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