Infrared and NMR Spectroscopic Fingerprints of the Asymmetric H7+O3 Complex in Solution

TL;DR

This study uses IR and NMR spectroscopy combined with QM/MM simulations to identify and analyze the asymmetric H7+O3 water cluster in solution, revealing its role in promoting irreversible proton transport in water clusters.

Contribution

It provides the first spectroscopic evidence and detailed simulation of the asymmetric H7+O3 water cluster, highlighting its significance in proton transport mechanisms.

Findings

H7+O3 exists in acetonitrile as an asymmetric protonated water trimer.

The core H7+O3 motif persists in larger water clusters up to at least 8 molecules.

H7+O3 promotes irreversible proton transport through structural evolution.

Abstract

Infrared (IR) absorption in the 1000-3700 cm-1 range and 1H NMR spectroscopy reveal the existence of an asymmetric protonated water trimer, H7+O3, in acetonitrile. The core H7+O3 motif persists in larger protonated water clusters in acetonitrile up to at least 8 water molecules. Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations reveal irreversible proton transport promoted by propagating the asymmetric H7+O3 structure in solution. The QM/MM calculations allow for the successful simulation of the measured IR absorption spectra of H7+O3 in the OH stretch region, which reaffirms the assignment of the H7+O3 spectra to a hybrid-complex structure: a protonated water dimer strongly hydrogen-bonded to a third water molecule with the proton exchanging between the two possible shared-proton Zundel-like centers. The H7+O3 structure lends itself to promoting irreversible proton transport in presence of even one additional water molecule. We demonstrate how continuously evolving H7+O3 structures may support proton transport within larger water solvates.