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Narvaez WA, Park SJ, Schwartz BJ. Competitive Ion Pairing and the Role of Anions in the Behavior of Hydrated Electrons in Electrolytes. J Phys Chem B 2022; 126:7701-7708. [PMID: 36166380 DOI: 10.1021/acs.jpcb.2c04463] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Experiments have shown that in the presence of electrolytes, the hydrated electron's absorption spectrum experiences a blue shift whose magnitude depends on both the salt concentration and chemical identity. Previous computer simulations have suggested that the spectral blue shift results from the formation of (cation, electron) contact pairs and that the concentration dependence arises because the number of cations simultaneously paired with the electron increases with increasing concentration. In this work, we perform new simulations to build an atomistic picture that explains the effect of salt identity on the observed hydrated electron spectral shifts. We simulate hydrated electrons in the presence of both monovalent (Na+) and divalent (Ca2+) cations paired with both Cl- and a spherical species representing ClO4- anions. Our simulations reproduce the experimental observations that divalent ions produce larger blue shifts of the hydrated electron's spectrum than monovalent ions with the same anion and that perchlorate salts show enhanced blue shifts compared to chloride salts with the same cation. We find that these observations can be explained by competitive ion pairing. With small kosmotropic cations such as Na+ and Ca2+, aqueous chloride salts tend to form (cation, anion) contact pairs, whereas there is little ion pairing between these cations and chaotropic perchlorate anions. Hydrated electrons also strongly interact with these cations, but if the cations are also paired with anions, this affects the free energy of the electron-cation interaction. With chloride salts, hydrated electrons end up in complexes containing multiple cations plus a few anions as well as the electron. Repulsive interactions between the electron and the nearby Cl- anions reduce the cation-induced spectral blue shift of the hydrated electron. With perchlorate salts, hydrated electrons pair with multiple cations without any associated anions, leading to the largest possible cation-induced spectral blue shift. We also see that the reason multivalent cations produce larger spectral blue shifts than monovalent cations is because hydrated electrons are able to simultaneously pair with a larger number of multivalent cations due to a larger free energy of interaction. Overall, the interaction of hydrated electrons with electrolytes fits well with the Hofmeister series, where the electron behaves as an anion that is slightly more able to break water's H-bond structure than chloride.
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Affiliation(s)
- Wilberth A Narvaez
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
| | - Sanghyun J Park
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
| | - Benjamin J Schwartz
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States
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Narvaez WA, Park SJ, Schwartz BJ. Hydrated Electrons in High-Concentration Electrolytes Interact with Multiple Cations: A Simulation Study. J Phys Chem B 2022; 126:3748-3757. [PMID: 35544344 DOI: 10.1021/acs.jpcb.2c01501] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Experimental studies have demonstrated that the hydrated electron's absorption spectrum undergoes a concentration-dependent blue-shift in the presence of electrolytes such as NaCl. The blue-shift increases roughly linearly at low salt concentration but saturates as the solubility limit of the salt is approached. Previous attempts to understand the origin of the concentration-dependent spectral shift using molecular simulation have only examined the interaction between the hydrated electron and a single sodium cation, and these simulations predicted a spectral blue-shift that was an order of magnitude larger than that seen experimentally. Thus, in this paper, we first explore the reasons for the exaggerated spectral blue-shift when a simulated hydrated electron interacts with a single Na+. We find that the issue arises from nonpairwise additivity of the Na+-e- and H2O-e- pseudopotentials used in the simulation. This effect arises because the solvating water molecules donate charge into the empty orbitals of Na+, lowering the effective charge of the cation and thus reducing the excess electron-cation interaction. Careful analysis shows, however, that although this nonpairwise additivity changes the energetics of the electron-Na+ interaction, the forces between the electron, Na+, and water are unaffected, so that mixed quantum/classical (MQC) simulations produce the correct structure and dynamics. With this in hand, we then use MQC simulations to explore the behavior of the hydrated electron as an explicit function of NaCl salt concentration. We find that the simulations correctly reproduce the observed experimental spectral shifting behavior. The reason for the spectral shift is that as the electrolyte concentration increases, the average number of cations simultaneously interacting in contact pairs with the hydrated electron increases from 1.0 at low concentrations to ∼2.5 near the saturation limit. As the number of cations that interact with the electron increases, the cation/electron interactions becomes slightly weaker, so that the corresponding Na+-e- distance increases with increasing salt concentration. We also find that the dielectric constant of the solution plays little role in the observed spectroscopy, so that the electrolyte-dependent spectral shifts of the hydrated electron are directly related to the concentration-dependent number of closely interacting cations.
