Laser-flash photolysis study of dithiobis (tetrazole); Reactivities of tetrazole-thio radical

The photo-cleavage of S - S bond of 5,5′-dithiobis (1-phenyl-1H-tetrazole) has been studied by the nanosecond-laser flash photolysis method. The transient absorption band at ca. 430 nm was attributed to 1-phenyl-1H-tetrazole-5-thio radical forming by the S - S bond fission. For the reaction with conjugated dienes, an addition reaction takes place forming the S - C bond, suggesting that unpaired electron of the radical localizes mainly on the S-atom. From the decay rates of the radical, the addition reaction rate constant for 2-methyl-1,3-butadiene is evaluated to be 5.5 × 10⁹ M^-1 s^-1 in THF at 23°C, which is as fast as diffusion controlled limit. The reactivity of the radical is ca. 100 times higher than that of the PhS. The reactivity of the thio radical to O₂ was too low to evaluate, which is one of the characteristics of a S-centered radical. The rate constant for 1,4-cyclohexadienene (1.4 × 10⁸ M^-1 s^-1) is larger than that of cyclohexene (2.8 × 10⁷ M^-1 s^-1), suggesting the hydrogen abstraction is a main reaction. The MO calculations have been performed for these radicals to reveal the reason of the high reactivity of the radical.