The electronic spectrum of phthalazine. Theory and experiment

Abstract

Abstract Ab initio configuration interaction-singles (CIS) calculations of the electronic spectra of phthalazine and pyridazine predict two neighbouring singlet ( π ∗ , n) states: 1A2 followed by 1B1 for phthalazine, 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 satisfactory results, and may be used in CIS calculations for the larger molecule, phthalazine. The lowest-energy π ∗ ← n transition for phthalazine is predicted to be forbidden, in agreement with experiment, but in conflict with most semiempirical calculations. Ab initio calculations are used to predict the vibrational frequencies of phthalazine and phthalazine-d4 at the HF/6–31G∗ and CIS/3–21G levels for the ground and excited electronic states, respectively. The ground-state calculations 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 initio, semiempirical and rotational-contour calculations. The first 6000 cm−1 of the spectrum is assigned to a forbidden π ∗ ← n transition, confirming the findings of crystal-state studies and the CIS/3–21G predictions. The higher-energy regions of this band system show no clear evidence of a second π ∗ ← n transition, despite theoretical predictions and circular-dichroism observations to the contrary.