Time-resolved ESR

Jent F, Paul H and Fischer H 1988 Two-photon processes in ketone photochemistry observed by time-resolved ESR spectroscopy Chem. Phys. Lett. 146 315-19... 

Verma N 0 and Fessenden R W 1976 Time resolved ESR spectroscopy. IV. Detailed measurement and analysis of the ESR time profile J. Chem. Phys. 65 2139-60... 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

In addition, Suehiro et al. (1987) were able to accurately determine the rates of dediazoniation of twenty substituted aryldiazenyl radicals formed from aryl(aryl-thio)diazenes (Scheme 8-36), in cyclopropane and in alkenic solvents at -48 to — 117°C, using a time-resolved ESR method. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

Kawai A, Shibuya K. Electron spin dynamics in a pair interaction between radical and electronically-excited molecule as studied by a time-resolved ESR method. J Photochem Photobiol C Photochem Rev 2006 7 89-103. 

Analysis of a CIDEP pattern with time-resolved ESR (TR ESR) spectra provides a solid conclusion to be made on the spin multiplicity of molecular precursors of polarized free radicals (a singlet or a triplet excited molecule) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry. Continuous wave TR ESR (CW TR ESR) devices are widely used for detection of photogenerated radicals. They usually consist of a pulsed ns laser with detection of transients by their ESR spectra with a X band ESR spectrometer in the direct detection mode (no hied modulation).Time-resolved Eourier transform ESR (FT ESR) has some advantages and drawbacks with respect to CW TR ESR. Rather sophisticated FT ESR devices have become available, and FT ESR studies become more common. 

As shown in reactions (2-1), (2-2), and (2-3), many radicals are produced in photochemical reactions. Electron spin resonance (ESR) is a good method to detect radicals. Especially, short-lived intermediate radicals have recently been measured with various time-resolved ESR (TRESR) techniques [3]. In this section, an introductory review of ESR will be given. As shown in Eq. (1-35), the magnetic moment (p ) due to an electron spin (s or S) is given by... 

Beckert D, Mehler K. (1983) Investigations of hydrogen atom addition to vinyl monomers by time resolved ESR spectroscopy. Ber Bunsen es Phys Chem 87 587-591. 

Wisniowski PB, Bobrowski K, Carmichael I, Hug GL. (2004) Bimolecular homolytic substitution (Sjjj) reaction with hydrogen atoms. Time resolved ESR detection in the pulse radiolysis of a-(methylthioacetamide). J Am Chem Soc 126 14668-14474. 

Wisniowski P, Bobrowski K, Filipiak P, Carmichael 1, Hug GL. (2005) Reactions of hydrogen atoms with a-(alkylthio)carbonyl compounds. Time-resolved ESR detection and DFT calculations. Res Chem Interm 31 633-641. 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

Carbonyl groups are most significant in triplet state photochemistry. Spectroscopic studies include time resolved ESR studies of enols of -hydroxybenzaldehyde 2 energy transfer from acetophenones to 9, 1 0-d ibr omoa n t hr a ce ne (S.,) which involves higher triplet states of the donor283, two photon excitation of substituted triplets of acetophenone , transient resonance Raman spectra of deuterated benzophenones , and diffuse reflectance of... 

Structure assignments to the various OH radical adducts of ascorbate have been further elucidated and/or confirmed using time-resolved ESR coupled with pulse radiolysis (37). The complexity of the system becomes apparent when one considers that OH can form the ascorbate anion radical by direct electron transfer or it can add to either end of... 

H-CIDNP and ESR techniques. A laser flash photolysis and time-resolved ESR study of the formation of phosphinoyl radicals from benzoyldiphenyl-phosphine oxide and 257 has appeared. The addition of dialkyIphosphoryl radicals to a fullerene system has also been studied. Interest in adducts of phosphine oxides with proton donors, notably phenols and other solvent guests ° has continued. 

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... 

A time resolved esr study of the magnetic and decay properties... 

The radical pairs in the diradical and dicarbene reaction intermediates have been extensively studied spectroscopically and lead to a consistent picture of the reaction mechanisms and of the ir-bond and radical electron structure of the intermediates. A new field of research has been opened by time resolved ESR spectroscopy of transient triplet states on the polymer chains. Spectroscopic worlc concerning triplet excited states is just at the very beginning. The correlation of the optical spectra (Figure 8), the transient absorption data well... 

The decomposition of a-GAV from the triplet excited state was monitored in 1 3 ethanokbenzene by time resolved ESR spectroscopy. A broad emissive resonance... 

Carbonyl triplet states continue to be significant subjects of photochemical research. Time-resolved ESR studies on excited triplet 2-methylacetylacetophenone and photoenolization are particularly significant since the enol triplets are not phosphorescent. 

Spin polarization conservation effects during triplet-triplet energy transfer in fluid solution has been observed by time-resolved ESR spectroscopy using the pyridinyl radical dimer and a sensitizer. 60 relaxation parameters for spin orbit coupling... 

Early radical kinetic values involved ESR methods, typically with continuous irradiation [16]. Much of the work had to be performed at low temperatures, and the resulting kinetic values at ambient temperatures could contain large errors. A modern variant of ESR spectroscopy removes the low-temperature requirement for relatively slow radical reactions and provides more precise data at ambient temperatures. The technique involves time-resolved ESR studies using pulsed photol-yses with continuous monitoring. It can be applied not only when the radicals of interest are formed instantly in the photochemical reaction but also when they are produced during the observing period from reaction of a first-formed transient [2]. 

Rate constants for some alkoxyl radical fragmentations are shown in Fig. 9. The tcrt-butoxyl radical (26) fragmentation to acetone and the methyl radical has been studied for years, but the rate constant shown below is from a very recent work that employed time-resolved ESR methods [2]. The cumyloxyl radical (27) fragmentation was studied directly by LFP methods, taking advantage of the IR and UV absorbances of this radical [57]. The rate constants for the reversible ring opening of the cyclopentyloxyl radical (28) were determined by competition kinetics [58], and one should note that the kinetic values are at 80 °C. 

Incidentally, this process was investigated not only by means of optical detection (e = 1.9 x 10 at 280 nm) but also by time-resolved ESR (aj,j 24.6 G aj3 8.9 G), 2 serving as an instructive example for the value of complementary experimental approaches. 

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