Microviscosity and wavelength effects on radical cage pair recombination

This study probed two aspects of the reactivity of geminate radical cage pairs formed by photolysis of (Cp′ = η⁵-C₅H₄CH₃). The first aspect studied examined whether the bulk viscosity has any predictive power in determining the magnitude of the cage recombination efficiency (F_cP). Although there is a clear relationship between the magnitude of F_cP and viscosity for systems where the bulk viscosity of the solution is altered by the addition of a non-macroscopic viscosity enhancer, the relationship is unclear for systems where the bulk viscosity is altered using polymeric viscosity enhancers. For this investigation, F_cP values were measured using femtosecond pump-probe transient absorption spectroscopy. The results clearly indicate that bulk viscosity can change drastically without affecting F_cP in systems containing small amounts of added polymers. The bulk viscosity is thus a poor parameter for predicting the cage effect in such systems. The second investigation looked at the effect of the photochemical excitation energy on F_cP for the [Cp′(CO)₃Mo, MoCp′(CO)₃] cage pair in hexane. The results showed that F_cP increased when the wavelength of irradiation was changed from 546 nm to 436 nm, and then remained constant as the wavelength of irradiation was changed from 436 nm to 404 nm to 366 nm. These results are somewhat surprising because the recombination efficiencies for diatomic and triatomic molecules have been shown to decrease monotonically with increasing excitation energy. Two explanations are offered for the reverse wavelength dependence observed in this study. The first explanation invokes the different dynamic behaviors of the two excited states involved at the selected wavelengths, and the second invokes the different speeds of radical separation following irradiation at the selected wavelengths.     

Re-examination of primary radical pair recombination in Rp. viridis with QA reduced

Charge recombination in the primary radical pair P⁺H⁻ of the purple photosynthetic bacterium Rp. viridis with pre-reduced secondary acceptor Q_A, has been studied by time-resolved photovoltage measurements and transient absorption spectroscopy with a time resolution of 1 ns. The lifetime of P⁺H⁻ was found to be 2.4–3 ns in intact membranes and ∼5 ns in isolated reaction centers, in contrast to a lifetime of ∼15 ns which has been widely accepted in the literature for reaction centers. The origin of the difference between lifetimes of P⁺H⁻ in membranes and isolated reaction centers is discussed.     

Macromolecular pair correlation functions from fluorescence depolarization experiments

Depolarization of fluorescence resulting from the transport of electronic excitations in chromophore-containing polymers is investigated as a technique for detecting deviations from ideal-chain statistics. An approximate expression for the fluorescence anisotropy that depends only on the pair correlation function of chromophore labels is presented. This approximation is shown to be accurate for particular cases of long-range correlations, short-range correlations, and no correlations among chromophore positions. The formalism allows fluorescence depolarization experiments to be used as a probe of macromolecular pair correlation functions.     

Influence of dipolar interactions on radical pair recombination reactions subject to weak magnetic fields

Monte Carlo simulations of the effects of weak magnetic fields on the recombination of interacting radical pairs undergoing free diffusion in solution have been performed, with the aim of determining the influence on the low field effect of the magnetic dipolar coupling between the radicals. The suppression of singlet-triplet interconversion in the radical pair by the dipolar interaction is found to be pronounced at magnetic field strengths comparable to the hyperfine interactions in the radicals, to the extent that the low field effect is completely abolished. The averaging of the dipolar coupling by the translational diffusion of the radicals around one another is relatively efficient in the presence of strong magnetic fields but becomes ineffective in weak applied fields where the strength of the dipolar interaction is independent of the orientation of the inter-radical vector. Low field effects are only likely to be observed if the motion of the radical pair is restricted in some way so as to increase the likelihood that, having separated to the large distance required for the dipolar interaction to have a negligible effect, the radicals subsequently encounter and have the opportunity to recombine.     

Theory of magnetic field modulation of radical recombination reactions. II. Short time behavior

The change of spin multiplicity in a radical pair, due to hyperfine interaction and depending on an external magnetic field, is treated by time-dependent perturbation theory. Analytic expressions, valid at short times, but at arbitrary field strengths, are derived which apply to radicals with any given hyperfine structure. The short time region deserves special interest, since here isotope effects in radical reactions, induced by differences in the nuclear magnetic moments rather than in masses, are shown to be much stronger than at longer times.     

