Showing papers on "Relaxation (NMR) published in 1971"

Fourier Transform Study of NMR Spin-Lattice Relaxation by "Progressive Saturation"

Abstract: When a nuclear spin system is subjected to a repetitive sequence of strong radiofrequency pulses, a steady state is established where there is a dynamic balance between the effect of the pulses and spin relaxation. Under certain readily satisfied pulse conditions, the deviation of the intensity of the free induction signal from its thermal equilibrium value is an exponential function of the pulse interval with time constant equal to the spin–lattice relaxation time. The determination is unaffected by spin–spin relaxation provided that the interval between pulses is long enough to permit all transverse components of magnetization to be eliminated, and provided precautions are taken to inhibit spin‐echo formation. Through Fourier transformation of the transient response, high resolution spectra with many component resonances may be studied, and the spin–lattice relaxation times of the individual lines determined. The technique lends itself particularly well to repeated accumulation of the transient signal for the purpose of improving sensitivity. It has been applied to the problem of determining the spin–lattice relaxation rates of the eight different carbon‐13 resonances in 3,5‐dimethylcyclohex‐2‐ene‐1‐one. The results span a range from 2.6 to 39 sec, and are in good agreement with those obtained by applying 180°–t–90° sequences to the same sample.

Natural Abundance Carbon‐13 Partially Relaxed Fourier Transform Nuclear Magnetic Resonance Spectra of Complex Molecules

Abstract: Proton‐decoupled partially relaxed Fourier transform (PRFT) NMR of carbon‐13 in natural abundance was used to determine spin–lattice relaxation times (T1) of individual carbons in solutions of cholesteryl chloride, sucrose, and adenosine 5′‐monophosphate (AMP) at 15.08 MHz and 42°C. With the exception of a few side‐chain groups, all protonated carbons have T1 values of less than 1 sec. Some side‐chain carbons on cholesteryl chloride show evidence of internal reorientation and have relaxation times of up to 2 sec. Nonprotonated carbons have T1 values in the range 2–8 sec. These relaxation times are sufficiently short to make ordinary Fourier transform NMR a very sensitive technique in the study of complex molecules without the need for spin‐echo refocusing schemes. Integrated intensities and nuclear Overhauser enhancements prove that, except for two of the three nonprotonated carbons in AMP, all 13C nuclei in these compounds relax mainly through 13C–1H dipolar interactions. Measured T1 values of protonated...

Nanosecond Time-Resolved Emission Spectroscopy: Spectral Shifts due to Solvent-Excited Solute Relaxation

Abstract: Nanosecond time‐resolved emission spectral techniques have been applied to the problem of the origin and nature of the well‐known temperature‐dependent spectral shifts characteristic of the aminophthalimides in alcohol solvents. It is demonstrated that the temperature‐dependent spectral shifts are in fact due to time‐dependent spectral shifts. At least two relaxation times characterize this phenomenon. One relaxation time is observed to be subnanosecond in character and may be associated with the exciplex that presumably is present in the system. The other relaxation time is presumably associated with the nonspecific dipolar reorientation although it has distinctly different characteristics from the solvent dielectric relaxation time. Wavelength‐dependent fluorescence decay that can be explained by the time dependence of the emission spectrum is also observed.

High-Resolution Study of NMR Spin Echoes: "J Spectra"*

Abstract: The spin–spin relaxation of individual lines in a high‐resolution NMR spectrum has been studied by a technique which extracts an essentially “monochromatic” component from the Carr–Purcell spin‐echo response by means of a narrow‐band filter. The resulting selectivity significantly reduces the complexity of the modulation of the spin‐echo amplitude due to homonuclear spin–spin coupling, and further simplification may be achieved by double irradiation experiments. Echo modulation has been used as a precise measure of the 0.051‐Hz long‐range coupling in 3‐bromothiophene‐2‐aldehyde, normally hidden by magnet inhomogeneity. It is proposed that modulation and spin–spin relaxation effects are best analyzed in the general case by calculating the Fourier transformation of the envelope of the spin‐echo peaks. The general term suggested for this new mode of presentation is the “spin‐echo spectrum.” It consists of a set of resonance responses with widths determined by the relevant spin–spin relaxation times, at freueqncies determined by the various modulation components of the spin‐echo decay. The present study concentrates on operating conditions (low pulse repetition rate and “first‐order” spin coupling) where the resonance frequencies correspond to one‐half the sums and differences of the spin coupling constants. This special case of a spin‐echo spectrum is called a “J spectrum.” When the spin‐echo response appropriate to a single group of chemically shifted nuclei is observed selectively, the transformation gives a “partial J spectrum” which can be used as an aid to the assignment of the full spectrum. J spectra are illustrated for a typical first‐order three‐spin system—the ring protons of methyl furoate.

