·4H2O is discussed with particular reference to the relatively short value of T1 in low applied fields. A preliminary experiment in the 1-dimensional system54Mn−(CH3)4NMnCl3 (TMMC) is also described. Pulsed NMRON measurements in54Mn−MnCl2·4H2O are outlined and the advant-systems with fast relaxation emphasized.

Comment on pulsed NMR thermometry at very low temperatures

At temperatures below 1 mK the rf power in pulsed NMR experiments is normally kept as low as possible in order to minimize eddy current heating in the metallic samples used for thermometry. However, for metallic samples with short Korringa constants a temperature dependence enters via the spin-lattice relaxation mechanism in addition to the paramagnetic dependence. It may cause systematic errors if the temperature of the conduction electrons is raised by eddy current heating due to the action of the rf pulse to a temperatureT e which is not very small compared to ω1κ, with κ being the Korringa constant and ω1 the rotation frequency of the nuclear moments in the rf field.

Mechanism of nuclear spin-lattice relaxation in insulators at very low temperatures.

Nuclear spin-lattice relaxation by paramagnetic impurities in insulating crystals is expected in conventional theory to be frozen out in the millikelvin temperature region owing to the freezing of the electron-spin orientation into the lowest-energy spin state in the presence of a static field. Experimental relaxation times are much shorter than the conventional theory predicts. The authors show that by extending the normal relaxation theory to include the slight change in electron-spin quantization arising from a nuclear spin flip, the electron effectively gains a new degree of freedom, which we call wobble, which takes much less energy to excite than a complete electron-spin flip. The electron-spin orientation is, therefore, effectively unfrozen even at millikelvin temperatures. This new mechanism is expected to dominate in high field at temperatures of tens of millikelvin and below.

Structural and electronic properties of hydrogen in the potassium-hydrogen-graphite ternary intercalation compound C8KH

A proton NMR study was carried out on a second-stage potassium-hydrogen-graphite intercalation compound, ${\mathrm{C}}_{8}$${\mathrm{KH}}_{\mathrm{x}}$ (x=0.55), in a powder form within the temperature range 1.5--77 K. An inhomogeneously broadened NMR signal (around 40 kHz in full width at half maximum) was observed, but the intrinsic linewidth of the spin isochromat, about 3.6 kHz, was determined from the spin-echo envelope. The shape of the echo envelope was explained by assuming a regular two-dimensional structure of dissociated hydride ions. The inhomogeneous broadening may possibly be ascribed to localized electron spins of the order of 100 ppm. The spin-lattice relaxation mechanisms were divided into two contributions, the metallic process and the process due to localized electron spins. Analysis of the metallic relaxation component revealed the existence of a small local density of states at the Fermi level at hydrogen sites, N(${E}_{F}$${)}_{\mathrm{H}}$=0.014 states per eV per spin per hydrogen atom. The hydrogen atomic plane was revealed to be weakly metallic. A possible electronic structure is discussed.

Decay processes in relaxed triplet states of self-trapped excitons in potassium halides

The temperature dependence of lifetimes of the π emission in KCl, KBr and KI has been studied in detail to clarify the spin-lattice relaxation mechanism among zero-field-split triplet sub-levels of the self-trapped excitons. It was concluded that the two-phonon process is very important in KCl and KBr instead of the one-phonon direct process which is dominant in KI.

Relaxation mechanisms and effects of motion in albite (NaAlSi3O8) liquid and glass: A high temperature NMR study

The nuclear magnetic relaxation of 23Na and 29Si in albite glass and liquid has been studied from 800 K to 1400 K. The dominant spin-lattice relaxation mechanism for 23Na is found to be nuclear quadrupole interaction arising from the Na+ diffusion. The activation energy of the Na diffusion is found to be 71±3 kJ/mol, in close agreement with the results on electrical conductivity and on Na self-diffusion from radio-tracer experiments. The correlation time of the Na motion is estimated to be about 8.5×10−11 s near the melting point (∼1390 K). Both nuclear dipole-dipole interaction and chemical shift anisotropy interaction are large enough to contribute to the 29Si relaxation. However, calculations based on a simplified model which employ single correlation time and exponential correlation function, with interactions typical of crystalline silicates, cannot completely account for the experimental data. NMR relaxation data also reveal that the Si motion is correlated to the Na motion and that the Si is relatively immobile. Several possible motions of SiO4 tetrahedra that can cause 29Si relaxation are suggested. The motion responsible for 29Si relaxation differs from that which is responsible for viscosity: the apparent activation energy for the former is much lower. Measurements of spin-spin relaxation times and linewidths are also presented and the significance of their temperature dependence is discussed.

