Showing papers on "Relaxation (NMR) published in 1987"

Parahydrogen and synthesis allow dramatically enhanced nuclear alignment

Abstract: The PASADENA effect is a method for transient high-sensitivity proton spin-labelling by molecular addition of dihydrogen. When the parahydrogen mole fraction differs from the high-temperature limit of 1/4, this population difference constitutes a form of spin order which can be converted to magnetization observable by NMR. Large NMR signals are observed, if subsequent to the hydrogen addition, the two protons experience magnetic inequivalence and spin-spin coupling and if observation is made before spin-lattice relaxation restores the equilibrium spin order. The analogous effect for D2 is also possible. The kinetic mechanisms of the homogeneous hydrogenation catalysts which permit the realization of the PASADENA effect have been the target of the experimental applications. The enhancement of the NMR transitions has facilitated the determination of true molecular rate constants. Ordinarily, the activity of a catalyst is assessed by dividing the observed rate by the total catalyst concentration. However, the question as to whether most of the catalytic rate is due to a tiny fraction of active species or a large fraction with a relatively low molecular rate is not clearly addressed by such an analysis. This ambiguity is entirely avoided in the PASADENA studies, since only active catalyst molecules can contribute to the enhanced signals from which all kinetic inferences are made. The sensitivity enhancement has also led to the identification of a novel intermediate in the mechanism for the Rh(DIPHOS)+ catalyzed hydrogenation of styrene. The rate of conversion of this species into product and starting material has been studied using two-dimensional NMR. The dramatically improved sensitivity should make it possible to observe key catalytic intermediates which do not build up in sufficient quantity to allow detection by conventional NMR arising from Curie-Law magnetization. The study of surface sites which bind pairwise with H2 is also a potentially fruitful area for future experimental work. The ambient temperature NMR spectroscopy of surfaces is not often feasible due to sensitivity limitations. Simulations have been performed using typical shift and coupling parameters in an effort to characterize the enhanced lineshapes which can be expected. The inverse of the PASADENA effect has also been proposed, whereby the spin order of a molecule containing hydrogen is probed by measuring the branching ratio to ortho and para dihydrogen. This RAYMOND phenomenon (radiowave application yields modulated ortho number desorbed) has the potential for measuring precursor NMR with extraordinary sensitivity, since it finesses the need for detection of radiowaves.

Transverse relaxation of solvent protons induced by magnetized spheres: application to ferritin, erythrocytes, and magnetite

Abstract: Since 1/T2 of protons of tissue water is generally much greater than 1/T1 at typical imaging fields, small single-ion contrast agents--such as Gd(DTPA), which make comparable incremental contributions and therefore smaller fractional contributions to 1/T2 compared to 1/T1--are not as desirable for contrast-enhancement as agents that could enhance 1/T2 preferentially. In principle, such specialized agents will only be effective at higher fields because the field dependence (dispersion) of 1/T1 is such that it approaches zero at high fields whereas 1/T2 approaches a constant value. The residual 1/T2 is called the "secular" contribution and arises from fluctuations in time--as sensed by the protons of diffusing solvent or tissue water molecules--of the component of the magnetic field parallel to the static applied field. For solutions or suspensions of sufficiently large paramagnetic or ferromagnetic particles (greater than or equal to 250 A diameter), the paramagnetic contributions to the relaxation rates satisfy 1/T2 much greater than 1/T1 at typical imaging fields. We examine the theory of secular relaxation in some detail, particularly as it applies to systems relevant to magnetic resonance imaging, and then analyze the data for solutions, suspensions, or tissue containing ferritin, erythrocytes, agar-bound magnetite particles, and liver with low-density composite polymer-coated magnetite. In most cases we can explain the relaxation data, often quantitatively, in terms of the theory of relaxation of protons (water molecules) diffusing in the outer sphere environments of magnetized particles. The dipolar field produced by these particles has a strong spatial dependence, and its apparent fluctuations in time as seen by the diffusing protons produce spin transitions that contribute to both 1/T1 and /T2 comparably at low fields; for the larger particles, because of dispersion, the secular term dominates at fields of interest. On the basis of the agreement of theory with data for solutions of small paramagnetic complexes, large magnetite particles, and liver containing low-density polymer-coated magnetite agglomerates, it is argued that the theory is sufficiently reliable so that, e.g., for ferritin--for which 1/T2 is unexpectedly large--the source of its large relaxivity must reside in nonideal chemistry of the ferritin core. For blood, it appears that diffusion through intracellular gradients determines 1/T2.

