Molecular Structure and Vibrational Spectra of 4-(4-Hydroxyphenylazo)phthalonitrile: DFT Study

Structural features, conformational manifold and isomeric forms of 4-(4-hydroxyphenylazo)phthalonitrile (p-HPhAPN) were studied by DFT calculations (B3LYP) using QTAIM, NPA, NBO, NCI, SAPT0 analysis. The influence of hydroxyl and nitrile groups on the geometric structure of the molecule was analyzed in comparison with the initial azobenzene and isomers differing in the location of the hydroxyl group - m-HPhAPN, o-HPhAPN. For the initial p-HPhAPN, 8 possible models were considered, for isomeres o-HPhAPN and m-HPhAPN - 32 and 16 models, respectively. The variety of the chosen models is due to the possibility of (a) different arrangement of hydrogen atoms of the hydroxyl group, determined by the corresponding torsion angle, (b) cisoid/transoid arrangement of nitrile groups with respect to the azo group, (c) cisoid/transoid arrangement of the hydroxyl group with respect to the azo group, (d) tautomerism, and (e) cis-trans isomerism. Assignment of vibrational modes of p-HPhAPN was carried out via potential energy distribution (PED) analysis among internal coordinates. The PED of most normal vibrations for p-HPhAPN, as well as for initial azobenzene, has a complex character. Changes in the IR spectra caused by the considered structural changes are noted.