Sequential microhydration of cationic 5-hydroxyindole (5HI⁺): infrared photodissociation spectra of 5HI⁺–W_n 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 A'' ground electronic state by infrared photodissociation (IRPD) spectroscopy of 5HI-W-L clusters (W = HO, L = Ar and N, 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 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 OHW hydrogen-bonds (H-bonds) are stronger than NHW H-bonds. Spectra of larger 5HI-W 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 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.

Microhydration of protonated 5-hydroxyindole revealed by infrared spectroscopy

“…All 5HIH + protomers exhibit competing H-bonding sites, and most important are the acidic functional OH (2) and NH groups acting as proton donors in OHÁ Á ÁO and NHÁ Á ÁO H-bonds to W. As already observed for related heterocyclic arene cations, 23,36,41,[51][52][53] p-stacking of W is rather unfavorable, e.g., D 0 = 33.3 kJ mol À1 for s5HIH + (C3)-W(p), and thus not considered further. Only one isomer with W attached to the protonated CH 2 group could be located, namely, s5HIH + (C4)-W(CH) with D 0 = 28.6 kJ mol À1 .…”

“…27,[31][32][33][34] This hybrid density functional with additive dispersion correction has proven to yield reliable results for related aromatic clusters. 11,22,23,[35][36][37][38][39] For example, the binding energies computed for the monohydrates of the benzene and naphthalene cations (D 0 = 3209 and 2773 cm À1 ) compare favorably with the experimental values (D 0 = 3290 AE 120 and 2800 AE 300 cm À1 ). 40,41 Similarly, the calculated binding energy of W 2 (D 0 = 1108 cm À1 ) matches the measured value (D 0 = 1105 AE 10 cm À1 ).…”

“…IRPD spectra are measured in the XH stretch range (X = N and O, 2600-3800 cm À1 ) with a quadrupole-octopole-quadrupole tandem mass spectrometer described elsewhere. [18][19][20][21][22][23][24][25][26] Briefly, protonated clusters are generated in an electron ionization source coupled to a pulsed molecular beam expansion. Solid 5HI (Sigma-Aldrich, 97%) is heated to 145 1C and the resulting vapor is seeded in Ar or N 2 carrier gas (8-9 bar) containing water.…”

“…Collision-induced dissociation in the octopole confirms the composition of mass-selected parent clusters. 22,23 The protonation sites in 5HIH + have already been examined in a previous study. 17 The 5HIH + protomers offer two competing H-bonding sites for W molecules, namely, their NH and OH functional groups.…”

“…Comparison to neutral and cationic s/a5HI-W clusters previously studied at the same level (B3LYP-D3/aug-cc-pVTZ) yields the effects of protonation. 22,23 Total binding energies (D 0 ) are derived by subtracting the zeropoint corrected energies of the corresponding monomers from that of the cluster: D 0 = E 0 (5HIH + -W n ) À E 0 (5HIH + ) À nÁE 0 (W). Binding energies of the Ar/N 2 tag in 5HIH + -W-Ar/N 2 are calculated in a similar way.…”