Broadband 2D IR spectroscopy reveals dominant asymmetric H5O2+ proton hydration structures in acid solutions

Abstract

Given the critical role of the aqueous excess proton in redox chemistry, determining its structure and the mechanism of its transport in water are intense areas of experimental and theoretical research. The ultrafast dynamics of the proton's hydration structure has made it extremely challenging to study experimentally. Using ultrafast broadband two-dimensional infrared spectroscopy, we show that the vibrational spectrum of the aqueous proton is fully consistent with a protonated water complex broadly defined as a Zundel-like HO motif. Analysis of the inhomogeneously broadened proton stretch two-dimensional lineshape indicates an intrinsically asymmetric, low-barrier O-H-O potential that exhibits surprisingly persistent distributions in both its asymmetry and O-O distance. This structural characterization has direct implications for the extent of delocalization exhibited by a proton's excess charge and for the possible mechanisms of proton transport in water.

Identifying Eigen-like hydrated protons at negatively charged interfaces

“…The peak position lies within the range generally assigned to embedded Eigen-like structures (i.e., an H 3 O + core, symmetrically solvated by three water molecules) 1,4,6,14 . However, this classification is controversial given the extremely diffuse vibrational signature of the hydrated proton in bulk solution 4,7 . Although the cryogenically cooled cluster behavior cannot be readily extrapolated to the bulk liquid at room temperature, their resolved features provide useful insight.…”

“…It represents the essence of a concept as widely used and popularized as pH, and is obviously fundamentally involved in all acid/base chemistry. Despite recent experimental [1][2][3][4][5][6][7] and theoretical advances 4,[8][9][10] , a comprehensive molecular understanding of hydrated protons in solution remains elusive. Vibrational spectroscopy, being sensitive to hydrogen bond strength and dynamics, has been the method of choice for determining the molecular structure of the hydrated proton 10 .…”

“…At room temperature, we require background subtraction and the deconvolution of vibrational spectral features to obtain indirect information of the structural motifs 4,15,16 . A precious insight has been provided by ultrafast two-dimensional infrared (2D-IR) spectroscopy, where by connecting the coupled resonances at different frequencies, distinct species have been identified in solution 6,7 . The current consensus leans towards the proton in the bulk liquid being primarily hydrated by two flanking water molecules in a Zundel-like configuration 4,7 .…”

Identifying Eigen-Like Hydrated Protons at Negatively Charged Interfaces

“…It represents the essence of a concept as widely used and popularized as pH, and is obviously fundamentally involved in all acid/base chemistry. Despite recent experimental [1][2][3][4][5][6][7] and theoretical advances, [8][9][10] a comprehensive molecular understanding of hydrated protons in solution remains elusive. Vibrational spectroscopy, being sensitive to hydrogen bond strength and dynamics, has been the method of choice for determining the molecular structure of the hydrated proton.…”

Transient Glass Formation around a Quadrupolar Photoexcited Dye in a Strongly H‐Bonding Liquid Observed by Transient 2D‐IR Spectroscopy

“…The long persistence of this broadening thus strongly contrasts with the equilibrium behavior of normal H‐bonding liquids like water, where the H‐bond network, albeit altered by solutes, remains highly dynamic even in the presence of ions at high concentrations . Likewise, the longest memory of inhomogeneity of the very strongly H‐bonded excess proton in water, was recently found to be only of the order of 200 fs …”