Hidden Hemibonding in the Aqueous Hydroxyl Radical

Abstract

The existence of a two-center, three-electron hemibond in the first solvation shell of OH(aq) has long been a matter of debate. The hemibond manifests in ab initio molecular dynamics simulations as a small-r feature in the oxygen radial distribution function (RDF) for H2O• • • OH, but that feature disappears when semilocal density functionals are replaced with hybrids, suggesting a self-interaction artifact. Using periodic simulations at the PBE0 + D3 level, we demonstrate that the hemibond is actually still present (as evidenced by delocalization of the spin density onto nearby water molecules) but is obscured by the hydrogen-bonded feature in the RDF, due to a slight elongation of the hemibond. Computed electronic spectra for OH(aq) are in excellent agreement with experiment and confirm that hemibond-like configurations play an outsized role in the spectroscopy due to an intense charge-transfer transition that is strongly attenuated in hydrogen-bonded configurations. Apparently, 25% exact exchange does not eliminate delocalization of unpaired spins.

Spectroscopic evidence of S∴N and S∴O hemibonds in heterodimer cations

“…19 Up to now, many experimental and theoretical studies on hemibonds have been performed. [20][21][22] However, most of the studies focused on the most common sulfur-sulfur hemibonds. [23][24][25][26] For instance, the existence of S'S hemibonds in solution as well as in the gas phase was first confirmed experimentally by Asmus and his collaborators by transient absorption spectroscopy and mass spectrometry.…”

Detection and Correction of Delocalization Errors for Electron and Hole Polarons Using Density-Corrected DFT

“…One of the most pernicious and long-standing problems in density functional theory (DFT) is that of self-interaction error (SIE), − which arises from the incomplete cancellation of Hartree self-repulsion by approximate exchange-correlation (XC) functionals. Among the manifestations of SIE is the overstabilization of fractionally charged systems, leading to an underestimation of reaction barrier heights and motivating the use of hybrid functionals. , When generalized gradient approximations (GGAs) are used to describe open-shell systems, SIE results in exaggerated delocalization of unpaired spins. − Conventional hybrid functionals with 20–25% exact exchange do not always fully eliminate this “delocalization error”. − Of particular interest in the present work are polaron defects in transition-metal and main-group oxides, where the extent of defect delocalization is sensitive to the fraction of exact exchange. − Herein, we demonstrate that density-corrected (DC-)­DFT can be used both to detect and to correct the overdelocalization of polaron defects.…”

Detection and correction of delocalization errors for electron- and hole-polarons using density-corrected DFT

“…6 When generalized gradient approximations (GGAs) are used to describe openshell systems, the result is over-delocalization of unpaired spins. [7][8][9][10][11][12][13][14][15][16][17] Conventional hybrid functionals with 20-25% exact exchange do not always fully eliminate this delocalization error. [16][17][18][19][20][21][22][23] Of particular interest in the present work are polaron defects in transition metal and maingroup oxides, where the extent of defect delocalization is sensitive to the fraction of exact exchange.…”

“…[7][8][9][10][11][12][13][14][15][16][17] Conventional hybrid functionals with 20-25% exact exchange do not always fully eliminate this delocalization error. [16][17][18][19][20][21][22][23] Of particular interest in the present work are polaron defects in transition metal and maingroup oxides, where the extent of defect delocalization is sensitive to the fraction of exact exchange. [18][19][20][21][22][23][24][25][26][27][28][29][30] We demonstrate that density-corrected (DC-)DFT can be used both to detect and to correct over-delocalization of polaron defects.…”

“…(Despite the limitations of the Mulliken procedure, these charges have proven to be an effective way to characterize localization or delocalization of a radical center. 16,17,63 ) For the DC-DFT cases in Table 2, this means that we perform the geometry optimization using ρ HF but then evaluate ρ BLYP at the optimized geometry, for demonstrative purposes. Results based on ρ HF represent the proper DC-DFT procedure.…”