Stochastic Search of the Quantum Conformational Space of Small Lithium and Bimetallic Lithium−Sodium Clusters

Energetics and spectroscopic studies of CNO (-) (H 2 O) n $$ {\mathbf{CNO}}^{\left(\hbox{-} \right)}{\left({\mathbf{H}}_{\mathbf{2}}\mathbf{O}\right)}_{\mathbf{n}} $$ clusters and the temperature dependencies of the isomers: An approach based on a combined recipe of parallel tempering and quantum chemical methods

A system associated with several number of weak interactions supports numerous number of stable structures within a narrow range of energy. Often, a deterministic search method fails to locate the global minimum geometry as well as important local minimum isomers for such systems. Therefore, in this work, the stochastic search technique, namely parallel tempering, has been executed on the quantum chemical surface of theCNO (-) (H 2 O) n $$ {\mathrm{CNO}}^{\left(\hbox{-} \right)}{\left({\mathrm{H}}_2\mathrm{O}\right)}_n $$ system forn = 1 $$ n=1 $$ -8 to generate global minimum as well as several number of local minimum isomers. IR spectrum can act as the fingerprint property for such system to be identified. Thus, IR spectroscopic features have also been included in this work. Vertical detachment energy has also been calculated to obtain clear information about number of water molecules in several spheres around the central anion. In addition, in a real experimental scenario, not only the global but also the local minimum isomers play an important role in determining the average value of a particular physically observable property. Therefore, the relative conformational populations have been determined for all the evaluated structures for the temperature range between 20K and 400K. Further to understand the phase change behavior, the configurational heat capacities have also been calculated for different sizes.

Coordination of Mercury(II) in Water Promoted over Hydrolysis in Solvated Clusters [Hg(H2O)1-6](aq)2+: Insights from Relativistic Effects and Free Energy Analysis

Understanding the nature of the interaction between mercury(II) ions, Hg2+, and water molecules is crucial to describe the stability and chemical behavior of structures formed during solvation, as well as the conditions that favor the Hg2+ coordination or inducing water hydrolysis. In our study, we explored exhaustively the potential energy surface of Hg2+ with up to six water molecules. We analyzed electronic and Gibbs free energies, binding, and nuclear magnetic resonance parameters. We used the zeroth-order regular approximation Hamiltonian, including scalar and spin-orbit relativistic corrections for free energy calculations and geometry optimizations to explore the interplay between electron correlation and relativistic effects. We analyzed intermolecular interactions with energy decomposition analysis, quantum theory of atoms in molecules, and natural bond orbital. Additionally, we used the four-component Dirac Hamiltonian to compute solvent effect on the magnetic shielding and J-coupling constants. Our results revealed that the water hydrolysis by Hg2+ requires a minimum of three water molecules. We found that the interaction between Hg2+ and water molecules is an orbital interaction due to relativistic effects and the most stable structures are opened-shape clusters, reducing the number of oxygen-mercury contacts and maximizing the formation of hydrogen bonds among water molecules. In these types of clusters, Hg2+ promotes the water hydrolysis over coordination with oxygen atoms. However, when we considered the change associated with the transfer of a cluster from the ideal gas to a solvated system, our solvation free energy analysis revealed that closed-shape clusters are more favorable, maximizing the number of oxygen-mercury contacts and reducing the formation of hydrogen bonds among water molecules. This finding suggests that, under room conditions, the coordination of Hg2+ is more favorable than hydrolysis. Our results have significant implications for understanding Hg2+ behavior in water, helping to develop targeted strategies for mercury remediation and management, and contributing to advancements in the broader field of environmental chemistry.

Electronic Structure of Heteronuclear Cerium-Platinum Clusters

Beyond the now well-known strong catalyst-support interactions reported for ceria-supported platinum catalysts, intermetallic Ce-Pt compounds exhibit fascinating properties such as heavy fermion behavior and magnetic instability. Small heterometallic Ce-Pt clusters, which can provide insights into the local features that govern bulk phenomena, have been less explored. Herein, the anion photoelectron spectra of three small mixed Ce-Pt clusters, Ce2OPt-, Ce2Pt-, and Ce3Pt-, are presented and interpreted with supporting density functional theory calculations. The calculations, which are readily reconciled with the experimental spectra, suggest the presence of numerous close-lying spin states, including states in which the Ce 4f electrons are ferromagnetically coupled or antiferromagnetically coupled. The Pt center is consistently in a nominal -2 charge state in all cluster neutrals and anions, giving the Ce-Pt bond ionic character. Ce-Pt bonds are stronger than Ce-Ce bonds, and the O atom in Ce2OPt- coordinates only with the Ce centers. The energy of the singly occupied Ce-local 4f orbitals relative to the Pt-local orbitals changes with cluster composition. Discussion of the results includes potential implications for Ce-rich intermetallic materials.

