Long‐Lived States of Magnetically Equivalent Spins Populated by Dissolution‐DNP and Revealed by Enzymatic Reactions

Hyperpolarization by dissolution dynamic nuclear polarization (D ‐DNP) offers a way of enhancing NMR signals by up to five orders of magnitude in metabolites and other small molecules. Nevertheless, the lifetime of hyperpolarization is inexorably limited, as it decays toward thermal equilibrium with the nuclear spin‐lattice relaxation time. This lifetime can be extended by storing the hyperpolarization in the form of long‐lived states (LLS) that are immune to most dominant relaxation mechanisms. Levitt and co‐workers have shown how LLS can be prepared for a pair of inequivalent spins by D ‐DNP. Here, we demonstrate that this approach can also be applied to magnetically equivalent pairs of spins such as the two protons of fumarate, which can have very long LLS lifetimes. As in the case of para‐hydrogen, these hyperpolarized equivalent LLS (HELLS) are not magnetically active. However, a chemical reaction such as the enzymatic conversion of fumarate into malate can break the magnetic equivalence and reveal intense NMR signals.     

Polymerization of aromatic nuclei. XXV. ESR and chemical studies on the radical cation nature of poly(p-phenylene)

The radical cation nature of poly(p-phenylene) (PPP) was examined by electron spin resonance (ESR) and chemical means. ESR studies revealed a radical concentration of 1.0 × 10²¹ spins/g for the crude polymer. Workup with aqueous acid decreased the value to 1.5 × 10¹⁸ spins/g. Reactions of the polymer with certain nucleophiles followed the half-regeneration mechanism, whereas with others, electron transfer mainly occurred. The origin of halogen in the polymer was found to arise from reaction of the radical cation with the oxidant, and not with halide during workup. Oxidation of PPP with various species increased the concentration of radical cations.     

Slow relaxation of longitudinal multispin orders in weakly and strongly coupled two‐spin systems

Longitudinal multispin order (LOMO) corresponds to a nonequilibrium population distribution in spin systems that exhibit scalar (J), dipolar, or quadrupolar coupling. We investigated the relaxation of longitudinal two‐spin order (2‐LOMO) in systems that had either weakly or strongly J‐coupled spins. Our results indicated longer relaxation times for the 2‐LOMO state compared with the corresponding longitudinal single‐spin state (1‐LOMO). Accessing nuclear spin states that have relaxation times longer than T₁, without the use of external contrast agents, is potentially useful for in vivo imaging and also for studying systems using dynamically hyperpolarized nuclear spins where longer life times are sought to increase the time available to study (bio)chemical events. Copyright © 2012 John Wiley & Sons, Ltd.     

Hyperpolarization of cis-15N2-Azobenzene by Parahydrogen at Ultralow Magnetic Fields**

The development of nuclear spins hyperpolarization, and the search for molecules that can be efficiently hyperpolarized is an active area in nuclear magnetic resonance. In this work we present a detailed study of SABRE SHEATH (signal amplification by reversible exchange in shield enabled alignment transfer to heteronuclei) experiments on ¹⁵N₂-azobenzene. In SABRE SHEATH experiments the nuclear spins of the target are hyperpolarized through transfer of spin polarization from parahydrogen at ultralow fields during a reversible chemical process. Azobenzene exists in two isomers, trans and cis. We show that all nuclear spins in cis-azobenzene can be efficiently hyperpolarized by SABRE at suitable magnetic fields. Enhancement factors (relative to 9.4 T) reach up to 3000 for ¹⁵N spins and up to 30 for the ¹H spins. We compare two approaches to observe either hyperpolarized magnetization of ¹⁵N/¹H spins, or hyperpolarized singlet order of the ¹⁵N spin pair. The results presented here will be useful for further experiments in which hyperpolarized cis-¹⁵N₂-azobenzene is switched by light to trans-¹⁵N₂-azobenzene for storing the produced hyperpolarization in the long-lived spin state of the ¹⁵N pair of trans-¹⁵N₂-azobenzene.     