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Affiliation(s)
- Wilberth A Narvaez
- Department of Chemistry and Biochemistry, University of California, Los Angeles Los Angeles, California 90095-1569 United States
| | - Sanghyun J Park
- Department of Chemistry and Biochemistry, University of California, Los Angeles Los Angeles, California 90095-1569 United States
| | - Benjamin J Schwartz
- Department of Chemistry and Biochemistry, University of California, Los Angeles Los Angeles, California 90095-1569 United States
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Shen Z, Glover WJ. Flexible boundary layer using exchange for embedding theories. I. Theory and implementation. J Chem Phys 2021; 155:224112. [PMID: 34911322 DOI: 10.1063/5.0067855] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Embedding theory is a powerful computational chemistry approach to exploring the electronic structure and dynamics of complex systems, with Quantum Mechanical/Molecular Mechanics (QM/MM) being the prime example. A challenge arises when trying to apply embedding methodology to systems with diffusible particles, e.g., solvents, if some of them must be included in the QM region, for example, in the description of solvent-supported electronic states or reactions involving proton transfer or charge-transfer-to-solvent: without a special treatment, inter-diffusion of QM and MM particles will eventually lead to a loss of QM/MM separation. We have developed a new method called Flexible Boundary Layer using Exchange (FlexiBLE) that solves the problem by adding a biasing potential to the system that closely maintains QM/MM separation. The method rigorously preserves ensemble averages by leveraging their invariance to an exchange of identical particles. With a careful choice of the biasing potential and the use of a tree algorithm to include only important QM and MM exchanges, we find that the method has an MM-forcefield-like computational cost and thus adds negligible overhead to a QM/MM simulation. Furthermore, we show that molecular dynamics with the FlexiBLE bias conserves total energy, and remarkably, sub-diffusional dynamical quantities in the inner QM region are unaffected by the applied bias. FlexiBLE thus widens the range of chemistry that can be studied with embedding theory.
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Affiliation(s)
- Zhuofan Shen
- NYU Shanghai, 1555 Century Ave., Shanghai 200122, China
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Park SJ, Narvaez WA, Schwartz BJ. How Water-Ion Interactions Control the Formation of Hydrated Electron:Sodium Cation Contact Pairs. J Phys Chem B 2021; 125:13027-13040. [PMID: 34806385 DOI: 10.1021/acs.jpcb.1c08256] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Although solvated electrons are a perennial subject of interest, relatively little attention has been paid to the way they behave in aqueous electrolytes. Experimentally, it is known that the hydrated electron's (eaq-) absorption spectrum shifts to the blue in the presence of salts, and the magnitude of the shift depends on the ion concentration and the identities of both the cation and anion. Does the blue-shift result from some type of dielectric effect from the bulk electrolyte, or are there specific interactions between the hydrated electron and ions in solution? Previous work has suggested that eaq- forms contact pairs with aqueous ions such as Na+, leading to the question of what controls the stability of such contact pairs and their possible connection to the observed spectroscopy. In this work, we use mixed quantum/classical simulations to examine the nature of Na+:e- contact pairs in water, using a novel method for quantum umbrella sampling to construct eaq--ion potentials of mean force (PMF). We find that the nature of the contact pair PMF depends sensitively on the choice of the classical interactions used to describe the Na+-water interactions. When the ion-water interactions are slightly stronger, the corresponding cation:e- contact pairs form at longer distances and become free energetically less stable. We show that this is because there is a delicate balance between solvation of the cation, solvation of eaq- and the direct electronic interaction between the cation and the electron, so that small changes in this balance lead to large changes in the formation and stability of e--ion contact pairs. In particular, strengthening the ion-water interactions helps to maintain a favorable local solvation environment around Na+, which in turn forces water molecules in the first solvation shell of the cation to be unfavorably oriented toward the electron in a contact pair; stronger solvation of the cation also reduces the electronic overlap of eaq- with Na+. We also find that the calculated spectra of different models of Na+:e- contact pairs do not shift monotonically with cation-electron distance, and that the calculated spectral shifts are about an order of magnitude larger than experiment, suggesting that isolated contact pairs are not the sole explanation for the blue-shift of the hydrated electron's spectrum in the presence of electrolytes.
