n→π* Transitions in a Diazine: Phthalazine

The absorption spectrum of phthalazine single crystals and mixed crystals with naphthalene at 4.2°K are presented and analyzed. The near‐uv region displays two ${}^1\mathrm{n\pi *}$ states each having $A_2$ electronic symmetry. The gap between the two states is 1657 cm⁻¹ in the crystal, 1672 in the mixed crystal, and 1685 cm⁻¹ for phthalazine‐$d_4$ in the mixed crystal. Both states display $a_1$ , $b_1$ , and $a_2$ vibrations that correlate well with naphthalene modes of known symmetry. It is shown that this interpretation of the spectra is at variance with MO theory, and the results most probably dramatize the effects of charge reorganization in the $\mathrm{n\pi *}$ excited states.