The photochemical transformation and tautomeric composition of matrix isolated benzotriazole †

Abstract

Photolysis of benzotriazole isolated in argon and nitrogen matrices at 12–14 K has been studied by means of UV–visible and IR absorption spectroscopy. Short wavelength irradiation resulted in N–NH bond scission to give diazoimine 3 and minor products: cyanocyclopentadiene 6 and ketenimine 5. The compound 3 was easily bleachable. With 420 nm light, the major pathway was cycloreversion to benzotriazole, but at shorter wavelengths photolysis of 3 led first to the ketenimine product and ultimately to cyanocyclopentadiene. The photoproducts were identified by means of their characteristic IR absorptions. Identification of 3 was additionally supported by comparison of experimental and computed IR transitions, which also indicated that 3 was generated predominantly as its E isomer. Detailed analysis of the benzotriazole IR spectra recorded prior to and after photolysis pointed towards the co-existence of both tautomeric forms, 1H-benzotriazole (1) and 2H-benzotriazole (2), frozen from the gas phase during matrix deposition. An estimate of 1.6∶1 was obtained for the gas-phase [1H]∶[2H] ratio at ca. 315 K, in fair agreement with another recent estimate. The benzotriazole tautomers exhibited different photoreactivity, but attempts to utilize this in identifying the IR bands of the individual tautomers were hampered by band overlap, and only a few unequivocal assignments could be made.