The calculated spectra of the azanaphthalenes

Abstract

A modified INDO (intermediate neglect of differential overlap) method is used to calculate the electronic spectra of naphthalene and some mono-, di-, and tetraazanaphthalenes. The technique is capable of reproducing the better classified bands of this series within a rms error of ∼1000 cm −1. The four lowest π-π ∗ bands of naphthalene are well represented; a fifth band reported at ∼52,600 cm −1, and generally assigned 1B 2u(π-π ∗) , may be 1B 3g(π-π ∗) borrowing intensity. The lowest excited singlet state calculated for quinoline is n → π ∗ , estimated nearly degenerate with the lowest π-π ∗ , while that for isoquinoline is calculated π-π ∗ ; experimental evidence supports the π-π ∗ assignment in both cases, but the corresponding absorption in quinoline appears complex. Of the diazanaphthalenes examined (with two nitrogens in one benzenoid ring) all are calculated to have one n-π ∗ transition before the first π-π ∗ except phthalazine, in which two n-π ∗ transitions are calculated to be the lowest lying. This is in accord with experimental evidence to date, although the nature of the observation in phthalazine is reinterpreted as one 1 B 2( n- π). 1,4,5,8-tetrazanaphthalene is predicted to have a group theoretically forbidden n-π ∗ excited state as low as 21,000 cm −1, and should prove interesting experimentally. Even though n-π ∗ excited states are often calculated to be the lowest lying, none of these compounds are predicted to have an “ n” orbital as homo. Further interpretations of the spectra of the azanaphthalenes are made in view of the calculations. Theoretical limitations of the method employed for this study are discussed.