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Affiliation(s)
- Sanghyun J Park
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569, United States
| | - Wilberth A Narvaez
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569, United States
| | - Benjamin J Schwartz
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569, United States
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Glover WJ, Schwartz BJ. The Fluxional Nature of the Hydrated Electron: Energy and Entropy Contributions to Aqueous Electron Free Energies. J Chem Theory Comput 2020; 16:1263-1270. [PMID: 31914315 DOI: 10.1021/acs.jctc.9b00496] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
There has been a great deal of recent controversy over the structure of the hydrated electron and whether it occupies a cavity or contains a significant number of interior waters (noncavity). The questions we address in this work are, from a free energy perspective, how different are these proposed structures? Do the different structures all lie along a single continuum, or are there significant differences (i.e., free energy barriers) between them? To address these questions, we have performed a series of one-electron calculations using umbrella sampling with quantum biased molecular dynamics along a coordinate that directly reflects the number of water molecules in the hydrated electron's interior. We verify that a standard cavity model of the hydrated electron behaves essentially as a hard sphere: the model is dominated by repulsion at short range such that water is expelled from a local volume around the electron, leading to a water solvation shell like that of a pseudohalide ion. The repulsion is much larger than thermal energies near room temperature, explaining why such models exhibit properties with little temperature dependence. On the other hand, our calculations reveal that a noncavity model is highly fluxional, meaning that thermal motions cause the number of interior waters to fluctuate from effectively zero (i.e., a cavity-type electron) to potentially above the bulk water density. The energetic contributions in the noncavity model are still repulsive in the sense that they favor cavity formation, so the fluctuations in structure are driven largely by entropy: the entropic cost for expelling water from a region of space is large enough that some water is still driven into the electron's interior. As the temperature is lowered and entropy becomes less important, the noncavity electron's structure is predicted to become more cavity-like, consistent with the observed temperature dependence of the hydrated electron's properties. Thus, we argue that although the specific noncavity model we study overestimates the preponderance of fluctuations involving interior water molecules, with appropriate refinements to correctly capture the true average number of interior waters and molar solvation volume, a fluxional model likely makes the most sense for understanding the various experimental properties of the hydrated electron.
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Affiliation(s)
- William J Glover
- NYU Shanghai , 1555 Century Ave. , Pudong, Shanghai , China 200122.,NYU-ECNU Center for Computational Chemistry at NYU Shanghai , 3663 Zhongshang Road , Shanghai , China 200062.,Department of Chemistry , New York University , New York , New York 10003 , United States
| | - Benjamin J Schwartz
- Department of Chemistry and Biochemistry , University of California, Los Angeles , 607 Charles E. Young Drive East , Los Angeles , California 90095-1569 , United States
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Structure and spectrum of the hydrated electron. A combined quantum chemical statistical mechanical simulation. J Mol Liq 2019. [DOI: 10.1016/j.molliq.2019.111300] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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Gillet N, Elstner M, Kubař T. Coupled-perturbed DFTB-QM/MM metadynamics: Application to proton-coupled electron transfer. J Chem Phys 2018; 149:072328. [PMID: 30134697 DOI: 10.1063/1.5027100] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We present a new concept of free energy calculations of chemical reactions by means of extended sampling molecular dynamics simulations. Biasing potentials are applied on partial atomic charges, which may be combined with atomic coordinates either in a single collective variable or in multi-dimensional biasing simulations. The necessary additional gradients are obtained by solving coupled-perturbed equations within the approximative density-functional tight-binding method. The new computational scheme was implemented in a combination of Gromacs and Plumed. As a prospective application, proton-coupled electron transfer in a model molecular system is studied. Two collective variables are introduced naturally, one for the proton transfer and the other for the electron transfer. The results are in qualitative agreement with the extended free simulations performed for reference. Free energy minima as well as the mechanism of the process are identified correctly, while the topology of the transition region and the height of the energy barrier are only reproduced qualitatively. The application also illustrates possible difficulties with the new methodology. These may be inefficient sampling of spatial coordinates when atomic charges are biased exclusively and a decreased stability of the simulations. Still, the new approach represents a viable alternative for free energy calculations of a certain class of chemical reactions, for instance a proton-coupled electron transfer in proteins.