Real-space analysis of the exchange-correlation energy

The exchange-correlation energy of a many-electron system may be written as the electrostatic interaction between the electron density at position r and the density of the exchange-correlation hole at position r + u. If we average the hole over the entire system, we find that the energy is uniquely decomposed into contributions from various electronic separations u. We may also decompose the hole into contributions from parallel and antiparallel spins. We give several exact conditions which this system-averaged, spindecomposed exchange-correlation hole satisfies. Local spin density (LSD ) and generalized gradient approximations (GGAS ), are more appropriate for u → 0 than for large u and more trustworthy for antiparallel spins than for parallel spins. We illustrate how good LSD is as u = 0 with explicit examples, but also note that, contrary to expectation, LSD is not exact for u=0, except in certain limiting cases. We show that the dramatic failure of the second-order gradient expansion for large u can be cured by a real-space cutoff procedure which generates a nonempirical GGA, the Pw91 functional. We conclude with some thoughts about the search for greater accuracy in the next 30 years of density functional theory. © 1995 John Wiley & Sons, Inc.     

Unitary symmetry and classification of the states of <Emphasis Type="Italic">n</Emphasis>-spin clusters. Magnetic and thermodynamic parameters

The unitary symmetry and classification of spin clusters by spin momenta S are considered on the basis of reduction of the full linear group to unitary groups U _{2s + 1} and orthogonal rotation group R ₃. Reduction of the permutation group P _n of n spins to the point group of the cluster is applied to the classification of the spatial states of a spin cluster with the use of permutation quantum numbers introduced in this work and the Young diagrams of the permutation group P _n . Examples of the classification of spin systems with spins s = 1/2, 1, 3/2, 2, and 5/2 with U _{2s + 1} × P _n groups (n = 5–15) are reported. This classification is common for all spin clusters and is the same for both cyclic clusters and 3D clusters with symmetry groups of a crystal. On the basis of this classification, the magnetic and thermodynamic parameters of a spin system are calculated as a function of the number of spins and temperature. For s = 1/2 clusters, the analytical formulas are derived for magnetic susceptibility, internal energy, heat capacity, and entropy as a function of quantum numbers for a cluster with any number of spins, and their dependences on temperature and the number of atoms are considered.     

Carbogenic Nanodots: Photoluminescence and Room‐Temperature Ferromagnetism

Herein, blue fluorescent carbogenic nanodots (CNDs) with room‐temperature ferromagnetism were synthesized by thermal decomposition of organic precursors at different temperatures. Photoluminescence (PL) studies show excitation‐wavelength‐dependent emission properties and PL excitation (PLE) studies confirm the triplet ground state of carbene at the zigzag edge as the fluorescent center. Room‐temperature magnetic studies reveal the ferromagnetic nature of CNDs and temperature‐dependent studies show the presence of an antiferromagnetic phase along with a ferromagnetic phase below 50 K. EPR studies reveal the presence of conduction electrons and localized spins with different g factors. Localized spins at zigzag edges are the origin of the unconventional magnetic behavior, whereas exchange coupling between conduction and localized spins are responsible for long‐range magnetic ordering.     

Synthesis,characterization, and photolysis of poly[3,5-di-tert-butyl-4-[(2,4,6-tri-tert-butylphenyl)oxalato]phenylacetylene], a photochemical polyradical precursor

A photochemical precursor to a pendant conjugated polyradical has been synthesized, poly[3,5-di-tert-butyl-4-[(2,4,6-tri-tert-butylphenyl)oxalato]phenylacetylene], 3 . Irradiation of 3 at 77 K in the solid state at < 300 nm yielded poly(3–5-di-tert-butyl-2-oxyphenyl acetylene), 2 , with 30–40% of the expected number of radical spins. Spin yields on the surface of solid samples appears to be considerably higher. Electron spin resonance experiments showed no evidence of cooperative exchange interaction between the pendant spins. Computational modeling indicated that a major reason for the failure of this and other polyphenylacetylenes to show ferromagnetic exchange between spins is the substantial twisting of the polyacetylene backbone required by steric interactions, leading to deconjugation and a loss of exchange interaction between pendant radicals along the chain. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2167–2176, 1997     

Model universal coulomb hole function for many-electron systems

In the framework of the homogeneous electron gas theory we give a model function G_c of the Coulomb hole that can be considered as an approximate universal correlation function for many-electron systems. The function G_c reflects the right asymptotic behavior of the correlation function of an electron gas in high and low density limits and enables one to reproduce experimental correlation energies of a number of atoms of the first and second periods in a local density approximation with the relative error 0.3–4.2%. The estimate of contributions of electrons with parallel or antiparallel spins into correlation energy shows that in the domain of densities typical for atoms of the first and second periods, the Coulomb correlation of electrons with parallel spins is in high extent suppressed by the Fermi correlation.     

