Electronic spectra of the protonated indole chromophore in the gas phase

International audienceThe electronic spectroscopy of cold protonated indole was investigated experimentally and theoretically. Two isomers have been observed in this experiment, the first isomer corresponds to the lowest energy isomer in the calculations, absorbing at ~350 nm and protonated on the C3 atom of the pyrrole ring. According to our calculations, the absorption of the other isomers protonated on carbon atoms (C2, C4, C5, C6 and C7) is in the visible region. Indeed, the absorption of the second isomer observed starts at 488 nm and has been assigned to protonation on the C2 carbon of the pyrrole ring. Since good agreement was obtained between calculated and experimental transitions for the observed isomers, reasonable ab initio transition energies may also be expected for the higher energy isomers protonated on other carbon atoms, which should also absorb in the visible. Protonation on the nitrogen atom leads to a blue-shifted transition with respect to that of the most stable isomer