Chemically Induced Dynamic Nuclear Polarization Effects Research Articles

New insights into the nature of short-lived paramagnetic intermediates of ketoprofen. Photo-CIDNP study

The short-lived paramagnetic particles formed during the UV irradiation of nonsteroidal anti-inflammatory drug - ketoprofen (KP) have been investigated using chemically induced dynamic nuclear polarization (CIDNP). This study answers the questions about the nature of paramagnetic particles which can be responsible for KP phototoxic effects. Phototoxic side effects of NSAIDs, according modern point of view, are associated with the action of free radicals, however, there is insufficient information regarding the nature of the radical species. In contrast, most ketoprofen photodegradation schemes include carbanion as a precursor of products. CIDNP effects analysis has shown that all the major products of KP photodegradation can form via radical pairs (RPs) involving benzyl (2′), ketyl (3′) and CO2H or CO2− free radicals and solvated electron. Radical ways of KP photodegradation include: decarboxylation via RP with benzyl radical formation (I) in non-aqueous solution and both, (I) and photoreduction with formation of ketyl radicals in the presence of water. Moreover, it was found that the photoinduced radical decarboxylation of KP represents a reversible process.

Level crossing analysis of chemically induced dynamic nuclear polarization: Towards a common description of liquid-state and solid-state cases.

Chemically Induced Dynamic Nuclear Polarization (CIDNP) is an efficient method of creating non-equilibrium polarization of nuclear spins by using chemical reactions, which have radical pairs as intermediates. The CIDNP effect originates from (i) electron spin-selective recombination of radical pairs and (ii) the dependence of the inter-system crossing rate in radical pairs on the state of magnetic nuclei. The CIDNP effect can be investigated by using Nuclear Magnetic Resonance (NMR) methods. The gain from CIDNP is then two-fold: it allows one to obtain considerable amplification of NMR signals; in addition, it provides a very useful tool for investigating elusive radicals and radical pairs. While the mechanisms of the CIDNP effect in liquids are well established and understood, detailed analysis of solid-state CIDNP mechanisms still remains challenging; likewise a common theoretical frame for the description of CIDNP in both solids and liquids is missing. Difficulties in understanding the spin dynamics that lead to the CIDNP effect in the solid-state case are caused by the anisotropy of spin interactions, which increase the complexity of spin evolution. In this work, we propose to analyze CIDNP in terms of level crossing phenomena, namely, to attribute features in the CIDNP magnetic field dependence to Level Crossings (LCs) and Level Anti-Crossings (LACs) in a radical pair. This approach allows one to describe liquid-state CIDNP; the same holds for the solid-state case where anisotropic interactions play a significant role in CIDNP formation. In solids, features arise predominantly from LACs, since in most cases anisotropic couplings result in perturbations, which turn LCs into LACs. We have interpreted the CIDNP mechanisms in terms of the LC/LAC concept. This consideration allows one to find analytical expressions for a wide magnetic field range, where several different mechanisms are operative; furthermore, the LAC description gives a way to determine CIDNP sign rules. Thus, LCs/LACs provide a consistent description of CIDNP in both liquids and solids with the prospect of exploiting it for the analysis of short-lived radicals and for optimizing the polarization level.

Impact of chirality on the photoinduced charge transfer in linked systems containing naproxen enantiomers.

The model reaction of photoinduced donor-acceptor interaction in linked systems (dyads) has been used to study the comparative reactivity of a well-known anti-inflammatory drug, (S)-naproxen (NPX) and its (R)-isomer. (R)- or (S)-NPX in these dyads is linked to (S)-N-methylpyrrolidine (Pyr) using a linear or cyclic amino acid bridge (AA or CyAA), to give (R)-/(S)-NPX-AA-(S)-Pyr flexible and (R)-/(S)-NPX-CyAA-(S)-Pyr rigid dyads. The donor-acceptor interaction is reminiscent of the binding (partial charge transfer, CT) and electron transfer (ET) processes involved in the extensively studied inhibition of the cyclooxygenase enzymes (COXs) by the NPX enantiomers. Besides that, both optical isomers undergo oxidative metabolism by enzymes from the P450 family, which also includes ET. The scheme proposed for the excitation quenching of the (R)- and (S)-NPX excited state in these dyads is based on the joint analysis of the chemically induced dynamic nuclear polarization (CIDNP) and fluorescence data. The (1)H CIDNP effects in this system appear in the back electron transfer in the biradical-zwitterion (BZ), which is formed via dyad photoirradiation. The rate constants of individual steps in the proposed scheme and the fluorescence quantum yields of the local excited (LE) states and exciplexes show stereoselectivity. It depends on the bridge's length, structure and solvent polarity. The CIDNP effects (experimental and calculated) also demonstrate stereodifferentiation. The exciplex quantum yields and the rates of formation are larger for the dyads containing (R)-NPX, which let us suggest a higher contribution from the CT processes with the (R)-optical isomer.

