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1. Exactly Fragment Additive Breakdown of Polarization for Energy Decomposition Analysis Based on the Self-Consistent Field for Molecular Interactions

   https://doi.org/10.1021/acs.jctc.3c00872  

   Shen, Hengyuan; Veccham, Srimukh Prasad; Head-Gordon, Martin (December 2023, Journal of Chemical Theory and Computation)

   Energy decomposition analysis (EDA) is a useful method to unravel an intermolecular interaction energy into chemically meaningful components such as geometric distortion, frozen interactions, polarization, and charge transfer. A further decomposition of the polarization (POL) and charge transfer (CT) energy into fragment-wise contributions would be useful to understand the significance of each fragment during these two processes. To complement the existing exact pairwise decomposition of the CT term, this work describes formulation and implementation of a non-perturbative polarization analysis that decomposes the POL energy into an exactly fragment-wise additive sum based on the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA). These fragment-wise contributions can be further decomposed into chemically intuitive molecular orbital pairs using complementary occupied-virtual pairs (COVP) analysis. A very useful phase convention is established for each COVP such that constructive interference of occupied and virtual corresponds to electron flow into that region, whilst destructive interference corresponds to electron outflow. A range of model problems are used to demonstrate that the polarization process is typically a collective behavior of the electrons that is quite different from the charge transfer process. This provides another reason in addition to their different distance-dependence on fragment separation for separating these two processes in EDA. 

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2. Dissociation of HCl in water nanoclusters: an energy decomposition analysis perspective

   https://doi.org/10.1039/d1cp04587c  

   Zech, Alexander; Head-Gordon, Martin (December 2021, Physical Chemistry Chemical Physics)

   As known, small HCl–water nanoclusters display a particular dissociation behaviour, whereby at least four water molecules are required for the ionic dissociation of HCl. In this work, we examine how intermolecular interactions promote the ionic dissociation of such nanoclusters. To this end, a set of 45 HCl–water nanoclusters with up to four water molecules is introduced. Energy decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA) is employed in order to study the importance of frozen interaction, dispersion, polarization, and charge-transfer for the dissociation. The vertical ALMO-EDA scheme is applied to HCl–water clusters along a proton-transfer coordinate varying the amount of spectator water molecules. The corresponding ALMO-EDA results show a clear preference for the dissociated cluster only in the presence of four water molecules. Our analysis of adiabatic ALMO-EDA results reveals a push–pull mechanism for the destabilization of the HCl bond based on the synergy between forward and backward charge-transfer. 

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3. Experimental study of the proton-transfer reaction C + H ₂ ⁺ → CH ⁺ + H and its isotopic variant (D ₂ ⁺ )

   https://doi.org/10.1039/D0CP04810K  

   Hillenbrand, Pierre-Michel; Bowen, Kyle P.; Dayou, Fabrice; Miller, Kenneth A.; de Ruette, Nathalie; Urbain, Xavier; Savin, Daniel W. (December 2020, Physical Chemistry Chemical Physics)

   We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of E r ∼ 0.01–10 eV. We used photodetachment of C − to produce a pure beam of atomic C in the ground electronic 3 P term, with statistically populated fine-structure levels. The H 2 + and D 2 + were formed in an electron impact ionization source, with well known vibrational and rotational distributions. The experimental work is complemented by a theoretical study of the CH 2 + electronic system in the reactant and product channels, which helps to clarify the possible reaction mechanisms underlying the ICS measurements. Our measurements provide evidence that the reactions are barrierless and exoergic. They also indicate the apparent absence of an intermolecular isotope effect, to within the total experimental uncertainties. Capture models, taking into account either the charge-induced dipole interaction potential or the combined charge-quadrupole and charge-induced dipole interaction potentials, produce reaction cross sections that lie a factor of ∼4 above the experimental results. Based on our theoretical study, we hypothesize that the reaction is most likely to proceed adiabatically through the 1 4 A′ and 1 4 A′′ states of CH 2 + via the reaction C( 3 P) + H 2 + ( 2 Σ+g) → CH + ( 3 Π) + H( 2 S). We also hypothesize that at low collision energies only H 2 + ( v ≤ 2) and D 2 + ( v ≤ 3) contribute to the titular reactions, due to the onset of dissociative charge transfer for higher vibrational v levels. Incorporating these assumptions into the capture models brings them into better agreement with the experimental results. Still, for energies ≲0.1 eV where capture models are most relevant, the modified charge-induced dipole model yields reaction cross sections with an incorrect energy dependence and lying ∼10% below the experimental results. The capture cross section obtained from the combined charge-quadrupole and charge-induced dipole model better matches the measured energy dependence but lies ∼30–50% above the experimental results. These findings provide important guidance for future quasiclassical trajectory and quantum mechanical treatments of this reaction. 

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4. Quantifying and elucidating the effect of CO ₂ on the thermodynamics, kinetics and charge transport of AEMFCs

   https://doi.org/10.1039/C9EE01334B  

   Zheng, Yiwei; Omasta, Travis J.; Peng, Xiong; Wang, Lianqin; Varcoe, John R.; Pivovar, Bryan S.; Mustain, William E. (September 2019, Energy & Environmental Science)

   It has been long-recognized that carbonation of anion exchange membrane fuel cells (AEMFCs) would be an important practical barrier for their implementation in applications that use ambient air containing atmospheric CO 2 . Most literature discussion around AEMFC carbonation has hypothesized: (1) that the effect of carbonation is limited to an increase in the Ohmic resistance because carbonate has lower mobility than hydroxide; and/or (2) that the so-called “self-purging” mechanism could effectively decarbonate the cell and eliminate CO 2 -related voltage losses during operation at a reasonable operating current density (>1 A cm −2 ). However, this study definitively shows that neither of these assertions are correct. This work, the first experimental examination of its kind, studies the dynamics of cell carbonation and its effect on AEMFC performance over a wide range of operating currents (0.2–2.0 A cm −2 ), operating temperatures (60–80 °C) and CO 2 concentrations in the reactant gases (5–3200 ppm). The resulting data provide for new fundamental relationships to be developed and for the root causes of increased polarization in the presence of CO 2 to be quantitatively probed and deconvoluted into Ohmic, Nernstian and charge transfer components, with the Nernstian and charge transfer components controlling the cell behavior under conditions of practical interest. 

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5. Model of charge transfer collisions between C ₆₀ and slow ions

   https://doi.org/10.1063/5.0100357  

   Smucker, J.; Montgomery, J. A.; Bredice, M.; Rozman, M. G.; Côté, R.; Sadeghpour, H. R.; Vrinceanu, D.; Kharchenko, V. (August 2022, The Journal of Chemical Physics)

   A semiclassical model describing the charge transfer collisions of C 60 fullerene with different slow ions has been developed to analyze available observations. These data reveal multiple Breit–Wigner-like peaks in the cross sections, with subsequent peaks of reactive cross sections decreasing in magnitude. Calculations of charge transfer probabilities, quasi-resonant cross sections, and cross sections for reactive collisions have been performed using semiempirical interaction potentials between fullerenes and ion projectiles. All computations have been carried out with realistic wave functions for C 60 ’s valence electrons derived from the simplified jellium model. The quality of these electron wave functions has been successfully verified by comparing theoretical calculations and experimental data on the small angle cross sections of resonant [Formula: see text] collisions. Using the semiempirical potentials to describe resonant scattering phenomena in C 60 collisions with ions and Landau–Zener charge transfer theory, we calculated theoretical cross sections for various C 60 charge transfer and fragmentation reactions which agree with experiments. 

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