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Affiliation(s)
- Natacha Gillet
- Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
| | - Marcus Elstner
- Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
| | - Tomáš Kubař
- Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
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Bishop KP, Roy PN. Quantum mechanical free energy profiles with post-quantization restraints: Binding free energy of the water dimer over a broad range of temperatures. J Chem Phys 2018; 148:102303. [DOI: 10.1063/1.4986915] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- Kevin P. Bishop
- Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
| | - Pierre-Nicholas Roy
- Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
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Zho CC, Farr EP, Glover WJ, Schwartz BJ. Temperature dependence of the hydrated electron’s excited-state relaxation. I. Simulation predictions of resonance Raman and pump-probe transient absorption spectra of cavity and non-cavity models. J Chem Phys 2017; 147:074503. [DOI: 10.1063/1.4985905] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Affiliation(s)
- Chen-Chen Zho
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California,
90095-1569, USA
| | - Erik P. Farr
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California,
90095-1569, USA
| | - William J. Glover
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California,
90095-1569, USA
- NYU-ECNU Center for Computational Chemistry at NYU Shanghai, Shanghai 200062, China
- NYU Shanghai, 1555 Century Avenue,
Shanghai 200135, China
| | - Benjamin J. Schwartz
- Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California,
90095-1569, USA
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Zho CC, Schwartz BJ. Time-Resolved Photoelectron Spectroscopy of the Hydrated Electron: Comparing Cavity and Noncavity Models to Experiment. J Phys Chem B 2016; 120:12604-12614. [PMID: 27973828 DOI: 10.1021/acs.jpcb.6b07852] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Affiliation(s)
- Chen-Chen Zho
- Department of Chemistry and
Biochemistry University of California, Los Angeles, Los Angeles, California 90095-1569, United States
| | - Benjamin J. Schwartz
- Department of Chemistry and
Biochemistry University of California, Los Angeles, Los Angeles, California 90095-1569, United States
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Coons MP, You ZQ, Herbert JM. The Hydrated Electron at the Surface of Neat Liquid Water Appears To Be Indistinguishable from the Bulk Species. J Am Chem Soc 2016; 138:10879-86. [DOI: 10.1021/jacs.6b06715] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Marc P. Coons
- Department of Chemistry and
Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
| | - Zhi-Qiang You
- Department of Chemistry and
Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
| | - John M. Herbert
- Department of Chemistry and
Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
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Casey JR, Schwartz BJ, Glover WJ. Free Energies of Cavity and Noncavity Hydrated Electrons Near the Instantaneous Air/Water Interface. J Phys Chem Lett 2016; 7:3192-3198. [PMID: 27479028 DOI: 10.1021/acs.jpclett.6b01150] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
The properties of the hydrated electron at the air/water interface are computed for both a cavity and a noncavity model using mixed quantum/classical molecular dynamics simulation. We take advantage of our recently developed formalism for umbrella sampling with a restrained quantum expectation value to calculate free-energy profiles of the hydrated electron's position relative to the water surface. We show that it is critical to use an instantaneous description of the air/water interface rather than the Gibbs' dividing surface to obtain accurate potentials of mean force. We find that noncavity electrons, which prefer to encompass several water molecules, avoid the interface where water molecules are scarce. In contrast, cavity models of the hydrated electron, which prefer to expel water, have a local free-energy minimum near the interface. When the cavity electron occupies this minimum, its absorption spectrum is quite red-shifted, its binding energy is significantly lowered, and its dynamics speed up quite a bit compared with the bulk, features that have not been found by experiment. The surface activity of the electron therefore serves as a useful test of cavity versus noncavity electron solvation.
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Affiliation(s)
- Jennifer R Casey
- Department of Chemistry and Biochemistry, University of California, Los Angeles , Los Angeles, California 90095-1569, United States
| | - Benjamin J Schwartz
- Department of Chemistry and Biochemistry, University of California, Los Angeles , Los Angeles, California 90095-1569, United States
| | - William J Glover
- NYU-ECNU Center for Computational Chemistry, New York University Shanghai , Shanghai 200122, China
- Department of Chemistry, New York University , New York, New York 10003, United States
- Department of Chemistry, East China Normal University , Shanghai 200062, China
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Turi L. Hydrated Electrons in Water Clusters: Inside or Outside, Cavity or Noncavity? J Chem Theory Comput 2016; 11:1745-55. [PMID: 26889512 DOI: 10.1021/ct501160k] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
In this work, we compare the applicability of three electron–water molecule pseudopotentials in modeling the physical properties of hydrated electrons. Quantum model calculations illustrate that the recently suggested Larsen–Glover–Schwartz (LGS) model and its modified m-LGS version have a too-attractive potential in the vicinity of the oxygen. As a result, LGS models predict a noncavity hydrated electron structure in clusters at room temperature, as seen from mixed one-electron quantum–classical molecular dynamics simulations of water cluster anions, with the electron localizing exclusively in the interior of the clusters. Comparative calculations using the cavity-preferring Turi–Borgis (TB) model predict interior-state and surface-state cluster isomers. The computed associated physical properties are also analyzed and compared to available experimental data. We find that the LGS and m-LGS potentials provide results that appear to be inconsistent with the size dependence of the experimental data. The simulated TB tendencies are qualitatively correct. Furthermore, ab initio calculations on static LGS noncavity structures indicate weak stabilization of the excess electron in regions where the LGS potential preferably and strongly binds the electron. TB calculations give stabilization energies that are in line with the ab initio results. In conclusion, we observe that the cavity-preferring pseudopotential model predicts cluster physical properties in better agreement with experimental data and ab initio calculations than the models predicting noncavity structures for the hydrated electron.
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Dale SG, Johnson ER. Counterintuitive electron localisation from density-functional theory with polarisable solvent models. J Chem Phys 2015; 143:184112. [DOI: 10.1063/1.4935177] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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