Triplet states, electron spin resonance

Comparison of the Computed and Observed 253 458 Triplet State Electron Spin Resonance Spin Hamiltonian Parameters for Thymine11... 

This report was quickly followed by two solid-state electron spin resonance (ESR) studies, which also indicated a bent structure with an angle of 136°. - By the end of the year, Herzberg reexamined his UV spectra and conceded that in fact triplet methylene is bent. His estimate of 136° is remarkably similar to the computed value. Theory had established a beachhead ... 

When, thermolysis of an aryl azide leads to an arylnitrene, the latter is generated in the singlet (182) state, which can be in equilibrium with the triplet (183). Electron spin resonance experiments have... 

Fessenden R W and Verma N C 1976 Time resolved electron spin resonance spectroscopy. III. Electron spin resonance emission from the hydrated electron. Possible evidence for reaction to the triplet state J. Am. Chem. Soc. 98 243-4... 

The paramagnetism of the triplet state can be observed by electron spin resonance spectroscopy. This is perhaps the most reliable means of determining the existence of a triplet state since the ESR signals can be predicted using the following Hamiltonian operator ... 

Electron spin resonance (ESR) studies of the urazole-bridged 1,3 diradicals 64 derived from the azoalkanes 63 confirm a triplet ground state for these species. The nearly zero symmetry parameter, that is, Elhc= 0.0004 0.0001 cm-1, for the triplet diradical 64 of the diphenyl azoalkane 63 establishes a planar conformation <1995JOC308, 1997JA10673>. 

Most stable ground-state molecules contain closed-shell electron configurations with a completely filled valence shell in which all molecular orbitals are doubly occupied or empty. Radicals, on the other hand, have an odd number of electrons and are therefore paramagnetic species. Electron paramagnetic resonance (EPR), sometimes called electron spin resonance (ESR), is a spectroscopic technique used to study species with one or more unpaired electrons, such as those found in free radicals, triplets (in the solid phase) and some inorganic complexes of transition-metal ions. 

Brinen, J. S. Application of electron spin resonance in the study of triplet states. III. Extinction coefficients of triplet-triplet transitions. J. Chem. Phys. 49, 586 (1968). 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

ESR spectroscopy (a) J. E. Wertz and J. R. Bolton, in Electron Spin Resonance Elementary Theory and Practical Applications McGraw-Hill, New York, 1972. (b) J. A. Berson, in The Chemistry of the Quinonoid Compounds, Vol. II, S. Patai and Z. Rappoport, Eds., John Wiley Sons, Inc., New York, 1988. (c) W. Gordy, in Theory and Applications of Electron Spin Resonance, Vol. 15, A. Weissberger series Ed., W. West, Ed., John Wiley Sons, Inc., New York, 1980, p. 589. (d) E. Wasseiman, W. A. Yager, and L. C. Snyder, Electron spin resonance (E.S.R.) of the triplet states of randomly oriented molecules, J. Chem. Phys. 1964, 41, 1763. 

In a few papers an attempt has been made to interpret quantum-mechanically the zero-field splitting parameters, which have been determined from the analysis of the electron spin resonance spectra on the triplet state of the pyrimidine bases. 

The photolysis of the 4-alkylidene-l-pyrazoline (94) gives rise to two isomeric methylene cyclopropanes (95 and 96).76 The available evidence points to the intermediacy of a trimethylenemethyl species (97) in the triplet state which can cyclize in three ways. The same species is postulated in the photolysis of a series of 4-alkylidene-l-pyrazoline-3-carboxylates.77 This appears to be a general route to derivatives of trimethylenemethyl trimethylenemethyl itself has been generated from 4-methylene-1-pyrazoline and the triplet nature of the intermediate identified by electron spin resonance (ESR) spectroscopy.78... 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

The involvement of the triplet state an unequivocally be studied by applying electron-spin resonance to the phosphorescent state. From this, the time of deactivation can also be obtain. 

In recent years the state of the art in ESR instrumentation has matured enough that the technique is being used more and more by photochemists to elucidate the structure and reactivity of free radicals and organic triplet molecules in photochemical systems. Indeed, a renewed and increasing interest in the field of free radical chemistry during the last decade can be attributed mainly to the rapid development of electron spin resonance... 

On the other hand, the cyanomethylene H—C—C=N triplet as well as a number of odd alternant methylene compounds, propargylene and its homologues, H— —C=CR (R = H, CH3, C6H5), and H—C— C=CC=CR [R = CH3, C(CH3)3, C6Hs] have been predicted to be linear on the basis of the zero-field splitting parameters from electron spin resonance (ESR) experiments (15). The linear nature of the first named H—C—C=N triplet carbene has been confirmed by a micro-wave study in the gas phase. However, if the potential well in which the linear configuration lies is shallow, the ground state may not show the effects of nonlinearity (142). 

Electron spin resonance measurements on thymine monoanion (triplet state) confirm the deprotonation of thymine from N -1 and/or N-3 atoms. 

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