Radicals, anions geometries

A number of radical anions of sulfur-containing aromatic compounds have been studied essentially by means of ESR spectroscopy and sometimes by electronic spectroscopy. The studied compounds include aromatic rings separated by the oxidized sulfur functionality. The effects caused by the latter depend on the geometry and topology of the aromatic systems as well as on the electron-withdrawing ability of the other substituents. 

O Malley, P., and S. J. Collins. 1996. Density functional studies of free radicals accurate geometry and hyperfine coupling prediction for semiquinone anions. Chem. Phys. Lett. 259, 296. 

S. Sinnecker, E. Reijerse, F. Neese and W. Lubitz, Hydrogen bond geometries from paramagnetic resonance and electron-nuclear double resonance parameters Density functional study of quinone radical anion-solvent interactions, J. Am. Chem. Soc., 2004, 126, 3280. 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

Reduction of the distannynes with sodium or potassium gives the radical anions. [Ar Sn=SnAr ] has the trans-bent geometry given in Table 11, and shows an ESR spectrum with g= 2.0691, (ll7Sn) 0.83 mT and (117Sn) = 0.85 mT, indicating a very low unpaired spin density on tin. The main line is about 0.6mT wide, and no proton coupling is resolved. 

A series of bicyclo[3.3.0]octanols are accessible by electroreductive tandem cyclization of linear allyl pentenyl ketones 189, as shown by Kariv-Miller et al. [189]. The electrolyses are carried out with an Hg-pool cathode and a Pt-flag anode. As electrolyte, tetrabutylammonium tetrafluororborate is used. The reaction is stereoselective, yielding only two isomers 192 and 193. In a competing reaction, a small amount of the monocyclic alcohol is formed. Since all the monocycles have the 1-allyl and the 2-methyl group in trans geometry it is assumed that this terminates the reaction. The formation of a bicyclic product requires that the first cyclization provides the cis radical anion which leads to cis-ring juncture [190] (Scheme 37). 

From the reaction of the radical anion of l,2-bis[(2,6-diisopropylphenyl)imino]acen-aphthene (dpp-bian) with i -PrMgCl, the persistent radical complex isopropylmagnesium dpp-bian (253) was isolated in yields up to 60% (equation 20). An X-ray crystal-structure determination of 253 showed that the magnesium atom has distorted tetrahedral coordination geometry as the result of the cr-bonded isopropyl group, one coordinate-bonded diethyl ether molecule and A, A -chelate bonding of the dpp-bian radical anion. The radical anionic character of the dpp-bian moiety is indicated by the relatively long Mg—N bond distances [Mg-N 2.120(2) and 2.103(2) A]. 

The difference between the lengths of the central CC bond in the equilibrium geometries of the radical cation of [ 1.1.1 Jpropellane (a one-electron bond, nearly the same length as in the neutral molecule) and of its radical anion (a three-electron bond, much longer than in the neutral molecule), well reproduced by ab initio calculations, emphasizes the inadequacy of the usual simplistic concepts of bonding based on Hiickel theory and neglect of overlap. An explicit introduction of overlap, however, permits a qualitative rationalization of the difference. 

Use the optimized geometry of CH3CCH for a single-point calculation of the energy of the anion CH3CCH (This is a radical anion). 

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