Spatial quantification of Mn2+ and Mn3+ concentrations in the Mn-catalyzed 1,4-cyclohexanedione/acid/bromate reaction using magnetic resonance imaging

This paper presents a magnetic resonance imaging (MRI) method for quantifying the spatial concentrations of the paramagnetic species Mn2+ and Mn3+, during pattern formation, in the manganese-catalyzed 1,4-cyclohexanedione-acid-bromate reaction. The relaxivity of each species was measured, which enabled their concentration to be calculated from nuclear magnetic resonance T2 relaxation times. Images were acquired using the fast spin-echo imaging method RARE. By determining the relation, in these experiments, between signal intensity and T2 relaxation time, it was possible to produce T2 maps from the images. These T2 maps were then used to produce concentration maps for the oxidized (Mn3+) and reduced (Mn2+) states of the catalyst.     

Nuclear magnetic resonance studies of convection in the 1,4-cyclohexanedione-bromate-acid reaction

The manifestation and development of convection during pattern formation in the 1,4-cyclohexanedione-acid-bromate reaction was investigated using pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) experiments. An apparatus was devised that enabled convection to be probed inside an NMR spectrometer and prevented hydrodynamic motion arising from extraneous sources, such as poor mixing or temperature gradients imposed by the experimental setup. PGSE experiments were performed concurrently with magnetic resonance imaging (MRI) experiments to show that convection arose spontaneously from inhomogeneities associated with the chemical patterns. Quantitative data on diffusion coefficients and hydrodynamic velocities are reported.     

Study of weak Mn2+ and Cu2+ complexes by a nuclear magnetic resonance method

An NMR method is described and applied, which allows the investigation of the first coordination sphere of certain transition metal ions. It is based on the measurement of the solvent proton nuclear magnetic relaxation times both as a function of magnetic field and of concentration of an admixed diamagnetic salt. This procedure enables an unambiguous separation of dynamic effects from ion-pair formation effects, which both can influence the relaxation times. As an application of this method, the complex formation of Mn²⁺ and Cu²⁺ with Br^–, I^–, ClO ₄ ^– , and SO ₄ ^2– in aqueous solution is studied. Thus the hydration numbersn _H2O of these cations as a function of anion concentration are obtained, allowing the detection of weak inner-sphere or outersphere complexes. Also several complex stability constants are derived and are compared with literature data.     

The role of the Mg2+ cation in ATPsynthase studied by electron paramagnetic resonance using VO2+ and Mn2+ paramagnetic probes

The electron paramagnetic resonance (EPR), electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectra of Mg2+-depleted chloroplast F1-ATPase substituted with stoichiometric VO2+ are reported. The ESEEM and HYSCORE spectra of the complex are dominated by the hyperfine and quadrupole interactions between the VO2+ paramagnet and two different nitrogen ligands with isotropic hyperfine couplings /A1/ = 4.11 MHz and /A2/ = 6.46 MHz and nuclear quadrupole couplings e2qQ1 approximately 3.89-4.49 MHz and e2qQ2 approximately 1.91-2.20 MHz, respectively. Aminoacid functional groups compatible with these magnetic couplings include a histidine imidazole, the epsilon-NH2 of a lysine residue, and the guanidinium group of an arginine. Consistent with this interpretation, very characteristic correlations are detected in the HYSCORE spectra between the 14N deltaM1 = 2 transitions in the negative quadrant, and also between some of the deltaM1 = 1 transitions in the positive quadrant. The interaction of the substrate and product ADP and ATP nucleotides with the enzyme has been studied in protein complexes where Mg2+ is substituted for Mn2+. Stoichiometric complexes of Mn x ADP and Mn x ATP with the whole enzyme show distinct and specific hyperfine couplings with the 31P atoms of the bonding phosphates in the HYSCORE (ADP, A(31Pbeta) = 5.20 MHz: ATP, A(31Pbeta) = 4.60 MHz and A(31Pgamma) = 5.90 MHz) demonstrating the role of the enzyme active site in positioning the di- or triphosphate chain of the nucleotide for efficient catalysis. When the complexes are formed with the isolated alpha or beta subunits of the enzyme, the HYSCORE spectra are substantially modified, suggesting that in these cases the nucleotide binding site is only partially structured.     

Fast inspection of fruits using nuclear magnetic resonance spectroscopy

High-resolution nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for obtaining chemical structure information. Its quantitative and noninvasive properties have led to its growing popularity as an analytical tool in many fields, including biology, chemistry, medicine, and food science. During transportation and storage, chemical reactions among the many nutrients lead to a loss of food quality. In these circumstances, portable NMR spectrometers can readily be used for food inspection and quality control. Because of the heterogeneous tissue distribution in food, a high-resolution NMR method is required for detailed food inspection. Therefore, in this study, we demonstrated the feasibility of using an intermolecular double-quantum coherence signal to obtain high-resolution metabolic profiles of several fruits, including grape, cantaloupe, tomato, and watermelon. The resulting high-resolution NMR spectra facilitate the identification of important metabolites, which can be used as biomarkers for food quality control. The method established here may be adapted for food inspection using portable NMR equipment.     