Mechanisms for the longitudinal and transverse relaxation of iron-57

For a separation of the contributions of different relaxation mechanisms to the longitudinal relaxation rate of 57Fe, the spin-lattice relaxation times T 1 of this nucleus in various iron complexes [Fe(CO) 5, ferrocene, (butadiene)Fe(CO) 3, (isoprene)Fe(CO) 3, (Z-) and (Epentadiene)Fe(CO)3] were determined in two widely different fields ( B 0 = 2.114 T and 9.395 T corresponding to 90 and 400 MHz proton Larmor frequency, respectively) at temperatures in the range 275 to 321 K. The spin-lattice relaxation rates are analyzed for a field-independent contribution, which is largely due to spin rotation, and a quadratically field-dependent one, which is due to chemical shift anisotropy combined with molecular reorientation. For the nearly spherically symmetric Fe(CO) 5 molecule, spin rotation is the only spin-lattice relaxation mechanism, whereas for all other Fe compounds under investigation, chemical shift anisotropy is the dominant process at B 0 = 9.4 T, while spin rotation dominates at 2.1 T. In addition to these mechanisms, the transverse relaxation, determined at 2.1 T, is influenced by a small amount of visible or colloidal particles, resulting from decomposition of the compounds under investigation. Adsorption of the iron complex on such a suspended particle results in a change of the 57Fe Larmor frequency by chemical exchange and/or scalar interaction. Such a change, for a time interval longer than the 57Fe Larmor period, influences only the transverse relaxation rate.

High-resolution 13C nuclear magnetic resonance studies of an unsaturated polyester in the solid state

Unsaturated resins crosslinked with styrene, have been investigated by magic angle spinning-cross polarization (MAS-CP) 13C n.m.r. and relaxation times in the rotating frame obtained. The relaxation times T 1 ϱ obtained ranged from 2 to 34 ms, and were found to change with cure temperature in the polyester resin with a maximum value observed for all carbons when curing proceeded in the temperature region 80°C–90°C. The relaxation times evaluated indicate that more complete crosslinking occurs at temperatures in this temperature region indicating that there was a correlation between T 1 ϱ and the cure temperature. There was also a common pattern for all carbon atoms and similarly a strong influence on relaxation time existed for all carbons in the cured polyester. The corresponding main-chain motions are mainly cooperative which, with a suggested spin-lattice relaxation mechanism will be a significant influence on the molecular mobility along with the cure temperature. This influence on the molecular mobility is dependent on the carbon chain polymer length and thereby on the number of endgroups in the carbon chain polymer.

1H n.m.r. spin—lattice relaxation study in bituminous coal

1H n.m.r. studies of spin-lattice relaxation of a bituminous coal were performed at temperatures between 128 and 308 K. The relaxation function was observed to be two-component over the whole temperature range. The component with the shorter relaxation time was assumed to be the molecular phase in coal and the component with the longer relaxation time to be the macromolecular phase. Mechanisms of spin-lattice relaxation through spin diffusion to paramagnetic centres and through modulation of dipolar interactions by molecular motions are suggested.

Nuclear magnetic resonance study of dynamics in oxide glasses above the glass-transition temperature

Abstract Nuclear magnetic resonance experiments on glasses of the AgI, Ag2O, B2O3 system are reported. We discuss the spin-lattice relaxation time T1 of boron atoms (11B) tetrahedrally coordinated to oxygen atoms above the glass-transition temperature T g. As expected, structural relaxation within the borate network becomes the main mechanism of spin-lattice relaxation at ∼ 150 K above T g. On the other hand, ∼ 50 K above T g the 11B spin-lattice relaxation rates increase rapidly with temperature and become independent of frequency. This behaviour is anomalous, since it implies that the dynamics involved in the spin-lattice relaxation process cannot be described in terms of a temperature-activated correlation time. Microscopic interpretations are presented which point to the collective vibration-like character of the motion responsible for the anomalous 11B spin-lattice relaxation.

Study of dynamics and spin lattice relaxation mechanism in the triphenylmethylperoxy radical using peroxy spin probes

The stable triphenylmethylperoxy radical Ph 3 COO· is obtained by gamma irradiation of triphenylacetic acid Ph 3 CCOOH in vacuum followed by exposure to oxygen. ESR spectra of the peroxy radicals were measured at X-band in the temperature range 77–300 K. The averaging of the g-anisotropy has been interpreted using the modified Bloch equations. Good agreement with experimental results is obtained by assuming a CO bond rotation mechanism with 120° jumps. The simulated spectra are very sensitive to the value used for the COO bond angle. The angle deduced from the simulations is 129±1°. ESR spectra of the peroxy radicals taken immediately after exposure of the irradiated samples to oxygen indicate the same motional mechanism as the annealed radicals, but a lower activation energy for the motion, 7.5 kJ/mol vs 9.2 kJ/mol. This is most likely due to the trapping of the radical initially on the surface. At low temperatures forbidden spin flip signals are detected, due to numerous protons in the system. The intensity ratio of the allowed to the forbidden transitions as a function of the microwave power indicates that the dominant spin-lattice relaxation mechanism in the peroxy radicals is the radial modulation of the electron-nuclear dipolar interaction by lattice modes.

