The coupling of the hydrated proton to its first solvation shell

Abstract

The Zundel ($${H}_{5}{O}_{2}^{+}$$ H 5 O 2 + ) and Eigen ($${H}_{9}{O}_{4}^{+}$$ H 9 O 4 + ) cations play an important role as intermediate structures for proton transfer processes in liquid water. In the gas phase they exhibit radically different infrared (IR) spectra. The question arises: is there a least common denominator structure that explains the IR spectra of both, the Zundel and Eigen cations, and hence of the solvated proton? Full dimensional quantum simulations of these protonated cations demonstrate that two dynamical water molecules and an excess proton constitute this fundamental subunit. Embedded in the static environment of the parent Eigen cation, this subunit reproduces the positions and broadenings of its main excess-proton bands. In isolation, its spectrum reverts to the well-known Zundel ion. Hence, the dynamics of this subunit polarized by an environment suffice to explain the spectral signatures and anharmonic couplings of the solvated proton in its first solvation shell.

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

“…(3) Recent ML-MCTDH simulations on the solvated hydronium or Eigen ion (H 9 O 4 + ) reveal the contribution of dozens of eigenstates that dominate the IR activation of the hydronium O–H stretch motion and show that the proton vibrations of the Eigen ion can be understood in terms of an embedded Zundel subunit. 35 Likewise, recent experimental studies indicate that the dynamics of protonated water clusters can be related to fluctuations of local electrical fields, 36 which also appear for solvated H 5 O 2 + . 9,37,38 …”

Accelerating Anharmonic Spectroscopy Simulations via Local-Mode, Multilevel Methods

“…Infrared and Raman spectroscopies provide detailed glimpses into the behavior of molecules and, implicitly, the potential energy and dipole surfaces that govern their vibrational dynamics. Recent progress in cold-ion sources and action spectroscopies has generated unprecedented insight into ion hydration, hydrogen bonding, and biomolecular structure. − Advanced theory and computational simulations have become critical partners in the interpretation of these modern experiments, ,,,,− and the present investigation aims to address the suitability of a combination of new computational approaches for molecules and clusters of the size accessed by these experiments. In this somewhat expanded Introduction section, the details of these methods will be reviewed, along with the reasons that this pairing of methods is particularly symbiotic for anharmonic simulations of large systems.…”

Calculating Vibrational Excited State Absorptions with Excited State Constrained Minimized Energy Surfaces

“…These beads evolve classically on a PES, and the bead distribution connects with the quantum probability density, thus describing nuclear quantum effects. As such, these path-integral methods can describe anharmonicity and provide relatively accurate vibrational spectra through reasonable approximations to the time autocorrelation functions. − An even more accurate method to describe vibrational spectra is through quantum nuclear dynamics, and a commonly used method is the multiconfigurational time-dependent Hartree (MCTDH) theory. − Although the underlying computational cost remains a major limiting factor, this theory has been shown to accurately describe the vibrational spectra of systems as large as water clusters with four water molecules …”

“…54−57 Although the underlying computational cost remains a major limiting factor, this theory has been shown to accurately describe the vibrational spectra of systems as large as water clusters with four water molecules. 58 In the past few years, our group has developed a new method for calculating vibrational spectra based on constrained minimized energy surfaces (CMESs). 59 The CMES is an effective PES for nuclei, but compared to the conventional PES, the CMES incorporates nuclear quantum effects, especially the zero-point effects, in the effective potential energy surface.…”