Figure 1.

(a) Gas-phase IRMPD spectrum of protonated indigo (magenta trace) with band centers for the main experimental bands indicated. The experimental spectrum is compared with computed spectra for (b) the cis- and (c) trans-isomer. The gray trace in (a) represents the aggregate spectrum of cis- and trans-isomers assuming fractional populations as suggested by the kinetic measurements described in the text. Computed intensities in km mol^–1 refer to the stick spectra. The optimized structures along with their relative Gibbs energies are shown. Atomic distances shown are in Å.