Intermolecular Association Radical Dimerization

Salts of the binary sulfur nitride cation radical [SsNa] (4.14, E = S) exhibit intermolecular association involving two S S interactions [d(S—S) 2.90 The SOMO-SOMO ( r - r ) interactions in the dication [S6N4] are illustrated in Eig. 4.9. The dimer adopts a transoid configuration. A similar structure [d(Se Se) 3.14 A] is found for the selenium analogue [Sc6N4] (4.14, E = Se).  

2-Dithiazolyl radicals have also attracted considerable attention recently as potential molecular conductors (Section 11.3.6). The advantage of these systems over 1,2,3,5-dithiadiazolyls lies in their relatively low disproportionation energies (Section 11.3.1). 

The S S distance in the dimer of the thiatrazinyl radical 4.16 (E = S) is 2.67 similar to the values observed for the intramolecular S S interactions in 4.12 (R = NMc2) and 4.1. Indeed this dimerization involves a interaction analogous to those depicted in Eigure 4.8 

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Crystals of dimers [22]2 (R1 = Me, Et R2 = H) are isostructural. The molecules adopt a normal slipped n-stackcd motif, in which the molecules appear to be linked laterally into ribbons, much as the cations are in the triflate salt above. However, closer inspection of the lateral intermolecular contacts reveals that, while the radicals are coupled at both ends by short four-center Se(l)—N(l ) contacts, they are actually fused at one end by a covalent Se(2)-Se(2 ) cr-bond (2.460(2) A for R1 = Me, 2.4628(8) A for R1 = Et). At the same time, the associated Se(2)-S(2) bond opens to a value (2.785(3) A for R1 = Me, 2.7847(9) A for R1 = Et) intermediate between the sum of the covalent radii and the expected van der Waals contact. Within this dimer or supermolecule there is a series of bond length changes relative to those seen in the cation [22]+, for example, a shortening of the C(3)-N(3) and C(4)-S(2) distances (Table 17), all of which are consistent with the closed-shell valence bond formulation [22]2. 

The effect of N-acetyl substitution in methionine on the nature of transients formed after one-electron oxidation was studied as a function of pH and NAM concentration. The observed absorption bands with X = 290 nm, 360 nm, and 490 nm were respectively assigned to a-(alkylthio)alkyl, hydroxysulfuranyl and dimeric radical cations with intermolecular three-electron bond between sulfur atoms. N-acetylmethionine amide (NAMA) (Chart 7) represents a simple chemical model for the methionine residue incorporated in a peptide. Pulse radiolysis studies coupled to time-resolved UV-Vis spectroscopy and conductivity detection of N-acetyl methionine amide delivered the first experimental evidence that a sulfur radical cation can associate with the oxygen of an amide function vide infra). ... 

Although the S3N2+ radical cation (13) has been well characterized in solution by its characteristic five-line (1 2 3 2 1) ESR signal (g = 2.01 aw = 3.15G), this S-N heterocycle is usually found in the solid state as the dimeric cation SeN4 + (7), in which two 7 7r-electron S3N2+ rings are associated via weak intermolecular S-S interactions [d S-S) = 300-310 pm]. 

These reactions show that dimers can be formed (and have been observed) if either peptide radicals or side chain radicals on the globin surface react intermolecularly. They also show that if these radicals are close enough to the iron center, electron transfer can occur leading to either reduction or oxidation, depending on the valence state, and to an associated color change. 

The most reliable mechanism for the dimer formation from PS proved to be associated with the intermolecular reaction of the benzyl radicals, which are formed from the secondary macroradicals, rather than the intramolecular 1,3-transfer of the secondary macroradical. 

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