Radical Pair Intermediates Research Articles

Effects of Spin-Locking of the Intermediate Radical Pair on the Product-Yield-Detected Electron Spin Resonance Spectrum for the Photoreduction of Anthraquinone in SDS Micellar Solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEffects of Spin-Locking of the Intermediate Radical Pair on the Product-Yield-Detected Electron Spin Resonance Spectrum for the Photoreduction of Anthraquinone in SDS Micellar SolutionsMasaharu Okazaki and Kazumi ToriyamaCite this: J. Phys. Chem. 1995, 99, 47, 17244–17250Publication Date (Print):November 1, 1995Publication History Published online1 May 2002Published inissue 1 November 1995https://doi.org/10.1021/j100047a031RIGHTS & PERMISSIONSArticle Views70Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (889 KB) Get e-Alerts

Internal return in the photochemistry of ring-substituted 1-(1-naphthyl)ethyl esters of phenylacetic acid

The photochemistry in methanol of the esters 12a–d has been studied in order to determine the importance of internal return of both ion pair and radical pair intermediates. The efficiency of internal return, determined by monitoring 18O exchange between the alcohol and carbonyl oxygens, was shown to be substituent dependent, varying from approximately 10% for the 4,7-dimethoxy substrate to nearly 50% for the 4-cyano case. The corresponding ground state solvolysis reactions gave about 10% internal return and, within experimental error, were substituent independent. Internal return was also examined by racemization of the chiral center in 12a and 12d. In summary, these combined results reveal that internal return probably occurs mainly through a contact (not solvent-separated) radical pair. More important, internal return has little effect on previously calculated electron transfer rate constants for converting the radical pair to the ion pair. Therefore, the previously reported Marcus' correlations are valid. Keywords: photochemistry of benzylic esters, internal return, photosolvolysis, electron transfer, radical pairs.

Time-Resolved ESR Investigation of Several Ketyl–Phenoxyl Radical Pairs in a Sodium Dodecyl Sulfate Micelle

Abstract The CIDEP (chemically induced dynamic electron polarization) spectra of quasi-stable radical pairs formed in the laser flash photolysis of xanthone and several phenol derivatives in SDS (sodium dodecyl sulfate) micellar solutions were studied. In consideration of the spectral patterns of CIDEP and the results of the magnetic-field effects on the reaction, it is concluded that the steric bulkiness of phenol derivatives obstructs the free diffusion of the reactants and the intermediate radical pairs in the micelle. The dependence of the number and position of the t-butyl substituents upon the kinetic parameters and the interaction as the radical pairs in the micellar environment are presented.

Product Yield Detected ESR Study on the Dynamic Behavior of Radical Pairs Generated in Photoreduction of Acetylenic Ketones in SDS Micellar Solution

Large magnetic-field and microwave effects were detected on the spin-adduct yield in the photoreduction of 1,5-diphenyl- 1,4-pentadiyn-3-0ne (I) and 1,3-dipheny1-2-propyn- 1 -one (11) in SDS micellar solution. These phenomena were interpreted with the “relaxation mechanism” for the intermediate radical pair. The dynamic behavior of the intermediate radical pair was elucidated by the pulse-mode product-yield-detected ESR (PYESR), in which the effect of microwave pulse on the spin adduct yield is observed. The rate constants for hydrogen abstraction, escape process of radical pair, and so on were successfully determined through the simulation of these microwave effects as functions of the delay period which is brought into the pulse sequences. The reactivity of excited ketones in the hydrogen abstraction increases significantly in the sequence I >> I11 > 11, where I11 represents benzophenone studied as a reference system. The results were compared with those estimated from the transient absorption observations.

Flip-angle effects in Fourier-transform EPR spectra of spin-correlated radical pairs

The EPR spectra of spin-correlated radical pairs in pulse-acquire Fourier-transform experiments are explored by means of spectral simulations. In particular, the dependence on the flip-angle (α) of the microwave pulse is examined. The spectra are seen to be linear combinations of two sub-spectra, with amplitudes proportional to sin α and sin 2α, which have distinctive shapes both for idealised dipolar-coupled radical pairs with isotropic g-values and no hyperfine couplings, and under the conditions found for radical pair intermediates in photosynthetic energy conversion. In cases of poor resolution, the two sub-spectra contain independent information on the structure and interactions of the radical pairs, which should supplement data available from the more routine time-resolved continuous-wave experiment.

Control of a Photochemical Reaction by Manipulating the Electron Spins of a Transient Radical Pair as Evidenced by HPLC Analysis

Abstract Product analysis was made for the photoreduction of anthraquinone in SDS micellar solution under the various magnetic fields with a simultaneous microwave irradiation. It has been confirmed that control of a chemical reaction is possible by inducing the ESR transitions of the intermediate radical pair.

