Showing papers on "Proton spin crisis published in 1980"

Dipolar relaxation and nuclear Overhauser effects in nonrigid molecules: The effect of fluctuating internuclear distances

Abstract: A simple treatment of the dipolar contribution to nuclear magnetic relaxation is developed for molecules in which conformational fluctuations modulate the relevant internuclear distances. Expressions are given for AX system spectral densities using a general model in which conformational fluctuations occur as random jumps among discrete conformations, while the molecule as a whole undergoes rotational diffusion as either a spherical or a symmetric top. Approximations valid for proton spin systems are given for cases in which the jumping rates are either fast or slow compared to rotational diffusion; the results are independent of the jumping rates. Similar results are obtained for cross‐correlation spectral densities. For complex spin systems, e.g., An1Mn2Xn3..., the cross‐relaxation constants σij which couple pairs of magnetizations, depend only upon autocorrelation, spectral densities and are thus easily obtained from the AX system results. Measurement of σij by time resolved Overhauser effect experimen...

Proton spin T1 relaxation dispersion in liquid H2O by slow proton‐exchange

Abstract: We have measured the longitudinal proton spin relaxation time T1 in pure water as a function of temperature (10, 37, 80°C) and Larmor frequency (50 Hz⩽ν0⩽50 kHz) by means of field‐cycling techniques. T1 becomes frequency dependent below ν0≈5 kHz due to the slow proton‐exchange between different oxygen environments (16O, 17O, 18O), which modulates the magnetic 1H–17O interaction. The proton‐exchange time τe was found to be Arrhenius‐like with activation energy 13.4 kJ/mol and pre‐exponential 1.5×10−6 s (37 °C:τe=2.6×10−4 s). This is in fairly good agreement with results obtained previously in the literature by more indirect NMR methods. The new access to proton‐exchange reduces the experiemtnal error limits and allows measurements at extremely small frequencies, where the familiar T1ρ technique no longer works.

Computer simulation of selective excitation in n.m.r. imaging

Abstract: It is attractive to apply selective excitation as part of a method for proton spin imaging of large objects. In the past, there has been some discussion on how selective excitation should be performed. The spin magnetization reacts in a nonlinear way to an r.f. (radio frequency) excitation pulse. This makes it difficult to give algebraic expressions for the excited transverse magnetization if one does not apply just a simpel rectangular r.f. pulse but, instead, an arbitrarily tailored pulse. We have, therefore, studied numerical solutions of the equations of motion (Bloch equations) for various forms of tailored pulses. Typical computation results will be shown and a brief discussion on practical aspects will be given.

Proton spin–lattice relaxation in meta‐carborane

Abstract: The proton spin lattice relaxation time, T1 has been measured at 30 MHz in meta‐carborane between 171 and 303°K. The temperature dependence of T1 in meta‐carborane is discussed.(AIP)

An experimental evaluation of simplified procedures for determining the proton spin–lattice relaxation rates of natural products

Abstract: A general discussion is given of the determination of the proton spin–lattice relaxation rates of natural products, with particular emphasis on use of the null-point method which, for the systems studied here, gives identical results with those obtained via the conventional (and relatively time consuming) computational method.

Proton spin lattice relaxation in lithium ammonium sulphate LiNH4SO4

Abstract: Lithium ammonium sulphate (LAS) undergoes a phase transition at TC1=459.5K from a paraelectric phase (phase I) to a ferroelectric phase (phase II) and again at TC2=283K to a polar ferroelastic phase (phase III). Proton spin lattice relaxation time measured at 10 MHz in powdered LAS in the temperature range 480 to 77K shows discontinuous changes at the two transitions.

An experimental evaluation of nonselective proton spin–lattice relaxation rates: analyses of data for the eight isomeric 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-hexopyranoses

Abstract: The nonselective spin–lattice relaxation rates (R1-values) have been determined for all of the ring protons of the eight isomers of 2,3,4-tri-O-acetyl-1,6-anhydro-(β-D-hexopyranose as 0.1 molar solutions in benzene-d6. The effects on the proton R1-values of changes in solvent, concentration, temperature, and proton impurities are documented and 13C R1-values are given to show that the first two sets of variations are due to changes in motional correlation times of the molecules. The proton relaxation data can be fitted by regressional analyses to a single set of interproton relaxation contributions, the numerical values of which accord with a 1C4 conformation for the pyranose ring somewhat distorted by the 1,6-anhydro bridge.

Proton spin-lattice relaxation in MBDBP

Abstract: 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.

