Time-resolved spectroscopy, hydrogen transfer

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

Zhang J, Grills DC, Huang K-W, Fujita E, Bullock RM. (2005) Carbon-to-metal hydrogen atom transfer Direct observation using time-resolved infrared spectroscopy. /Hot Chem Soc 127 15684-15685. 

The physical aspects underlying the reactivity of P-centred radicals are essential in order to benefit from the full potential offered by phosphorus-containing compounds. Hydrogen atom transfers (HATs), halogen atom abstractions, cyclisations and additions are the tool box of the widely applied free radical chemistry to date. Several authors have determined key rate constants through various physical methods, including time-resolved ESR spectroscopy and flash photolysis.3... 

The electron-transfer reaction as an initiation mechanism of the copolymerization of maleimide and vinyl ethers was investigated by time-resolved optical spectroscopy and FT EPR. Maleimide vinyl ether resins polymerize upon UV irradiation without the addition of a photoinitiator. The hrst step of initiation is an electron transfer from the ground-state vinyl ether molecule to the triplet maleimide. The maleimide radical-anion was detected by FT EPR in systems with hydroxybutyl vinyl ether and thiocyanate as electron donors whereas the maleimide hydrogen-adduct radical was found with 2-propanol as triplet quencher. 

Two and Three Component Photoinitiating Systems Based on Coumarin Derivatives. AUonas, X. Fouassier, J.P. Kaji, M. Miyasaka, M. Hidaka, T. Departement de Photochimie Genetale, UMR No. 7525, Ecole Nationale Superieure de Chimie, Mulhouse, Fr. Polymer (2001), 42 (18), 7627-7634 (Eng.). Several coumarin or ketocoumarin/additives combinations (bisimidazole deriv., mercapto-benzoxazole, titanocene, oxime ester) can initiate quite efficiently radical polymn. The interactions between the excited states of coumarins or aketocoumarin and various additives have also been studied by laser absorption spectroscopy, time resolved fluorescence and photocond. the redox potentials of these different compds. have been measured and allowed the calcn. of free enthalpy variations for a possible electron transfer reaction. The whole results explain the interaction mechanism and show that the coumarins are able to form radicals through an electron transfer reaction with the different additives whereas the ketocoumarin leads to an energy transfer with bisimidazole and to an hydrogen abstraction with the benzoxazole deriv. 

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