Dissecting Bonding Interactions in Cysteine Dimers

Neutral (n) and zwitterionic (z) forms of cysteine monomers are combined in this work to extensively explore the potential energy surfaces for the formation of cysteine dimers in aqueous environments represented by a continuum. A simulated annealing search followed by optimization and characterization of the candidate structures afforded a total of 746 structurally different dimers held together via 80 different types of intermolecular contacts in 2894 individual non-covalent interactions as concluded from Natural Bond Orbitals (NBO), Quantum Theory of Atoms in Molecules (QTAIM) and Non-Covalent Interactions (NCI) analyses. This large pool of interaction possibilities includes the traditional primary hydrogen bonds and salt bridges which actually dictate the structures of the dimers, as well as the less common secondary hydrogen bonds, exotic X⋯Y (X = C, N, O, S) contacts, and H⋯H dihydrogen bonds. These interactions are not homogeneous but have rather complex distributions of strengths, interfragment distances and overall stabilities. Judging by their Gibbs bonding energies, most of the structures located here are suitable for experimental detection at room conditions.

Multiple Facets of Modeling Electronic Absorption Spectra of Systems in Solution

In this Perspective, we outline the essential physicochemical aspects that need to be considered when building a reliable approach to describe absorption properties of solvated systems. In particular, we focus on how to properly model the complexity of the solvation phenomenon, arising from dynamical aspects and specific, strong solute-solvent interactions. To this end, conformational and configurational sampling techniques, such as Molecular Dynamics, have to be coupled to accurate fully atomistic Quantum Mechanical/Molecular Mechanics (QM/MM) methodologies. By exploiting different illustrative applications, we show that an effective reproduction of experimental spectral signals can be achieved by delicately balancing exhaustive sampling, hydrogen bonding, mutual polarization, and nonelectrostatic effects.

Microsolvation of electrons by a handful of ammonia molecules

Microsolvation of electrons in ammonia is studied here via anionic NH_{3 n} ^- clusters with n = 2-6. Intensive samplings of the corresponding configurational spaces using second-order perturbation theory with extended basis sets uncover rich and complex energy landscapes, heavily populated by many local minima in tight energy windows as calculated from highly correlated coupled cluster methods. There is a marked energetical preference for structures that place the excess electron external to the molecular frame, effectively coordinating it with the three protons from a single ammonia molecule. Overall, as the clusters grow in size, the lowest energy dimer serves as the basic motif over which additional ammonia molecules are attached via unusually strong charge-assisted hydrogen bonds. This is a priori quite unexpected because, on electrostatic grounds, the excess electron would be expected to be in contact with as many protons as possible. Accordingly, a full quantum mechanical treatment of the bonding interactions under the tools provided by the quantum theory of atoms in molecules is carried out in order to dissect and understand the nature of intermolecular contacts. Vertical detachment energies reveal bound electrons even for n = 2.

Effect of nanostructuring on the interaction of CO2 with molybdenum carbide nanoparticles

Transition metal carbides are increasingly used as catalysts for the transformation of CO₂ into useful chemicals. Recently, the effect of nanostructuring of such carbides has started to gain relevance in tailoring their catalytic capabilities. Catalytic materials based on molybdenum carbide nanoparticles (MoC_y) have shown a remarkable ability to bind CO₂ at room temperature and to hydrogenate it into oxygenates or light alkanes. However, the involved chemistry is largely unknown. In the present work, a systematic computational study is presented aiming to elucidate the chemistry behind the bonding of CO₂ with a representative set of MoC_y nanoparticles of increasing size, including stoichiometric and non-stoichiometric cases. The obtained results provide clear trends to tune the catalytic activity of these systems and to move towards more efficient CO₂ transformation processes.

