Vibrational Spectra of the Ground and the Singlet Excited ππ* State of 6,7-Dimethyl-8-ribityllumazine

J. Phys. Chem. B, 2011, DOI: 10.1021/jp112126a, 115 (13), pp 3689–3697 published on 16.03.2011
J. Phys. Chem. B, online article
6,7-Dimethyl-8-ribityllumazine serves as fluorophore in lumazine proteins (LumP) of luminescent bacteria. The molecule exhibits several characteristic vibrational absorption bands between 1300 and 1750 cm-1 in its electronic ground state. The IR-absorption pattern of the singlet excited ππ* state was monitored via ultrafast infrared spectroscopy after photoexcitation at 404 nm. The comparison of experimentally observed band shifts for a number of isotopologues allows for a clear assignment of several absorption bands—most importantly the two carbonyl bands. This assignment is confirmed by normal-mode calculations by means of either density functional theory (DFT) calculations for the ground state or the configuration interaction singles (CIS) method for the excited singlet state. A good agreement between experiment and calculation is obtained for models including explicitly a first solvation shell. The results provide a basis for further investigations of lumazine protein and demonstrate the necessity of proper accounting for explicit hydrogen bonding in case of strongly polar molecular systems.

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