Eugene N. Step

Flash photolysis and time-resolved electron spin resonance studies of triplet benzophenone quenching by hindered amine light stabilizers (HALS). A comparison of HALS amines and aminoethers as electron and hydrogen atom donors

Abstract Transient absorption spectroscopy and time-resolved electron spin resonance (TR-ESR) were employed to understand the electron and hydrogen atom transfer quenching of triplet benzophenone ( 3 BP) in acetonitrile and benzene by amine and aminoether derivatives of 2,2,6,6-tetramethylpiperidine (hindered amine light stabilizers, HALS). The amines ( I and II ) are efficient quenchers of 3 BP. The observed quenching rate constants were found to be in the range 6 × 10 8 –6 × 10 9 M −1 s −1 and depend on the structure of the amine and the solvent. On the other hand, the aminoethers ( III and IV ) do not exhibit any quenching at all. Reactions of 3 BP with amines I and II in benzene yield ketyl radicals (detected by transient absorption spectroscopy) and the corresponding aminyl or aminomethyl radicals (detected by TR-ESR). Neither experimental method provided any evidence for products of the reactions of 3 BP with aminoethers III and IV . The experimental results are discussed from the viewpoint of the mechanism of polymer stabilization by HALS.

Spin catalysis of the radical recombination reaction

Abstract Rate constant of the recombination of alkyl radicals with nitroxide biradicals in dioxane and ethyl benzene exceed rate constants with ‘parent’ mono radicals by 10%–50%. The difference in reactivity is attributed to spin catalysis of the recombination reaction which occurs in the encounter pair of an alkyl radical with one of the biradical termini by the second spin-carrying terminus.

Singlet state photochemistry of dibenzyl ketone and its o-tolyl derivatives

The singlet state contribution of α-cleavage in the photochemistry of dibenzyl ketone (DBK) and o-tolylmethyl benzyl ketone (TBK) was investigated. The fitting of a master Stern—Volmer equation for the simultaneous quenching of the S1 and T1 states of DBK and TBK leads to the conclusion that in both ketones α-cleavage occurs in the singlet state, but to a lesser (20%–30%) extent than in the triplet. Comparison of the relative fluorescence and α-cleavage quantum yields of DBK and TBK is also consistent with the presence of singlet state α-cleavage in both ketones. The introduction of a methyl group in the ortho position of the phenyl ring in TBK introduces a second singlet state ketone reaction, δ-hydrogen-atom abstraction. This reaction is faster than α-cleavage in the S1 state, but proceeds with a low quantum efficiency. A δ-hydrogen-atom-induced internal conversion is proposed to be a major mechanism for the deactivation of the S1 state of TBK. This deactivation of S1 serves to reduce the efficiency of intersystem crossing and competes effectively with α-cleavage in S1. For TBK, o,o′-dimethyl TBK and mesitylmethyl benzyl ketone, which all show fast singlet state hydrogen-atom abstraction, the quantum yields of fluorescence and α-cleavage decrease. On the basis of the assumption of a certain (20%) participation of singlet state α-cleavage and fast δ-hydrogen-atom abstraction, a general mechanism and rate constants of each process were estimated for DBK and TBK.

Magnetic effects in the photolysis of micellar solutions of phenacylphenylsulfone

Abstract The photolysis of phenacylphenylsulfone (PhCOCH2SO2Ph, (1) in aqueous SDS solution proceeds through a primary β cleavage to produce a germinate triplet radical pair (RP) of benzoylmethyl and phenylsulphonyl radicals. The photolysis of 1 in micellar systems has been investigated by product studies, by CIDNP and CIDEP spectroscopy, and by magnetic isotope effects (13C and 33S). In addition, magnetic field effects (MFE) on the converson of 1 and on the yields of products have been monitored. The MFE of acetophenone, the escape product and the MFE of an unstable product of head-to-tail recombination of the geminate RP (a light-absorbing transient, LAT) have the same shape, but both differ from the shape of the MFE of the conversion of the staring ketone. It is proposed that the primary geminate pair (RP1) undergoes recombination to generate the starting material and is subjected to a MFE in which hyperfine coupling is the dominant intersystem crossing (ISC) mechanism. The primary geminate pairs, which do not recombine to form a starting ketone, yield a different geminate pair (RP2), which has a different diffusional history than RP1 and is located in a different micellar environment. RP2 is proposed to form LAT in high yield and to give benzoylmethyl radicals, which escape into the bulk phase and yield acetophenone. The results are consistent with paramagnetic relaxation as the dominant mechanism for ISC of RP2.

29Si magnetic isotope effects in the photolysis of silyl ketones

Abstract Silicon isotope selection induced by magnetic isotope effects has been discovered in the photolysis of a silyl-containing ketone, PhCH 2 COSi(CH 3 ) 2 Ph. Silicon and carbon atom isotope selection occurs most efficiently when the photolysis is directed via the triplet radical-pair mechanism. This conclusion is supported by CIDNP data.

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

Abstract 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 P r , 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.

Wavelength effects in the photolysis of ketones: intramolecular hydrogen abstraction of o-methyl-dibenzyl ketones vs. α-cleavage in micellar solution

Abstract The chemical yields of indanols, the products of intramolecular δ-hydrogen abstraction produced in the photolysis of o -methyldibenzyl ketone ( o -MeDBK) and o , o′ -dimethyldibenzyl ketone ( o , o′ -Me 2 DBK) in SDS micelles are sensitive to the wavelength of excitation: the yields of indanols are about two times smaller for excitation at 254 nm than at ca . 320 nm. At the same time the magnitude of the secondary cage effect, arising from coupling of geminate benzyl radical pairs is increased slightly for excitation at 254 nm relative to 300 nm. In contrast to the results for micellar solution, in hexane no wavelength effect on the yields of indanols is observed. A kinetic analysis of the reaction scheme shows that the decrease in the yield of indanol in micelles for shorter wavelength excitation is caused mainly by a decrease in the recombination probability P r of the primary phenacyl/benzyl radical pair resulting from α-cleavage. This decrease in probability for recombination of the primary geminate pair causes an increase in the formation of the secondary geminate benzyl/benzyl pair and a higher relative yield of geminate cage recombination. Intramolecular hydrogen abstraction as well as further cyclization of the biradical to indanol are not wavelength dependent processes. A Salem correlation diagram for hydrogen abstraction provides theoretical support for the proposed mechanism.

Magnetic isotope effect in the reaction of disproportionation of radical pairs

Abstract The photolysis of methyldeoxybenzoin was carried out in aqueous SDS solution inthe earths field and in a field of 1500 G. Benzaldehyde and styrene are produced by disproportionation of benzoyl—sec-phenethyl geminate radical pair. Both benzaldehyde and the recovered methyldeoxybenzoin are enriched in 13 C. These results represent the first direct evidence that both disproportionation and recombination of a radical pair are identically selective to the magnetic isotope effect, a results consistent with the conclusions that the reaction pathways which involve a triplet geminate radical pair are selected after the completion of intersystem crossing.