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.

Enhancement or suppression of CIDNP intensities by a second magnetic nucleus: Interplay between g factor difference, external magnetic field, and hyperfine coupling constants

AbstractThe enhancement factors of the 1H polarization induced in radical pairs generated by α‐cleavage of 12CO‐ and CO‐labelled ketones deviate from the ratios expected on the basis of the relative abundances of the respective carbon isotopomers. For dibenzyl ketone, the ratio of absolute CIDNP intensities of the 1H(13C) doublet (90% 13CO) to the 1H(12C) singlet (10% 12CO) was found to be >9:1 at 58·3 kG and <9:1 at 21 kG. Similar deviations were observed for other 13C labelled ketones, viz. deoxybenzoin (13CO) and acetophenone (13CO or 13CH3). This novel enhancement or suppression effect on the CIDNP enhancement can be ascribed to the presence of a second hyperfine coupled nucleus in the intermediate radical pair. The individual contributions of the g factor difference (Δg), the external magnetic field strength (Ho), and the hfc constants of the observed (aA) and the interacting nucleus (ax) in the enhancement‐suppression are exemplified. The experimental results are consistent with theoretical calculations of the mutual effect of Δg, H0, aA and ax on the CIDNP enhancement factor of the observed nucleus.

Study of the magnetic-field-dependent behaviour of radicals generated photochemically from benzil in micellar media

Excited benzil, encaged in sodium dodecylsulphate (SDS) surfactant in aqueous solution, generates a radical pair (benzil ketyl radical and SDS radical) on hydrogen abstraction from the micelle. The lifetime of this triplet-born radical pair is dependent on the magnetic field, reaching saturation at fields greater than 1 kG. An intermediate radical pair, as demonstrated by the occurrence of a magnetic field effect, is also observed on ππ* excitation of this fairly labile molecule. In addition, Cu 2+ ions quench the effect of the magnetic field on the decay of the radical pair. From the temporal response and the magnetic field effect, it is concluded that radicals initially generated by nπ* and ππ* excitation are different.

Study of the linewidth of the CIDNP-detected ESR spectrum

Microwave-power dependence of the linewidth of the CIDNP-detected ESR spectrum in the photolysis of ]-benzoquinone has been investigated. The linewidth of the dynamic nuclear polarization spectrum increases with increase of the microwave power, whereas that of the stimulated nuclear polarization spectrum is almost independent of the power. These observations indicate that the linewidth of the dynamic nuclear polarization (DNP) spectrum is governed by the microwave power, and that the broad feature of the stimulated nuclear polarization (SNP) spectrum comes from the short lifetime of the intermediate radical pair.

Porphyrin–tyrosine interactions: Photo‐CIDNP and NMR studies

AbstractStrong 1H and 13C nuclear polarizations are observed when aqueous solutions of synthetic water‐soluble porphyrins are irradiated in the presence of tyrosine and some of its derivatives. These polarizations are strongly pH dependent. Evidence is also shown for the formation, in the dark, of a complex between the reactants. The association constants are evaluated from the NMR chemical shifts on the aromatic tyrosine protons induced by the presence of the porphyrins. The nature of the intermediate radical pair generating the CIDNP effects is discussed. An electron‐transfer reaction from tyrosine to the excited triplet state of the porphyrin, or within the porphyrin–tyrosine excited complex, is expected to be the primary step in the reaction. It is followed by subsequent proton transfer within the initial ion radical pair. The spin polarizations arise principally from the back‐transfer step, as the reactants are the only products which are polarized.

A theoretical MNDO and AM1 SCF-MO study of dihydrogen transfer reactions

The mechanisms of concerted and stepwise dihydrogen transfer reactions have been studied for a wide variety of hydrogen donors and acceptors using the MNDO and AM1 semiempirical SCF-MO methods. The calculated barriers for concerted [1,2]dihydrogen transfer from cis-di-imide were sensitive to the strain and geometry of the π acceptor but less so to polar substituents, and with the exception of sterically hindered alkenes and systems with silicon and phosphorus double bonds, the energy differences between this route and an alternative stepwise process were surprisingly constant. The AM1 method tends to favour a concerted and essentially synchronous transfer of hydrogen whereas MNDO tends to favour a stepwise mechanism, with signficiant differences also for sterically hindered systems, for the barriers to the concerted hydrogen transfer, and in the disproportionation energies of the intermediate radical pairs involved in the stepwise mechanistic pathway. Highly strained alkene acceptors induce asymmetry in the concerted transfer, and such alkenes also form hydrogen bonded complexes with diimide. Transition state frontier orbitals are shown to give a simple interpretation of selectivity and reactivity, particularly for strained systems and allene it acceptors. The entropy of activation is suggested to be important in rationalising cis/trans alkene selectivity. Bis-α(oxy-anion) substituents on the donor significantly decrease the activation barriers, but induce a stepwise transfer of hydride anion rather than a concerted dihydrogen transfer. Hydrogen kinetic isotope effects show significantly different behaviour for the concerted and stepwise transfer. The concerted [1,4] reduction of butadiene proceeds antarafacially with a much higher barrier than [1,2] reduction. Dimethyl transfer from diazomethane proceeds with double inversion rather than with double retention of configuration at the methyl groups.

