Geminate Recombination Of Radicals Research Articles

General relationships between the kinetic characteristics of bulk and geminate recombination of radicals in solutions

General relationships between the kinetic characteristics of bulk and geminate recombination of radicals in solutions

Femtosecond dynamics of geminate recombination of radicals upon photodissociation of aromatic disulfides

The dynamics of the geminate recombination of thiyl radicals formed upon photodissociation of aromatic disulfides and the effect of the intramolecular relaxation on this recombination are studied using pico-and femtosecond kinetic spectroscopy. It is shown that, in terms of a phenomenological model, the geminate recombination of phenylthiyl radicals in neutral solvents can be satisfactorily described by a biexponential dependence. The model suggests the occurrence of primary geminate recombination in a solvent cage formed around an original molecule and secondary recombination controlled by the diffusional motion of the radicals of a pair. The primary geminate recombination, whose characteristic time (9 ps) is close to the characteristic times of intermolecular vibrational relaxation of complex molecules in solvents at room temperatures, masks the manifestation of thermalization processes in the time kinetics. The direct geminate recombination of aminophenylthiyl radicals with the formation of original molecules virtually does not occur because of the intramolecular charge transfer. In connection with this, the intermolecular vibrational relaxation manifests itself in the kinetics of the induced optical density in the region of the absorption band maximum of radicals as a growing component with a characteristic time of 6 ps.

Preliminary observations on the copolymerisation of acceptor monomer:donor monomer systems under microwave irradiation

AbstractThe mechanistic rationalisation of specific ‘microwave effects’ previously reported in a range of chemical reactions suggests that they may be observable in the freeradical copolymerisation of comonomer pairs capable of forming donor-acceptor complexes. Polymerisation under microwave irradiation is carried out for several comonomer pairs with weak donor-acceptor interactions, and no acceleration in rate or change in degree of alternation attributable to changes in propagation are observed. An increase in reaction rate of 150–200 % is observed for all systems, with trends in molecular weight suggesting this was due to an increase in radical flux. This is consistent with earlier reports of rate enhancement for free-radical polymerisations using the initiator 2,2′-azobis-isobutyronitrile, and it is proposed that microwave irradiation may reduce the amount of geminate radical recombination.

Molecular dynamics simulation of the geminate radical recombination in solution: effects of the large dipole moment of the radicals

Molecular dynamics simulation of a simple model system of the geminate radical pair in solution has been performed to elucidate the dynamic behavior of radicals. The diffusion process of radicals in the microscopic region was simulated and the effect of the dipole of the radicals was investigated in both the nonpolar and polar solvents. We found that the dipole-dipole interaction stabilized the radical pair with a small separation. The dipole pair can be the precursor of the experimentally observed sandwich radical dimer. The conformation of the dipole pair may not be favorable for the recombination of the radicals, which can be the reason of the high escape probability observed for the p-aminophenylthiyl radical.

Magnetic field effects on radical pairs photochemically derived from 2,3,6,7-dibenzofluorenone

2,3,6,7-Dibenzofluorenone triplets react with triethylamine, 2,4,6-trimethylphenol and ascorbyl-palmitate in micellar solutions to yield radical pairs which are sensitive to magnetic field effects whenever separation is inefficient. The three substrates afford three distinctively different scenarios with respect to the confinement of daughter radicals within the micellar phase. Triethylamine and corresponding radicals are soluble in water, which results in rapid entry—exit equilibria with the micellar phase, and negligible magnetic field effects. Conversely, ascorbyl-palmitate and its radicals are virtually insoluble in water, thus being confined to the micellar phase. Application of an external magnetic field attenuates the geminate recombination of radicals, probably through slowing down T +T 0 and T −T 0 interconversions. The case of 2,4,6-trimethylphenol is intermediate, with the proportion of radicals that escape geminate recombination increasing with increasing applied field. The relevance of these reactions in relation to the use of 2,3,6,7-dibenzofluorenone or molecules of comparable solubility as a triplet probe in biological systems is discussed.

Geminate Recombination of the p-Aminophenylthiyl Radical Pair Produced by the Photodissociation of p-Aminophenyl Disulfide in Nonpolar Solvents

Photodissociation of p-aminophenyl disulfide and the following geminate recombination of the p-aminophenylthiyl radical pair as well as radical dimer formation in nonpolar solvents have been investigated by using picosecond transient absorption spectral measurement techniques. Temperature effects of the geminate recombination and the sandwich dimer formation were measured and compared with the results of the molecular orbital calculation. The spectral broadening of the radical band observed in the short delay times was considered to be an indication of the radical−radical interaction in the geminate pair and was analyzed to obtain the time dependence of the separation distance between the radicals. The exponential narrowing in time was found for the p-aminophenylthiyl radical band in n-hexane, which should carry information about the diffusion process in the solvent cage and is compared with the results of the simulation. The speed of the increase in the radical−radical separation distance shows a larger temperature effect than for the geminate radical recombination to form the parent molecule. The characteristic feature of the dynamic behavior of the p-aminophenylthiyl radical pair, the formation of the sandwich dimer, and the slow recombination of the linear dimer should be due to the large dipole moment of the radical, which not only prevents the recombination but keeps the radicals in close proximity with each other.

