Rearrangements of a Water Molecule in Both Directions between Two Hydrogen-Bonding Sites of 5-Hydroxyindole Cation: Experimental Determination of the Energy Threshold for the Rearrangement.

Rearrangements of a water molecule in both directions between two hydrogen-bonding (H-bonding) sites of the 5-hydroxyindole (5HI) cation was investigated in the gas phase. IR-dip spectra of jet-cooled 5HI-(H2O)1 revealed that two structural isomers, 5HI(OH)-(H2O)1 and 5HI(NH)-(H2O)1, in which a water molecule is bound to either the OH group or the NH group of 5HI, were formed in the S0 state. The IR photodissociation spectrum of [5HI-(H2O)1]+ generated by two-color resonant two-photon ionization (2C-R2PI) via the S1–S0 origin of 5HI(NH)-(H2O)1 clearly showed that [5HI(OH)-(H2O)1]+ and [5HI(NH)-(H2O)1]+ coexist in the D0 state. The appearance of [5HI(OH)-(H2O)1]+ after R2PI via the S1–S0 origin of 5HI(NH)-(H2O)1 is explained by isomerization of [5HI(NH)-(H2O)1]+ to [5HI(OH)-(H2O)1]+, which corresponds to the rearrangement of the water. In addition, isomerization in the opposite direction was also observed when [5HI-(H2O)1]+ was generated via the S1–S0 origin of 5HI(OH)-(H2O)1. The upper limit of the energy...