Spin Chemistry Investigation of Peculiarities of Photoinduced Electron Transfer in Donor–Acceptor Linked System

Photoinduced intramolecular electron transfer in linked systems, (R,S)- and (S,S)-naproxen-N-methylpyrrolidine dyads, has been studied by means of spin chemistry methods [magnetic field effect and chemically induced dynamic nuclear polarization (CIDNP)]. The relative yield of the triplet state of the dyads in different magnetic field has been measured, and dependences of the high-field CIDNP of the N-methylpyrrolidine fragment on solvent polarity have been investigated. However, both (S,S)- and (R,S)-enantiomers demonstrate almost identical CIDNP effects for the entire range of polarity. It has been demonstrated that the main peculiarities of photoprocesses in this linked system are connected with the participation of singlet exciplex alongside with photoinduced intramolecular electron transfer in chromophore excited state quenching.

A Tribute to Renad Z. Sagdeev on the Occasion of His 70th Birthday

Renad Sagdeev stands in a tradition of great Russian scientists in magnetic resonance and the chemistry of free radicals. He was a student of Yuri Molin, who himself had done his PhD with V.V. Voevodsky, one of the early leaders of the Institute of Chemical Kinetics and Combustion (ICK&C) in Novosibirsk. Voevodsky, in turn, came from the school of the famous scientist and Nobel laureate N.N. Semenov. Shortly after the discovery of chemically induced dynamic nuclear polarization (CIDNP) [1, 2] and its explanation in terms of the radical pair mechanism (RPM) [3, 4], Sagdeev and Molin started to explore several consequences of the RPM such as the effect of magnetic isotopes and magnetic fields on free radical reactions [5, 6]. A particularly interesting development was the stimulated nuclear polarization (SNP) experiment [7]. This experiment involves microwave irradiation of transient radical pairs in an auxiliary magnet outside the nuclear magnetic resonance (NMR) spectrometer and allows the detection of the electron spin resonance spectrum of these short-lived species via the modulation of their CIDNP effects. Altogether, these novel results were crucial in establishing the field of Spin Chemistry as it is nowadays called and put the Novosibirsk group firmly on the map. They earned Renad Sagdeev the Lenin Prize in 1986 (together with Molin, Salikhov, Buchachenko, and Frankevich) and in 1994 the State Prize in Science and Engineering of the Russian Federation. My own interactions with Renad and the Novosibirsk group started in the late 1970s and led to frequent exchanges in the 1970s and 1980s. My visits to Akademgorodok, the lovely science town near Novosibirsk, were extremely gratifying, both scientifically and socially. I remember vividly the stimulating intellectual atmosphere at ICK&C but also the social activities such as frequent barbecues and other outings. Kev Salikhov’s teachings on fishing in the Ob lake and picking edible mushrooms (strictly forbidden in the Netherlands) were lessons for life. At the institute, financial conditions were not lavish and modern commercial equipment was difficult to obtain. However, this was more than compensated by the great experimental skills and the instrument building capacity of the group. Also, chemicals were hard to come by, so before my trips to Novosibirsk Renad would send me considerable grocery lists of chemicals to bring to Russia (fortunately unnoticed by Russian Customs!). These visits and return visits by Renad and his coworker Alexei Podoplelov to Groningen (and later Utrecht) yielded several joint publications [8–11]. In the late 1980s, Renad took a very important initiative to establish the International Tomography Centre (ITC), in which a high level of research was combined with entrepreneurial activities such as NMR and magnetic resonance imaging services. The establishment of the ITC and its relation with Bruker Spectrospin were very important during the difficult times resulting from the collapse of the Soviet Union in 1991. Under Renad’s leadership, the ITC was adopted by the Russian Academy of Sciences and grew out to the premier center for research in magnetic resonance and Spin Chemistry. Apart from his qualities as a scientist and organizer, Renad Sagdeev is also involved in science policy: as the vice-president of the Siberian Branch of the Russian Academy of Sciences, he plays an important role in shaping science in Siberia. Finally, I should mention his great human qualities: he is an energetic and inspiring leader, but also genuinely interested in his co-workers and colleagues. Renad, I wish you a very happy birthday and many healthy years to come!

