Singlet Radical Pair Research Articles

ChemInform Abstract: CONCERNING THE STEREOCHEMICAL COURSE AND MECHANISM OF THE PHOTOCHEMICAL 1,3‐ACETYL SHIFT IN A β,Γ‐UNSATURATED KETONE. PRELIMINARY COMMUNICATION

AbstractDie zwischen ‐45 und +50°C beobachtete Isomerisie‐ . rung der Ketone (I) und (II) wird von einer temperaturunabhängigen Änderung der Enantiomerenverteilung begleitet, die einer Racemisierung durch Rekombination von sekundären Singulett‐ und Triplett‐Radikalpaaren zugeschrieben wird.

Concerning the Stereochemical Course and Mechanism of the Photochemical 1,3‐Acetyl Shift in a β,γ‐Unsaturated Ketone. Preliminary Communication

AbstractEnantiomerically enriched samples of 1,2‐dimethyl‐3‐(2H3)methyl‐2‐cyclopentenyl (1) and 1‐(2H3)methyl‐2,3‐dimethyl‐2‐cyclopentenyl methyl ketones (2) have been irradiated at 313 nm in methanol in the temperature range +50 to −45°. The 1,3‐acetyl shift, which interconverts the two isomeric ketones, occurs with a small change in the enantiomeric composition and independently of temperature in the range studied. This change corresponds to an upper limit of approximately 20% reaction with racemization. It is proposed that reaction occurs from both the S1(n, π*) and T2(n, π*) excited states with stereospecific reaction from S1 (rapid primary geminate recombination of a singlet radical pair, with a possible contribution by a concerted 1,3 shift) dominating throughout the temperature range, but with the proportion of reaction from T2 increasing as the temperature is lowered. The racemization results from secondary geminate recombination of the singlet and triplet radical pairs and the random recombination of free radicals. Viscosity effects are proposed to explain the independence of the racemization on temperature.

Micelles, magnets and molecular mechanisms. Application to cage effects and isotope separation

This report reviews the general problem of cage reactions of radical pairs and diradicals. Based on Wigner's spin conservation rule and the expectation that only singlet radical pairs can undergo cage reactions, a natural consequence is the expectation that it should be possible to observe magnetic effects on the reactions of radical pairs. It is shown that two types of magnetic effects should be possible: (1) Effects due to the application of the magnetic field of a laboratory magnet during a reaction; (2) Effects due to the interactions of nuclear magnetic momemts and electron magnetic moments. The experimental consequences of these magnetic effects are separation of isotopes based on differing nuclear magnetic moments and variation of the extent of cage reaction by variation of an applied field or by variation of nuclear magnetic moments. Striking experimental examples of these magnetic effects are provided by the photolysis of ketones in micellar solution and by the thermolysis of endoperoxides.

Magnetic field affects the fluorescence yield in reaction center preparations from Rhodopseudomonas sphaeroides R-26

Purified photochemical reaction centers from Rhodopseudomonas sphaeroides R-26 were reduced with Na 2S 2O 4 so as to block their photochemical electron-transfer reactions. The magnetic field induced an increase in the emission yield. Our results support the hypothesis that under these conditions, charge recombination in the singlet radical pair composed of the oxidized primary donor and reduced primary acceptor predominantly generates the excited singlet state of the reaction center bacteriochlorophyll. The maximum relative fluorescence change and the value of the magnetic field at which half-saturation of the effect is achieved ( B 1 2 ) at room temperature are 5.5% and 75 G, respectively. For the whole cells of Rps. sphaeroides R-26 these parameters are 1.2% and 120 G. The relative fluorescence change at 600 G, ΔF F(600) , and B 1 2 are studied as functions of temperature. The temperature dependencies of ΔF F(600) for reaction centers and whole cells of Rps. sphaeroides R-26 are qualitatively the same, with the maximum effect (8% for reaction centers) occurring at 230 K. However, the B 1 2 curves for the two preparations are different.

Regio- and stereo-selectivity correlation of the Wittig rearrangement

The [1,2]- and [1,6]-rearrangements of s-octyl benzhydryl ether gave products in equal amounts whose formation occurred with 20% retention of configuration, and with complete recemization, for the [1,2]- and the [1,6]-products respectively; these results can be rationalized either by the formation of different non-equilibrating singlet radical pair intermediates, or by an important product formation after diffusion of the intermediate radical pairs.