Kinetic and hydrodynamic studies of the complex of proflavine with poly A-poly U.

Abstract: Relaxation kinetic experiments reveal general similarity between the mechanism of binding of proflavine to poly A·poly U and DNA. There are differences in detail, however. For example, the rate constants are roughly an order of magnitude smaller for the former, and the thermodynamic parameters of the individual steps are also different. The total heat and free energy for intercalation of free dye are quite similar in the two cases. As was the case with DNA, considerable dye (up to 25% of the bound form) is attached externally to the double helix, even in the strong binding region of the isotherm. Sedimentation measurements on small, rodlike fragments of poly A·poly U reveal a length increase on binding proflavine of a magnitude similar to that found with DNA. This length increase seems to become smaller under conditions (high temperature) where the relaxation measurements indicate a higher fraction of externally bound dye.

Nuclear spin relaxation and self-diffusion in the binary system, dimethylsulphoxide (DMSO)+ water

Abstract: Spin-lattice (T1) and, where appropriate, tranverse (T2) relaxation times are reported for the protons in DMSO + water mixtures over a range of temperature and composition. Self-diffusion coefficients are also measured. A minimum in T1 for a d6-DMSO + H2O sample enables rotational correlation times for water molecules to be evaluated in mixtures with mol fractions of water, x(H2O), ⩽0.7. The low temperature (170-250 K) behaviour of T1 for DMSO + D2O mixtures with x(D2O)≃0.7 is interpreted in terms of end-over-end rotation of DMSO molecules and internal rotation of methyl groups. Under certain assumptions rotational correlation times for the DMSO molecules are obtained from the intramolecular contribution to T1 derived from measurements on ternary mixtures of the type d6-DMSO + DMSO + D2O. These are found to be very similar to those for the water molecules at the same temperature and compositions suggesting that the DMSO and water molecules reorient together. All measurements indicate a minimum in molecular mobility, both rotational and translational, at around a mol fraction of water of 0.65.

Local order in liquid tellurium

Abstract: Structural data and electronic properties argue against the existence of independent chains in the liquid state of tellurium. It is proposed here that the local order consists of sites with 2 or 3 first neighbours, joined by bonds with a strong covalent character. In space, that local order persists up to the second neighbours. In time, it is of very short duration : the relaxation times for these « networks » are typical of a normal liquid ∼ 10-12 s. Some of the valence electrons are localized in these bonds, with the others forming a metallic electron gas. The value of the corresponding electronic density of states at the Fermi level is discussed ; its variation with temperature is related to the structural evolution of the liquid.

Nuclear relaxation studies of lecithin bilayers.

Abstract: BILAYER phases of lipids have broad proton magnetic resonance (PMR) spectra, which have been attributed to incomplete averaging of the dipole–dipole interactions between various protons as a result of slow molecular motion1,2. Two pulsed nuclear magnetic resonance (NMR) studies, however, have proposed internal magnetic field gradients as the source of spectral broadening. Kaufman et al. concluded that these gradients led to inhomogeneity broadening3, whereas Hansen and Lawson proposed molecular diffusion through them4. These conclusions were based on the apparent dependence of the spectral width on the applied magnetic field5 as well as abrupt changes in transverse relaxation time (T2) without concomitant changes in the longitudinal relaxation time (T1) at the bilayer phase boundaries4,5. The dependence of T2 on pulse spacing in a Carr–Purcell experiment6 has also been taken as evidence for molecular diffusion through internal magnetic field gradients4.

