Microhydration of protonated 5-hydroxyindole revealed by infrared spectroscopy

Abstract

Controlled microsolvation of protonated aromatic biomolecules with water is fundamental to understand proton transfer reactions in aqueous environments. We measured infrared photodissociation (IRPD) spectra of mass-selected microhydrates of protonated 5-hydroxyindole (5HIH⁺–W_n, W = H₂O, n = 1–3) in the OH and NH stretch ranges (2700–3800 cm⁻¹), which are sensitive to the spectroscopic characteristics of interior solvation, water network formation, and proton transfer to solvent. Analysis of the IRPD spectra by dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ) reveals the coexistence of C3- and C4-protonated carbenium ions, 5HIH⁺(C3) and 5HIH⁺(C4), as well as the O-protonated oxonium ion, 5HIH⁺(O). Monohydrated 5HIH⁺–W clusters are formed by hydrogen-bonding (H-bonding) of the first water to the most acidic functional group, namely, the NH group in the case of 5HIH⁺(C3), the OH group for 5HIH⁺(C4), and the OH₂ group for 5HIH⁺(O). The latter benefits from its twofold degeneracy and the outstandingly high binding energy of D₀ ∼ 100 kJ mol⁻¹. Larger 5HIH⁺–W_2/3 clusters preferably grow (i) by H-bonding of the second water to the remaining vacant functional group and and/or (ii) by formation of W₂ water chains at the respective most acidic functional group. Our IRPD spectra of 5HIH⁺–W_n do not indicate any proton transfer to the solvent up to n = 3, in line with the proton affinities of 5HI and W_n. Comparison of 5HIH⁺–W_n to neutral 5HI–W and cationic 5HI⁺–W_n clusters elucidates the impact of different charge states on the topology of the initial solvation shell. Furthermore, to access the influence of the size of the arene ion and a second functional group, we draw a comparison to microhydration of protonated phenol.