Chemically induced dynamic nuclear polarization and the mechanism of the reaction of Et3Al with CCl4 in the presence of transition metal complexes

Integral effects of chemically induced 1H and 13C nuclear polarization are reported for the reaction of Et3Al with CCl4 catalyzed by Pd(acac)2, Cu(acac)2, and Cp2TiCl2; for the reaction of (n-C8H17)3Al with CCl4 in the absence of a catalyst and in the presence of Ni(acac)2; and for the reaction of the cyclic organoaluminum compound 1-ethyl-3-butylaluminacyclopentane with CCl4 in the presence of Pd(acac)2. A scheme of the catalytic cycle of this reaction predicting the formation of both radical and nonradical products is derived from the observed chemically induced dynamic nuclear polarization (CIDNP) effects and from data on the products of the reaction between Et3Al and CCl4 in the presence of Pd(acac)2. According to the results of qualitative analysis of the CIDNP effects, the reactions of the trialkylalanes and the cyclic organoaluminum compound with CCl4 in the presence of various metal complexes proceeded via similar mechanisms.

TR-CIDNP as tool for quantitative analysis of hyperfine couplings in elusive radicals

For many years Chemically Induced DynamicNuclear Polarization (CIDNP) has been successfullyused for investigating the kinetics of fast reactions ofradicals [1]. The origin of CIDNP is the nuclear spinselective recombination of radical pairs. This case isgiven when the electronic singlettriplet conversion isthe bottleneck for the radical pair recombination. As aconsequence, radical pairs have different reactivity indifferent nuclear spin states, with the result that thediamagnetic reaction products have nonthermal populations of their nuclear spin energy levels. CIDNPmanifests itself in anomalous phase and intensity ofthe NMR spectra of the diamagnetic molecules. Thetimeresolved (TR) version of the CIDNP techniquehas a particularly high content of information abouttransient radicals [2] as it can differentiate primary andsecondary reaction steps. First, CIDNP spectrarecorded right after recombination of the geminateradical pairs (geminate CIDNP) are a fingerprint ofthe transient radicals. Second, CIDNP effects can bedetected in liquids at ambient conditions, whereasconventional EPR spectroscopy often allows one toinvestigate the elusive radical species only in frozenmatrix at cryogenic temperatures. Third, the CIDNPtime dependence (kinetics) gives information aboutthe rates of radical termination, intramolecular electron migration and nuclear spin relaxation in transientradicals.In the present work, we will investigate theoretically and experimentally the possibility of extractinghyperfine coupling constants (HFCCs) in radicalsfrom the geminate CIDNP spectrum recorded at high

Olefin Isomerization via Radical‐Ion Pairs in Triplet States Studied by Chemically Induced Dynamic Nuclear Polarization (CIDNP)

AbstractGeometric isomerizations of olefins following photoinduced electron transfer (PET) are classified according to the relative energetic positions of the radical‐ion pairs and the reactant triplets. Each class exhibits characteristic CIDNP (chemically induced dynamic nuclear polarization) effects, for which typical examples are presented. Time‐resolved CIDNP experiments on the system triphenylamine/fumarodinitrile (= (2E)‐but‐2‐enedinitrile), where formation of the olefin triplet is impossible, show that there is also no isomerization of the olefin radical anion. With triisopropylamine or fumarodinitrile as the reaction partner for 4,4′‐dimethoxystilbene (= 1,1′‐[(1E)‐ethane‐1,2‐diyl]bis[4‐methoxybenzene]), both oxidative and reductive quenching give almost mirror‐image CIDNP spectra because of the pairing theorem; reverse electron transfer of the triplet radical‐ion pairs populates the stilbene triplet only, which then isomerizes. With anethole (= 1‐methoxy‐4‐(prop‐1‐enyl)benzene; M), the competition between electron return of triplet pairs $\rm{{}^{{3}}\overline {{{M}}^{ \bullet + } {{X}}^{ \bullet - } } }$ to give either M + 3X or 3M + X was studied by using a second isomerizable olefin (diethyl fumarate (= diethyl (2E)‐but‐2‐enedioate) or cinnamonitrile (= (2E)‐3‐phenylprop‐2‐enenitrile)) as the reaction partner X. Classes can be changed by employing PET sensitization. With ACN (anthracene‐9‐carbonitrile) as the sensitizer, anethole does not produce any directly observable polarizations, but a substitution of ACN.− by the radical anion of 1,4‐benzoquinone (= cyclohexa‐2,5‐diene‐1,4‐dione) or fumarodinitrile within the lifetime of the spin‐correlated radical‐ion pairs leads to very strong CIDNP signals that reflect the effects of both pairs.