Investigations on the electron paramagnetic resonance parameters for Mn2+ in the ABF3 fluoroperovskites

The electron paramagnetic resonance (EPR) parameters (g factor, the hyperfine structure constant A and the superhyperfine parameters A' and B') for Mn(2+) in the fluoroperovskites ABF(3) (A=K and Cs; B=Zn, Mg, Cd and Ca) are theoretically investigated from the perturbation formulas of these parameters for a 3d(5) ion under ideal octahedra. In the above treatments, not only the crystal-field mechanism but also the charge transfer mechanism is considered uniformly on the basis of the cluster approach. The theoretical EPR parameters are in good agreement with the experimental data. The charge transfer contribution to the g-shift Δg (≈g-g(s), where g(s)≈2.0023 is the spin-only value) is opposite (positive) in sign and comparable in magnitude to the crystal-field one. Nevertheless, the charge transfer contribution to the hyperfine structure constant shows the same sign and about 10% that of the crystal-field one. So, the conventional argument that the charge transfer contributions to the zero-field splittings are negligible for 3d(5) ions under low symmetrically distorted fluorine octahedra is proved no longer valid for the Δg analysis of ABF(3):Mn(2+) in view of the dominant second-order charge transfer perturbation terms. The unpaired spin densities of the fluorine 2s, 2p σ and 2p π orbitals are determined from the quantitative dependences upon the related molecular orbital coefficients, rather than obtained by fitting the observed superhyperfine parameters in the previous works.     

Quadrupolar nuclear magnetic resonance spectroscopy in solids using frequency-swept echoing pulses

The acquisition of ideal powder line shapes remains a recurring challenge in solid-state wideline nuclear magnetic resonance (NMR). Certain species, particularly quadrupolar spins in sites associated with large electric field gradients, are difficult to excite uniformly and with good efficiencies. This paper discusses some of the opportunities that arise upon departing from standard spin-echo excitation approaches and switching to echo sequences that use low-power, frequency-swept radio frequency (rf) pulses instead. The reduced powers demanded by such swept rf fields allow one to excite spins in different crystallites efficiently and with orientation-independent pulse angles, while the large bandwidths of interest that are needed by the measurement can be covered, thanks to the use of broadband frequency sweeps. The fact that the spins' evolution and ensuing dephasing starts at the beginning of such rf manipulation calls for the use of spin-echo sequences; a number of alternatives capable of providing the desired line shapes both in the frequency and in the time domains are introduced and experimentally demonstrated. Sensitivity- and lineshape-wise these experiments are competitive vis-a-vis current implementations of wideline quadrupolar NMR based on hard rf pulses; additional opportunities that may derive from these ideas are also briefly discussed.     

Study of Mn2+ and Fe2+ in the structure of monticellite by the electron paramagnetic resonance method

Journal of Structural Chemistry -     

Theoretical investigation of electron paramagnetic resonance spectra and local structure distortion for Mn2+ ions in CaCO3:Mn2+ system: a simple model for Mn2+ ions in a trigonal ligand field

This paper reports on a novel application of a ligand field model for the detection of the local molecular structure of a coordination complex. By diagonalizing the complete energy matrices of the electron-electron repulsion, the ligand field and the spin-orbit coupling for the d5 configuration ion in a trigonal ligand field, the local distortion structure of the (MnO6)10- coordination complex for Mn2+ ions doped into CaCO3, have been investigated. Both the second-order zero-field splitting parameter b(0)2 and the fourth-order zero-field splitting parameter b(0)4 are taken simultaneously in the structural investigation. From the electron paramagnetic resonance (EPR) calculations, the local structure distortion, DeltaR=-0.169 A to -0.156 A, Deltatheta=0.996 degrees to 1.035 degrees for Mn2+ ions in calcite single crystal, DeltaR=-0.185 A to -0.171 A, Deltatheta=3.139 degrees to 3.184 degrees for Mn2+ ions in travertines, and DeltaR=-0.149 A to -0.102 A, Deltatheta=0.791 degrees to 3.927 degrees for Mn2+ ions in shells are determined, respectively. These results elucidate a microscopic origin of various ligand field parameters which are usually used empirically for the interpretation of EPR and optical absorption experiments. It is found that the theoretical results of the EPR and optical absorption spectra for Mn2+ ions in CaCO3 are in good agreement with the experimental findings. Moreover, to understand the detailed physical and chemical properties of the doped CaCO3, the theoretical values of the fourth-order zero-field splitting parameters b(0)4 for Mn2+ ions in travertines and shells are reported first.     

Infrared spectroscopy of Mn+ (H2O) and Mn2+ (H2O) via argon complex predissociation

Singly and doubly charged manganese-water cations, and their mixed complexes with attached argon atoms, are produced by laser vaporization in a pulsed nozzle source. Complexes of the form Mn(+)(H(2)O)Ar(n) (n = 1-4) and Mn(2+)(H(2)O)Ar(4) are studied via mass-selected infrared photodissociation spectroscopy, detected in the mass channels corresponding to the elimination of argon. Sharp resonances are detected for all complexes in the region of the symmetric and asymmetric stretch vibrations of water. With the guidance of density functional theory computations, specific vibrational band resonances are assigned to complexes having different argon attachment configurations. In the small singly charged complexes, argon adds first to the metal ion site and later in larger clusters to the hydrogens of water. The doubly charged complex has argon only on the metal ion. Vibrations in all of these complexes are shifted to lower frequencies than those of the free water molecule. These shifts are greater when argon is attached to hydrogen and also greater for the dication compared to the singly charged species. Cation binding also causes the IR intensities for water vibrations to be much greater than those of the free water molecule, and the relative intensities are greater for the symmetric stretch than the asymmetric stretch. This latter effect is also enhanced for the dication complex.     

Neutron spectroscopy and magnetic relaxation of the Mn6 nanomagnets

Inelastic neutron scattering has been used to determine the microscopic Hamiltonian describing two high-spin variants of the high-anisotropy Mn₆ nanomagnet. The energy spectrum of both systems is characterized by the presence of several excited total-spin multiplets partially overlapping the S = 12 ground multiplet. This implies that the relaxation processes of these molecules are different from those occurring in prototype giant-spin nanomagnets. In particular, we show that both the height of the energy barrier and resonant tunnelling processes are greatly influenced by low-lying excited total-spin multiplets.