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

Infrared (IR) absorption in the 1000-3700 cm^-1 range and ¹ H NMR spectroscopy reveal the existence of an asymmetric protonated water trimer, H₇⁺ O_3, 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.