Solvent‐Dependent Structure of the I3− Ion Derived from Photoelectron Spectroscopy and Ab Initio Molecular Dynamics Simulations

Ab initio molecular dynamics (MD) simulations of the solvation of LiI₃ in four different solvents (water, methanol, ethanol, and acetonitrile) are employed to investigate the molecular and electronic structure of the I₃^? ion in relation to X‐ray photoelectron spectroscopy (XPS). Simulations show that hydrogen‐bond rearrangement in the solvation shell is coupled to intramolecular bond‐length asymmetry in the I₃^? ion. By a combination of charge analysis and I 4 d core‐level XPS measurements, the mechanism of the solvent‐induced distortions has been studied, and it has been concluded that charge localization mediates intermolecular interactions and intramolecular distortion. The approach involving a synergistic combination of theory and experiment probes the solvent‐dependent structure of the I₃^? ion, and the geometric structure has been correlated with the electronic structure.     

Direct Ab Initio Dynamics Study on the Reaction of CH3CHF2 (HFC‐152a) with the Cl Atom

A direct ab initio dynamics method is used to investigate the hydrogen‐abstraction reaction CH₃CHF₂+Cl. One transition state is located for α‐H abstraction, and two are identified for β‐H abstraction. The potential‐energy surface (PES) is obtained at the G3(MP2)//MP2/6‐311G(d, p) level. Furthermore, the rate constants of the three channels are evaluated by using canonical variational transition‐state theory (CVT) with small‐curvature tunneling (SCT) contributions over a wide temperature range of 200–2500 K. The dynamic calculations show that the reaction proceeds mainly by α‐H abstraction over the whole temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.     

Cover Feature: Ab Initio Molecular Dynamics Study of H2 Dissociation Mechanisms on Cu13 and Defective Graphene-supported Cu13 Clusters: Active Sites,Energy Barriers and Adsorption States (ChemPhysChem 19/2023)

Ab initio molecular dynamics calculations were performed to study H₂ dissociation mechanisms on Cu₁₃ and defective graphene-supported Cu₁₃ clusters. The study reveals that seven types of corresponding dissociation processes are found on the two clusters. The average dissociation energy barriers are 0.51 eV on the Cu₁₃ cluster and 0.12 eV on the defective graphene-supported Cu₁₃ cluster, which are lowered by ~19 % and ~81 % compared with the pristine Cu(111) surface, respectively. Furthermore, compared with the pure Cu₁₃ cluster, the average dissociation energy barrier on the defective graphene-supported Cu₁₃ cluster is substantially reduced by about 76 %. The preferred dissociation mechanisms on the two clusters are H₂ located at a top-bridge site with the barrier heights of 0.30 eV on the Cu₁₃ cluster and −0.31 eV on the defective graphene-supported Cu₁₃ cluster_. Analysis of the H−Cu bond interactions in the transition states shows that the antibonding-orbital center shifts upward on the defective graphene-supported Cu₁₃ cluster compared with the one on the Cu₁₃ cluster, which explains the reduction of the dissociation energy barrier. The average adsorption energy of dissociated H atoms is also greatly enhanced on the defective graphene-supported Cu₁₃ cluster, about twice that on the Cu₁₃ cluster.     

Magnetic Relaxation in Single‐Electron Single‐Ion Cerium(III) Magnets: Insights from Ab Initio Calculations

Detailed ab initio calculations were performed on two structurally different cerium(III) single‐molecule magnets (SMMs) to probe the origin of magnetic anisotropy and to understand the mechanism of magnetic relaxations. The complexes [Ce^III{Zn^II(L)}₂(MeOH)]BPh₄ ( 1 ) and [Li(dme)₃][Ce^III(cot′′)₂] ( 1 ; L=N,N,O,O‐tetradentate Schiff base ligand; 2 ; DME=dimethoxyethane, COT′′=1,4‐bis(trimethylsilyl)cyclooctatetraenyldianion), which are reported to be zero‐field and field‐induced SMMs with effective barrier heights of 21.2 and 30 K respectively, were chosen as examples. CASSCF+RASSI/SINGLE_ANISO calculations unequivocally suggest that m_J|±5/2〉 and |±1/2〉 are the ground states for complexes 1 and 2 , respectively. The origin of these differences is rooted back to the nature of the ligand field and the symmetry around the cerium(III) ions. Ab initio magnetisation blockade barriers constructed for complexes 1 and 2 expose a contrasting energy‐level pattern with significant quantum tunnelling of magnetisation between the ground state Kramers doublet in complex 2 . Calculations performed on several model complexes stress the need for a suitable ligand environment and high symmetry around the cerium(III) ions to obtain a large effective barrier.     