Analysis of deuterium nuclear magnetic resonance line shapes in anisotropic media

Abstract: Methods for the analysis of motional effects on 2H NMR solid state line shapes are described. Several simple models of anisotropic motions in solids are presented from which line shapes and relaxation experiments are calculated. Using these methods order parameters of fast limit spectra can be explicitly evaluated. These methods are extended to include analysis of intermediate exchange spectra in terms of specific motional models. The effects of differential T2’s arising from exchange broadening on the line shape in powders are described. Methods for the calculation and analysis of T1 anisotropy in partially relaxed spectra of powders for both fast and intermediate exchange regimes are presented.

Ultrasonic studies of silicate melts

Abstract: The ultrasonic velocity and absorption of 32 silicate melts have been determined as a function of temperature (1175–1925 K) and frequency (3–12 MHz). The samples include the components SiO2, TiO2, Al2O3, FeO, MgO, CaO, SrO, BaO, Li2O, Na2O, K2O, Rb2O, and Cs2O, and range from simple alkali silicates to multicomponent synthetic and natural melts. The relaxed sound speed varies systematically with substitution of the alkali and alkaline earth oxides, decreasing monotonically with increased cation radius. An algorithm for the compositional dependence of the ultrasonic velocity at 1673 K:1/c=∑ (xi,υ/c¯i), where xi,υ is the volume fraction of component i and c¯i is independent of composition, is capable of predicting relaxed melt velocities to within 4%. The temperature dependence of the sound speeds in the nondispersive region is quite small, ≈1 part in 104 K−1. A simple linear model for the compositional dependence of the derived property (δV/δP)T has an uncertainty of 13%, approximately equal to that expected from the propagation of errors involving sound speed, density, thermal expansion, and heat capacity. The contributions of the changes in sound speed and density with temperature are nearly the same in their influence on the temperature dependence of the bulk modulus of natural melts. The relaxation properties c/c0 (ratio of the sound speed to the low-frequency value) and αλ (absorption per wavelength) of all of the liquids studied are very similar when plotted against ωτ, the product of the angular frequency and the shear relaxation time. Both properties are consistent with τ equal to 1% of the product of the low-frequency shear viscosity and bulk modulus. Dispersion begins when ωτ ≃ 0.1 and the limiting high-frequency sound speed c∞ is about 2.3 times c0. The product αλ attains a maximum value of about 1.4 when ωτ ≃ 0.8. The data cannot be explained by a single relaxation time in the melts; rather, a spectrum of relaxation times is indicated.

NMR relaxation times of blood: dependence on field strength, oxidation state, and cell integrity

Abstract: The variation with field strength or interecho interval of the T1 and T2 relaxation times of oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), and methemoglobin (MHb) in either intact or lysed red blood cells was studied with a variable field (0.19-1.4 T) nuclear magnetic resonance spectroscopy unit. The T2 relaxation time of intracellular HbO2 decreased slightly with increasing field strength and interecho interval. The T2 relaxation times of intracellular Hb and MHb decreased markedly with increasing field strength and interecho interval. This T2 proton relaxation enhancement increased as the square of the applied field strength and was 1.6 times stronger for intracellular MHb than for intracellular Hb. The T2 relaxation enhancement is secondary to the loss of transverse phase coherence of water protons that diffuse across cellular magnetic field gradients. These field gradients occur when an external field is applied to a region with gradients of magnetic susceptibility. The heterogeneity of magnetic susceptibility is caused by the heterogeneous distribution (only intracellular) of the paramagnetic molecules (Hb or MHb). The T2 relaxation times of red cell lysates (homogeneous magnetic susceptibility) were independent of field strength or interecho interval. There was a decrease in the T1 relaxation times when the red cells were lysed. This may be due to an increase in the slow motional components of water molecules, because of the decrease in the average distance between water and hemoglobin molecules in the lysate. The T1 relaxation times of all the MHb samples were shortened because of proton-electron dipolar-dipolar relaxation enhancement. All the T1 relaxation times increased with increasing field strength.

