Electronic Spectra of the Azide Ion, Hydrazoic Acid, and Azido Molecules

The electronic absorption spectra of aqueous hydrazoic acid solutions have been studied as a function of concentration. The spectra are analyzed on the basis of complementary absorption by the azide ion and the associated acid species, and the acid dissociation constant has been determined. The absorption of azide ion in hydrazoic acid solutions is found to be identical to that of solutions of the azide salts. The aqueous solution spectrum of associated hydrazoic acid is shown to be the same in the near‐ultraviolet region as that of hydrazoic acid gas and very similar to that of n‐amylazide. Vibrational analyses have been carried out on the structured regions of the absorption spectra of NH₃ gas. The absorption spectra of HN₃ and n‐amylazide have been determined in the vacuum ultraviolet and four series of Rybderg transitions have been identified in the spectrum of HN₃. Ionization potentials of 11.5 and 12.6 eV have been determined for HN₃. MWH‐type computations have been performed on the azide ion and on hydrazoic acid. These computations, coupled with experimental data and with the knowledge of the spectra of other isoelectronic molecules and ions, are used to assign all molecular transitions observed in the spectra of the azide ion and of the azido molecules. The electronic states, in order of increasing energy, of N₃⁻, $D_{\mathrm{\infty h}}(\mathrm{R}–{\mathrm{N}}_3{\mathrm{,\; C}}_s)$ are: ${}^1{\Sigma }_g^{+}{(}^1{\mathrm{A\prime )\hspace{0.17em}<\hspace{0.17em}}}^1{\Sigma }_u^{-}{(}^1{\mathrm{A″)\hspace{0.17em}<\hspace{0.17em}}}^1{\Delta }_u{(}^1{\mathrm{A″\hspace{0.17em}<\hspace{0.17em}}}^1{\mathrm{A\prime )\hspace{0.17em}<\hspace{0.17em}}}^1{\Pi }_{\mathrm{(g,u)}}{(}^1{\mathrm{A″\hspace{0.17em}<\hspace{0.17em}}}^1{\mathrm{A\prime )\hspace{0.17em}<\hspace{0.17em}}}^1{\Sigma }_u^{+}{(}^1\mathrm{A\prime )}$ .