Ab initio HF-SCF study of naphthazarin: Geometries, isomerism, hydrogen bonding, and vibrational spectrum

The structural properties and intramolecular hydrogen bonding of a series of structures of naphthazarin molecule were investigated by ab initio HF-SCF methods. The geometries of theC 2v ,C 2h ,D 2h , andC s symmetry structures were optimized using split-valence basis sets. MP2/6-31G*// HF/6-31G single-point energy calculations indicate that theC 2v isomer (5,8-dihydroxy-1,4-naphthoquinone) is the lowest energy structure of the molecule and that theC 2h symmetry one (4,8-dihydroxy-1,5-naphthoquinone), lying 37 kJ/mol above theC 2v form, is the other stable isomer of naphthazarin. At the HF/6-31G level, the intramolecular proton exchange between two equivalentC 2v structures is a two-step process where each proton can be independently transferred through an unsymmetrical potential having a 1,5-quinone intermediate, theC 2h symmetry structure, and two equivalent transition states ofC s symmetry, with a barrier height equal to 38 kJ/ mol (MP2/6-31G*//HF/6-31G). The study of naphthazarin molecule is flanked by a theoretical investigation on theC 2v andC 2h isomers of the parent naphthoquinone and dihydroxynaphthalene molecules. The SCF vibrational spectrum of the ground state of naphthazarin, harmonic frequencies, and infrared and Raman band intensities were computed at the HF/6-31G level. The results of the calculations are compared with the matrix isolation FT-IR spectroscopy measurements and with the infrared and Raman spectra of the crystal molecule.