Stimulated Nuclear Polarization Research Articles

Influence of nitroxides on CIDNP of micellised radical pairs and short-lived biradicals

The effects of stable nitroxide radicals on stimulated nuclear polarization (SNP) and chemically induced dynamic nuclear polarization (CIDNP) in short-lived consecutive biradicals and radical pairs in homogeneous solutions as well as spin-correlated radical pairs in micelles were studied in high and low magnetic fields. It is shown that experimentally observed effects of nitroxide additions on CIDNP and SNP can be well simulated taking into account only the increase in the rates of relaxation in the paramagnetic species constituting radical pairs or biradicals. Effects of coherent spin evolution in three-spin systems under study seem to be of negligible importance.

The investigation of consecutive micellised radical pairs by the method of stimulated nuclear polarization

The main features of stimulated nuclear polarization (SNP) formation in consecutive micellised radical pairs (RP) have been studied both experimentally and theoretically. The photolyses of dibenzyl ketone (DBK), α-methyldibenzyl ketone (MDBK), and 2,2′,4,4′-tetramethyldimethyl acetone dicarboxylate (PEST) in aqueous solutions of alkylsulfate micelles have been used as model photoreactions. The rate constants of relaxation and RP escape from micelles have been obtained. SNP spectra of consecutive micellised RPs have been compared to the spectra of consecutive biradicals with similar structure of radical centers and chain length equal to the inner diameter of the micelle. It has been shown that the significant difference in the SNP spectra of micellised RPs and biradicals is determined by the difference in their molecular dynamics and rate of reencounters.

ChemInform Abstract: Dynamic and Stimulated Nuclear Polarization in Photochemical Radical Reactions

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Temperature and Salt Addition Effect on the Micellized Radical Pairs Recombination Studied by Stimulated Nuclear Polarization

It was found that the shape of the 13C stimulated nuclear polarization (SNP) spectra, detected in the photolysis of dibenzyl ketone (DBK) and α-methyldeoxybenzoine (MDB), exhibits a noticeable temperature dependence. For the relatively large sodium dodecylsulfate (SDS) micelles, the temperature increase leads to the increase in the line width of SNP spectra, while in the smaller sodium octylsulfate (SOS) micelles the splitting in the SNP spectra decreases. From the comparison of the experimental data with the model calculations based on the numerical solution of the Liouville equation, the influence of the temperature and salt additions on the collision rate of radicals in micelle interior was evaluated. The obtained activation energy of the translational diffusion of small radicals is Ea = 6.7 ± 0.25 kcal/mol and Ea = 5.6 ± 0.2 kcal/mol for SDS and SOS micelles, respectively. Additionally, the influence of temperature and salt additions on the lifetime of the micellized radical pairs (RPs) formed in the ...

DNP and CIDEP Study of Cross-Relaxation Processes in Short-Lived Radicals in Solution

Dynamic nuclear polarization (DNP) and chemically induced electron polarization (CIDEP) of transient radicals in solution are measured at L- and X-band frequencies to investigate their electron−nuclear cross-relaxation. It is demonstrated experimentally and theoretically, that a strong multiplet-type electron spin polarization leads to a multiplet-type DNP spectrum with the same relative line intensities as they are observed in the CIDEP spectrum. DNP spectra are found in cases such as this to be separable from the always superimposed SNP (stimulated nuclear polarization) spectra via measurements at different microwave powers. Examination of radicals (CH3)2XC• with X = CH3, OH, D, and C(O)CH3 shows the existence of efficient electron−nuclear flip-flop (Δm = 0) cross-relaxation for t-butyl and 2-hydroxy-2-propyl radicals, whereas for 2-acetyl-2-propyl radicals DNP is absent. The results indicate that phase inversions, observed in time-resolved CIDEP spectra of t-butyl and 2-hydroxy-2-propyl radicals, are a...

Theoretical and experimental stimulated nuclear polarization investigation of consecutive biradicals

The characteristic features of stimulated nuclear polarization (SNP) in consecutive short-lived biradicals have been investigated theoretically and experimentally. For calculations of SNP of consecutive biradicals, a model has been developed based on the numerical solution of the stochastic Liouville equation, which accounts explicitly for the distance-dependent exchange interaction and molecular mobility of the biradical chain. The SNP kinetics and spectra of acyl-alkyl and bis(alkyl) biradicals generated upon photolysis of tetramethyl-substituted cycloalkanones with 11–15 carbons in the chain were studied and analyzed in the range of magnetic fields of 11–50 mT.