Background-Free Detection of Spin-Exchange Dynamics at Ultra-Low Magnetic Field

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) inherently suffer from a low signal-to-noise ratio due to the small thermal polarization of nuclear spins. Transfer of polarization from a pre-polarized spin system to a thermally polarized spin system via the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) could potentially be used to overcome this limitation. SPINOE is particularly advantageous at ultra-low magnetic field, where the transferred polarization can be several orders of magnitude higher than thermal polarization. Here we demonstrate direct detection of polarization transfer from highly polarized ¹²⁹Xe gas spins to ¹H spins in solution via SPINOE. At ultra-low field, where thermal nuclear spin polarization is close to background noise levels and where different nuclei can be simultaneously detected in a single spectrum, the dynamics of the polarization transfer can be observed in real time. We show that by simply bubbling hyperpolarized ¹²⁹Xe into solution, we can enhance ¹H polarization levels by a factor of up to 151-fold. While our protocol leads to lower enhancements than those previously reported under extreme Xe gas pressures, the methodology is easily repeatable and allows for on-demand enhanced spectroscopy. SPINOE at ultra-low magnetic field could also be employed to study ¹²⁹Xe interactions in solutions.     

The Fock Matrix Analysis for Atomic Orbitals in Molecular Orbitals II. The Electronic Structure of N2 Molecules

Weinhold's natural hybrid orbitals can be chosen as the molecular adapted atomic orbitals to build the canonical molecular orbitals of N₂ molecules. The molecular Fock matrix expanded in the natural hybrid orbitals can reveal deeper insight of the electronic structure and reaction of the N₂ molecule. For example, the magnitude of F_ab can signify the bonding character of the paired electrons as well as the diradical character of the unpaired electrons for both σ‐ and π‐types. Discarding the concept of the overlap between non‐orthogonal atomic orbitals, the different orbitals for different spins in the unrestricted Hartree‐Fock wavefunction reveal that there are three pairs of opposite spin density flows between two atoms, which proceed until the bonding molecular orbitals form.     

Electron spin resonance studies on Ziegler-Natta type catalyst systems

Electron spin resonance spectra of Ziegler-Natta type catalyst systems (titanium trichloride and alkyl aluminums) have been studied. When triethylaluminum was added to titanium trichloride, an absorption with a g value of 1.96 was observed from the solid phase of the system. The number of spins of this signal is equal to that of the active centers for propylene polymerization. The solid phase from the mixture of titanium trichloride and diethylaluminum chloride also gives the same signal, which suggests the identity of the action of two alkyl aluminums on titanium trichloride surface. The signals obtained from the liquid phase of the both systems are all ascribed to the reaction products of the reaction of alkyl aluminum with titanium tetrachloride occluded in solid.     

Asymmetric NMR spectra of weakly coupled nuclei: A common characteristic in the 13C NMR spectra of [U-13C]-labeled molecules and in natural abundance protoncoupled 13C spectra

Typically encountered proton-decoupled spectra of [U-¹³C]-labeled molecules or proton-coupled spectra of natural abundance or singly labeled molecules contain many nuclei which satisfy the weak coupling approximation. The spectra of such nuclei are frequently highly asymmetric and often appear to exhibit the skewed intensity distribution characteristic of strongly coupled spins. Analysis of typical cases indicates that such effects arise in the presence of at least one moderately strong coupling interaction in the spin system (Jδν ～ 1/3), and the apparent intensity asymmetry reflects small differences in the spacing of unresolved components of the observed resonance. This effect is analogous to the case of ‘virtual coupling’ in which weakly coupled spins cannot be analyzed as first order spectra; however; the ABX spin system which generally serves as a model for such systems does not predict the existence of spectral asymmetry for the X resonances. Prediction of this asymmetry requires a spin system of at least four spins with at least ABMX complexity, and can be treated generally using the effective Hamiltonian approach of Poeple and Schaefer.     

Computer-aided Modeling of the Time Dependences of the Current during Anisotropic Growth of Product Nuclei

A fast computer model, intended for the calculation of the overall reaction rate (current) of anisotropic or nonhomotetic growth of a new-phase nuclei on the basis of the Voronoi diagram, is designed. The model is used for studying the kinetics of a heterogeneous reaction in the conditions where hemispherical nuclei of the new phase acquire a semiellipsoid shape in the course of an anisotropic growth. The calculation of current transients (potentiostatic i vs. t dependences) is substantially complicated in the initial stage of reaction, where the size of growing nuclei exceeds the critical value by less than an order of magnitude. If semiellipsoid nuclei overlap, the overall reaction rate is not determined by variations in the overall area of the reaction surface, as opposed to the growth of hemispherical nuclei. The kinetics of a nonhomotetic nuclei growth may be described by models designed for an isotropic growth of hemispherical nuclei.     