Quantum‐Chemical Study of the FeNCN Conversion‐Reaction Mechanism in Lithium‐ and Sodium‐Ion Batteries

We report a computational study on 3d transition‐metal (Cr, Mn, Fe, and Co) carbodiimides in Li‐ and Na‐ion batteries. The obtained cell voltages semi‐quantitatively fit the experiments, highlighting the practicality of PBE+U as an approach for modeling the conversion‐reaction mechanism of the FeNCN archetype with lithium and sodium. Also, the calculated voltage profiles agree satisfactorily with experiment both for full (Li‐ion battery) and partial (Na‐ion battery) discharge, even though experimental atomistic knowledge is missing up to now. Moreover, we rationalize the structural preference of intermediate ternaries and their characteristic lowering in the voltage profile using chemical‐bonding and Mulliken‐charge analysis. The formation of such ternary intermediates for the lithiation of FeNCN and the contribution of at least one ternary intermediate is also confirmed experimentally. This theoretical approach, aided by experimental findings, supports the atomistic exploration of electrode materials governed by conversion reactions.     

A First‐Principles Study of Lithium Adsorption on a Graphene–Fullerene Nanohybrid System

The mechanism of Li adsorption on a graphene–fullerene (graphene–C₆₀) hybrid system has been investigated using density functional theory (DFT). The adsorption energy for Li atoms on the graphene–C₆₀ hybrid system (?2.285 eV) is found to be higher than that on bare graphene (?1.375 eV), indicating that the Li adsorption on the former system is more stable than on the latter. This is attributed to the high affinity of Li atoms to C₆₀ and the charge redistribution that occurs after graphene is mixed with C₆₀. The electronic properties of the graphene–C₆₀ system such as band structure, density of states, and charge distribution have been characterized as a function of the number of Li atoms adsorbed in comparison to those of the pure graphene and C₆₀. Li adsorption is found to preferentially occur on the C₆₀ side due to the high adsorption energy of Li on C₆₀, which imparts a metallic character to the C₆₀ in the graphene–C₆₀ hybrid system.     

Ab Initio Study of H-Bond Dynamics in Three-Component Crystals Comprising (DABCOH+)n Polycationic Chains

In this work, the dynamic character of hydrogen-bond (H-bond) networks in two three-component crystals comprising polycationic chains was described. The first studied system was 1,4-diazabicyclo[2.2.2]octan-1-ium (DABCOH⁺) sulfamate monohydrate, known for its large negative linear compressibility. The second analyzed material was the newly obtained polar salt co-crystal: 1,4-diazabicyclo[2.2.2]octan-1-ium sulfamate urea. X-ray diffraction measurements enabled us to study the H-bond systems in both crystals using the graph set analysis. Obtained structures served as the initial models for Born-Oppenheimer molecular dynamics computations. A detailed study of intermolecular interactions and power spectra was conducted. The analysis of time and space correlations between the changes in H-bonds enabled the detection of proton transfer occurring in both systems at 300 K. Further study of those dynamic phenomena was done using the Energy Decomposition Analysis for selected trajectory fragments. Our work should improve the understanding of dielectric and ferroelectric properties of hybrid organic-inorganic materials.     

Ab Initio Molecular Dynamics Study of Hydrogen Cleavage by a Lewis Base [tBu3P] and a Lewis Acid [B(C6F5)3] at the Mesoscopic Level—Dynamics in the Solute–Solvent Molecular Clusters

With the help of state‐of‐the‐art ab initio molecular dynamics methods, we investigated the reaction pathway of the {tBu₃P + H₂ + B(C₆F₅)₃} system at the mesoscopic level. It is shown that: i) the onset of H₂ activation is at much larger boron???phosphorus distances than previously thought; ii) the system evolves to the product in a roaming‐like fashion because of quasi‐periodic nuclear motion along the asymmetric normal mode of P???H?H???B fragment; iii) transient configurations of a certain type are present despite structural interference from the solvent; iv) transient‐state configurations with sub‐picosecond lifetime have potentially interesting infrared activity in the organic solvent (toluene) as well as in the gas phase. The presented results should be helpful for future experimental and theoretical studies of frustrated Lewis pair (FLP) activity.     

Cover Picture: QM/MM Car‐Parrinello Molecular Dynamics Study of the Solvent Effects on the Ground State and on the First Excited Singlet State of Acetone in Water (ChemPhysChem 11/2003)

The cover picture shows …?‥the solvent effect of water upon the lowest‐lying singlet excitation in acetone. The transition, which involves the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), is blue‐shifted in water (sol) with respect to the gas phase (vac), since the HOMO is stronger stabilized in water than the LUMO. The authors calculate the absorption and fluorescence spectra of acetone in water with a hybrid Car–Parrinello quantum chemical/classical molecular dynamics approach and investigate the influence of the solvent. The lower part of the picture shows the excitation energy during a simulation. One configuration with a very high excitation energy (three hydrogen bonds, short C?O bond) and one configuration with a very low excitation energy (two hydrogen bonds, long C?O bond) are shown in detail. Find out more in the article by Rothlisberger and co‐workers on pages pp. 1177–1182.     

