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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2016-03, Vol.120 (11), p.1825-1832
• Main Authors: Ikeda, Takamasa, Sakota, Kenji, Sekiya, Hiroshi
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.5b12111

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 threshold for the rearrangement of the water in [5HI­(NH)-(H2O)1]+ was experimentally determined to be 2127 ± 30 cm–1 from the adiabatic ionization energy of 5HI­(NH)-(H2O)1. Above the energy threshold, the water molecule in [5HI-(H2O)1]+ may fluctuate between the two preferential H-bonding sites of 5HI+.