Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data

In this work we demonstrate that low-field chemically induced dynamic nuclear polarization (CIDNP) is strongly affected by re-distribution of polarization, which is formed in the course of spin evolution in transient radical pairs, in diamagnetic reaction products. This phenomenon is of importance when the spins of the reaction product are coupled strongly meaning that spin–spin interactions between them are comparable to the differences in their Zeeman interactions with the external magnetic field. In this case, polarization transfer relies on a coherent mechanism; as a consequence, spins can acquire significant polarization even when they have no hyperfine coupling to the electron spins in the radical pairs, i.e., cannot be polarized directly by CIDNP. This is demonstrated by taking CIDNP of n-butylamine as an example: in this case only the α-CH₂ protons are polarized directly, which is confirmed by high-field CIDNP, whereas the β-CH₂, γ-CH₂ and δ-CH₃ protons get polarized only indirectly due to the transfer of polarization from the α-CH₂ protons. These results show that low-field CIDNP data should be interpreted with care to discriminate between the effects of spin evolution in transient radical pairs and in diamagnetic reaction products.     

Field-dependent nuclear relaxation of spins 1/2 induced by dipole-dipole couplings to quadrupole spins: LaF3 crystals as an example

A general theory of spin-lattice nuclear relaxation of spins I=1/2 caused by dipole-dipole couplings to quadrupole spins S1, characterized by a non-zero averaged (static) quadrupole coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through dipole-dipole couplings with certain transitions of quadrupole spins. The averaged quadrupole coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual dipole-dipole coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the 'classical' Solomon relaxation theory.     

NMR multiple-quantum processes in a doubly rotating frame in solids

Multiple-quantum transitions between nuclear spin Zeeman levels of fluorine nuclei in a CaF₂ single crystal are studied by a rotary-saturation method in NMR. The theoretical studies are performed for the system consisting of spins interacting with each other through dipole-dipole coupling. The analysis of the behavior of nuclear magnetization, the second quantization of a perturbing field, and the calculation of the nth-order timedependent perturbation are all performed in the reference frame of a doubly rotating system. We found evidence that (1) one type of single spin-n quantum transitions is taking place, but another type of two spin-n quantum transitions is not confirmed yet in this crystal; (2) the linewidths of the multiple-quantum absorptions are extremely narrow at the magic angle but do not completely vanish because of the presence of a perturbing field; (3) the effects of the energy level mixing through the dipole-dipole interactions on the multiple-quantum transitions are negligible over a wide range around the magic angle; and (4) the inhomogeneity of the RF field used to quantize the spin system in the rotating frame affects considerably the results of the rotary-saturation experiments.     

How to measure diffusion constant and absolute susceptibility of charge carriers at the same time

Multiple spin echos can be excited for spin-1/2 systems by two radiofrequency pulses with tipping angles unequal to 180°, if the absolute magnetization is sufficiently large. If a magnetic field gradient is applied, the attenuation of the primary echo reveals the diffusion constant of these spins, whereas the ratio of primary and secondary echo amplitudes indicates the absolute magnetic susceptibility of the resonating spins. This potentiality is examined here numerically and by application of nuclear magnetic resonance to liquid samples containing protons. We analyse the complications that have to be overcome, if this technique is used for the study of the charge and spin carriers in low-symmetry quasi-one-dimensional organic conductors. Reasonable accuracy is obtained for the parameters derived for the conduction electrons of perylene radical cation salts.     

NMR study of fluorine and proton motion in the ionic conductor NH4Sn2F5

The nuclear spin relaxation times of protons and fluorine atoms in the ionic conductor NH₄ Sn₂ F₅ show a particular temperature behaviour: they decrease as the temperature decreases below 200 K and have the same temperature and frequency dependences.Cross relaxation experiments show an important coupling between fluorine atoms and protons. NMR results as well as crystallographic and conductivity data on hydrogenated and deuterated compounds, allow to determine two dominant kinds of motion: at low temperature the reorientation of the ammonium ion is observed and above 250 K the diffusion of the fluorine atoms is responsible for the relaxation. In the high temperature range, the activation energies of the motion deduced from conductivity and NMR measurements are in good agreement.     