Two independent isotope effects and their magnetic field dependences in the photoreduction of menadione in deuterium labeled SDS micellar solutions. A spin trapping study

Spin adduct yields in the photoreduction of menadione (2-methylnaphthoquinone) in isotope labeled micellar solutions were measured as functions of the external magnetic field. As surfactants, ordinary SDS, perdeuteriated SDS, and a mixture of the two were employed. In a mixed micellar solution, a normal isotope effect on the spin adduct yield was observed. On the other hand, in the pure micellar solutions with each of the former two surfactants, a reversed isotope effect was observed. These two apparent isotope effects depend on the external magnetic field and were separated into two independent isotope effects: one is the primary isotope effect on the hydrogen abstraction step and the other is the secondary magnetic isotope effect on the intersystem crossing step of the intermediate radical pair.

Isotope enrichment by electron spin resonance transitions of the intermediate radical pair

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIsotope enrichment by electron spin resonance transitions of the intermediate radical pairMasaharu Okazaki, Takeshi Shiga, Shigeru Sakata, Ryusei Konaka, and Kazumi ToriyamaCite this: J. Phys. Chem. 1988, 92, 6, 1402–1404Publication Date (Print):March 1, 1988Publication History Published online1 May 2002Published inissue 1 March 1988https://doi.org/10.1021/j100317a008RIGHTS & PERMISSIONSArticle Views61Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (374 KB) Get e-Alerts

Magnetic resonance methods in the mechanistic photochemistry of ketones, olefins, and oximes

Abstract

Magnetic Field Effect as a Probe for Radical Pair Intermediates in the Photoreactions of Nitroaromatic Ethers

Abstract Intermediacy of a radical ion pair in the photo-Smiles rearrangement of N-[2-(4-nitro-1-naphthoxy)ethyl]aniline in CH3CN was suggested on the basis of magnetic field effects on the photoreaction of its higher homolog.

ChemInform Abstract: Arene‐Iminium Salt Electron‐Transfer Photochemistry. Mechanistically Interesting Photoaddition Processes.

AbstractSeveral photochemical reactions are studied such as a) quenching of the fluorescence of the pyrrolinium perchlorates (II) by the arenes (I) and (VIIa) (R2: ‐H), b) quenching of the fluorescence of the perchlorates (IV) and (V) by the arenes (III) and (VIIa), c) photochemical reaction of perchlorate (VI) with the toluenes (VII) leading to the products (VIII)‐(X) (solvent MeCN) or (VIII)‐(XII) (solvent MeOH) in yields depending on arene (VII) and solvent, d) the same photochemical reactions of (IIa) with the toluenes (VII) (solvents MeCN and MeOH) to analogous products, or of (IV) with the toluenes (VIIa) (solvents MeCN and MeOH).

Studies of various mechanisms of nuclear polarisation due to a resonant high-frequency field in radical reactions

Two different mechanisms of creation of nuclear polarisation in radical reaction products are studied by means of the high-frequency field influence on intermediate radical pairs (stimulated nuclear polarisation, SNP) and on intermediate short-lived radicals (dynamic nuclear polarisation, DNP). Criteria are formulated for distinguishing the contributions of DNP and SNP effects, and experimental demonstration of this possibility is presented for photoinduced reactions of quinones.

Surface photochemistry: the amide Photo-Fries rearrangement

The Photo-Fries rearrangement of eight anilides on the surface of dry silica gel has been examined. The rearrangement occurs quite cleanly, giving somewhat more of the para isomer than that obtained in methanolic solution. A test for intermolecularity on the silica gel surface showed that, as in solution, the radical pair intermediates do not separate. This contrasts with the behaviour of a previously reported dibenzyl ketone derivative.

Electronic energy transfer processes and symmetry conservation in reactions in inclusion compounds

Abstract The results are presented of a study on the electronic energy transfer (EET) processes in inclusion host-guest compounds irradiated by gamma or UV photons. The host cavity site symmetry is correlated with the guest molecular symmetry and with the stereocontrol of the reaction products formed under irradiation; a specific “heteroexcimer” or “excithrate” is suggested as an excited host-guest intermediate radical-pair; in particular, symmetry correlations are suggested in order to explain reaction pathways into cavities of inclusion compounds in the excited state.