Based on the new electrostatic solvation energy expression of the nonequilibrium state by means of dielectric continuum theory and constrained equilibrium method, a novel formula for the absorption/emission spectral shift is deduced within the dipole-in-a-sphere approximation. The absorption spectra in the electronic transition of three tautomers: pyridazine, pyrimidine and pyrazine in vacuum and the dipole moments of the tautomers in five polar solvents are calculated at the level of CASSCF(6,5)/6-31＋＋G**. The variation tendency of the blue shifts in the transition, due to the variational dipole moments of solutes and polarities of solvents, is discussed.

Key words: nonequilibrium solvation, constrained equilibrium, diazine, absorption spectra, spectral shift