A THEORETICAL STUDY OF THE PHOTOINDUCED INTRAMOLECULAR PROTON TRANSFER IN 2-(2′-HYDROXYPHENYL)-IMIDAZOLINE

摘要

A relatively simple molecule, 2-(2′-hydroxyphenyl)-imidazoline (HPI), was used as a model compound for the investigation of a proton transfer in the first singlet excited state. Multi-configurational SCF wavefunctions (all the single-, double-, triple- and quadruple-excited configurations included) with AM1 Hamiltonian were employed for optimizing the geometry during the proton transfer. A four-level molecular electronic system, containing asymmetric double-minimum potential energy surfaces in the ground and the singlet excited states was obtained without any additional assumptions. In the excited state the keto minimum is situated over 5 kcal/mol below the energy minimum for the enol tautomeric form. In the ground state the enol minimum lies almost 20 kcal/mol lower than the minimum for the keto form. After Franck-Condon excitation the minimal energy of the enol tautomer of molecule is over 5 kcal/mol above the potential curve minimum for that tautometer and after deexcitation of the keto tautomer the molecular system is about 7 kcal/mol above the local minimum for the ground state. In both situations the excess of the vibrational energy leads to a very fast proton transfer, which is consistent with the experimental findings for similar compounds. The maximum of the calculated absorption band agrees very well with the observed value, just as the large Stokes' shift of the fluorescence spectrum.