Electronic Spectra and Electronic Structures of Benzonitrile N-Oxide and Its Derivatives

The electronic spectra of 2,6-dimethylbenzonitrile N-oxide and its some derivatives were recorded in n-heptane and were compared with those of benzonitrile and phenylacetylene. The analyses of the electronic spectra of aromatic nitrile N-oxides were done by the following procedures. First, simple molecular orbitals (SMO) of benzonitrile and phenylacetylene were calculated using some parameters determined by the use of the overlap integrals and valence state ionization potentials. Then, by comparing the eigenvalues of the SMO thus obtained with the observed spectra the spectroscopic exchange integrals have been obtained for 1A1 and 1B1 bands, where interactions between two configurations were taken into consideration for the 1B1 bands. The SMO of benzonitrile N-oxide was next calculated on reference to the physicochemical nature of aromatic nitrile N-oxides studied before. The parameters were chosen so as to fit with the observed values of dipole moments of benzonitrile N-oxide and its some methyl derivatives. Electronic spectra of the nitrile N-oxides were then analyzed using the above SMO’s and spectroscopic exchange integrals. The substituent effect on the electronic spectrum of benzonitrile N-oxide was also discussed by the similar treatment. Characteristic strong band appearing in the near UV region was assigned to the 1A1 band having a change transfer (CT) nature, where the CT from the oxygen atom to the residual conjugated system makes a larger contribution than that of the reverse direction. In addition, the physicochemical nature and reactivity of nitrile N-oxides were discussed on the basis of their molecular diagrams, which were obtained from the SMO’s mentioned above.