Electronic Spectra of the Protonated Indole Chromophore in the Gas Phase

Abstract

The electronic spectroscopy of 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 is in the visible region. Indeed the second isomer observed absorbs in the blue (starting 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 isomers observed, reasonable ab-initio transition energies may also be expected for the higher energy isomers protonated on carbon atoms, which should also absorb in the visible. Protonation on the nitrogen atom leads to a blue shifted transition.