Hidden role of intermolecular proton transfer in the anomalously diffuse vibrational spectrum of a trapped hydronium ion

Abstract

Significance Understanding the origin of the extremely diffuse vibrational spectrum of an excess proton in water presents a grand challenge for contemporary physical chemistry. Here, we report the key observation that such diffuse bands occur even when the hydronium ion is held in the binding pocket of a rigid crown ether scaffold at 10 K. The broadening is traced to the zero-point vibrational displacements of the ion in the crown. The diffuse spectra are therefore an intrinsic property of the system and mimic the action of thermal fluctuations at elevated temperatures. We treat the mechanics underlying this phenomenon with a vibrationally adiabatic ansatz, from which emerges a qualitative picture that emphasizes the hidden role of vibrationally driven, intermolecular proton transfer.