Dielectric Relaxation and Molecular Structure. V. Application of the Single Frequency Method to Systems with two Debye Dispersions

Abstract: There are four Debye equations for dilute solutions which can be used to determine the relaxation time of a polar solute molecule. This paper offers a critical examination of these equations, especially two representative formulas which are mutually independent. If the solute molecule has a non-rigid configuration and has two relaxation times, τ1 and τ2, for overall and internal rotations, respectively, average relaxation times will be obtained from these equations based on measurements at a single frequency. However, the analysis of this paper shows that under a certain condition (if a proper frequency for the measurement and a suitable equation for the calculation are employed) one may make a crude estimate regarding one of the two relaxation times. Further, the ratio of two relaxation times seems to afford a clue for investigating the mechanism of internal rotation. In addition, Cole-Cole dispersion and atomic polarization problems are also discussed.

Theory of Molecular Reorientation in Liquids: Magnetic Spin Resonance Line Shapes

Abstract: The Debye model of rotational Brownian motion and the rotational random jump model have been extended to allow for time fluctuations of the rotational diffusion constant and the jump rate constant, respectively. The treatment is applicable to experimental situations which are conventionally described in terms of a distribution of correlation times and to systems where molecular reorientation is complicated, e.g., by rapid chemical exchange processes. Correlation functions for spin relaxation are discussed in some detail and are compared with the corresponding correlation functions for dielectric relaxation. In particular, the ESR (or NMR) line shape is given for a simple example of very slow reorientation where the rotational correlation time is comparable with the spin relaxation time. In this case, the Debye model and the jump model yield different line shapes, and further changes occur if environmental fluctuations are considered. Details are given for fluctuations described by an Uhlenbeck–Ornstein process.

Nuclear magnetic resonance studies of molecular relaxation mechanisms in polymers

Abstract: N RECENT YEARS substantial progress has been made in the understanding of the Istructures and dynamics of high polymers. Crystal structures and chemical composition are well known for most common polymers [1, 2] . The fringed micelle model for crystalline-amorphous organization has been supplemented by the discovery and elucidation of chain folding and the relationship of chain-folded lamellae to spherulitic structures [2, 3]. Relaxation phenomena have been related to molecular motions, and qualitative and semiquantitative interpretations of diverse relaxation experiments show gratifying consistency [4, 5]. In this paper I

Dielectric relaxation in non aqueous solutions. Part 2.—Solutions of tri(n-butyl)ammonium picrate and iodide in polar solvents

Abstract: The permittivity and loss of solutions of tri(n-butyl)ammonium picrate and iodide in various polar solvents have been measured at a number of frequencies between 0.2 and 3.0 GHz. The relative static permittivities of the pure solvents used range from 3.37 to 20.7 at 25°C. For all the solutions investigated, the observed dispersion of permittivity is adequately described by a single relaxation time, which for a given solvent depends on the concentration as well as the nature of the solute. Apparent dipole moments evaluated from the dispersion amplitude by means of Bottcher's theory, on the assumption that the dipole occupies a spherical cavity, are 12.3±0.4 D and 11.2±0.6 D for the picrate and iodide respectively. These values indicate that the relaxing dipole is an ion pair. Dielectric relaxation times corrected for internal field effects by different relations are compared with those calculated from the partial molar volume of the solute and the viscosity of the solvent by means of the Debye-Stokes equation. Density data for solutions of the two salts in 1,2-dichloroethane are included and from these data, partial molar volumes of the salts have been evaluated.

Relaxation Processes in Water. The Spin–Lattice Relaxation of the Deuteron in D2O and Oxygen‐17 in H217O

Abstract: An experimental study of the deuteron relaxation time T1 has been made over a temperature range − 18–178°C. The equation for the temperature dependence of the relaxation time is of the form − lnT1 = ln(aeb/T + ced/T). The data are interpreted in terms of an equilibrium leading to a species which relaxes by an isotropic rotational diffusion process. Using transition rate theory and a quadrupole coupling constant derived for the relaxing species from dielectric data, heats and entropies are calculated for both the equilibrium and rate processes. For the equilibrium ΔH = 6.8 ± 0.2 kcal mole−1 and ΔS = 24.8 ± 0.9 e.u. mole−1. For the rate, ΔH* = 2.50 ± 0.06 kcal mole−1 and ΔS* = 3.6 ± 0.1 e.u. mole−1. Similar measurements for the oxygen‐17 relaxation time T1 over the temperature interval − 14–180°C yield for the equilibrium ΔH = 5.6 ± 0.3 kcal mole−1 and ΔS = 20.7 ± 1.4 e.u. mole−1. For the rate, ΔH* = 2.43 ± 0.08 kcal mole−1 and ΔS* = 3.9 ± 0.2 e.u. mole−1. The results are discussed in terms of models for the water structure, the species present in the liquid, the relaxation process and molecular motion in the liquid.

