Rearrangements of a Water Molecule in Both Directions between Two Hydrogen-Bonding Sites of 5-Hydroxyindole Cation: Experimental Determination of the Energy Threshold for the Rearrangement

Microhydration of the Pyrrole Cation (Py+) Revealed by IR Spectroscopy: Ionization-Induced Rearrangement of the Hydrogen-Bonded Network of Py+(H2O)2

Microhydration of heterocyclic aromatic molecules can be an appropriate fundamental model to shed light on intermolecular interactions and functions of macromolecules and biomolecules. We characterize herein the microhydration process of the pyrrole cation (Py⁺) by infrared photodissociation (IRPD) spectroscopy and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Analysis of IRPD spectra of mass-selected Py⁺(H₂O)₂ and its cold Ar-tagged cluster in the NH and OH stretch range combined with geometric parameters of intermolecular structures, binding energies, and natural atomic charge distribution provides a clear picture of the growth of the hydration shell and cooperativity effects. Py⁺(H₂O)₂ is formed by stepwise hydration of the acidic NH group of Py⁺ by a hydrogen-bonded (H₂O)₂ chain with NH···OH···OH configuration. In this linear H-bonded hydration chain, strong cooperativity, mainly arising from the positive charge, strengthens both the NH···O and OH···O H-bonds with respect to those of Py⁺H₂O and (H₂O)₂, respectively. The linear chain structure of the Py⁺(H₂O)₂ cation is discussed in terms of the ionization-induced rearrangement of the hydration shell of the neutral Py(H₂O)₂ global minimum characterized by the so-called "σ-π bridge structure" featuring a cyclic NH···OH···OH···π H-bonded network. Emission of the π electron from Py by ionization generates a repulsive interaction between the positive π site of Py⁺ and the π-bonded OH hydrogen of (H₂O)₂, thereby breaking this OH···π hydrogen bond and driving the hydration structure toward the linear chain motif of the global minimum on the cation potential.

Real-time observation of photoionization-induced water migration dynamics in 4-methylformanilide-water by picosecond time-resolved infrared spectroscopy and ab initio molecular dynamics simulations

A novel time-resolved pump-probe spectroscopic approach that enables to keep high resolution in both the time and energy domain, nanosecond excitation-picosecond ionization-picosecond infrared probe (ns-ps-ps TRIR) spectroscopy, has been applied to the trans-4-methylformanilide-water (4MetFA-W) cluster. Water migration dynamics from the CO to the NH binding site in a peptide linkage triggered by photoionization of 4MetFA-W is directly monitored by the ps time evolution of IR spectra, and the presence of an intermediate state is revealed. The time evolution is analyzed by rate equations based on a four-state model of the migration dynamics. Time constants for the initial to the intermediate and hot product and to the final product are obtained. The acceleration of the dynamics by methyl substitution and the strong contribution of intracluster vibrational energy redistribution in the termination of the solvation dynamics is suggested. This picture is well confirmed by the ab initio on-the-fly molecular dynamics simulations. Vibrational assignments of 4MetFA and 4MetFA-W in the neutral (S₀ and S₁) and ionic (D₀) electronic states measured by ns IR dip and electron-impact IR photodissociation spectroscopy are also discussed prior to the results of time-resolved spectroscopy.

Microhydration of protonated 5-hydroxyindole revealed by infrared spectroscopy

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+-Wn, 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+-W2/3 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+-Wn do not indicate any proton transfer to the solvent up to n = 3, in line with the proton affinities of 5HI and Wn. Comparison of 5HIH+-Wn to neutral 5HI-W and cationic 5HI+-Wn 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.

Real-time observation of the photoionization-induced water rearrangement dynamics in the 5-hydroxyindole-water cluster by time-resolved IR spectroscopy

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

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

The hydrogen-bonding properties of the acidic OH and NH groups of the 5-hydroxyindole cation are probed by infrared spectroscopy and DFT calculations of its microhydrated clusters.