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 us ed to study the electronic spectrum of imidazole and the imidazolium i on. The calculations comprise a large number of, both singlet and trip let, valence and Rydberg excited states. A newly developed continuum m odel has been used to compute solvatochromic shifts. In the gas phase the first and second pi --> pi excited singlet valence states of imid azole are computed at 6.72 and 7.15 eV, and they shift to 6.32 and 6.5 3 eV upon solvation. The gas-phase values are somewhat too large (appr oximate to 0.3 eV) due to an erroneous valence-Rydberg mixing in the C ASSCF wave function. The first and second it pi --> pi excited single t 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 solutio ns. 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 a nother possible (but less probable) interpretation of the solution spe ctrum, 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) sta te 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 im idazole.