Shock‐Tube Studies of HF Vibrational Relaxation

Abstract: The vibrational relaxation of HF has been studied behind incident shock waves in the temperature range 1350–4000°K by monitoring the 2.7−μ infrared emission. Relaxation times reduced to standard pressure were obtained for mixtures containing 1%–10% HF in argon and for mixtures containing N2, He, and D2. These data were used to calculate the relaxation times of HF due to these various gases. The relaxation times can be summarized by the expressions PτHF‐HF=1.02×10−2 exp(34.39/T1/3) μsec· atm, PτHF‐Ar=1.62×10−3 exp(111.97/T1/3) μsec· atm, PτHF‐He=1.52×10−4 exp(133.3/T1/2) μ sec· atm, and Pτ HF‐D2=5.1×10−4 exp(96.6/T1/3) μsec· atm. The values of PτHF‐N2 fell between those for helium and deuterium. The relaxation time of HF due to itself is compared to the predictions of several theories of V‐T and V‐R energy transfer, and to published experimental data for other hydrogen halides. The HF rates of this study, together with the measured rate at room temperature, suggest the possibility of an attractive potential playing a dominant role in energy transfer at low temperature.

The lanthanide cations as probes in biological systems. Proton relaxation enhancement studies for model systems and lysozyme.

Abstract: A study has been made over a range of temperatures at 20 MHz and 35 MHz of the spin relaxation rates of water protons in aqueous solutions of ethylene glycol bis-(aminoethyl)-tetraacetic acid containing Gd(III) and Eu(II) ions. The results suggest that the dominant relaxation process arises from rotation of the solvated complexes. Relaxation enhancement has been observed for Gd(III) in the presence of lysozyme and values of ɛ*, Kd and τc have been determined. An application of these results to obtain distances of substrate analogues from the active site of this and other enzymes is discussed.

Structures of the Excited States of Benzil and Related Dicarbonyl Molecules

Abstract: Nonhindered aromatic α‐dicarbonyl molecules show abnormal spectroscopic behavior when compared to other molecules containing carbonyl groups. Taking benzil as a typical example, its photoabsorption and emission spectra cannot be accounted for by the normal Jablonski scheme. The spectral data are consistent with a mechanism involving molecular reorientation. It is postulated that photoexcitation of benzil causes it to change from its normal skew configuration into one in which the dicarbonyl system becomes coplanar while it is in the excited singlet state. As a consequence photoemission (both fluorescence and phosphorescence) takes place from this configuration, resulting in a ground‐state molecule with the same nuclear configuration as the excited state; relaxation then occurs to the normal configuration. Formation of a trans configuration is most likely to occur on photoexcitation, but a cis configuration cannot be ruled out. The formation of a coplanar dicarbonyl system must involve movement of the phenyl groups into positions out of the dicarbonyl plane. Additional support for this mechanism is obtained from the energetics of the various processes.

Quenching the Nuclear Overhauser Effect in NMR Spectra of Carbon‐13

Abstract: Fourier transform techniques have been employed to determine the spin‐lattice relaxation time and the enhancement of the intensity of the carbon‐13 signal of formic acid upon strong proton irradiation—the nuclear Overhauser effect. Addition of paramagnetic hexaquochromic perchlorate is found to decrease the relaxation time and the enhancement factor in the same proportion. Beyond a concentration of about 0.1M, the Overhauser enhancement is effectively quenched, line broadening is only just becoming apparent, and the loss of intensity can be fully recovered by readjustment of the pulse conditions to take advantage of the faster relaxation. The carbon‐13 spin‐lattice relaxation in neat formic acid is shown to proceed predominantly through dipolar interaction with the protons, and when paramagnetic ions are added they are found to act directly on the carbon nuclei rather than via the proton spins, a negligible ``three‐spin effect.'' The quenching technique has been employed to establish uniform intensities for the four carbon‐13 resonances of vinyl acetate, where normally there is a wide disparity in signal strength due to unequal Overhauser effects.