A Comprehensive Picture of the Structures, Energies, and Bonding in the Alanine Dimers

High level quantum mechanical computations and extensive stochastic searches of the potential energy surfaces of the Alanine dimers uncover rich and complex structural and interaction landscapes. A total of 416 strongly bound (up 13.4 kcal mol^-1 binding energies at the DLPNO-CCSD(T)/6-311++G(d,p) level corrected by the basis set superposition error and by the zero point vibrational energies over B3LYP-D3 geometries), close energy equilibrium structures were located, bonded via 32 specific types of intermolecular contacts including Y⋅⋅⋅H-X primary and Y⋅⋅⋅H-C secondary hydrogen bonds, H⋅⋅⋅H dihydrogen contacts, and non conventional anti-electrostatic Y δ - ⋯ X δ - interactions. The putative global minimum is triply degenerate, corresponding to the structure of the common dimer of a carboxylic acid. All quantum descriptors of chemical bonding point to a multitude of weak individual interactions within each dimer, whose cumulative effect results in large binding energies and in an attractive fluxional wall of non-covalent interactions in the interstitial region between the monomers.

Structure, energy, and bonding in anionic water tetramers obtained by exhaustive search

An analysis of the structures, some energy related properties, and key aspects of the bonding nature of the microsolvated electron with four water molecules is presented. The study is based on an exhaustive potential energy surface scan of the ground state of (H₂O)₄ ^- at the UCCSD(T)/6-311(3+,4+)G(d,p)//UMP2/6-311(3+,4+)G(d,p) level. A total of 18 structures, most of them not reported before, spanning in an energy range of 8.8 kcal mol^-1 were found. The energetic stability of the clusters is dictated by the effect of the excess electron on their structures, on their partial fragmentation, and on the hydrogen bonds' framework. Vertical detachment energies depend on the number of water molecules holding the excess electron in "direct contact" to their two protons at the same time and, to a lesser extent, also depend on the hydrogen bond sequence in the rest of the structure. In general, hydrogen bonds in (H₂O)₄ ^- are of closed shell character, and there are other less common interactions assisted by the excess electron.

An efficient and robust procedure to calculate absorption spectra of aqueous charged species applied to NO2. Phys Chem Chem Phys 2021;23:14857-14872. [PMID: 34223573 DOI: 10.1039/d1cp00652e]

Accurate calculation of absorption spectra of aqueous NO2- requires rigorously sampling the quantum potential energy surfaces for microsolvation of NO2- with at least five explicit water molecules and embedding the resulting clusters in a continuum solvent accounting for the statistical weighted contributions of individual isomers. This method, which we address as ASCEC + PCM, introduces several desired features when compared against MD simulations derived QM/MM spectra: comparatively fewer explicit solvent molecules to be treated with expensive QM methods, the identification of equilibrium structures in the quantum PES to be used in further vibrational spectroscopy, and the unequivocal identification of cluster orbitals undergoing electronic transitions and charge transfer that originate the spectral bands.

A molecular twist on hydrophobicity

A thorough exploration of the molecular basis for hydrophobicity with a comprehensive set of theoretical tools and an extensive set of organic solvent S/water binary systems is discussed in this work. Without a single exception, regardless of the nature or structure of S, all quantum descriptors of bonding yield stabilizing S⋯water interactions, therefore, there is no evidence of repulsion and thus no reason for etymological hydrophobicity at the molecular level. Our results provide molecular insight behind the exclusion of S molecules by water, customarily invoked to explain phase separation and the formation of interfaces, in favor of a complex interplay between entropic, enthalpic, and dynamic factors. S⋯water interfaces are not just thin films separating the two phases; instead, they are non-isotropic regions with density gradients for each component whose macroscopic stability is provided by a large number of very weak dihydrogen contacts. We offer a definition of interface as the region in which the density of the components in the A/B binary system is not constant. At a fundamental level, our results contribute to better current understanding of hydrophobicity.

Notwithstanding the very weak nature of individual contacts, it is the cumulative effect of a large number of interactions (green NCI surfaces) which provides macroscopic stability to the interfaces.