Photolysis of ketones in oxygen-saturated micellar solution: oxygen scavenging of C-centered radicals in microheterogeneous media

The oxygen scavenging of C-centered radicals in microheterogenous media was studied by investigation of the photolysis of several ketones (dibenzyl ketone (DBK), o- and p-methyldibenzyl ketone (o-MeDBK and p-MeDBK) and d,1-2,4-diphenylpentan-3-one (d,1-DPP)) in oxygen-saturated sodium dodecylsulfate (SDS) micellar solution. The acyl- and benzyl-type radicals initially formed during the photolysis of these ketones can undergo geminate micellar and random bulk aqueous recombination or can react with oxygen in the bulk or micellar phases to yield peroxy radicals. The efficiency of radical scavenging by oxygen was calculated on the basis of three experimental parameters, i.e. the recombination probability Pr, of a primary acyl-benzyl radical pair (RP), the secondary cage effect for benzyl-benzyl RP recombination and the chemical yield of dibenzyls. Under a partial pressure of 1 atm, oxygen was found to scavenge only those radicals which enter the bulk aqueous phase if the lifetime of the geminate radicals inside the micelles is relatively short (about 20 ns). However, for micellized geminate radicals with a longer lifetime (about 70–80 ns), scavenging by oxygen inside the micelles successfully competes with geminate radical recombination. These results are compared with other investigations involving the dynamics and partitioning of oxygen in SDS micelles.

Direct observation of photodissociation of tetraphenylhydrazine and its derivatives in the solution phase: picosecond study of nitrogen-nitrogen bond rupture in the fluorescence state

The photodissociation of tetraphenylhydrazine and tetratolylhydrazines has been investigated by picosecond transient absorption and fluorescence measurements. We have confirmed that the formation of the product radical is observed in the tens of picoseconds time region and that the geminate radical recombination is not observed in the time region of 10 ps to several nanoseconds. The slow N-N bond rupture occurs in the fluorescence state of tetraarylhydrazines, which can be explained by the slow internal conversion from the fluorescence state to the dissociative state

Picosecond photodissociation and geminate radical recombination in alkane solutions of tetraphenylhydrazine

Tetraphenylhydrazine is observed to dissociate in less than 1 ps in alkane solution after 288 nm excitation. The diphenylaminyl radicals produced undergo rapid geminate recombination at a rate proportional to solvent shear viscosity. A strong magnetic field decreases the recombination yield. A model for the recombination is proposed which involves initial separation of nitrogen atoms and recombination of the radicals by rotational rather than translational motion alone. In a different approach the rates of radical recombination are also well described by Kramers' model of barrier crossing.

Magnetic field effects in radical pair recombination. I. CIDNP and CIDEP in geminate recombination

The method of analytic solution of the stochastic Liouville equation (SLE) describing geminate radical pair recombination in a magnetic field is developed. The method permits taking into account the exchange interaction J(r) and diffusive relative motion of the radicals. It is based on the sudden perturbation approximation and is valid when ξ=( Q/Dα 2) 1 2 <1, where Q is the characteristic magnetic interaction (hfi, etc.), D is the relative diffusion coefficient and α −1 is the characteristic length factor in the exponential dependence J(r). The solution of the SLE is found as an expansion in ξ up to terms ∼E 2. This solution is applied to derive simple matrix expressions for magnetic field effects (MFEs) such as chemically induced dynamic nuclei (electron) polarization (CIDN(E)P). In the high magnetic field limit in ST 0 approximation the simple analytical formulae for CIDN(E)P are obtained. The formulae derived are quite accurate in a wide range of values of the parameters available in the theory. The high accuracy of these formulae is demonstrated by comparison with the numerical calculations. A number of experimental investigations are analyzed by the formulae of the theory. The analysis shows that in general the SLE formalism makes it possible to describe quantitatively the experiments on MFEs in geminate recombination of radicals.

Radical pair mechanism of CIDEP: comparison of the cage and free-diffusion models

A simple analytical expression for chemically induced dynamic electron polarization (CIDEP) in geminate recombination of radicals, undergoing relative diffusion in an inter-radical attractive potential, is obtained. The potential is assumed to have a well in which the cage is formed. The expression for CIDEP is derived by the approximate but accurate solution of the stochastic Liouville equation. It is shown that under general conditions the shape of the CIDEP spectra (the dependence of the intensities of hyperfine components on their numbers) is quite universal in the important cases of (1) a short-lived cage; (2) a long-lived cage; (3) rapid free diffusion; (4) slow free diffusion. In all these cases, the CIDEP spectra are calculated and their significant differences, sufficient for experimental identification, are found.

Surface photochemistry: 13C enrichment by photolysis on silica gel

The enrichment of the 13C isotope by partial photolysis of dibenzyl ketone on silica gel has been demonstrated. The 13C enrichment is substantial and is parallel to the increase in the probability of geminate radical recombination on a silica surface as compared with that in solution. A substantial 13C enrichment was also observed for the rearranged starting material, 1-phenyl-4′-methylacetophenone. The single-stage isotope separation factors are different for the two ketones although both are isomers derived from the same benzyl–phenylacetyl radical pairs.

Surface photochemistry: generation of benzyl radical pairs on dry silica gel

Singlet and triplet benzyl radical pairs have been generated on silica gel by photolysis of a benzyl phenylacetate, a dibenzyl ketone, and a dibenzyl sulfone. The extent of geminate radical recombination has been measured and requires that translational motion of radicals occur on the silica gel surface. This motion was affected by the radical pair multiplicity and the photolysis temperature, but was relatively insensitive to the state of hydration of the silica gel and the presence of coadsorbates. The presence of certain rearranged starting materials, which are not formed in solution, amongst the products from photolysis of both dibenzyl ketone and dibenzyl sulfone on silica gel indicates the restrictions on radical movement on the surface on the shorter timescale of the benzyl–phenylacetyl and benzyl – benzyl sulfonyl radical pairs.

Diffusion-controlled reactions in free radical polymerization—III: An estimate of the importance of geminate recombinations of radicals

Models are described for the discussion of the time taken for the encounter of diffusing particles. The extent of geminate recombination of free radicals produced by the decomposition of initiator is estimated and is 10 per cent or less.