Electron spin resonance of X-irradiated single crystal of calcium tartrate tetrahydrate

The electron spin resonance spectra of an irradiated single crystal of calcium tartrate tetrahydrate grown from silica gel have been investigated. Only one species of free radical was observed at room and liquid nitrogen temperatures. The free radical was found to be the result of the splitting of a CH bond adjacent to both the hydroxyl and carboxyl groups. For some orientations of the crystal in the external magnetic field two unresolved doublets, due to two noequivalent protons, was observed. The g factor was found to be almost isotropic, with a value of 2.0032 ± 0.0005. Couplings with two H nuclei are believed to be the result of the proton of the hydroxyl group attached directly to the unsaturated asymmetric carbon atom and of the proton attached directly to the other asymmetric carbon atom of the molecule. The principal elements of the nuclear coupling of these protons are. 5.8, 7.9, 3.7 and 6.8, 7.0, 17.3 G respectively. The radical was found to be very stable, the ESR pattern being undiminished for more than half a year after the irradiation.     

Transfer of CIDNP among coupled spins at low magnetic field

Coherent polarization transfer among groups of dynamically polarized spins is explored and applied to field cycling experiments where spin evolution proceeds at low magnetic field while observation is performed at high field. The case of two nonequivalent spins-1/2 with scalar spin coupling is considered theoretically in detail for the cases of sudden and adiabatic field change. The criterion for efficient polarization transfer is derived theoretically and consistently confirmed experimentally for three photochemical reactions, involving spin systems of increasing complexity that exhibit chemically induced dynamic nuclear polarization: (1) the two polarized protons of the purine base of adenosine monophosphate; (2) four coupled indole protons of tryptophan; and (3) long-range polarization transfer among the aliphatic protons of cycloundecanone. The importance of polarization transfer in other cases with non-equilibrium population of the nuclear spin levels and the possibility of its utilization in field cycling NMR studies are discussed.     

Theoretical Study of Dipolar Relaxation of Coupled Nuclear Spins at Variable Magnetic Field

A theoretical study was made of magnetic field-dependent dipolar relaxation in two- and three-spin systems. The results for the nuclear magnetic relaxation dispersion (NMRD) curves were compared with those for the simpler model of fluctuating local fields. For both models it was found that at low fields spins tend to relax with a common T ₁-relaxation time. Sharp features in the NMRD curves coming from nuclear spin level anti-crossings are also predicted by both models. However, the simple model fails to describe the behavior of so-called long-lived spin states (LLS). We have studied the LLS as function of magnetic field and molecular geometry and simulated experimental results for the LLS in histidine amino acid obtained at the laboratory of Prof. H.-M. Vieth (Free University Berlin, Germany). In addition, we described polarization transfer in a three-spin system where two spins are protons, which are initially hyperpolarized by para-hydrogen induced polarization (PHIP), while the third spin is a spin ½ hetero-nucleus, which acquires polarization in the course of cross-relaxation.     

Muon-fluorine entangled states in molecular magnets

The information accessible from a muon-spin relaxation experiment can be limited due to a lack of knowledge of the precise muon stopping site. We demonstrate here the possibility of localizing a spin polarized muon in a known stopping state in a molecular material containing fluorine. The muon-spin precession that results from the entangled nature of the muon spin and surrounding nuclear spins is sensitive to the nature of the stopping site. We use this property to identify three classes of sites that occur in molecular magnets and describe the extent to which the muon distorts its surroundings.     

EPR spectroscopy of fullerene adducts

EPR spectroscopy has been used to identify and characterize the paramagnetic adducts of small free radicals with C₆₀ through measurements of the hyperfine interactions of protons and¹³C nuclei. The initially formed mono-radical adducts (RC₆₀) have unpaired spin density localized near the point of attachment of the radical. Generally, they are in thermal equilibrium with their diamagnetic dimer, and have a surprisingly large barrier to internal rotation about the new bond. The tri- and penta-radical adducts have electronic structures similar to those of allyl and cyclopentadienyl radicals, respectively.NRCC No. 35262     

Phase modulation in dipolar-coupled A2 spin systems: effect of maximum state mixing in H NMR in vivo

Coupling constants of nuclear spin systems can be determined from phase modulation of multiplet resonances. Strongly coupled systems such as citrate in prostatic tissue exhibit a more complex modulation than AX connectivities, because of substantial mixing of quantum states. An extreme limit is the coupling of n isochronous spins (A_n system). It is observable only for directly connected spins like the methylene protons of creatine and phosphocreatine which experience residual dipolar coupling in intact muscle tissue in vivo. We will demonstrate that phase modulation of this “pseudo-strong” system is quite simple compared to those of AB systems. Theory predicts that the spin-echo experiment yields conditions as in the case of weak interactions, in particular, the phase modulation depends linearly on the line splitting and the echo time.     