Application of a quantum genetic algorithm and QTAIM analysis in the study of structural and electronic properties of neutral bimetallic clusters NaxLiy (4 ≤ x + y ≤ 10)

Alloy clusters of Na_xLi_y (4 ≤ x + y ≤ 10) are studied by exploring the potential energy surface in the ab initio MP2 level with the support of a quantum genetic algorithm (QGA). In some cases, the structures have been also refined with DFT and coupled-cluster methods. The general trends of sodium-lithium structures are in line with previous studies. The ionization potentials and polarizabilities to all structures were calculated with MP2 method and the average error between these two properties compared with experimental data was 6% and 13%, respectively. The topological analysis based on quantum theory of atoms in molecules (QTAIM) showed that by increasing the cluster size of the diatomic system there was a decrease of atomic interaction energies. The degree of degeneracy from D3BIA aromaticity index and the analysis of the atomic charges showed the influence (by charge transfer) of the chemical element in lower quantity in the cluster with respect to the other atoms. Our achievements of comparing our theoretical results with available experimental data have demonstrated that our approach can also predict satisfactorily quantum atomic and alloy clusters properties, at least, for low nuclearities.

Analyzing ZnO clusters through the density-functional theory

The potential energy surface of Zn _n O _n clusters (n = 2, 4, 6, 8) has been explored by using a simulated annealing method. For n = 2, 4, and 6, the CCSD(T)/TZP method was used as the reference, and from here it is shown that the M06-2X/TZP method gives the lowest deviations over PBE, PBE0, B3LYP, M06, and MP2 methods. Thus, with the M06-2X method we predict isomers of Zn _n O _n clusters, which coincide with some isomers reported previously. By using the atoms in molecules analysis, possible contacts between Zn and O atoms were found for all structures studied in this article. The bond paths involved in several clusters suggest that Zn _n O _n clusters can be obtained from the zincite (ZnO crystal), such an observation was confirmed for clusters with n = 2 - 9,18 and 20. The structure with n = 23 was obtained by the procedure presented here, from crystal information, which could be important to confirm experimental data delivered for n = 18 and 23.

Microsolvation of F−

The effective charge in F⁻ is strong enough to induce water dissociation even for small molecularities in the gas phase.

Initial stage of the degradation of three common neonicotinoids: theoretical prediction of charge transfer sites

Tautomerization of acetamiprid gives alternatives to search new pathways for its degradation in water.

Microsolvation of NO3−: structural exploration and bonding analysis

A rich and complex structural diversity is uncovered in the microsolvation of the nitrate anion.

Hydrogen bonds in methane–water clusters

Water–methane clusters are stable at low temperatures as those found in Mars. Water cages enveloping methane are stable, although they present small probability to occur.

Microsolvation of methylmercury: structures, energies, bonding and NMR constants (199Hg,13C and17O)

Hartree–Fock (HF) and second order perturbation theory (MP2) calculations within the scalar and full relativistic frames were carried out in order to determine the equilibrium geometries and interaction energies between cationic methylmercury (CH₃Hg⁺) and up to three water molecules.

Theoretical tools to distinguish O-ylides from O-ylidic complexes in carbene–solvent interactions

Orbital interactions and bond indices are among the theoretical tools suitable to distinguishO-ylides fromO-ylidic carbene–solvent complexes.

Potential energy surfaces of WC6 clusters in different spin states

Stochastic explorations of the structural possibilities of neutral WC6 clusters in several spin states lead to very rich and complex potential energy surfaces, with geometries quite different from those of pure carbon clusters at the PBE0/def2-TZVP level. The global minimum is predicted to be a triplet-state semicyclic C6 conformation having every carbon in direct coordination to the W atom. Interaction energies are comparable to those of C7 clusters, revealing very strong W-C bonding. Our results suggest that C-C interactions in the clusters should be considered as intermediate between single and double bonds.

Heavy periodane

The potential energy surface of the hypothetical NaMgAlSiPSCl system (heavy periodane) is exhaustively analyzed via the gradient embedded genetic algorithm (GEGA) in combination with density functional theory (DFT) computations. The electronegativity differences among the elements in both the second and third rows of the periodic table indicate that low-energy heavy periodane structures are obtained when highly electronegative and electropositive elements are bound together, but the global minimum of the heavy periodane system is completely different to its second-row analog (LiBeBCNOF).