Determination of electrical transport properties using a novel magnetic field‐dependent Hall technique

Abstract: A novel technique is presented for interpreting magnetic field‐dependent Hall data at magnetic fields below the range at which Shubnikov–de Haas oscillations occur. The technique generates a ‘‘mobility spectrum’’ in which the maximum carrier density or maximum conductivity is determined as a continuous function of mobility. Examples of the use of the technique with synthetic data as well as data from HgCdTe and GaAs/AlGaAs samples are provided. Other uses of the procedure, including measurement of Fermi surface shapes and direct measurement of the distribution of relaxation times, are discussed.

Virtual and solution conformations of oligosaccharides.

Abstract: The possibility that observed nuclear Overhauser enhancements and bulk longitudinal relaxation times, parameters measured by 1H NMR and often employed in determining the preferred solution conformation of biologically important molecules, are the result of averaging over many conformational states is quantitatively evaluated. Of particular interest was to ascertain whether certain 1H NMR determined conformations are "virtual" in nature; i.e., the fraction of the population of molecules actually found at any time within the subset of conformational space defined as the "solution conformation" is vanishingly small. A statistical mechanics approach was utilized to calculate an ensemble average relaxation matrix from which (NOE)'s and (T1)'s are calculated. Model glycosidic linkages in four oligosaccharides were studied. The solution conformation at any glycosidic linkage is properly represented by a normalized, Boltzmann distribution of conformers generated from an appropriate potential energy surface. The nature of the resultant population distributions is such that 50% of the molecular population is found within 1% of available microstates, while 99% of the molecular population occupies about 10% of the ensemble microstates, a number roughly equal to that sterically allowed. From this analysis we conclude that in many cases quantitative interpretation of NMR relaxation data, which attempts to define a single set of allowable torsion angle values consistent with the observed data, will lead to solution conformations that are either virtual or reflect torsion angle values possessed by a minority of the molecular population. On the other hand, calculation of ensemble average NMR relaxation data yields values in agreement with experimental results. Observed values of NMR relaxation data are the result of the complex interdependence of the population distribution and NOE (or T1) surfaces in conformational space. In conformational analyses, NMR data can therefore be used to test different population distributions calculated from empirical potential energy functions.

Dielectric relaxation time and structure of bound water in biological materials

Abstract: The dielectric behavior of living tissues and a number of biological materials was examined by new equipment of the time domain reflectometry method in a wide frequency range of 10/sup 7/-10/sup 10/ Hz. The authors found two peaks of Debye absorption around 100 MHz and 20 GHz for all the materials. The low-frequency absorption is probably due to bound water while the high-frequency absorption to free water. From the observed relaxation times of bound water a hypothesis is ventured on the structure of bound water and its relaxation mechanism.

Evidence for two pairing energies from nuclear spin-lattice relaxation in superconducting Ba2YCu3O7- delta.

Abstract: We report /sup 63/Cu nuclear spin-lattice relaxation data for Ba/sub 2/YC/sub 3/O/sub 7-//sub delta/ in the superconducting state obtained by nuclear quadrupole resonance in zero applied magnetic field. The temperature dependences of the relaxation rates reveal striking differences in the excitation spectra for quasiparticles on the chain-forming Cu1 and planar Cu2 lattice sites, corresponding to substantially different pairing energies for electrons on the planes and chains. Values of the gap ratio 2..delta../kT/sub c/ 8.3 and 2.4 are obtained for the Cu1 and Cu2 sites, respectively.

Proton Spin Relaxation in the Superconducting State of (TMTSF)2ClO4

Abstract: The proton spin lattice relaxation rate (1 / T 1 ) in the superconducting state of(TMTSF) 2 ClO 4 has been measured at zero magnetic field using the field cycling technique. It was found that the enhancement of 1 / T 1 just below T c , commonly observed in typical BCS superconductors, was absent and 1 / T 1 varied as T 3 . These results indicate that the superconductivity in this system is associated with the anisotropic order parameter vanishing along lines on the Fermi surface.

Progress in the non-equilibrium vibrational kinetics of hydrogen in magnetic multicusp H - ion sources

Abstract: The non-equilibrium vibrational kinetics of H 2 in multicusp magnetic discharges has been studied by improving a previous model developed by our groups. In particular, a complete set of V-T (vibrational translation) rates involving H-H 2 ( v ) collisions, calculated by using a three-dimensional dynamics approach, has been inserted into our self-consistent model for better representing the corresponding relaxation. Different experimental situations are simulated with special emphasis on the temporal scales necessary for the different distributions (electron energy and vibrational distributions) to reach stationary values. Finally, a comparison between theoretical and experimental quantities such as vibrational temperature, electron temperature, electron number density and concentration of negative ions (H − ) shows a satisfactory agreement, thus indicating the basic correctness of our model.