Photoinitiated oxidation of NADH catalyzed by horseradish peroxidase studied by chemically induced dynamic nuclear polarization

The photoinitiated oxidation of β-NADH catalyzed by horseradish peroxidase (Per3+) was studied by time-resolved photoinitiated chemically induced dynamic nuclear polarization (CIDNP). The polarization observed on protons at the C(4) atom of the β-NADH molecule is evidence for the reversible one-electron transfer between the radical cation NADH+· and the ferroperoxidase intermediate (Per2+). A new approach based on electron transitions in the (NADH+· Per2+) pair was proposed to describe the formation of CIDNP effects in systems including quartet (Q)—doublet (D) electron transitions.

CIDNP Determination of the Rate of In-Cage Deprotonation of the Triethylamine Radical Cation

CIDNP (chemically induced dynamic nuclear polarization) effects in the sensitized photoreactions of triethylamine DH (sensitizers A, 9,10-anthraquinone and 2,7-dinitrofluorenone) are investigated in a series of aprotic solvents of widely varying relative permittivity ε, including mixtures of acetonitrile and chloroform. The dependence of the polarization pattern, that is, the relative polarization intensities of the olefinic α- and β-protons in the reaction product N,N-diethylvinylamine, on ε is studied quantitatively by evaluating the polarization ratio r of these protons. The factor responsible for the change of the polarization pattern is the rate, relative to the kinetic window of CIDNP, of in-cage deprotonation of the triethylamine radical cation by the sensitizer radical anion (rate constant kdep), which converts the initially formed radical ion pair into a neutral radical pair . A theory of pair substitution taking place solely during encounters of the radicals is presented. Based on this, an expression for the dependence of r on kdep is derived, which in turn allows the extraction of kdep from the experimentally observed polarization ratio. The accessible range of kdep in these systems is from about 108 to about 1010 M-1 s-1.

Radical pair dynamics in micelles investigated by Monte Carlo simulations

The reencounter functions (conditional probability densities of first reencounter) of radical pairs are central quantities for the calculation of CIDNP (chemically induced dynamic nuclear polarization) effects. A computer program was developed to obtain the reencounter functions of radical pairs and radical ion pairs in micelles by Monte Carlo simulations of diffusional trajectories. With these computer experiments, the influence of micelles on the radical pair dynamics was investigated. By varying the initial locations it was found that unless at least one of the radicals starts its diffusional trajectory extremely near the micelle boundary the reencounter function is essentially the same as in the case of free diffusion. It was concluded that pronounced changes of the reencounter function occur only if the boundary disturbs the short-time diffusional motion.

Time resolved CIDNP study of electron transfer reactions in proteins and model compounds

Intramolecular electron transfer (IET) from tyrosine to tryptophan cation radicals is investigated using time resolved chemically induced dynamic nuclear polarization (CIDNP) spectroscopy in combination with laser flash photolysis. In both the tryptophan-tyrosine dipeptide and the denatured state of hen lysozyme in aqueous solution, the transformation TrpH+ → TyrO by IET leads to an increase in the tyrosine radical concentration, growth in the tyrosine CIDNP signal, fast decay of the tryptophan CIDNP, and inversion of the phase of the CIDNP of the photosensitizing dye, 2,2′-dipyridyl. IET effects are not observed for mixtures of the amino acid or for the native state of lysozyme. The steady state CIDNP effects seen for denatured lysozyme thus depend not only on the accessibility of the amino acid residues on the surface of the protein but also on the reactivity of the radical intermediates.

Field Dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) in the Photoreaction of N-Acetyl Histidine with 2,2‘-Dipyridyl in Aqueous Solution

Chemically induced dynamic nuclear polarization (CIDNP) effects for the amino acid-dye (histidine-dipyridyl) photoreaction system are measured in the range between 0 and 7 T using a novel mechanical field cycling unit with fast digital positioning of a high-resolution NMR probe in a spatially varying magnetic field. 1H CIDNP effects are observed for the CH2 protons in β-position and for two protons (H-2 and H-4) at the imidazole ring. For the protons in β-position a multiplet effect is observed having a polarization pattern that changes with the magnetic field. By analysis of the spin nutation, the non-Boltzmann population differences among the nuclear levels are determined. At a field below 20 mT “zero-field character” of the multiplet effect prevails corresponding to preferentially populated states with symmetric spin wave functions. Likewise, for the two histidine ring-protons strong polarization with an emission/absorption multiplet pattern is found between 20 and 300 mT changing below 20 mT to zero-f...

