Theoretical study of the electronic spectrum of imidazole

The complete active space (GAS) self-consistent field (SCF) method and multireference second-order perturbation theory (CASPT2) have been used to study the electronic spectrum of imidazole and the imidazolium ion. The calculations comprise a large number of, both singlet and triplet, valence and Rydberg excited states. A newly developed continuum model has been used to compute solvatochromic shifts. In the gas phase the first and second pi --> pi* excited singlet valence states of imidazole are computed at 6.72 and 7.15 eV, and they shift to 6.32 and 6.53 eV upon solvation. The gas-phase values are somewhat too large (approximate to 0.3 eV) due to an erroneous valence-Rydberg mixing in the CASSCF wave function. The first and second it pi --> pi* excited singlet valence states of the imidazolium ion are computed at 5.72 and 6.94 eV in the gas phase and shifted to 5.86 and 6.83 eV in aqueous solutions. The present results are in agreement with the observed absorption band maxima in aqueous solution, 6.0 and 6.5 eV for imidazole and 6.0 and 6.9 eV for the imidazolium ion. The computed intensities suggest another possible (but less probable) interpretation of the solution spectrum, where both species are simultaneously present. In this case the lowest band at 6.0 eV is assigned to a transition to the 2(1)A(1) state in protonated imidazole, while the second band observed in neutral solution at 6.5 eV corresponds to excitation to the 3(1)A' state in imidazole.