Monte Carlo investigation of the electron-hole-interaction effects on the ultrafast relaxation of hot photoexcited carriers in GaAs.

Abstract: The roles of the electron-hole (e-h), electron-electron (e-e), hole-hole (h-h), and screened electron-phonon (e-ph) interactions on the ultrafast relaxation of photoexcited carriers in GaAs are examined. Theoretical expressions for the various scattering rates are obtained, and these are used in an ensemble Monte Carlo calculation. At low carrier concentrations the e-ph interaction is the main energy-loss channel for hot electrons, while at high carrier concentrations the e-h interaction is the primary energy-loss channel. This latter result follows from the high e-h scattering rate, the screening of the e-ph interaction, and the high efficiency of hole-phonon scattering through the unscreened deformation-potential interaction. The electron energy-loss rates through the e-h interaction increase as the excitation energies and intensities are increased. For excitation by an excess photon energy of 130 meV, for example, it is found that the e-e interaction slows the cooling rates at all excitation levels, while the h-h interaction enhances the cooling rate of the holes.

A NMR technique for the analysis of pore structure: Application to materials with well-defined pore structure

Abstract: The application of NMR spin—lattice relaxation measurements of water contained in porous solids is investigated as a possible tool for the determination of pore size distributions. NMR methods have several potential advantages over conventional porosimetry/adsorption techniques including the study of wet porous solids, a wide range of sample sizes that may be studied, and the fact that no pore shape assumption is required. In principle, water contained in a pore will relax faster than bulk water. This decrease in the observed relaxation rate decay constant, T1, is directly related to the pore volume to surface area ratio (i.e., hydraulic radius) according to the “two-fraction, fast-exchange model.” However, the suitability of this model and NMR spin—lattice relaxation measurements for the determination of pore size distributions has not been previously demonstrated. Pore size distributions have been determined for two series of porous solids by mercury porosimetry and a NMR method. A series of four controlled-pore glasses with very narrow pore size distributions and a mean radius in the range 3.4 to 17.6 nm were studied at a frequency of 20 MHz and at four temperatures. By using four samples with similar surface chemistry/ pore shape, the validity of the two-fraction, fast-exchange model was assessed. Experiments were conducted using a 180°–τ-90° pulse sequence and the distribution of T1 was calculated using a nonnegative leastsquares routine (NNLS) presented in a previous paper. The two-fraction model described the change of mean T1 with pore radius at a given temperature. The change of the surface relaxation decay constant, T1s, fit an Arrhenius expression with an activation energy of 1.8 kcal/mole. Agreement between the mercury porosimetry and NMR derived pore size distributions is excellent. T1 measurements at both 20 and 300 MHz and 303 K were also made on a series of porous solids fabricated by pelleting submicron silica spheres. These solids exhibit afairly wide pore size distribution (⋍ one order of magnitude). The two-fraction, fast-exchange model satisfactorily described the relationship between pore size and T1. Agreement between mercury porosimetry and NMR pore size distributions is good. However, the NMR derived distributions consisted of a series of narrow peaks overlapping the single broad porosimetry peak. The surface relaxation effect, and, hence, the overall pore size sensitivity, is increased by a factor of 3 when the proton frequency is decreased from 300 to 20 MHz.

Magnetic resonance imaging of stationary blood: a review.

Abstract: The magnetic resonance imaging appearance of blood, as with other body tissues, is affected strongly by magnetic relaxation rates of the water protons. For blood containing only oxyhemoglobin, as for most tissues, the relaxation times are determined by diamagnetic effects related primarily to protein content. However blood containing either deoxyhemoglobin or methemoglobin exhibits additional paramagnetic relaxation effects, which have important consequences for magnetic resonance imaging of hematomas. First, the field inhomogeneity created by the concentration of paramagnetism in the red blood cells lowers the effective T2. This effect depends on field strength, and so is more striking at high fields, and is greater if gradient echoes are used. In fact, the observation of a difference in T2 with the two different echo methods provides an unequivocal indication of field inhomogeneity such as is produced by erythrocytes. A second paramagnetic relaxation effect is the direct interaction of protons with the electron spin of methemoglobin, which markedly lowers both T1 and T2. This effect is important in the imaging of hematomas that are at least several days old, after significant conversion of hemoglobin to the met form has taken place.