Path integral based calculations of symmetrized time correlation functions. I

In this paper, we examine how and when quantum evolution can be approximated in terms of (generalized) classical dynamics in calculations of correlation functions, with a focus on the symmetrized time correlation function introduced by Schofield. To that end, this function is expressed as a path integral in complex time and written in terms of sum and difference path variables. Taylor series expansion of the path integral's exponent to first and second order in the difference variables leads to two original developments. The first order expansion is used to obtain a simple, path integral based, derivation of the so-called Schofield's quantum correction factor. The second order result is employed to show how quantum mechanical delocalization manifests itself in the approximation of the correlation function and hinders, even in the semiclassical limit, the interpretation of the propagators in terms of sets of guiding classical trajectories dressed with appropriate weights.     

Path integral based calculations of symmetrized time correlation functions. II

Schofield's form of quantum time correlation functions is used as the starting point to derive a computable expression for these quantities. The time composition property of the propagators in complex time is exploited to approximate Schofield's function in terms of a sequence of short time classical propagations interspersed with path integrals that, combined, represent the thermal density of the system. The approximation amounts to linearization of the real time propagators and it becomes exact with increasing number of propagation legs. Within this scheme, the correlation function is interpreted as an expectation value over a probability density defined on the thermal and real path space and calculated by a Monte Carlo algorithm. The performance of the algorithm is tested on a set of benchmark problems. Although the numerical effort required is considerable, we show that the algorithm converges systematically to the exact answer with increasing number of iterations and that it is stable for times longer than those accessible via a brute force, path integral based, calculation of the correlation function. Scaling of the algorithm with dimensionality is also examined and, when the method is combined with commonly used filtering schemes, found to be comparable to that of alternative semiclassical methods.     

Incorporation of anthracene into zeolites: confinement effect on the recombination rate of photoinduced radical cation-electron pair.

FT-Raman spectrometry in combination with diffuse reflectance UV/Vis absorption (DRUVv) and fluorescence emission indicate that complete anthracene (ANT) sorption as intact molecules takes place over 6 months in the medium pores of non-Br?nsted acidic M(n)ZSM-5 zeolites (n=0.0, 3.4, 6.6; M=Na+, K+, Rb+, Cs+) with 1 ANT per unit cell loading. The combined effect of confinement and electrostatic field induced by bulky cations (Rb+, Cs+) leads to specific changes in the occluded ANT Raman spectra after very long organization periods (one year). The laser photolysis (266 nm, 355 nm) of ANT@M(n)ZSM-5 equilibrated samples generates long-lived charge separated species in aluminum rich zeolites (n=3.4, 6.6). The very long-lived radical pairs are characterized by conventional DRUVv and CW-EPR spectroscopy. The direct charge recombination rates of ANT.+-electron pairs are dispersive, extending over a broad range of timescales. The kinetic constant values are found to increase dramatically with the aluminum content and increase markedly with M+ according to the following order Na+ < K+ < Rb+ < Cs+. The small reorganization energy (lambda) of ZSM-5 zeolite pores coupled with large negative free energy changes (-DeltaG degrees ) between the ground state ANT oxidation potential and Fermi level of aluminum rich M(n)ZSM-5 explain the observed trends of the ANT.+@M(n)ZSM-5.- charge recombination rates.     

Rate constants for radical recombination. III. The trifluoromethyl radical

Products of the radical reactions arising from t-Bu₂O₂, CF₃I, and CH₃I at 146°C in the vapor phase have been measured over a 33-fold range of CH₃I/CH₃I ratios and shown to be governed by the rapidly established equilibrium Together with K estimated by thermochemical methods, the results yield, for the rate of recombination for CF₃· radicals, k_r = 10^{9.7 ± 0.5} M^?1 sec^?1.     

Radio frequency magnetic field effects on a radical recombination reaction: a diagnostic test for the radical pair mechanism

The photoinduced electron-transfer reaction of chrysene with isomers of dicyanobenzene is used to demonstrate the sensitivity of a radical recombination reaction to the orientation and frequency (5-50 MHz) of a approximately 300 muT radio frequency magnetic field in the presence of a 0-4 mT static magnetic field. The recombination yield is detected via the fluorescence of the exciplex formed exclusively from the electronic singlet state of the radical ion pair Chr*+/DCB*-. Magnetic field effects are simulated using a modified version of the gamma-COMPUTE algorithm, devised for the simulation of magic angle spinning NMR spectra of powdered samples. The response of a chemical or biological system to simultaneously applied radio frequency and static or extremely low-frequency magnetic fields could form the basis for a diagnostic test for the operation of the radical pair mechanism that would not require prior knowledge of the nature and properties of the radical reaction.     