Comparison of 13C- and 1H-Magnetic Resonance Spectroscopy as Techniques for the Quantitative Investigation of Dynamic Processes. The cope rearrangement in bullvalene

The potential advantages of ¹³C-(¹H-noise decoupled) spectroscopy (in the Fourier transform mode) over ¹H-spectroscopy for the quantitative investigation of molecular dynamic process is discussed. The Cope rearrangement in bullvalene, an example of complex exchange of spins over different magnetic sites, has been studied by both kinds of spectroscopy as a test.     

Solid-state photochemical generation of polymeric polyradicals: Poly(4-vinylphenoxy) and copolymers with styrene

Synthesis of poly(4′-vinyl-2,4,6-tri-tert-butyl-diphenyloxalate) and its 65 : 35 and 19 : 81 copolymers with styrene are described. Rigid phase broad-band photolysis of the homopolymer with a quartz filtered xenon arc at 77 K results in production of up to 25% of the theoretical number of spins/mol expected for quantitative production of pendant phenoxyl radicals, and shows no major loss of radical signal in the ESR at temperatures below 100 K. Curie law analysis of the temperature dependence of the ESR radical signal intensity for the neat photolyzed homopolymer 1 shows curvature consistent with antiferromagnetic pairing of radical spins at low temperatures. Since through-bond conjugation of radical spins is not possible in this system, the antiferromagnetic interaction is interpreted in terms of intrachain and/or interchain through-space exchange interactions.     

Examination of the Hund rule in closed-shell systems: Investigation of spin correlation effects

The validity of the Hund rule in atomic orbitals (AO s) of the carbon atoms inside closed-shell molecules, such as acetylene, ethylene, and ethane, is examined. Electron-pair populations and contributions of the two-electron covalent structures with parallel (?) and antiparallel (↑↓) spins are calculated by multielectron population analysis of MO + CI wave functions. Such an analysis, which allows the visualization of various cooperative electronic effects in some target AO s, is extended on the basis of (strictly orthogonal) hybrid orbitals. Although the HF level shows, incorrectly, that the Hund rule is not satisfied, the CI clearly shows a preference for (?) spins to those of (↑↓): This holds for both the electron-pair populations [those of (↑↓) spins diminish with the CI more drastically than those of (?) spins], as well as for contributions of the two-electron covalent structures [those of (?) spins increase with the CI more drastically than do those of (?) spins]. The calculation of the correlation functions (or dependent functions) in AO space allows the comparison of repulsive or “attractive” behaviour of (?) and (↑↓) spins in various AO couples. Mutual dependence of the two electrons inside the sigma system increases in the series ethane < ethylene < acetylene. Also found is that parallel spins in (pure) AO s of sigma systems are preferred to the antiparallel spins when going from ethane to acetylene. The preference parallel–antiparallel spins in AO s belonging to two different atoms, including hydrogens, is also examined. © 1993 John Wiley & Sons, Inc.     

Molecular design,synthesis, and magnetic characterization of poly(phenylacetylene) with pi-toporegulated pendant nitronyl nitroxide radicals as models for organic superpara- and ferromagnets

The present article describes the synthesis and magnetic properties of poly(phenylacetylene) ( 3 ) with pi-toporegulated pendant stable 4,4,5,5-tetramethylimidazolin-3-oxide-1-yloxyl radicals topologically participating in the pi-conjugated system of poly(phenylacetylene). Polyradical 3 was prepared by the condensation reaction of poly(p-ethynylbenzaldehyde) with 2,3-bis(hydroxylamino)-2,3-dimethylbutane followed by oxidation with lead dioxide. The spin concentration of 3 determined by the ESR spectroscopic method was 1.1 × 10²¹ spins/g. This value approximately corresponds to 0.5 unpaired electron spin per the repeating unit. A powder ESR spectrum of 3 gave a 100% Lorentzian single line showing spin-exchange narrowing. The vibrating sample magnetometer (VSM) measurement of 3 afforded a straight line with a positive slope, suggesting that 3 has predominant paramagnetic properties within an experimental error.