Spin and Molecular Dynamics of Biradicals as Studied by Low Field Nuclear Polarization at Variable Temperature

1H and 13C chemically induced dynamic nuclear polarization (CIDNP) and time-resolved stimulated nuclear polarization (SNP) have been applied to the investigation of spin and molecular dynamics in biradicals, generated in the photolysis of 2,2,12,12-tetramethylcyclododecanone, at low magnetic field in the temperature range from 200 to 360 K. This reaction involves two consecutive biradical stages, the primary acyl−alkyl and the secondary bis-alkyl biradical, with different reaction products. Characteristic differences in the CIDNP field dependencies and SNP decay times for nuclei of different reaction products are seen at high temperatures. Upon cooling, the emission maximum position shifts to low field and the width of the 1H CIDNP field dependencies of the acyl−alkyl biradical products decreases. Below 243 K these effects reverse direction, furthermore, a significant slowing down of the SNP decay is observed. Calculations of the 1H and 13C CIDNP field dependencies and the SNP kinetics for the primary biradical have been performed based on the numerical solution of the stochastic Liouville equation for the biradical spin density matrix. They use a model of restricted diffusion for the description of the molecular dynamics and take into account the distance dependence of the exchange interaction. Good quantitative agreement between the calculations and experimental data is found for the whole temperature range when using the appropriate dependence of some of the parameters on temperature. In particular, it was necessary for the quantitative simulation in the low temperature range to introduce a temperature dependence of the biradical reactivity. The model calculations qualitatively confirm that the low-temperature broadening of the CIDNP field dependencies, and the increase of the SNP decay time are caused by the slowing down of the molecular mobility and the decreasing of reactivity. For the products of the secondary biradical, a qualitative interpretation is given showing that in comparison with the products of the primary biradical spin−orbit interaction is of less importance for the biradical spin dynamics, that the influence of the molecular mobility under the temperature variation is more pronounced, that the lifetime is longer, and that the nuclear polarization is more efficiently formed.

The manifestation of degenerate electron exchange in stimulated nuclear polarization at high magnetic fields

A detailed theoretical and experimental investigation of the degenerate electron exchange (DEE) effect on the stimulated nuclear polarization (SNP) intensity and spectra at high magnetic fields has been made. The theoretical description of DEE processes is performed in two different ways: (i) The numerical solution of equations of noncorrelated frequency migration theory and (ii) the analytical solution for simple two-site model for the dependence of the SNP intensity on the rate of DEE at low microwave (mw) field amplitudes ( B 1 ⪡ a, a is a constant of hyperfine interaction (HFI) of radical-ions) and low DEE rates ( aτ e ⪢ 1, τ e is the characteristic time of DEE). It is shown that the increase of DEE rate leads to the decrease of splitting in SNP spectra, and also to a substantial decrease of the SNP signal intensity due to an overlapping of hyper-fine structure lines with opposite signs. A numerical evaluation of DEE rate constants can be done by comparison of experimental and calculated spectrum shape in the case of fast DEE rates and from the dependence of SNP intensity in resonance on the concentration of diamagnetic molecules.

Temperature dependence of low field CIDNP and time resolved SNP in cyclic ketones

1H CIDNP and time resolved Stimulated Nuclear Polarization (SNP) in the photolysis of 2,2,12,12-tetramethylcyclododecanone have been investigated at low magnetic field in the temperature range from −70 to +90°C. Considerable differences for protons of different reaction products are seen in the CIDNP field dependences and SNP decay times at high temperatures. For disproportion products of acyl-alkyl biradicals the width of the CIDNP field dependence and the position of the emission maximum exhibit a pronounced temperature dependence with the largest shift towards low field around −30°C. At lower temperatures a significant slowing down of the SNP decay is observed. The results are discussed in terms of a superposition of contributions from acyl-alkyl and bis(alkyl) biradicals to the nuclear spin polarization.

The SNP measurement of the electron transfer reaction from quadricyclane to tetrafluoro-p-benzoquinone

The stimulated nuclear polarization (SNP) [1] is measured in the photoinduced electron transfer reaction from quadricyclane to tetrafluoro-p-benzoquinone. We conclude that the radicalion pair in this system involves quadricyclane cation radical and that the isomerization reaction from quadricyclane cation radical to norbornadiene cation radical mainly occurs outside the cage of radical-ion pair.

Low and zero field stimulated nuclear polarization in cyclic ketones

13C stimulated nuclear polarization (SNP) in acyl-alkyl biradicals has been investigated at a low magnetic field B 0. For carbons in the carbonyl position, which have a strong hyperfine interaction (hfi), a significant SNP effect is observed at low fields B 0 < hfi, in contrast to carbons in the α-position with smaller hyperfine coupling. A qualitative analysis using a two-state biradical model shows that this effect has a strong dependence onthe non-spin selective channels of the biradical decay.

A Time-Resolved Stimulated Nuclear Polarization Study of Biradicals in Low Magnetic Field

Stimulated nuclear polarization (SNP) in acyl−alkyl biradicals of different size formed as a result of α-cleavage of cyclic ketones in the triplet state has been investigated in low magnetic field around 10 mT. Time resolution was achieved by pulsing the light and the rf source and monitoring the SNP signal as function of the rf pulse delay. The SNP amplitude reaches its maximum for a delay around 40 ns and decays exponentially for longer delays. The decay time decreases with increasing molecular chain length. The different influence of triplet quencher cis-piperylene and radical scavenger CBrCl3 shows that the decay is mainly determined by the lifetime of the photoexcited ketone triplet state. The SNP maximum position can be attributed to the competition between spin-dependent and spin-independent channels of intersystem crossing.