Light emission originating from photosystem II radical pair recombination is sensitive to zeaxanthin related non-photochemical quenching (NPQ)

We have used chlorophyll fluorescence, delayed luminescence and thermoluminescence measurements to study the influence of an artificial DeltapH in the presence or absence of zeaxanthin on photosystem II reactions. Energization of the pea thylakoid membranes induced non-photochemical fluorescence quenching and an increase in the overall luminescence emission of PSII during delayed luminescence and thermoluminescence measurements. This DeltapH-induced overall luminescence increase was caused by a strongly enhanced delayed luminescence in the seconds range before sample heating. In the subsequent thermoluminescence measurements the intensity of the B-band decreased after one and increased after two or more single turnover flashes. We propose that strong membrane energization shifted the redox potential of photosystem II radical pairs to more negative values causing the high delayed luminescence. The zeaxanthin-dependent non-photochemical fluorescence quenching component, however, did not alter thermoluminescence B-bands but decreased the delayed luminescence intensity by 30%. To our knowledge this is the first report that the radiative radical pair recombination, exhibited as delayed luminescence but not thermoluminescence emission, is sensitive to the antenna located zeaxanthin related non-photochemical fluorescence quenching. Our data can be interpreted within the frame of the exciton/radical pair equilibrium model that describes photosystem II as a shallow trap and incorporates the transfer of energy from the re-excitated reaction centre to the antenna of photosystem II.     

Rate constants for alkyl radical recombination. II. The isopropyl radical

Products of radical combination from the free-radical buffer system \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$${{\rm R}^{\rm .} + {\rm R}^{\rm '} {\rm I}\mathop {\leftrightharpoons}\limits^{{\rm K}_{{\rm RR}}}{\rm RI} + {\rm R}^{'}}$$\end{document}. have been analyzed for the two cases, R = Me, R′ = iPr and R = Et, R′ = iPr. Results are consistent with the previously examined system where R = Me, R′ = Et, and give a value of k_P for iPr· combination of 10^8.6±1.1 M^?1 sec^?1.     

A supramolecular "ship in bottle" strategy for enantiomeric selectivity in geminate radical pair recombination

[reaction: see text] Reactions in which zeolites are modified with chiral inductors to serve as media for chiral induction are often limited by the propensity of both substrate and inductor to occupy the same supercage. Herein, we report a "ship in bottle" strategy utilizing the thermal decomposition of dioxetanes obtained from oxazolidinone-substituted enecarbamates for the enantioselective generation of methyl desoxybenzoin (MDB). Photoexcitation of the supramolecular geminate molecular pair results in enrichment of the opposite enantiomer of MDB.     

Forward-backward semiclassical and quantum trajectory methods for time correlation functions

Forward-backward trajectory formulations of time correlation functions are reviewed. Combination of the forward and reverse time evolution operators within the time-dependent semiclassical approximation minimizes phase cancellation, giving rise to an efficient methodology for simulating the dynamics of low-temperature fluids. A quantum mechanical version of the forward-backward formulation, based on the hydrodynamic formulation of time-dependent quantum mechanics, is also available but is practical only for small systems.     

On the calculation of time correlation functions by potential scaling

We present and analyze a general method to calculate time correlation functions from molecular dynamics on scaled potentials for complex systems for which simulation is affected by broken ergodicity. Depending on the value of the scaling factor, correlations can be calculated for times that can be orders of magnitude longer than those accessible to direct simulations. We show that the exact value of the time correlation functions of the original system (i.e., with unscaled potential) can be obtained, in principle, using an action-reweighting scheme based on a stochastic path-integral formalism. Two tests (involving a bistable potential model and a dipeptide bond-vector orientational relaxation) are exemplified to showcase the strengths, as well as the limitations of the approach, and a procedure for the estimation of the time-dependent standard deviation error is outlined.     

Magnetic field effect studies indicate reduced geminate recombination of the radical pair in substrate-bound adenosylcobalamin-dependent ethanolamine ammonia lyase

The apparent conflict between literature evidence for (i) radical pair (RP) stabilization in adenosylcobalamin (AdoCbl)-dependent enzymes and (ii) the manifestation of magnetic field sensitivity due to appreciable geminate recombination of the RP has been reconciled by pre-steady-state magnetic field effect (MFE) investigations with ethanolamine ammonia lyase (EAL). We have shown previous stopped-flow MFE studies to be insensitive to magnetically induced changes in the net forward rate of C-Co homolytic bond cleavage. Subsequently, we observed a magnetic-dependence in the continuous-wave C-Co photolysis of free AdoCbl in 75% glycerol but have not done so in the thermal homolysis of this bond in the enzyme-bound cofactor in the presence of substrate. Consequently, in the enzyme-bound state, the RP generated upon homolysis appears to be stabilized against the extent of geminate recombination required to observe an MFE. These findings have strong implications for the mechanism of RP stabilization and the unprecedented catalytic power of this important class of cobalamin-dependent enzymes.