Nuclear relaxation and molecular motion of pyridine-N-oxide in carbon tetrachloride

Abstract: Proton spin lattice relaxation of pyridine-N-oxide-2,6-d2 in dilute solution (0·2 M) with carbon tetrachloride has been measured at -12·6, 6·5, and 26·6°C. Using 2H and 14N decoupling, eight transitions were resolved and recovery was monitored after non-selective inversion. In absence of 14N decoupling, scalar relaxation produced clearly visible effects on the proton recovery curves. Combined with measurements of deuterium relaxation of the 2,6-d2 and 4-d derivatives, proton relaxation data (including effects of dipolar cross correlation) were used to determine all three rotational diffusion constants at each temperature. The in-plane spinning motion was found to be ∼four-fold more strongly dependent on viscosity than predicted by the slip rotational diffusion model, while motion about the other two principal axes proceeds at rates within ∼40 per cent of slip predictions. 14N spin lattice relaxation data were combined with the anisotropic reorientation rates in efforts to determine liquid phase values of ...

Proton spin-lattice relaxation rate studies. 1-Aryl-4,4-dimethyl-2-methylthio-2-imidazolin-5-ones

Abstract: Proton spin-lattice relaxation rates (R1 values) have been measured, at 270 MHz, for a series of 1-aryl-4,4-dimethyl-2-methylthio-2-imidazolin-5-ones, using both null-point and non-linear regression methods; the two procedures give the same rates. A normalization procedure has been used to enable comparisons to be made between the R1 values of different compounds; this is necessary because the experimental values are affected by changes in motional correlation times associated with changes in molecular structure. A substantial (five-fold) dynamic range of R1 values has been observed for the protons of individual compounds, and the values have been correlated with the molecular environments of the nuclei. Evidence for anisotropic molecular motion and inter-ring proton relaxation in these compounds has been considered.

Proton Spin-Lattice Relaxation in the Critical Regime of a Nematic Liquid Crystal

Abstract: We present here the proton spin-lattice relaxation rates in MBBA measured over a large temperature range above the nematic-isotropic transition and at a number of frequencies. From the observed temperature and frequency dependences and using a recent theory, we demonstrate that these proton relaxation rates can be well understood when we consider: T 1)−1 expt = T 1)−1 CF + T 1)−1 0, where the first part arises from the critical fluctuations (CF) and the second from the diffusion-controlled mechanism(s). All the frequency dependence observed in the ranges of frequencies and temperatures measured is shown to arise from T 1)−1 CF which, according to the theory, consists of two parts—T 1)−1 CFI and T 1)−1 CFN. Using the frequency dependences of these two parts, they are separated and compared with the theory. The agreement is reasonably good.

Proton concentration dependence of the proton spin-lattice relaxation time in copper tutton salts

Abstract: The proton spin-lattice relaxation time, T1p, was measured in a series of single crystals of the Tutton salt: (Cu,Zn)Cs2(SO4)2.6(H,D)2O. The measurements were performed at fields in the region from 5 to 15 kOe and at liquid- helium temperatures. We investigated samples with 12, 25, 50 and 100% protons in the waters of hydration, all having a Cu2+-concentration of about 0.5%. In these samples, the proton relaxation path is bottlenecked, under most experimental conditions, by the thermal contact between the proton Zeeman system and the electron dipole-dipole interaction system, provided by the proton three-spin process. This was always the case in our previously investigated 6%-1H sample [1, 2]. For higher proton concentrations, however, the role of the electron spin-lattice relaxation in the proton relaxation path becomes more important. The results of the 50 and 100%-1H sample at 1.3 K show clearly that at high fields the proton three-spin process bottlenecks the proton relaxation path, while with decreasing field the electron spin-lattice relaxation process takes over. Supplementary measurements in two samples with 12%-1H and different Cu2+-concentrations show a strong dependence of T1p on this concentration. Finally, an attempt has been made to determine the role of the proton spin diffusion in the relaxation path.

NMR Proton Relaxation Investigation of the Nematic-Isotropic Phase Transition on Homologues of the PAA Series

Abstract: It is by now well-established that proton spin T1 relaxation in both the nematic and isotropic phase of liquid crystals is mainly determined by director order fluctuations (OFD) and by self-diffusion (SD) [1]. However, in the vicinity of the nematic to isotropic phase transition temperature the relaxation process shows a more complex behaviour and is less well understood. Evidently there exists another T1 mechanism! We have performed extensive measurements of the Larmor frequency and temperature dependence of the proton T1 for several homologues in the PAA series (PAA, PAP, PAB, HAB) below the transition points, to find out the properties of the addidional ‘third’ relaxation rate in the nematic state. The third contribution was found to depend critically upon the temperature, whereas its connection with Larmor frequency and molecular chain length proved rather small. Deuteration of PAA considerably enhanced its magnitude.