Structural distortion of pyridazine in the 1(n, pi) excited state: evidence for local excitation.

Orbital interaction is fundamentally important in understanding molecular structure, reaction mechanism, and chemical reactivity. The extent of orbital interaction is, however, hardly known quantitatively, and resulting molecular properties have often been inferred from general conceptual rules of thumb which cannot be validated. One way to find out the extent of orbital interactions is to induce local perturbation via the external field. That is, as the orbital interaction gets stronger, electronic delocalization would be more efficient after the local perturbation is set to work. In this sense, molecular structures in the excited or ionic states are quite informative since localization of electrons would result in structural distortion. Diazabenzenes have received a lot of attention as model systems for this situation. There has long been a well-conceived idea about the extents of orbital interactions and their effects on the properties of diazabenzenes. Among these diazabenzenes, the electronic delocalization in the excited pyrazine is considered to be relatively weak, and thus many authors have suggested that structural distortion occurs in the S1 state due to the local (n,p*) excitation, though its through-bond interaction is known to be significant. According to Hoffmann's suggestions, and judging from relative values for the electronic excitation and ionization energies, the orbital interaction in pyridazine is considered to be much stronger than that of pyrazine or pyrimidine. Therefore, the geometrical distortion in the excited state has been considered hardly to occur, because the two closely spaced lone-pairs of nitrogen atoms interact so strongly in pyridazine to give efficient electronic delocalization. This latter conjecture, taken for granted for a long time by most experimentalists, has essentially blocked the investigation of photophysics and photochemistry of pyridazine for the last several decades. As a result, even the vibronic assignments still remain controversial, despite a number of spectroscopic studies on the S1 state of pyridazine. [2,5,7±13] Two nonbonding orbitals of pyridazine generate two mixed orbitals, n+ and n , of which the latter is higher in energy. The lowest singlet transition is therefore the (n ,p*) transition, giving the B1 symmetry for the S1 state in C2v. The resonance-