The calculated spectra of the azanaphthalenes

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 ¹B₂( 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.