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

Abstract

Mild thermolysis (at 110–150°C) of 1,3,2,4-benzodithiadiazine 1 and its carbocyclic substituted derivatives 2–15 in hydrocarbon solvents quantitatively yields stable 1,2,3-benzodithiazolyl π-radicals 1^•–15^•. Kinetics 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, k₀ = 10^{6.4 ± 1.1} s⁻¹ for 1 in squalane. Room-temperature photolysis of 1 in hydrocarbon solvents also affords radical 1^• in nearly quantitative yield. Quantum yield of photolysis is wavelength dependent and is equal 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 those calculated at the B3LYP/CC-pVDZ level of theory. Spin density distribution in 1^•–15^• is in striking contrast to that of isomeric 1,3,2-benzodithiazolyls but resembles the distribution in correspondingly substituted benzyl radicals. ESR linewidths of radicals 1^•-15^• display some features likely related to spin-rotational relaxation.