Physical Properties of 1,3,2,4-Dithiadiazolylium Salts

The nmr data available for 1,3,2,4-dithiadiazolylium salts are compiled in Table X. Studies by Passmore and co-workers have shown the ring- 

Chemical shifts referenced to Me4Si CFC13 Me4Si Me.N02  

Heterocyclic-ring assignments are given in accordance with the following diagram  

A recent study [93JCS(D)967] of aryl-l,3,2,4-dithiadiazoIylium salts has shown that they typically show a half-wave reduction potential in the region +0.29 Em +0.4 V (with reference to the standard calomel electrode). As found for the 1,2,3,5-isomers, these salts show a Hammett relationship between 1/2 and the a value for the para substituent on the aryl group. The greater reaction constant p, for the 3+/3 redox system, compared with 2+/2, is in agreement with MO calculations—which indicate some unpaired spin density on the heterocyclic carbon, allowing delocalization onto the aryl substituent. (The nodal plane associated with 2 prohibits, to a first-order approximation, delocalization with the aryl functionality.) 

Because of isomerization of 3 to the more stable 2 (Sections XXI. A. 1 and XXIII.G.2,3), cyclic voltammograms of 3+ salts tend to show weak electroactive peaks due to the redox system 2+/2. Such effects are most prominent when using slow scan rates with radicals that rearrange significantly at ambient temperatures. 

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