State-resolved infrared spectrum of the protonated water dimer: Revisiting the characteristic proton transfer doublet peak

TL;DR

This study provides detailed insights into the complex IR spectrum of the protonated water dimer, revealing previously overlooked spectral features and emphasizing the importance of high-accuracy quantum simulations for understanding fluxional molecules.

Contribution

The paper introduces a comprehensive analysis of the protonated water dimer's IR spectrum using advanced potential energy surfaces and quantum simulations, uncovering subtle spectral features.

Findings

Identification of low-intensity satellite peaks in the IR spectrum.

Demonstration of the sensitivity of spectral features to energetic changes.

Validation of high-accuracy simulations for spectral assignment.

Abstract

The infrared (IR) spectra of protonated water clusters encode precise information on the dynamics and structure of the hydrated proton. However, the strong anharmonic coupling and quantum effects of these elusive species remain puzzling up to the present day. Here, we report unequivocal evidence that the interplay between the proton transfer and the water wagging motions in the protonated water dimer (Zundel ion) giving rise to the characteristic doublet peak is both more complex and more sensitive to subtle energetic changes than previously thought. In particular, hitherto overlooked low-intensity satellite peaks in the experimental spectrum are now unveiled and mechanistically assigned. Our findings rely on the comparison of IR spectra obtained using two highly accurate potential energy surfaces in conjunction with highly accurate state-resolved quantum simulations. We demonstrate that these high-accuracy simulations are important for providing definite assignments of the complex IR signals of fluxional molecules.