Vibronic analyses of the lowest singlet-singlet and singlet-triplet band systems of pyridazine

The first singlet-singlet and singlet-triplet band systems of the absorption spectrum of pyridazine vapour are analysed using ab initio and vibronic coupling calculations. The lowest singlet-triplet absorption involves a comparatively unperturbed (π(*),n)³B₁ state, and contrasts with the highly perturbed singlet-singlet spectrum. The major source of vibronic perturbation in the singlet-singlet absorption is attributed to coupling between near-resonant (π(*),n)¹A₂ and (π(*),n)¹B₁ states, with the former being slightly lower in energy. Many features of this complex and unusual spectrum, and its associated single vibronic level fluorescence spectrum, can be explained using a simple vibronic model. This provides experimental support for recent relaxed CASPT2 and EOM-CCSD calculations, but contrasts with earlier assignments of the spectrum. Theory and experiment suggest that the spacing between the lowest A₂ and B₁ states is larger in the triplet manifold, leading to a simpler spectrum. (C) 2000 Published by Elsevier Science B.V.