Examination of the Exchange Interaction through Micellar Size. 3. Stimulated Nuclear Polarization and Time Resolved Electron Spin Resonance Spectra from the Photolysis of Methyldeoxybenzoin in Alkyl Sulfate Micelles of Different Sizes

Stimulated nuclear polarization (SNP) and time resolved electron spin resonance (TR ESR) spectra were recorded during the laser flash photolysis of [sup 13]C carbonyl labeled [alpha]-methyldeoxybenzoin solubilized in a series of alkyl sulfate micelles of different sizes. While the SNP spectra show a decrease in the splitting of the two hyperfine lines with decreasing micelle size, this decrease in hyperfine splitting is not seen in the experimental TR ESR spectra The qualitatively different variations between the SNP and TR ESR spectra, as a function of micelle size were interpreted in terms of the stochastic Liouville equation as applied to the model of the microreactor. 28 refs., 6 figs., 2 tabs.

Theory of multiquantum SNP spectra of radical pairs

The present paper deals with a theoretical study of the spectral peculiarities appearing in stimulated nuclear polarization (SNP) when multiquantum resonances in a recombining radical pair (RP) take place. For nonviscous solutions, a correspondence between the SNP formalism and the theory of optically detected ESR in the liquid phase is established on the basis of kinematic approximation separating the RP spin dynamics and the kinematics of the relative motion of reagents. It is shown that in the case of nonequilibrium initial nuclear distribution a new central component in multiquantum SNP spectra can appear. It is suggested that a simplified spectral pattern of multiquantum resonances should be used to experimentally obtain the Green function of the relative motion of RP reagents. For parabenzoquinone in CD 3OD the appearance of a double-quantum SNP signal at the rf field amplitude exceeding 15 Oe is predicted.

Studies of photochemical reaction by CIDNP-detected ESR spectrum

CIDNP-detected ESR spectroscopy was applied to the study of photochemical reaction. In the CIDNP-detected ESR spectroscopy, the spectrum due to intermediate radical pairs and the spectrum due to free (escaped) radicals can be separately observed. The spectrum due to radical pairs is called stimulated nuclear polarization (SNP), and the spectrum due to free radicals is called dynamic nuclear polarization (DNP). The method was applied to the study of photochemical reaction of 2,6-dichlorobenzoquinone and the isomerization of quadricyclane assisted by photoinduced electron transfer, and the chemical species of the intermediate radical pair and of the radical were clarified.

Calculation of SNP effects in weak magnetic fields

A theory of stimulated nuclear polarization (SNP) in weak magnetic fields has been advanced. An analytic expression providing the calculation of the SNP spectrum has been derived for the radical pair (RP) having one magnetic nucleus with spin I= 1 2 . It is shown that in weak magnetic fields, in contrast to strong magnetic fields, additional spectral lines appear. It is concluded that SNP, generated at high and low power of mw pumping, is due to different types of singlet-triplet transitions. A qualitative interpretation of the spectra is given. The two-quantum resonance caused by the absorption of one quantum by each of the pair's radicals is predicted. As an illustration, the calculated and experimental data on SNP in photolysis of chinones are compared.

The mechanisms of oxidation of NADH analogues 3. Stimulated nuclear polarization (SNP) and chemically induced dynamic nuclear polarization (CIDNP) in low magnetic fields in photo-oxidation reactions of 1,4-dihydropyridines with quinones

Radical particles formed during the photo-oxidation of 1,4-dihydropyridines (1,4-DHPs) by quinones in benzene and acetone have been characterized using stimulated nuclear polarization (SNP) and low-magnetic-field chemically induced dynamic nuclear polarization (CIDNP) techniques. The experimentally obtained hyperfine coupling constants of the 1,4-DHP radical ion and neutral radical qualitatively agree with those calculated using the INDO method. It has been found that the disproportionation reaction of two neutral nitrogen-centred 1,4-DHP radicals provides the main contribution to CIDNP in low magnetic fields.

ChemInform Abstract: Kinetic and Mechanistic Aspects of Stimulated Nuclear Polarization

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Investigation of Micellized Radical Pairs in the Photolysis of Ketones by Time-Resolved Stimulated Nuclear Polarization*

Article Investigation of Micellized Radical Pairs in the Photolysis of Ketones by Time-Resolved Stimulated Nuclear Polarization* was published on January 1, 1993 in the journal Zeitschrift für Physikalische Chemie (volume 182, issue 1-2).

Calculation of stimulated nuclear polarization in short-lived biradicals and comparison with experiment

A theoretical model is presented which allows one to calculate stimulated nuclear polarization (SNP) (and RYDMR) in short-lived flexible biradicals. It is based on the numerical solution of the stochastic Liouville equation for spin density matrix and takes into account distance-dependent exchange interaction. For description of polymethylene chain conformational motions the “restricted diffusion” model is used. This approach is shown to allow adequate description of SNP spectra of acyl—alkyl biradicals formed in the photolysis of cycloalkanones. The influence of different parameters on calculated SNP spectra is considered. From the comparison with experimental data it is concluded that SNP technique is very sensitive to the magnitude of exchange interaction. Parameters of the latter are discussed.