Hyperfine structure radical

Eriksson, L. A., Malkina, O. L., Malkin, V. G., Salahub, D. R., 1994, The Hyperfine Structures of Small Radicals from Density Functional Calculations , J. Chem. Phys., 100, 5066. 

A complex EPR spectrum detected (47) in the y-irradiated Vahrenkamp molecule Mn2 (CO) 8(/i-AsPh2) 2 is thought to belong to the cation of the molecule. From an analysis of the 55Mn and 75As hyperfine structure it was concluded that the d6d5 dimer radical has its single unpaired electron in a o MO composed of Mn 3(1 2 y2 orbitals. 

Nickel carbonyl radicals show an even greater tendency than cobalt carbonyls to cluster in a krypton matrix. Three binuclear nickel carbonyls have been detected by EPR spectroscopy in the products of y-irradiated Ni(CO)4 in Kr, yet no mononuclear species has been positively identified (65). 13C hyperfine structure has... 

Hyperfine splitting. As was discussed above, one consequence of placing a free electron onto a molecule is to alter its 0-value. Another is that the electron spin comes under the influence of any magnetic nuclei present in the radical, with the result that the spectrum is split into a number of lines centred on the position of the single resonance expected for the simple /transition discussed above. This hyperfine structure is the most useful characteristic ofepr spectra in the identification of an unknown radical species. 

It follows that the hyperfine structure is characteristic of the molecular symmetry and of great value in identifying the parent radical species. 

Figure 8.6 EPR spectrum of the dibenzo[a,e]cyclooctene radical cation showing hyperfine structure. The radical was generated by UV irradiation of the neutral parent molecule. From Gerson, F., Felder, P., Schmidlin, R. and Wong, H. N. C., J. Ghent. Soc., Ghent. Gommun., 1659-1660 (1994). Reproduced by permission of The Royal Society of Chemistry.

Thirdly, from the EPR hyperfine structure, it is possible to compute the structure of the radical (both the atomic and the electronic structures). 

Electron-spin resonance (e.s.r.) spectra with characteristic hyperfine structure have been recorded during the initial stages of the Maillard reaction between various sugar and amino compounds. The products responsible for the spectra appear to be IV, Af -disubstituted pyrazine radical cations. The pyrazine derivatives are assumed to be formed by the bimolecular condensation of two- and three-carbon enaminol compo-... 

EPR studies showed that RsSi radicals (R = Me, Et, i-Pr and t-Bu), obtained by hydrogen abstraction from the parent silanes by photogenerated t-BuO radicals, add to Ceo to form the corresponding adducts [29], The spectra of MesSi- and t-Bus Si-adducts have hyperfine structure due to 9 and 27 equivalent protons, respectively, at 27 °C suggesting free rotation of Si—C bonds on the EPR time scale. On the other hand, the middle members of the series, EtsSi-and z-Pr3 Si-adduct radicals, showed a unexpected hyperfine manifold which has been accommodated with free rotation about the Si—Ceo and frozen rotation about the Si—R bonds on the EPR time scale. 

ESR methods unambiguously establishes the presence of species bearing unpaired electrons (ion-radicals and radicals). The ESR spectrum quantitatively characterizes the distribution of electron density within the paramagnetic particle by a hyperfine structure of ESR spectra. This establishes the nature and electronic configuration of the particle. A review by Davies (2001) is highly recommended as a guide to current practice for ESR spectroscopic studies (this quotation is from the title of the review). The ESR method dominates in ion-radical studies. Its modern modifications, namely, ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) and special methods to observe ion-radicals by swiftness or stealth are described in special literatures (Moebius and Biehl 1979, Kurreck et al. 1988, Werst and Trifunac 1998). 

Besides BuCgo, a series of other monoalkyl- and also monoaryl-radical adducts have been similarly investigated [1, 12, 15-17, 20-23]. The observed hyperfine structures are those expected for the corresponding group R. An unusual long-... 

Attempts have been made by various groups to detect EPR signals from this radical ion. On ZnO, a symmetric EPR spectrum with g = 2.003 and A H s3G can be observed on adsorption of oxygen. When 170-enriched oxygen is used to produce this EPR spectrum, no hyperfine structure is observed (3). It is possible that this spectrum can be assigned to OJ which is undergoing a rapid exchange reaction ... 

In order to appreciate the size of the basis sets required for fully converged calculations, consider the interaction of the simplest radical, a molecule in a electronic state, with He. The helium atom, being structureless, does not contribute any angular momentum states to the coupled channel basis. If the molecule is treated as a rigid rotor and the hyperfine structure of the molecule is ignored, the uncoupled basis for the collision problem is comprised of the direct products NMf ) SMg) lnii), where N = is the quantum number... 

The new spectrum has a well resolved hyperfine structure. It seems to be the ESR spectrum of the monosubstituted intermediate l-methoxy-2.4.6-triphenyl-phosphorin radical cation. 

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