The carbon-13 chemical shifts and the analysis of the relaxation times T1 and long range 13C− 1H coupling constants of quinoline and of 1-(X-quinolyl)ethyl acetate derivatives

The 13C chemical shifts and the 13C− 1H coupling constants of quinoline (1-(X-quinolyl)ethyl acetate derivatives (where X=−CH(OAc)CH 3 substituted at positions 2,4,5–8) are reported. Substituent chemical shift (SCS) effects for the ethyl acetate group are additive at all positions. A substantial upfield shift of 4.5 and 4.8 ppm was observed at C-4 and C-5, arising from the peri interaction of 5- and 4-ethyl acetate substituents respectively. A vicinal ( peri) 3 J CCCH coupling constant of approximately 5 Hz is observed between both C 5−H 4 and C 4−H 5. Carbon-13 relaxation times ( T 1) and nuclear Overhauser enhancements (η) have been measured for quinoline and its derivatives, and the contributions of dipolar, T 1 DD, and spin rotation, T 1 SR, relaxation have been determined. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism whenever protons are bound directly to the carbons under investigation. Non-protonated ring carbons are relaxed by both DD and SR mechanisms. Anisotropic motion has an easily observable effect on the DD contribution to T 1, and can form the basis for spectral assignments, as in 1-phenylethyl acetate. Long-range 13C− 1H coupling constants were observed both between ring carbons and between ring carbons with ring side-chain hydrogens. These results have been used for the structure determination of the title compounds.

Luminescent Lifetimes and Decay Processes of Hetero-Nuclear Relaxed Excitons [ClI-(VK)+e] in KCl:I and RbCl:I

Lifetimes, emission yields, and their temperature dependence of the BG emission from localized excitons with the configuration of [ClI - +e] are investigated in KCl:I and RbCl:I from 1.9 K to 30 K. Below 4 K, the emission in KCl:I decays with two exponential components: τ s ∼8.0 ms and τ f ∼120 µs. In raising temperature, τ s decreases rapidly to approach a constant value ∼170 µs, while τ f remains constant up to ∼9 K where this component vanishes abruptly and is replaced by an appearance of a delayed `build-up'. Similar results are obtained in RbCl:I. Analysis is made in terms of phonon-induced transitions among zero-field-split triplet sub-levels. Concerning the spin-lattice relaxation mechanism, importance of ralaxation process of Orbach type is suggested in place of the one-phonon direct process which has been successful for the π-emission in KI.

Tissue relaxation time: in vivo field dependence.

Relaxation times (T1 and T2) were measured in vivo in mongrel dogs at fields of 0.3, 0.5, 1.0, 1.35, and 1.5 tesla (T). T1 was measured using nine values of inversion time ranging from 10 to 1,280 msec. T2 was measured with a four-point multiple spin-echo sequence. Relaxation times were calculated for muscle, kidney cortex, spleen, and adipose tissue. T2 is independent of field. A linear fit to the field dependence of T1 yields slopes of 400-500 msec/T for tissues in which the primary source of protons is water. The lower slope of adipose (approximately 150 msec/T) reflects the different mechanism of spin-lattice relaxation of the -CH2 protons.

Shielding anisotropy in tin compounds: spectroscopic studies of alkyltin cations in strong acid solutions

We have observed that the 119Sn nmr spectra of dialkyltin cationic species in highly acidic solutions are strongly field dependent. Relaxation time measurements at three different magnetic fields have established that the dominant spin-lattice relaxation mechanism for these species at high magnetic field is shielding anisotropy. A comparison of T1−1 and T2−1 values indicates that at room temperature a rapid chemical exchange process is occurring. In the case of the (CH3)2Sn(SO3F)2–HSO3F system variable temperature 119Sn nmr reveals the presence of three tin species which are involved in this exchange process.

Proposed structure for the noncovalently associated heme prosthetic group of dissimilatory nitrite reductases. Configuration of substituents of acrylochlorin.

