CNNC conformational isomers of 2′-hydroxyacetophenone azine: FTIR matrix isolation and DFT study

2′-hydroxyacetophenone azine (APA) has been studied by matrix isolation infrared spectroscopy and quantum chemical calculations. The DFT/B3LYP/6-311++G(2d,2p) calculations demonstrated the existence of two conformers for the lowest energy E/E configuration of APA, a s-trans and a gauche ones. The conformers are characterized by similar energies and differ in the value of a CNNC angle, that was calculated to be 180° for a planar s-trans conformer and 155° for a non-planar gauche one. The calculated barrier for conformational interconversion is also very low, ca. 1 kJ mol−1 for the conversion from a gauche conformer to a trans one. The FTIR spectra of an argon matrix doped with APA from a vapour above solid sample evidence the presence of both conformers that exhibit reversible interconversion at matrix temperatures. The comparison of the theoretical spectra with the experimental ones and reversible temperature dependence of the experimental spectra allowed for unambiguous spectroscopic characterization of the trans and gauche conformers. The experiment also demonstrated that a gauche conformer is more stable than a trans one. The spectra analysis indicates that transformation from a trans conformer to a gauche one weakens the intramolecular OH⋯N bonds in the molecule.