Chemically induced dynamic nuclear polarization during the photoreaction of triethylamine with carbon tetrachloride

Enhanced absorption CIDNP is observed for the chloroform proton and for the terminal olefinic protons of NN-diethylvinylamine during photolysis of the triethylamine–carbon tetrachloride charge transfer complex in methanol and acetonitrile; this points to the formation of the singlet radical pair Et2NĊHMe–ĊCl3 as a primary reaction step.

Theory of the magnetic field modulated geminate recombination of radical ion pairs in polar solvents: Application to the pyrene–N,N-dimethylaniline system

Pairs of radical ions generated in polar solvents by photoinduced electron transfer either recombine within a few nanoseconds or separate. The (geminate) recombination process is governed by a hyperfine-coupling-induced coherent motion of the unpaired electron spins which can be modulated by weak external magnetic fields. The process which also generates the well-known CIDNP and CIDEP effects is described theoretically by a stochastic Liouville equation comprising for realistic systems a large set of coupled diffusion equations. For the integration of these equations a finite-difference algorithm with space and time discretization is developed. By comparison with exact solutions of the Liouville equation for model systems, it is demonstrated that an approximate Liouville equation which entails only two coupled diffusion equations for singlet and triplet radical pairs, respectively, suffices to predict the geminate recombination yields accurately. The approximate Liouville equation is employed then to study on the basis of known hyperfine coupling constants, second-order recombination rate constants, diffusion coefficients, and dielectric constants, the solvent, temperature, concentration, and magnetic field dependence of the geminate (singlet and triplet) recombination yields for the system pyrene–N,N-dimethylaniline. The effect of deuteration upon the recombination yield and its magnetic field dependence is also studied. Furthermore, the influence of an exchange interaction acting at small separations of the radicals is investigated for a model system.

Reaction mechanism and chemical polarization of phosphorus nuclei when diethyl phenylazophosphonate is reacted with sodium ethylate

The action of ethoxide anion on diethyl phenylazophosphonate leads to nucleophilic substitution of the phenyldiazenyl anion, the cleavage of nitrogen from it, and subsequent electron transfer from the formed phenyl anion to the starting phosphonate molecule. Singlet-triplet evolution in the singlet radical pair that is formed here and its recombination give diethyl diphenylhydrazophosphates, which exhibit the31P CPN effect.

Polarisation nucleaire induite chimiquement: Decomposition thermique du peracide dodecanoique dans l'hexachloroacetone

Undecyl chloride formed by decomposition of perdodecanoic acid in hexachloroacetone showed CIDNP signals. Thermal decomposition of aliphatic peracids was studied by this way. The first step is the formation of a singlet radical pair of undecyl and hydroxyl radicals which do not recombine into ROH and give exclusively transfer on the solvent.

Chemical polarization of nuclei in reactions of cyclohexadienone carbenes with halogen-containing organic compounds

The thermolysis of 2,6-di-tert-butyl-p-benzoquinone diazide in a medium of halogenated hydrocarbons was investigated by the method of chemically induced polarization of nuclei. The cyclohexadienone carbene formed during thermolysis reacts with the medium in the singlet form, while the observed polarization is the result of secondary processes, associated with the diffusion of radicals from the primary singlet radical pairs into the volume.

CIDNP Evidence for Divergent Behavior of Singlet and Triplet Radical Pair Encounters in the Photo‐Claisen Rearrangement

CIDNP Evidence for Divergent Behavior of Singlet and Triplet Radical Pair Encounters in the Photo‐Claisen Rearrangement

Photochemical α‐Cleavage of β, γ‐Unsaturated Aryl Ketones: Competition between recombination/disproportionation and dissociation in geminate singlet and triplet radical pairs

AbstractThe photolysis of (R)‐(+)‐phenyl and (R)‐(+)‐p‐anisyl 1, 2, 3‐trimethylcyclopent‐2‐enyl ketone (1, 2) and the corresponding rac‐1‐ and 3‐desmethyl analogs (3, 4) led to isomerization due to formal 1, 3 aroyl migration and to formation of aryl aldehydes (7, 8), dienes (9, 10) and dimers (5, 6) of the cyclopentenyl radical. Evidence obtained from a chiroptical and mass spectrometric analysis of a crossing experiment and from photolytic CIDNP measurements including the use of CCl4 as a free radical scavenger, supports the conclusion (1): that the ketones undergo photochemical α‐cleavage predominantly in the triplet state; (2): that recombination and disproportionation reactions within the geminate singlet and triplet aroyl/allyl radical pairs (11) compete with the dissociation into free radicals (12): (3): that ketone isomerization by paths not involving polarizable radical intermediates is unimportant; (4): that no triplet oxa‐di‐π‐methane type rearrangement products are formed.