90°-domain wall relaxation in tetragonally distorted ferroelectric ceramics

Abstract: A phenomenological model of a vibrating 90°-domain wall in an electric and mechanic stress field is presented. The model allows to separate the domain wall and the volume contribution to the complex dielectric, piezoelectric and elastic constants. The results are compared with representative measurements of the above mentioned six material parameters on a Fe-doped PZT sample. It turns out that the absolute values and the temperature dependencies of the intrinsic contributions are comparable to those obtained by a thermodynamical treatment. The ratio S0/P0 (spontaneous deformation/spontaneous polarization) which can also be derived in our model from the measured losses, is in good agreement with other measuring methods. It follows that, although there is as yet no physical interpretation for the loss mechanism, the conception of a vibrating 90°-domain wall under the above described conditions is a useful path towards a better insight into the material properties of ferroelectric ceramics.

Low-temperature magnetic spectroscopy of a dilute magnetic semiconductor

Abstract: A newly developed integrated SQUID magnetic spectrometer yields direct high-resolution measurements of the optically induced magnetization in a 10 \ensuremath{\mu}m-diam sample of ${\mathrm{Cd}}_{0.8}$${\mathrm{Mn}}_{0.2}$Te. Both the magnitude and the picosecond dynamics of the magnetic response have been studied and are seen to be dramatically dependent on the energy and polarization of the optical excitation. The data show that the overall sample magnetization changes upon illumination, and that the perturbed spins equilibrate through spin-lattice relaxation.

An MRI phantom material for quantitative relaxometry.

Abstract: Most phantom media in current use exhibit T1 relaxation times that are significantly dependent on both temperature and operating frequency. This can introduce undesirable variability into relaxation measurements due to temperature fluctuations, and complicates direct comparison of imagers operating at different magnetic field strengths. Our investigations of a nickel-doped agarose gel system have demonstrated near independence of the proton relaxation rates to a wide range of temperatures and frequencies. We therefore propose the adoption of Ni2+ as a relaxation modifier for phantom materials used as relaxometry standards.

Phenylene motion in polycarbonate and polycarbonate/additive mixtures

Abstract: Pulsed deuteron NMR line shapes have been analysed to characterize type and time scale of the phenylene group motion in glassy bisphenol-A polycarbonate. The motional mechanism involvesπ-flips about theC1C4 axis augmented by small angle fulctuations about the same axis, reaching a rms amplitude of ±35‡ at 380 K. The distribution of correlation times for theπ-flips is heterogeneous in nature and can be described either by a log-Gaussian or an asymmetric distribution with a more rapid decay at high correlation times comparable to the Williams-Watts distribution. From both distributions essentailly the same mean activation energy of 37 kJ/mol is obtained, whereas the temperature dependent width of the highly asymmetric distribution is somewhat smaller compared to the log-Gaussian distribution. Time scale and activation energy of theπ-flip motion are correlated to secondary mechanical relaxations. Low molecular mass additives, which suppress the mechanical relaxation, also hinder the phenylene motion for a substantial fraction of phenylene groups. The effect of additives is not only to shift the mean value of the distribution of correlation times to higher values but also to increase drastically the width of the distribution. The results of this work strongly suggest that the secondary mechanical relaxation and the large amplitude motions of the phenylene groups in polycarbonate are related.

Evolution of rheology during chemical gelation

Abstract: Rheology is a sensitive measure of the evolving molecular structure in a crosslinking polymer. Dynamic mechanical experiments in small amplitude oscillatory shear give the storage modulus G′(ω, p) and the loss modulus G″(ω, p) as a function of frequency ω. The extent of crosslinking, p(t), changes with reaction time. Dynamic mechanical experiments allow detection of the gel point (GP) and give a macroscopic description of the critical gel state (network polymer at GP). This critical gel state is used as a reference for describing the entire evolution of rheology. The most surprising discovery of these experiments was that critical gels exhibit stress relaxation in a power law, i. e. the relaxation modulus is given as G()=St −n. The relaxation exponent, n, depends on network structure. The power law behavior is an expression of mechanical self similarity (fractal behavior). The range of self similarity is defined between an upper and a lower frequency limit. The lower frequency limit (reciprocal of characteristic relaxation time) corresponds to an upper scaling length, the correlation length, which is of the order of the linear size of the largest molecular cluster (of pre-gel) or of the largest remaining percolation cluster (of post-gel). High frequencies probe relaxation within single chains. The upper frequency limit corresponds to a lower scaling length, the glass length, which is given by the dimension of the molecular network units responsible for glassy behavior. The correlation length and, hence, the characteristic relaxation time increase in the approach of the gel point, diverge to infinity at the gel point, and then decrease again with increasing extent of crosslinking. The critical gel has no characteristic length or time scale. All observations are restricted to polymers at a temperature above the glass transition temperature and at frequencies much below the glass frequency.

