Dithiazolyl dimerisation

Theoretical calculations on the dithiazolyl radical 4 (R=CF3) have recently shown that n -n dimerisation was unfavourable but association of two such dimers via electrostatic interactions generated a thermodynamically stable tetramer consistent with single crystal X-ray studies. Thus while the value of [AE-P ] may favour (or disfavour) dimer formation, the van der Waals, dipole contributions and electrostatic interactions to the lattice enthalpy should not be underestimated in assessing the thermodynamic stability or instability of these... 

However at elevated temperatures (T2 > Tj, Figure 9) the increased entropy (TAS) associated with an open shell structure overcomes the ti —ti enthalpy of dimerisation associated with these distorted Ti-stacked structures and they undergo a solid-solid phase transition (Figure 9) The high temperature phase is typically associated with a Ti-stack of regularly spaced radicals which exhibit longer inter-radical S- S contacts (ca. 3.7 A). This process was first observed by Oakley60 in the DTA radical thiadiazolopyrazine-l,3,2-dithiazolyl 26, and a number of other derivatives have subsequently been identified which exhibit similar behaviour. These are compiled in Table 1. 

Only 5 derivatives have been isolated to date. While the non-fused system forms a 71 -ji dimer (<7S s 3.27 A),86 structural studies on the more delocalised derivatives indicate a reduced tendency to dimerise. However, rather than forming optimal n-stacked motifs, there is a clear tendency to adopt slipped n-stack structures which minimise orbital overlap. As a consequence although the Hubbard energy is low, conductivity measurements show poor conductivities (10 5 S cm-1) similar to those observed in either the isomeric 1,3,2-dithiazolyl (4) or 1,2,3,4-dithiadiazolylradicals (2).84... 

Theoretical calculations on the dithiazolyl radical 4 (R=CF3) have recently shown that dimerisation was unfavourable but association of two such... 

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