Amino→Imino Tautomerization upon in Vacuo Sublimation of 2-Methyltetrazole-Saccharinate as Probed by Matrix Isolation Infrared Spectroscopy

The amino-imino tautomerization of the nitrogen-linked conjugate 2-methyltetrazole-saccharinate (2MTS) was observed upon sublimation of the compound in vacuo. As shown previously by X-ray diffraction [Ismael, A.; Paixao, J. A.; Fausto, R.; Cristiano, M. L. S. J. Mol. Struct., 2011, 1023, 128-142], in the crystalline phase the compound exists in an amino-bridged tautomeric form. Infrared spectroscopic investigation of a cryogenic matrix prepared after sublimation of a crystalline sample of 2MTS and deposition of the sublimate together with argon (in similar to 1:1000 molar ratio) onto an IR-transparent cold (15 K) substrate, revealed that the form of 2MTS present in the matrix corresponds to the theoretically predicted most stable imino-bridged tautomer. In this tautomer, the labile hydrogen atom is connected to the saccharine nitrogen, and the two heterocyclic fragments are linked by an imino moiety in which the double-bond is established with the carbon atom belonging to the saccharyl fragment. The observed isomeric form of this tautomer is characterized by a zusammen (Z) arrangement of the two rings around the C=N bond of the bridging group and an intramolecular NH center dot center dot center dot N hydrogen bond. The experimental IR spectrum of the matrix-isolated 2MTS has been fully assigned based on the calculated spectra for the two most stable conformers of this tautomer. A mechanism for the conversion of the tautomeric form existing in the crystal into that present in the gas phase is proposed. As a basis for the interpretation of the experimental results, a detailed theoretical [at the DFT(B3LYP) level of approximation with the 6-31++G(d,p) and 6-311++G(3df,3pd)] study of the potential energy surface of the compound was performed.