1,2,3-Benzodithiazolyl radicals formed by thermolysis and photolysis of 1,3,2,4-benzodithiadiazines

Mild thermolysis (at 110-150 degreesC) of 1,3,2,4-benzodithiadiazine 1 and its carbocyclic substituted derivatives 2-15 in hydrocarbon solvents quanti tatively yields stable 1,2,3-benzodithiazolyl pi -radicals 1(.)-15(.). Kine tics of this reaction can be described as a first-order process. Arrhenius parameters of the effective rate constant are E-a = 80 +/- 8 kJ mol(-1), k( 0) = 10(6.4 +/- 1.1) s(-1) for 1(.) in squalane. Room-temperature photolysi s of 1(.) in hydrocarbon solvents also affords radical 1(.) in nearly quant itative yield. Quantum yield of photolysis is wavelength dependent and is e qual to 0.08 +/- 0.01 at 313 nm in benzene. Experimental hyperfine coupling (hfc) constants in the ESR spectra of 1(.)-15(.) agree fairly well with th ose calculated at the B3LYP/CC-pVDZ level of theory. Spin density distribut ion in 1(.)-15(.) is in striking contrast to that of isomeric 1,3,2-benzodi thiazolyls but resembles the distribution in correspondingly substituted be nzyl radicals. ESR linewidths of radicals 1(.)-15(.) display some features likely related to spin-rotational relaxation.