title: UV-induced hydrogen-atom transfer in 3,6-dithiopyridazine and in model compounds 2-thiopyridine and 3-thiopyridazine.
abstract: Monomeric 3,6-dithiopyridazine (3-mercapto- 6(1H)-pyridazinethione) was studied using the matrix-isolation method combined with quantum chemical calculations. The monomers of 3,6-dithiopyridazine, trapped from the gas phase into a low-temperature Ar matrix, were found to adopt the thione-thiol structure. In agreement with this experimental observation, the thione-thiol form was predicted (at the QCISD level) to be more stable by 13.5 kJ mol(-1) and by 39.6 kJ mol(-1) than the dithiol and the dithione tautomers, respectively. Monomers of 3,6-dithiopyridazine isolated in Ar matrixes were then irradiated with broadband UV (λ > 335 nm) light. Upon such irradiation, the thione-thiol form of the compound converted into the dithiol tautomer. The same phototransformation was observed when monochromatic λ = 385 nm laser light was used for irradiation. This allowed a first observation and spectral characterization of the dithiol form of 3,6-dithiopyridazine. Subsequent irradiation of the UV-generated dithiol tautomer with shorter-wavelength UV (λ > 275 nm) light led to partial repopulation of the thione-thiol form. Spectral signatures of the analogous photoreversibility were also found for the phototautomeric transformation in the model compound 3-thiopyridazine. The reliability of the QCISD predictions of relative energies of thiol and thione tautomeric forms was tested on the archetype example of 2-thiopyridine. For this compound, the comparison of the computed relative energy 10.9 kJ mol(-1) with the experimental estimate 10.0 ± 1.5 kJ mol(-1) (both in favor of the thiol form) was more than satisfactory.
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Warsaw, Poland.
authors.forename: Hanna
authors.initials: H
authors.lastname: Rostkowska
authors.forename: Leszek
authors.initials: L
authors.lastname: Lapinski
authors.forename: Igor
authors.initials: I
authors.lastname: Reva
authors.forename: Bruno J A N
authors.initials: BJ
authors.lastname: Almeida
authors.forename: Maciej J
authors.initials: MJ
authors.lastname: Nowak
authors.forename: Rui
authors.initials: R
authors.lastname: Fausto