THE ELECTRONIC-SPECTRUM OF PHTHALAZINE - THEORY AND EXPERIMENT

Ab initio configuration interaction-singles (CIS) calculations of the electronic spectra of phthalazine and pyridazine predict two neighbour ing singlet (pi, n) states: (1)A(2) followed by B-1(1) for phthalazin e, and the reverse for pyridazine. A comparison of the 3-21G and 6-31G basis sets for pyridazine shows that the 3-21G basis set gives satis factory results, and may be used in CIS calculations for the larger mo lecule, phthalazine. The lowest-energy pi <-- n transition for phthal azine is predicted to be forbidden, in agreement with experiment, but in conflict with most semiempirical calculations. Ab initio calculatio ns are used to predict the vibrational frequencies of phthalazine and phthalazine-d(4) at the HF/6-31G and CIS/3-21G levels for the ground and excited electronic states, respectively. The ground-state calculat ions for phthalazine are in good agreement with experiment, including the results of several SERS studies. The vapour absorption spectrum of phthalazine is reported for the first time and analysed using ab init io, semiempirical and rotational-contour calculations. The first 6000 cm(-1) of the spectrum is assigned to a forbidden pi <-- n transition , confirming the findings of crystal-state studies and the CIS/3-21G p redictions. The higher-energy regions of this band system show no clea r evidence of a second pi <-- n transition, despite theoretical predi ctions and circular-dichroism observations to the contrary.