Paramagnetism matrix resonance spectroscopy

Since the phenoxyls possess an S = ground state, they have been carefully studied by electron paramagnetic spectroscopy (EPR) and related techniques such as electron nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM). These powerful and very sensitive techniques are ideally suited to study the occurrence of tyrosyl radicals in a protein matrix (1, 27-30). Careful analysis of the experimental data (hyperfine coupling constants) provides experimental spin densities at a high level of precision and, in addition, the positions of these tyrosyls relative to other neighboring groups in the protein matrix. 

The problem of bringing a large magnet into the field for ambient measurements has been overcome in electron paramagnetic resonance (EPR, also called electron spin resonance, ESR) by Mihelcic, Helten, and coworkers (93-99). They combined EPR with a matrix isolation technique to allow the sampling and radical quantification to occur in separate steps. The matrix isolation is also required in this case because EPR is not sensitive enough to measure peroxy radicals directly in the atmosphere. EPR spectroscopy has also been used in laboratory studies of peroxy radical reactions (100, 101). 

Seventeen-electron species have also been found to form complexes with noble gases. For example, the two paramagnetic radicals RrMn(00)5 and [KrFe(00)5]+ have been detected by EPR spectroscopy by Morton, Perutz, and co-workers following the y-radiolysis of HMn(00)5 and Fe(00)5 in laypton matrices at 77 and 20 K, respectively (37). Evidence for the interaction of Kr with the unpaired electron on the metal center came from the observation of hyperfine couphng with a single Kr nucleus in the EPR spectra of these species. As an example, the EPR spectrum obtained from y-radiolysis of HMn(CO)5 in a matrix of krypton enriched to 42% in the isotope Kr (I = ) is shown in Fig. 5. The spectrum shows the resonances of the Mn(CO)5 radical with characteristic decets of satellites due to hyperfine interaction between the unpaired spin on Mn and a Kr nucleus. 

We have used two spectroscopic methods to investigate the triplet state of localized diradicals. On the one hand, matrix electron paramagnetic resonance (EPR) spectroscopy was employed, which affords the zero-field splitting (zfs) parameters D and E and gives valuable information on the electronic structure [5-7]. On the other hand, time-resolved transient... 

Electron paramagnetic resonance (EPR) spectroscopy is a powerful method for investigating interrelations between electronic and structural features of systems with unpaired electrons, such as radicals, coordination compounds and paramagnetic sites in solids [107-109]. Along with the hyperfine coupling constants, the electronic g matrix (often called g tensor ) is the fundamental quantity furnished by EPR spectroscopy. Nevertheless, it was only in the past few years that g tensors attracted significant attention of the research community that deals with high-level quantum chemical calculations [86,110-113]. 

Stepwise generation of mononitrene, dinitrene, and trinitrene by the matrix photolysis of 2,4,6-triazido-l,3,5-triazine 42 (cyanuric triazide) was observed by matrix IR and electron paramagnetic resonance (EPR) spectroscopy (Scheme 5). The generated species were identified by comparison of their matrix IR spectra with DET computational results <2004JA7846>. 

Once synthesized, the nature of the encaged complex needs to be confirmed by a combination of characterization techniques. Spectroscopies such as UV-vis, IR, and Raman can be very useful as the zeolite matrix does not usually obstruct the interesting zones of the spectra. The former is used to probe the metal, while the latter two are more ligand-based techniques. Results are usually compared to the pure, unsupported, complex. Solid-state NMR can also give precious information but with enriched samples or analyses of heteroatoms, electron paramagnetic resonance (EPR) spectroscopy has also been used, especially with Co(II) complexes. X-ray photoelectron spectroscopy... 

In radiolysis reactions, excited molecules, cations, free electrons, anions, and radicals are the main intermediates. For the study of radical anions, cations and radicals formed in a solid matrix, e.g., in polymers, electron paramagnetic resonance (EPR) spectroscopy is used since the 1950s. Anion and cation species can also be studied by UV spectroscopy. Absorption spectra of many organic radical anions and cations were measured in tetrahydrofiiran or in halogenated hydrocarbon matrices (Shida 1988). 

Nuclear Magnetic Resonance (NMR) Spectroscopy This techni-que85-89 utilizes solid-state NMR to analyze nanoscale dispersion for the overall sample. The iron in the montmorillonite structure facilitates the relaxation of nearby protons, which provides information on the dispersion of the clay in the polymer matrix. In the cases reported, a signal in the polymer is identified and its relaxation time (fy) is measured the relaxation time depends on how close the proton is to a paramagnetic iron atom. On average, the protons of the polymers will be closer to the iron in the clay in a well-exfoliated system and... 

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