Elevation of the Energy Threshold for Isomerization of 5-Hydroxyindole-(tert-butyl alcohol)1 Cluster Cations.

Abstract

Isomerization between two hydrogen-bonded (H-bonded) isomers of 5-hydroxyindole-(tert-butyl alcohol)1 cluster cations ([5HI-(t-BuOH)1]+) was investigated in the gas phase. In the S0 state, jet-cooled 5HI-(t-BuOH)1 has two structural isomers, 5HI(OH)-(t-BuOH)1 and 5HI(NH)-(t-BuOH)1, in which the t-BuOH molecule is bound to the OH or the NH group of 5HI. The IR photodissociation spectrum of [5HI-(t-BuOH)1]+ generated by two-color resonant two-photon ionization (2C-R2PI) via the S1-S0 origin of 5HI(NH)-(t-BuOH)1 provided evidence of both [5HI(OH)-(t-BuOH)1]+ and [5HI(NH)-(t-BuOH)1]+ coexisting in the D0 state, indicating that [5HI(NH)-(t-BuOH)1]+ isomerizes to [5HI(OH)-(t-BuOH)1]+ after 2C-R2PI of 5HI(NH)-(t-BuOH)1. The lower limit of the energy threshold for the isomerization of [5HI(NH)-(t-BuOH)1]+ to [5HI(OH)-(t-BuOH)1]+ was experimentally determined to be 3362 ± 30 cm-1, and the corresponding energy threshold for the isomerization of [5HI(NH)-(H2O)1]+ to [5HI(OH)-(H2O)1]+ has been reported to be 2127 ± 30 cm-1. Thus, the energy threshold for the isomerization is elevated by at least 1200 cm-1 when the solvent molecule changes from H2O to t-BuOH. The elevation of the energy threshold is explained by the difference in the stabilization energies of [5HI-(t-BuOH)1]+ and [5HI-(H2O)1]+ in the initial and transition states owing to the larger proton affinity of t-BuOH than H2O.