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

Abstract

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.

Microhydration of protonated 5-hydroxyindole revealed by infrared spectroscopy

“…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 ).…”

Microhydration of PAH+ cations: evolution of hydration network in naphthalene+-(H2O)n clusters (n ≤ 5)

“… 87 , 114 These may be probed in the future for Np-W n clusters by time-resolved pump-probe IR spectroscopy, 114 , 115 a technique recently applied to monitor solvent rearrangement reactions in related aromatic clusters in real time. 116 – 120 Although Np – is an unstable anion because of its negative electron affinity, stable microhydrated Np – -W n clusters can be produced in supersonic expansions and have been characterized by photoelectron ( n ≤ 8) 76 – 78 and IR ( n ≤ 6) 79 spectroscopy. The computational analysis of these IR spectra 79 yields multiple π H-bonded structures for Np – -W n for n ≤ 4, in which H-bonded W n clusters are attached to a single side of Np.…”

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

“…This wide variety of the migration time-constants have been rationalized by the differences in excess energies and potential energy surfaces for both the initial and final binding sites, such as (a) repulsive → bound, (b) flat → bound, and (c) bound → bound. 23,25 The slow migration lifetime in the bound → bound potential surface in 5hydroxyindole-water is well reproduced by RRKM theory, and thus the intracluster vibrational energy redistribution (IVR) is one of the key factors to understand the variety of the hydration dynamics at the molecular level. Analysis of the fast migration regime relies mostly on MD simulations.…”

“…This technique of UV-UV ( ′ ) -IR ps time-resolved IR (ps-TRIR) spectroscopy combined with mass spectrometry has been successfully applied to a variety of monohydrated clusters, including the single water CO → NH migration around the peptide linkage in trans-acetanilide (tAA) 20,21 and trans-formanilide (tFA), 24 the CN → NH migration around 4-aminobenzonitrile, 22 and the NH → OH migration in 5-hydroxyindole. 25 Surprisingly, the lifetimes of these single water migrations vary over four orders of magnitude, ranging from 4 ps to 53 ns. This wide variety of the migration time-constants have been rationalized by the differences in excess energies and potential energy surfaces for both the initial and final binding sites, such as (a) repulsive → bound, (b) flat → bound, and (c) bound → bound.…”

“…19 To circumvent the problem of inhomogeneous water molecules, we have been working on hydration dynamics at a single solvent molecular level by probing solvated clusters in the gas phase using time-resolved infrared (TRIR) spectroscopy and advanced ab initio molecular dynamics (MD) simulations. [20][21][22][23][24][25] This approach offers size-and isomer-selectivity as well as full control over solvation structure and energy in the cluster.…”