Microhydration of the Pyrrole Cation (Py + ) Revealed by IR Spectroscopy: Ionization-Induced Rearrangement of the Hydrogen-Bonded Network of Py + (H 2 O) 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 + (H 2 O) 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 + (H 2 O) 2 is formed by stepwise hydration of the acidic NH group of Py + by a hydrogen-bonded (H 2 O) 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 + H 2 O and (H 2 O) 2 , respectively. The linear chain structure of the Py + (H 2 O) 2 cation is discussed in terms of the ionization-induced rearrangement of the hydration shell of the neutral Py(H 2 O) 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 (H 2 O) 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.