Quantum-chemical investigation of the π-electronic structure of 3-hydroxy-4-phenylazoquinoline and 4-hydroxy-3-phenylazoisoquinoline

The total bond energies, the atomization energies, the molecular diagrams in the ground and first excited states, the interatomic distances in the ground state, and the electronic spectra of the azo and quinonehydrazone forms of 3-hydroxy-4-phenylazoquinoline and 4-hydroxy-3-phenylazoisoquinoline were calculated by the MO LCAO method within the Pariser-Parr-Pople approximation with the utilization of optimization of the internuclear distances with respect to the minimum of the atomization energy. It follows from an analysis of the ΔH values that the azo form in the first case is energetically more advantageous than the quinonehydrazone form, while the opposite is true in the second case. The calculated interatomic distances and bond orders of the tautomers correspond to those in the azo and quinonehydrazone structures. The absorption bands in the electronic spectra were assigned. The long-wave band in the absorption spectra of the quinonehydrazone tautomers is due primarily to charge transfer from the bridge-amino-nitrogen atom to the quinoid system.