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.