Imaging of spatial radiation dose distribution in agarose gels using magnetic resonance

Abstract: Radiation dose distributions are conventionally measured using ionization chambers or diodes in liquid phantoms, or in two dimensions using film. This work describes a new application of magnetic resonance imaging to radiation dose planning. Agarose gels containing ferrous sulfate, sulfuric acid, and benzoic acid have been irradiated with /sup 137/Cs gamma rays and 6-14 MeV electrons, to doses of up to 20 Gy. The dose distributions have been imaged by magnetic resonance, making use of the effect on the T1 proton relaxation times of the radiolytic Fe/sup 3 +/. The image intensity was proportional to doses of up to 10 Gy, and images were stable for at least 24 h postirradiation. The G value for Fe/sup 3 +/ production was about 100 (molecules per 100 eV absorbed).

Dynamic behavior and some of the molecular properties of water molecules in pure water and in magnesium chloride solutions

Abstract: From the study of the relaxation rates of /sup 2/H, /sup 17/O, and /sup 1/H due to /sup 17/O in pure water the values of the interaction constants were derived and it is demonstrated that on a picosecond time scale water reorients isotropically for temperatures between -10 and 53/sup 0/C. In MgCl/sub 2/ solutions it is shown that in the cationic hydration shell the water molecule reorients anisotropically and that the effective correlation times and coupling constants of the relaxation rates are significantly changed. The anisotropic motion of the hydration water is interpreted in concordance with recent neutron diffraction results. The concentration and temperature dependence of these effects are discussed.

Transverse Nuclear Spin Relaxation in Phospholipid Bilayer Membranes

Abstract: Experimental proof is presented that some of the motions responsible for transverse relation (T/sub 2/) in deuterium magnetic resonance (/sup 2/H NMR) experiments on acyl chains of a model membrane in the liquid-crystalline phase are extremely slow on the /sup 2/H NMR time scale being characterized by a correlation time tau/sub 2/ >> 10/sup -5/ s. The experiments used to investigate these slow motions involve a form of the Carr-Purcell-Meiboom-Gill pulse sequence modified so as to be suitable for /sup 2/H NMR. The most plausible mechanism responsible for T/sub 2/ relaxation is the gradual change in the average molecular orientation due to lateral diffusion of the phospholipid molecules along curved membrane surfaces. A procedure for separating contributions to T/sub 2/ relaxation due to slow and fast motions is described.

The anomalous relaxivity of Mn3+ (TPPS4).

Abstract: It was recently reported (C-W. Chen et al., FEBS Lett.168, 70 (1984)) that water solutions of Mn3+(TPPS4) have a surprisingly high relaxivity at 20 MHz and 37oC, greater than most Mn2+ complexes including the hexaaquoion. Because Mn3+(TPPS4) is highly stable, and porphyrins in general are tumor-seeking, we have sought to understand the origin of the large relaxivity by comparing the 1/Tl NMRD profiles (magnetic field dependence of 1/T1 of solvent protons) of Mn3+(TPPS4) solutions with those of a number of other small Fe3+ and Mn2+ complexes. By relating the measured NMRD profiles to the theory of relaxation by magnetic dipolar interactions, in a form appropriate for small paramagnetic solute molecules, we establish that the theory affords an excellent quantitative description of the relaxation behavior of all the samples, and confirm that the relaxivity of Mn3+(TPPS4) is anomalously high. The effect is attributed, in part, to the anisotropy of the ground-state wavefunction of Mn3+ in the porphyrin complex, effectively bringing the spin density of the Mn3+ ions closer to the protons of the coordinated water molecules than would a spherically symmetric S-state ion. In addition, the paramagnetic relaxation time of the Mn3+ spins, though short, is longer than would be anticipated for a non-S-state ion, and increases substantially with magnetic field above about 2 MHz. In this regard, Mn3+(TPPs4) may be one of a class of molecules with properties particularly favorable for use as contrast-enhancing agents in magnetic resonance imaging. © 1987 Academic Press, Inc.