Vibrational Signatures of Electronic Properties in Oxidized Water: Unraveling the Anomalous Spectrum of the Water Dimer Cation

Abstract

The water dimer cation, (H 2 O) 2 + , has long served as a prototypical reference system for water oxidation chemistry. In spite of this status, a definitive explanation for the anomalousand dominantfeatures in the experimental vibrational spectrum [Gardenier, G. H.; McCoy, A. B. J. Phys. Chem. A, 2009, 113, 4772−4779] has not been determined, and harmonic analyses qualitatively fail to reproduce these features. In this computational study, accurate quantum chemistry methods are combined with a fully coupled, six-dimensional anharmonic model to show that the unassigned bands are the result of resonant mode interactions and strong anharmonic coupling. Such coupling is fundamentally due to the unique electronic structure of this open-shell ion and the manner in which auxiliary modes affect the natural charge-transfer properties of the shared-proton stretch. These unique vibrational signatures provide a key reference point for modern spectroscopic and mechanistic analyses of water-oxidation catalysts.

Ionized water clusters , n = 2 to 6: A high‐accuracy study of structures and energetics

“…We have optimized two cationic dimers, in hemibonded (HB) and proton‐transferred (PT) forms, at the DF‐OLCCD/aDZ level. These structures are in agreement with the previous theoretical studies . Relative energies of the dimers, at the CBS level, are reported in Table .…”

“…Relative energies of cationic dimer clusters (Table ) at CCSD(T)/CBS level differ as much as 8.7 kcal mol −1 , which is in very good agreement with the value of 8.8 kcal mol −1 reported by Schaefer and coworkers at the CCSD(T)/aug‐cc‐pVQZ level. As it was discussed in previous studies, the PT form (dimer‐2) is significantly more stable than the HB form (dimer‐1). Mean absolute errors (MAEs) of other considered methods with respect to CCSD(T) are 0.6 (MP2), 1.9 (MP3), 0.2 (DF‐OLCCD), and 1.2 (CCSD) kcal mol −1 .…”

“…Ionized water clusters, , have been of remarkable interest due to their crucial roles in many chemical and biological processes, such as catalytic chemistry, radiotherapy, and the energy relaxation of photoexcited DNA base pairs . Small cationic water clusters , n = 2 to 6 serve as reasonable models for understanding the nature of the ionized water.…”

Signatures of Size-Dependent Structural Patterns in Hydrated Copper(I) Clusters, Cu+(H2O)n=1–10

“…The detailed, inner-sphere redox mechanisms 10 of many of these catalysts remain experimental challenges, however, which hampers further catalyst development. Recently, our research group has provided detailed computational analyses of the first oxidation step in bare water, including structures and spectra of (H 2 O) 2 + , 11 (H 2 O) 1−5 + , 12 and (H 2 O) 6−21 + . 13 In addition to explaining the unique vibrational signatures of the initially formed ion-radical contact pair (H 3 O + •••OH • ), these studies and others 14−17 demonstrated a propensity for ion-radical pair separation for n ≥ 5, which continues through n = 21 (and likely beyond 18 ).…”

Infrared Signatures of Isomer Selectivity and Symmetry Breaking in the Cs+(H2O)3 Complex Using Many-Body Potential Energy Functions

“…Gauss-Hermite quadrature was used for evaluation of the potential energy and dipole matrix elements. [124][125][126] The theoretical methods used in these calculations included harmonic normal-mode analysis; localmode vibrational self-consistent field theory (VSCF); 127,128 second-order, local-mode vibrational degenerate perturbation theory (VDPT2) 129 on VSCF states; the local-monomer approximation (LMon); [130][131][132][133][134][135][136][137] and a perturbatively corrected local-monomer model (cLMon) that included twomode, cross-monomer couplings. 138 Across all models, the small system sizes allowed ten quanta of excitation to be included along each mode.…”