6533b850fe1ef96bd12a81b3

RESEARCH PRODUCT

Theoretical Study of the Electronic Spectrum of Imidazole

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The complete active space (CAS) 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 π → π* 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 (≈0.3 eV) due to an erroneous valence−Rydberg mixing in the CASSCF wave function. The first and second π → π* 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...