Radical Mechanisms of the Decomposition of Peroxynitrite and the Peroxynitrite–CO2 Adduct and of Reactions with l-Tyrosine and Related Compounds as Studied by 15N Chemically Induced Dynamic Nuclear Polarization

Enhanced absorption is observed in the 15N NMR spectra of 15NO−3 during decomposition of peroxynitrite and the peroxynitrite–CO2 adduct at pH 5.25, indicating the formation of 15NO−3 in radical pairs [15NO•2, HO•] and [15NO•2, CO•−3]. During the reaction of peroxynitrite and the peroxynitrite–CO2 adduct with l-tyrosine, the 15N NMR signal of the nitration product 3-nitrotyrosine exhibits emission showing a radical pathway of its formation. The nuclear polarization is built up in radical pairs [15NO•2, tyr•] generated by free radical encounters of nitrogen dioxide and tyrosinyl radicals. The 15N NMR signal of 15NO−2 formed during reaction of peroxynitrite with l-tyrosine appears in emission. It is concluded that tyrosinyl radicals are generated by reaction of nitrogen dioxide with l-tyrosine. In contrast to this, 15NO−2 does not show 15N chemically induced dynamic nuclear polarization (CIDNP) during reaction of the peroxynitrite–CO2 adduct with l-tyrosine, indicating a different reaction mechanism, which is assumed to be a hydrogen transfer between CO•−3 and l-tyrosine. Emission is also observed in the 15N NMR signals of 2-nitro-4-fluorophenol, 3-nitro-4-hydroxyphenylacetic acid, 2-nitrophenol, and 4-nitrophenol during reaction of 4-fluorophenol, 4-hydroxyphenylacetic acid, and phenol with peroxynitrite and the peroxynitrite–CO2 adduct. 3-Nitro-4-hydroxyphenylacetic acid is also observed in emission during reaction of phenylacetic acid with peroxynitrite, but is not formed with the peroxynitrite–CO2 adduct. The magnitude of the 15N CIDNP effect during reaction of peroxynitrite with 4-fluorophenol and of the peroxynitrite–CO2 adduct with 4-fluorophenol and phenol is determined. It excludes the occurrence of nonradical reactions. Only weak emission signals are observed during the reaction of peroxynitrite with phenol in 15NO−2, 2-nitrophenol, and 4-nitrophenol. 2-Nitrophenol is only formed in traces, and 4-nitrophenol is only formed in higher yields. The latter might be generated in part via a nonradical pathway.

Electron–electron–nuclear three-spin mixing in spin-correlated radical pairs

Electron–electron–nuclear three-spin mixing occurs in radical pairs in solid-state matrices if the radicals feature a significant electron–electron spin coupling and an anisotropic hyperfine coupling. The perturbation of nuclear frequencies by the electron–electron spin coupling and the three-spin mixing have to be generally considered in the calculation of transition frequencies and probabilities in such radical pairs. Analytical descriptions of three-spin mixing for different ratios of the spin Hamiltonian parameters are introduced. It is found that nuclear frequencies are strongly perturbed if the difference of the Zeeman energies of the two electron spins is matched to half the hyperfine coupling and that three-spin mixing is maximum, if also the nuclear Zeeman frequency matches the former two interactions. Such double matching situations may be encountered for pairs of organic radicals under the conditions where transient electron spin resonance (ESR) experiments are usually performed. If three-spin mixing is significant, spin-correlated radical pairs are born in a state that features nuclear coherence in addition to the electron spin zero-quantum coherence that is created irrespective of this mixing. The possibility is discussed to detect such chemically induced nuclear coherence (CINC) in transient electron spin resonance experiments by selective microwave irradiation. It is shown that subsequent electron transfer reactions can yield chemically induced dynamic nuclear coherence (CIDNC) in isolated radicals and chemically induced dynamic nuclear polarization (CIDNP) in diamagnetic products if three-spin mixing is significant. The novel CINC, CIDNC, and CIDNP effects in the solid state might be used in the structure determination of spin-correlated radical pairs with applications to photosynthesis research.