密度泛函理论方法研究锂离子电池电解液体系分子-离子结构

采用密度泛函理论方法,研究锂离子电池碳酸丙烯酯（PC）基电解液体系中锂盐离子与溶剂分子静电相互作用形成的可能结构. 计算结果表明,电解液中溶剂分子-离子的结构取决于体系的溶剂分子数. 在PC基电解液,Li⁺最多只能与4个PC溶剂分子相结合,锂盐阴离子与带正电的PC分子烷基基团相结合,而不以自由离子形式存在. 本文的计算结果能很好地解释文献报道的实验结果.     

Combined Crossed Molecular Beams and Ab Initio Study of the Bimolecular Reaction of Ground State Atomic Silicon (Si; 3P) with Germane (GeH4; X1A1)

The chemical dynamics of the elementary reaction of ground state atomic silicon (Si; ³P) with germane (GeH₄; X¹A₁) were unraveled in the gas phase under single collision condition at a collision energy of 11.8±0.3 kJ mol⁻¹ exploiting the crossed molecular beams technique contemplated with electronic structure calculations. The reaction follows indirect scattering dynamics and is initiated through an initial barrierless insertion of the silicon atom into one of the four chemically equivalent germanium-hydrogen bonds forming a triplet collision complex (HSiGeH₃; ³ i1 ). This intermediate underwent facile intersystem crossing (ISC) to the singlet surface (HSiGeH₃; ¹ i1 ). The latter isomerized via at least three hydrogen atom migrations involving exotic, hydrogen bridged reaction intermediates eventually leading to the H₃SiGeH isomer i5 . This intermediate could undergo unimolecular decomposition yielding the dibridged butterfly-structured isomer ¹ p1 (Si(μ-H₂)Ge) plus molecular hydrogen through a tight exit transition state. Alternatively, up to two subsequent hydrogen shifts to i6 and i7 , followed by fragmentation of each of these intermediates, could also form ¹ p1 (Si(μ-H₂)Ge) along with molecular hydrogen. The overall non-adiabatic reaction dynamics provide evidence on the existence of exotic dinuclear hydrides of main group XIV elements, whose carbon analog structures do not exist.     

The Aqueous Ca2+ System,in Comparison with Zn2+, Fe3 +, and Al3 +: An Ab Initio Molecular Dynamics Study

Herein, we report on the structure and dynamics of the aqueous Ca²⁺ system studied by using ab initio molecular dynamics (AIMD) simulations. Our detailed study revealed the formation of well‐formed hydration shells with characteristics that were significantly different to those of bulk water. To facilitate a robust comparison with state‐of‐the‐art X‐ray absorption fine structure (XAFS) data, we employ a 1st principles MD‐XAFS procedure and directly compare simulated and experimental XAFS spectra. A comparison of the data for the aqueous Ca²⁺ system with those of the recently reported Zn²⁺, Fe³⁺, and Al³⁺ species showed that many of their structural characteristics correlated well with charge density on the cation. Some very important exceptions were found, which indicated a strong sensitivity of the solvent structure towards the cation′s valence electronic structure. Average dipole moments for the 2nd shell of all cations were suppressed relative to bulk water.     

Electronic Structures of LixV2O5 (x=0.5 and 1): A Theoretical Study

The electronic properties of α‐Li_xV₂O₅ (x=0.5 and 1) are investigated using first principle calculations based on density functional theory with local density approximation. Different intercalation sites for Li in the V₂O₅ lattices are considered, showing different influences on the electronic structures of Li_xV₂O₅. The lowest total energy is found when Li is only intercalated along the c axis between two bridging oxygen ions of sequential V₂O₅ layers. The intercalation of Li into V₂O₅ does not change the electron transition property of V₂O₅, which is an indirect band gap semiconductor, but leads to a reduction of vanadium ions and an increase of the Fermi level of Li_xV₂O₅ arising from the electron transfer from the Li 2 s orbital to the initially empty conduction band of the V₂O₅ host.     

The Nature of [PdCl2(C2H4)(H2O)] as an Active Species in the Wacker Process: New Insights from Ab Initio Molecular Dynamics Simulations

First‐principles molecular dynamics coupled with metadynamics have been used to gain a deeper insight into the reaction mechanism of the Wacker process by determining the nature of the active species. An explicit and dynamic representation of the aqueous solvent, which was essential for modeling this reaction, was efficiently included into the simulations. Prompted by our earlier results, which showed that the configuration of the catalytically active species [PdCl₂(H₂O)(C₂H₄)] was crucial in the subsequent steps of the Wacker process, herein we focused on the preceding equilibria that led to the formation of both the cis and trans isomers. Starting from the initial catalyst, [PdCl₄]^2?, the free‐energy barriers for the forward and backward reactions were calculated. These results confirmed the relevance of the trans intermediate in the reaction mechanism, whilst conversely, they showed that the cis configuration played no role in it. This sole participation of the trans intermediate has some very important implications; besides the mechanistic interpretation of the initial steps in the Wacker reaction mechanism, the analysis of these equilibria provides additional information about the chemical nature of these ligand‐substitution processes.