Structure and absorption sepctra of 3-phenyl-4-hydroxyisoquinoline

Abstract

It was shown by IR spectroscopy that 3-phenyl-4-hydroxyisoquinoline exists in the hydroxy form in solution in organic nonpolar solvents. The splitting of the vOH band in the IR absorption spectra was assigned to the S-cis and S-trans orientations of the OH group relative to the phenyl ring. An intramolecular hydrogen bond is formed in the cis form of 3-phenyl-4-hydroxyisoquinoline due to interaction of the hydroxyl hydrogen atom with the π-electron system of the phenyl ring. An interpretation of the first two absorption maxima in the electronic spectra of the neutral and ionic forms of the 4-hydroxyisoquinoline and 3-phenyl-4-hydroxyisoquinoline molecules is given within the framework of the MO method and the Pariser-Parr-Pople approximation. It is shown that the introduction of a phenyl group in the 3 position of 4-hydroxyisoquinoline, protonation of the ring nitrogen atom, the ionization of the exocyclic Β-hydroxyl group affect the energy of the upper occupied molecular orbital, leaving the lower vacant molecular orbital of 4-hydroxyisoquinoline almost unchanged.