Physicochemical characterization of environmental mutagens: 3-nitro-6-azabenzo[a]pyrene and its N-oxide derivative

Molecular geometries, nucleus independent chemical shifts, electronic, infrared and Raman spectra, vertical ionization energy and electron affinity, dipole moment, and electronic polarizabilities of 3-nitro-6-azabenzo[a]pyrene and its N-oxide derivative were calculated for the gas phase using semiempirical and density functional theory. The influence of the N-oxidation on structural, electronic, and spectroscopic properties is discussed. The infrared spectral region around 1,300 cm(-1) involving the azabenzene N-O and the NO2 stretching vibrations is potentially useful to distinguish 3-nitro-6-azabenzo[a]pyrene from its N-oxide derivative. The calculated vertical ionization energy for 3-nitro-6-azabenzo[a]pyrene N-oxide is smaller than that of its parent compound, whereas the vertical electron affinity is higher in agreement with the experimental electrochemical reduction potentials. Both dipole moment and polarizability increase on passing from 3-nitro-6-azabenzo[a]pyrene to its N-oxide derivative, the contribution from the N-O bond formation being substantial. The electric property enhancements are explained through atomic charges and spectral shifts. Present results support the higher mutagenic activity for the N-oxide derivative in comparison with its parent compound.