Sequential microhydration of cationic 5-hydroxyindole (5HI+): infrared photodissociation spectra of 5HI+–Wn clusters (W = H2O, n ≤ 4)

Abstract

Most biochemical processes occur in aqueous solution. Here, we characterize the initial microhydration steps of the 5-hydroxyindole cation (5HI⁺) in its ²A′′ ground electronic state by infrared photodissociation (IRPD) spectroscopy of 5HI⁺–W_n–L_m clusters (W = H₂O, L = Ar and N₂, n ≤ 4, m ≤ 2) in a molecular beam and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Characteristic size- and isomer-dependent XH stretch frequencies (X = O, N) of 5HI⁺–W_n reveal information about the preferred cluster growth and solvation energies. The IRPD spectrum of 5HI⁺–W is a superposition of the spectra of two isomers, in which W is H-bonded to the acidic NH or OH group, whereby OH⋯W hydrogen-bonds (H-bonds) are stronger than NH⋯W H-bonds. Spectra of larger 5HI⁺–W_n clusters (n ≥ 2) elucidate the competition between interior ion solvation and the formation of H-bonded water networks. The nature and strengths of the competing H-bonds are quantified by the noncovalent interaction approach. Comparison to results for neutral 5HI–W and 5HI⁺–L_n clusters with nonpolar ligands reveals the effects of ionization and ligand type on the intermolecular interaction potential and cluster growth. Comparison to corresponding microhydrated clusters of the phenol, indole, and pyrrole cations illustrates the effects of substitution of functional groups and addition of aromatic rings on the hydration process.