Vibrational Spectra of the OH Radical in Water: Ab Initio Molecular Dynamics Simulations and Quantum Chemical Calculations Using Hybrid Functionals

Apostolidou, Christina

doi:10.1002/adts.202000174

Cited by 9 publications

(88 citation statements)

References 136 publications

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“…To further investigate this issue, we performed ai MD simulations of OH(aq) using periodic DFT, in a unit cell [OH(H 2 O) 63 ] that is thought to be large enough to avoid finite-size artifacts. 29,32 One simulation employs the PBE0 + D3 functional at T = 370 K, consistent with the most recent hybrid DFT ai MD simulations of OH(aq), 35 except that the unit cell is twice as large in the present work. (Details can be found in Section S1 of the Supporting Information.)…”

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confidence: 66%

“…The feature at smaller values of r O * O that is is typically associated with the hemibond is absent from the periodic simulation at T = 370 K, consistent with previous simulations at the same temperature but using a smaller unit cell. 35 At T = 310 K, however, one can just barely observe the appearance of a feature at r O * O = 2.2Å. Even at the higher temperature, the RDF is not zero at r O * O = 2.2Å although there is no distinct feature prior to the main hydrogen-bonding peak.…”

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confidence: 92%

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Hidden Hemibonding in the Aqueous Hydroxl Radical

Rana

Herbert

2021

Preprint

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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.

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confidence: 66%

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confidence: 92%

Hidden Hemibonding in the Aqueous Hydroxl Radical

Rana

Herbert

2021

Preprint

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“…This will definitely show how this thesis is a turning point for both system types as well as for other complicated systems. 36,41,42…”

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confidence: 99%

OH radical in water from ab initio molecular dynamics simulation employing hybrid functionals

Apostolidou

2019

The Journal of Chemical Physics

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In this thesis, complicated solvation phenomena that are impossible to observe via laboratory experiments are comprehensively investigated by molecular dynamics (MD) simulations. The studied system types are the OH radical in aqueous systems and keratin in ionic liquids (ILs), which are both important key contributors to the environment. The OH radical effectively cleanses the atmosphere and is also used in wastewater cleaning processes. Regenerated keratin is a biodegradable material that has been recently used as an ingredient in bioplastics. Its regeneration minimizes environmental pollution and a regeneration process by means of a greener route with ILs underpins the urgency of this research area.Within the last 15 years, only representative ab initio MD (AIMD) simulation studies with the generalized gradient approximation (GGA) functionals can be found for the OH radical in aqueous environments. However, these aforementioned functionals include the self-interaction error (SIE) that leads to artificial delocalization of unpaired electrons. Thus, a comprehensive, representative and accurate Born-Oppenheimer (BO) MD simulation study of OH -wn (w=water, n=1-5) clusters and OH -w31 was performed by employing hybrid functionals and the accurate diffuse basis set DZVP-MOLOPT-SR-GTH. In particular, one important research area was to investigate the existence of the hemibonded structure. This hemibonded structure is a threeelectron two-center interaction between the oxygen atoms of the OH radical and water with a characteristic low O -O distance of around 238 pm and a delocalization of the unpaired electron over these two centers. With the aid of this highly accurate BOMD simulation, the absence of the long-debated hemibonded configuration is shown. This clearly conveys that the hemibonded structure is an artifact from the SIE. Furthermore, this thesis reveals that a less accurate basis set such as the 6-31G basis set and a too low simulation temperature can also lead to the occurrence of the hemibonded structure. Since these two features were also used in recent AIMD simulation studies, this thesis paves the way for a more accurate description of this system type and thus sets a clear milestone within the whole topic. Finally, also the orientation of the OH radical in the gas phase up to bulk liquid water was studied by means of these BOMD simulations. It is shown that the OH radical has a less propensity for the bulk phase and that the highest occurrence is given by the OH -w1 complex in which the OH acts as a hydrogen bond donor toward water and which will therefore be a focal point for future tropospheric reactions.Additionally, a BOMD simulation of pure liquid water, involving 32 water molecules and simulated at the B3LYP-D3 and PBE0-TC-LRC-D3 levels of theory, showed very precise structural features that were in very good agreement with highly accurate experimental data.The former BOMD simulations of the OH radical in aqueous systems by using the B3LYP-D3/DZVP-MOLOPT-SR-GTH level of theory were further used to ...

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“…[29][30][31][32] Perhaps for these reasons, the possibility of a hemibonded solvation motif seems not to have been considered in the analysis of recent x-ray experiments on OH(aq). 33 Recent ai MD simulations using hybrid functionals report no evidence of the feature traditionally associated with this motif, [34][35][36] namely, a peak at 2.2-2.3 Å in the radial distribution function (RDF) for oxygen. 26 In the present work, we use periodic DFT simulations at the PBE0 + D3 level 37,38 to demonstrate that the disappearance of this feature in hybrid DFT does not imply the absence of a hemibonded solvation motif.…”

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confidence: 99%

“…Hemibonding in • OH(aq) was predicted in some of the earliest ab initio molecular dynamics ( ai MD) simulations using density functional theory (DFT), − but these predictions were quickly rejected as artifacts of self-interaction error (SIE) associated with the use of semilocal functionals, since SIE is known to overstabilize 2c–3e bonds. , More recently, it has been suggested that the hemibonds observed in those early simulations might instead be artifacts of a too-small periodic simulation cell. − Perhaps for these reasons, the possibility of a hemibonded solvation motif does not seem to have been considered in the recent analysis of ultrafast X-ray experiments on • OH(aq) . The latest ai MD simulations using hybrid functionals report no evidence of the feature traditionally associated with this motif, − namely, a peak at 2.2–2.3 Å in the radial distribution function (RDF) for oxygen . In the present work, we use periodic DFT simulations at the PBE0+D3 level , to demonstrate that the disappearance of this feature in hybrid DFT does not imply the absence of a hemibonded solvation motif.…”

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confidence: 99%