The configuration of the substituents around the macrocycle core has been determined for acrylochlorin, the reduced porphyrin that is the iron-free form of heme d1. This has been accomplished by two independent techniques. Nuclear Overhauser enhancements on the proton resonances of acrylochlorin d3-methyl ester were measured to determine nearest-neighbor substituents and mesoprotons. A lanthanide shift reagent, tris-(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-di onato) europium(III), was found to bind in the vicinity of the carbonyl oxygens of the propionate and acrylate methyl esters. Differential chemical shift effects on the proton resonances of acrylochlorin provided evidence on relative distances from the lanthanide-binding site. Since the substituents are constrained along the circumference of the macrocycle, this provided information on configuration. Assuming that the dominant mechanism for spin-lattice relaxation in acrylochlorin in deuterated chloroform solution is proton dipole-dipole interaction, measured values of spin-lattice relaxation times could be interpreted as being consistent with the proposed configuration. The configuration bears sufficient similarity to that of protoporphyrin IX so that a common nomenclature may be used (Smith K. M. (ed) (1975) Porphyrins and Metalloporphyrins, pp. 3-5, Elsevier/North-Holland Biomedical Press, Amsterdam). The deduced configuration of the acrylochlorin methyl ester is: 1-methyl, 1-hydroxymethyl, 2-methyl, 2-hydroxymethyl, 3-methyl formate, 4-methyl formate, 5-methyl, 6-methyl propionate, 7-methyl acrylate, 8-methyl. The naturally occurring heme d1 has free carboxylic acids at the 3-, 4-, 6-, and 7-positions.

ENDOR Enhancements and relaxation properties of the −OOCĊHCOO − radical in a KH malonate single crystal

ENDOR enhancements and linewidths of the a proton in the −OOCĊHCOO − radical included in a potassium hydrogen malonate single crystal were determined in the three principal planes of the hyperfine tensor. In two planes the ENDOR intensities are very anisotropic, whereas in the third a nearly constant intensity is found. Very strong ENDOR signals (up to 15% of the EPR intensity) are detected near the crossing points of the “ββ” and “βα” energy levels. ENDOR enhancements were calculated by the method developed by Freed and co-workers, taking into account the anisotropies of transition moments and spin-lattice transition probabilities. Calculated intensities are in good agreement with the experimental ones. The dependence of ENDOR intensities on transition moments is very small, whereas they are strongly dependent on spin-lattice transition probabilities. Orientationally dependent W N values must be introduced in the calculations. A strong contribution to W N by the modulation of the dipolar hyperfine field due to the electron relaxation must be assumed. The electron spin-lattice relaxation mechanism is isotropic, being the phonon modulation of the spin-orbit coupling. The W E values giving the best agreement between experimental and calculated enhancements are in the range 10 2 to 5 × 10 2 s −1.

Proton spin-lattice relaxation mechanisms and the metal-insulator transition in cerium hydrides

Nuclear-magnetic-resonance (NMR) experiments have been done on cerium hydride ($\mathrm{Ce}{\mathrm{H}}_{x}$) samples to search for correlations between NMR properties and known electrical conductivity changes as a function of hydrogen concentration and temperature. Data are presented for the $^{1}\mathrm{H}$ spin-lattice relaxation rate ${R}_{1} (=\frac{1}{{T}_{1}})$ and some line shapes for $2.10\ensuremath{\le}x\ensuremath{\le}2.92$ for temperatures from about 100 to 375 K. Although two $^{1}\mathrm{H}$ resonances are observed at some temperatures, proton spin-lattice relaxation is characterized by a single relaxation time at each $x$ and $T$. To a good approximation ${R}_{1}=\frac{A}{T}+R$, where $\frac{A}{T}$ is attributed to direct dipolar coupling between protons and the electronic magnetic dipole moment of ${\mathrm{Ce}}^{3+}$, and $R$ is an essentially temperature-independent term attributed to indirect [Ruderman-Kittel-Kasuya-Yosida (RKKY)] coupling to the ${\mathrm{Ce}}^{3+}$ moment. The $\frac{A}{T}$ term is so large that for most experiments the proton-proton dipolar and proton---conduction-electron couplings are negligible. The $x$ dependence of the constant $A$ is consistent with the dipolar coupling. The constant $R$ decreases in a steep manner as $x$ is increased above $x\ensuremath{\approx}2.65$ just below the regime $2.75&lt;x&lt;2.85$, where the metal-semiconductor transition occurs in $\mathrm{Ce}{\mathrm{H}}_{x}$. It is proposed that $R\ensuremath{\propto}{N}_{d}({E}_{F})$ and that the RKKY interaction includes coupling through the $d$-band density of states. The marked decreases in ${R}_{1}$ and in the electrical conductivity that are associated with the concentration-dependent transition are thus attributed to the vanishing electron density of states at the Fermi surface. No temperature-dependent transition in ${R}_{1}$ is found. Results are consistent with a Mott transition model in which the electron donors are hydrogen vacancies.