Effect of enzymatic desialylation of human serum amyloid P component on surface exposure of laser photo CIDNP (chemically induced dynamic nuclear polarization) - reactive histidine, tryptophan and tyrosine residues

The human pentraxin serum amyloid P component (SAP) exhibits no microheterogeneity in its complex di-antennary glycan. To elucidate whether the removal of sialic acids from this glycoprotein might affect the accessibility of certain amino acid residues of the protein we employed the laser photo CIDNP approach as a sensitive tool. The CIDNP effect is created by the interaction of a photoexcited dye with reactive amino acids and results in enhanced absorption- or emission-signals which can be observed for the three aromatic amino acids histidine, tryptophan, and tyrosine if they are accessible to the dye. Therefore, this technique can be applied to explore surface exposure of these residues. The respective spectra of SAP and enzymatically desialylated SAP were determined. Six tryptophan/histidine signals and one tyrosine signal are present in the aromatic part of the CIDNP difference spectrum of SAP. The corresponding spectrum of desialylated SAP shows remarkable alterations. The chemical shift of one tryptophan/histidine-characteristic signal is decreased by 0.1 ppm. One tryptophan/histidine signal disappeared and a new one was formed in the CIDNP difference spectrum of desialylated SAP, while the other signals were unaffected. The tyrosine signal has a clearly enhanced intensity in desialylated SAP. Therefore, the removal of sialic acid moieties from the single N-glycan of each monomer apparently affects surface presentation of distinct CIDNP-reactive amino acids of SAP.

Triplex promoted intersystem crossing of ion–radical pairs in the photosensitized valence isomerization of quadricyclane: chemically induced dynamic nuclear polarization (CIDNP) evidence

Photosensitized valence isomerization of quadricyclane to norbornadiene by dibenzoylmethanatoboron difluoride in the presence of durene shows a CIDNP effect which is opposite in direction to that occurring in the absence of durene, demonstrating possible participation of triplexes in the durene co-sensitized reaction.

Flash CIDNP measurement of triplet lifetimes and investigation of two-photon processes

The photolysis of aliphatic ketones using laser pulse excitation (λ = 308 nm) was investigated by the time-resolved flash chemically induced dynamic nuclear polarization (CIDNP) method. The contributions of geminate and homogeneous processes to the CIDNP effects were separated. It was shown that the lifetimes of the triplet molecules can be determined from an analysis of CIDNP formation kinetics and from the Stern—Volmer dependence of the amplitude of geminate polarization in the presence of quencher (piperylene). Irradiation of the samples with high-intensity laser light revealed the presence of two-photon processes which are demonstrated by a change in the correlation of CIDNP line intensities of different products with variations in laser radiation energy and by the non-linear character of the Stern—Volmer dependence. Increased irradiation intensity leads to the appearance of radicals resulting from the decomposition of highly excited triplet molecules of aliphatic ketones. Model calculations of CIDNP kinetics and the Stern—Volmer dependence were performed in terms of a kinetic scheme involving short-lived triplet states of aliphatic ketones. The calculations enable an estimation to be made of the contribution of two-photon processes to the photolysis and the molar extinction coefficients of the triplet state for methylethylketone and cyclododecanone.

The Anion Radicals of Pheophytin a and its Derivatives Studied by Means of CIDNP Technique

Abstract We applied the Chemically Induced Dynamic Nuclear Polarization (CIDNP) technique to the photochemical reaction of pheophytin a (P1) and 4-methoxyphenol (1) in benzene-d6. The observed CIDNP effects were explained in terms of reversible electron-transfer reactions from 1 to 3(P1)* and from (P1)\ewdot to (1)\underset.+. The polarized signals of P1 were induced from the in-cage recombination of the triplet radical ion pair.

CIDNP of radical-ion pairs in arbitrary magnetic fields

Numerical calculations based on the one-nuclear radical pair model with spin I = 1 2 and oppositely charged partners bound by Coulomb interaction have been carried out. The stochastic Liouville equation for the spin density matrix is solved by the finite difference method. The chemically induced dynamic nuclear polarization (CIDNP) in a wide range of magnetic fields is calculated. The dependences of CIDNP on the magnitude of Coulomb and exchange interactions, diffusion parameters, and precursor multiplicity and recombination constant are studied. The influence of the volume accessible for diffusion on the sign and magnitude of CIDNP is derived. It is shown that, under CIDNP effects, the Coulomb interaction mainly changes the partners' total residence time in the reaction zone.