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.

Ionization-Induced π → H Site Switching in Resorcinol-Arn (n = 1 and 2) Clusters Probed by Infrared Spectroscopy

Infrared (IR) spectra of resorcinol (Rs)-Ar_n clusters (n = 1 and 2) have been measured in the neutral and cationic ground states (S₀ and D₀) by IR dip and resonance-enhanced multiphoton ionization (REMPI)-IR spectroscopy. The OH stretching vibrations in S₀ keep their frequency regardless of the number of Ar atoms and the conformation of the OH groups in Rs (rotamers RsI and RsII), demonstrating that the Ar atoms are attached to the aromatic π-ring (π-bound structure) in S₀. In the D₀ state, the IR spectra of Rs⁺-Ar_n reflect the difference in the Rs conformations (RsI⁺ and RsII⁺). For n = 1, the IR spectra of both rotamers are almost the same as those of the corresponding monomer cations, indicating that Ar ligands essentially remain π-bonded after ionization. In contrast, the IR spectra of Rs⁺-Ar₂ show hydrogen-bonded and free OH stretching vibrations, demonstrating that for a significant fraction of the clusters, the Ar atoms migrate from the π-bound site to the OH groups. The ionization-induced π → H migration yields are not unity for both rotamers RsI⁺-Ar₂ and RsII⁺-Ar₂. This result is in sharp contrast to phenol⁺-Ar₂, in which one of the Ar atoms migrates to the OH site with 100% yield. The mechanism leading to the nonunity yield in Rs⁺-Ar₂ is discussed in terms of the number of OH binding sites and Franck-Condon factors. The ionization excess energy dependence of the IR spectra of Rs⁺-Ar₂ and its Rs⁺-Ar fragments is discussed in terms of the Ar binding energies estimated from the photoionization and photodissociation efficiency spectra.

Photoionization-induced π ↔ H site switching dynamics in phenol+–Rg (Rg = Ar, Kr) dimers probed by picosecond time-resolved infrared spectroscopy

Picosecond time-resolved infrared spectroscopy of phenol–rare gas dimer cations reveal delocalization of a wavepacket of the single rare gas atom above and below phenol in around 100 ps.

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol–Ar cluster near the ionization threshold

A new spectroscopic method reveals the barrier and the crucial role of direct photoionization in the π → H site switching in phenol–Ar.