Vector model of electron spin echo envelope modulation due to nuclear hyperfine and zeeman interactions

The transverse electron spin magnetization of a paramagnetic center with effective spinS=1/2 interacting with nonquadrupolar nuclei may be presented as a function of pairs of nuclei magnetization vectors which precess around the effective magnetic field directions. Each vector of the pair starts its precession perpendicular to both effective fields. The free induction decay (FID) signal is proportional to the scalar product of the vectors for nuclear spinI=1/2. The electron spin echo (ESE) signal can be described with two pairs of magnetization vectors. The ESE shape is not equal to two back-to-back FID signals except in the absence of ESE envelope modulation. A recursion relation is obtained which allows calculation of ESE signals for larger nuclear spins in the absence of nuclear quadrupole interaction. This relation can be used to calculate the time course of the ESE signal for arbitrary nuclear spins as a function of the nuclear magnetization vectors. While this formalism allows quantitative calculation of modulation from nuclei, it also provides a qualitative means of visualizing the modulation based on simple magnetization vectors.     

Determination of the relative signs of proton spin coupling constants by double irradiation

The method of Evans [1] for determining relative signs of nuclear spin coupling constants by double irradiation experiments has been extended to the case where the three coupled nuclei are all protons. It is thus possible to demonstrate from the high resolution proton magnetic resonance spectrum of the ring protons of 2-Furoic acid that the three spin coupling constants are all of like sign.     

Determination of the relative signs of proton spin coupling constants by double irradiation

The method of Evans [1] for determining relative signs of nuclear spin coupling constants by double irradiation experiments has been extended to the case where the three coupled nuclei are all protons. It is thus possible to demonstrate from the high resolution proton magnetic resonance spectrum of the ring protons of 2-Furoic acid that the three spin coupling constants are all of like sign.     

Origin of the NMR signal in liquid water

An experimental method for testing alternative hypotheses about the origin of the NMR signal in liquid water, i.e., whether it is generated by (a) protons 1H with spin 1/2 of H₂O molecules or (b) orthomolecules with spin 1, containing parallel spins and constituting three-quarters of the total number of H₂O molecules, is proposed.     

Measurement of the nuclear spin diffusion barrier around Eu2+ ions in CaF2

The NMR spectrum of discrete ¹⁹F nuclear spins surrounding paramagnetic Eu²⁺ impurities in CaF₂ is observed at 1.6 K by high-sensitivity NMR. All but the very nearest nuclei are found to have an appreciable thermal contact with the bulk nuclei. A microscopic analysis of the electron-nuclear and nuclear-nuclear spin coupling mechanisms gives results in accord with this observation.     

Electron spin resonance in X-irradiated single crystal of strontium tartrate tetrahydrate

The electron spin resonance spectra of an irradiated single crystal of strontium tartrate tetrahydrate (SrC₄H₄O₆·4H₂O) grown from silica gel have been investigated. Two species of free radical were observed at room and liquid nitrogen temperatures. The free radicals were found to be the result of the splitting of a C-H bond of the tartrate ions contained in the unit cell. The unpaired electron of each radical specie interacts with two nonequivalent protons giving rise to hyperfine splittings of relatively small magnitude. The g factors of the radicals were found to be almost isotropic and have similar values and it was very difficult to determine the ESR parameters from experimental patterns directly. A least-squares method, proposed earlier, was used to analyze the observed experimental patterns at various orientations.The existence of two nonequivalent radicals is believed to be the result of the nonplanarity of the two halves of the tartrate ions. The H nuclei giving rise to the hyperfine splittings are the protons of the hydroxyl groups attached directly to the unsaturated carbon atoms and those attached directly to the other asymmetric carbon atoms (β protons). The principal elements of the hyperfine splittings for the observed radicals were: Radical I: ?9.02, ? 2.84, ?0.47 and ?1.66, ?1.54, ?0.89. Radical II: ?10.16, ?7.83, ? 1.51 and 0.00, ?1.58, ?1.06. The radicals were found to be very stable, the ESR patterns being undiminished for more than six months after the irradiation.     

Determination of signs of hyperfine coupling constants for an S = ½ system through E.P.R., ENDOR and double ENDOR

A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of ⁷⁵As in the AsO₄ ^4- radical in KH₂AsO₄.