1.2.5- Thiadiazole anion radicals

ESR parameters of the anion radical of [l,2,5]thiadiazolo[3,4-c][l,2,5]thiadiazole show good agreement with those calculated by McLachlan s perturbation method using two models for the sulfur atom (sp/spd). The HMO method does not differentiate between the p -orbital and d-orbital models for the thiadiazole anion radicals. Due to the presence of a plane of symmetry through the sulfur atom, the contribution of the 3d-orbital is absorbed... 

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

Anion radicals of benzo-2,l,3-thia(and selena)diazole have been produced by metal reduction in several solvents at low temperatures, and their electron spin resonance has been studied. The reductive desulphurization of substituted benzo-2,l,3-thiadiazoles by stannous chloride in concentrated hydrochloric acid gives o-phenylenediamines in 80—90% yields. The stability of the heterocyclics against reduction increases with the number of chlorine substituents. ... 

Simple 1,2,3-thiadiazoles show three absorption bands in the ultraviolet (UV) 211-217 (emax 4380-5300), 249-253 (1460-2100), 290-294 (195-245) nm <1996CHEC-II(4)289>. The ESR spectrum for the radical anion generated by the electrochemical reduction of the 1,2,3-thiadiazolium ion 8 has been reported. A number of 5-substituted derivatives were also examined and the splitting constants in the ESR spectrum were analyzed <1998MRC8>. 

Discussion on the ESR spectra of 1,2,5-thiadiazoles and 2,1,3-benzothiadiazole radical anions has appeared in both CHEC(1984) <1984CHEC(6)513> and CHEC-II(1996) <1996CHEC-II(4)355>. 

ESR spectra of the radical anions of 3-phenoxy-, 3,4-diphenoxy-, and 3,4-dichloro-l,2,5-thiadiazole and of the radical cations of various 3-aryloxy-4-morpholino-l,2,5-thiadiazoles have been obtained by the electrochemical generation of the ions in 3 x 10-3M solutions of the thiadiazoles in the system MeCN/Et4NC104 (ca. 0.1 M) on a platinum helix electrode directly in an ESR resonator at first wave potentials <2003RJC806>. 

The carbon-poor sulfur-nitrogen bicyclic [l,2,5]thiadiazolo[3,4-f][l,2,5]thiadiazole 6, its radical anion 7, and its dianion 8 have been the subject of a number of theoretical studies <1997JP033, 2004JA11202, 2005IC7194>. 

The redox behaviour has been determined, in acetonitrile solution at a mercury and platinum electrode, of 3,4-disubstituted and fused 1,2,5-thiadiazoles, including 2,1,3-benzothiadiazoles. All ring systems and their derivatives are reversibly reduced initially in a one-electron step to their radical anion, but nitro- and bromo-derivatives are reduced preferentially at the substituent group. ... 

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