Electron Paramagnetic Relaxation and EPR Line Shapes of Manganous Ion Complexes in Aqueous Solutions. Frequency and Ligand Dependence

Abstract: Line shapes of electron paramagnetic resonance spectra of manganous ion in various ligand environments have been examined at two microwave frequencies, 9.1 and 35 GHz. From the observed differences in line shape at two microwave frequencies and the theoretical expression for relaxation of sextet state ions, the correlation times for processes modulating the zero‐field splitting (zfs) interaction and the magnitude of the effective zfs were evaluated. For all complexes which were examined the correlation times were in the range 3×10−12−9×10−12sec at 300°K. Two types of zfs are distinguished, a transient zfs which is related to the relaxation processes and a static zfs. In this regard there appears to be no simple relation between the transient zfs as obtained from relaxation studies and the static zfs as obtained from solid‐state spectra. The large static zfs for MnEDTA gives quasi‐solid‐state features to the solution EPR spectra of this complex.

Lattice Relaxation and Small-Polaron Hopping Motion

Abstract: The three-dimensional analog of Holstein's molecular crystal model is utilized as the basis for a study of the relaxation of the lattice after a small-polaron hop. In particular, it is shown explicitly that the time-dependent activationlike energy arising in the previously developed theory of correlated small-polaron hopping motion is directly related to the actual relaxation of the lattice from the distorted configuration it must assume to facilitate a small-polaron hop. That is, the time dependence of this "activation energy" and the concomitant relaxation of the hop-related lattice displacements are governed by a single entity denoted as the relaxation function. Furthermore it is demonstrated that this function is directly expressible in terms of the transfer of vibrational energy from the initially distorted sites to successive (initially undistorted) neighbor sites. In fact, for the most part, the relaxation of the lattice after a hop is associated with the transfer of vibrational energy to only nearest-neighbor sites, this being essentially a local phenomenon independent of the periodic nature of the lattice. Finally, although the lattice relaxation for our three-dimensional model is found to proceed much faster than in the previously developed one-dimensional model, its effect on small-polaron hopping motion may not be inconsequential. In particular, the small-polaron drift mobility is shown to be significantly affected by lattice relaxation effects when the mean time between small-polaron hops is less than or comparable to the lattice relaxation time; this time being essentially the reciprocal of the optical-phonon bandwidth parameter.

N.M.R. relaxation time studies in liquid methyl iodide

Abstract: The spin-lattice relaxation times of the various nuclei in methyl iodide, methyl iodide-d 3, and carbon-13 methyl iodide (13C, 1H, 2D) were measured between 210 and 350 K. The separation of the proton-proton intermolecular relaxation was accomplished by a dilution study in methyl iodide-d 3; the resulting intermolecular contribution agreed well with the existing theories for this mechanism. It was found that the spin-rotation interaction contributed significantly to the intramolecular relaxation of both the protons and the carbon-13. For both nuclei the separation of the spin-rotation interaction from the intramolecular dipole-dipole interaction was accomplished without making any assumptions about the temperature dependence of the spin-rotation relaxation time. The resulting spin-rotation relaxation times for both carbon-13 and protons offer evidence that the large spin-rotation effects are due to the methyl group reorientation.

Conductivity and Dielectric Relaxation in Concentrated Aqueous Lithium Chloride Solutions

Abstract: This paper describes a study of the recently recognized phenomenon of conductivity relaxation in liquid electrolytes as it is observed in the system LiCl+water. Advantage has been taken of the supercooling ability of 6–20m solutions in this system to reduce solution temperatures to the vicinity of −100°C and thereby to increase the relaxation time characteristic of the conductance process sufficiently to permit its study with conventional ac conductance bridges operating in the frequency range 0.2–2 000 kHz. Extensive data are presented for four solutions in the concentration range 8.3–11.9m. When the frequency‐dependent conductance and capacitance data are analyzed in the dielectric modulus notation developed by Macedo, Bose, and Litovitz, mean relaxation times for conductance are obtained which have a non‐Arrhenius temperature dependence identical to that of the dc conductivity. Earlier studies of dielectric relaxation in these solutions, carried out at room temperature and gigahertz frequencies, are re...