Bond Critical Points Research Articles

Machine learning-based prediction of bioactivity in HIV-1 protease: insights from electron density analysis.

Aim: To develop a model for predicting the biological activity of compounds targeting the HIV-1 protease and to establish factors influencing enzyme inhibition.Materials & methods: Machine learning models were built based on a combination of Richard Bader's theory of Atoms in Molecules and topological analysis of electron density using experimental x-ray 'protein-ligand' complexes and inhibition constants data.Results & conclusion: Among all the models tested, logistic regression achieved the highest accuracy of 0.76 on the test set. The model's ability to differentiate between less active and highly active classes was relatively good, as indicated by an AUC-ROC score of 0.77. The analysis identified several critical factors affecting the biological activity of HIV-1 protease inhibitors, including the electron density contribution of hydrogen atoms, bond-critical points and particular amino acid residues. These findings provide new insights into how these molecular factors influence HIV-1 protease inhibition, emphasizing the importance of hydrogen bonding, glycine's flexibility and hydrophobic interactions in ligand binding.

Shannon entropy variation as a global indicator of electron density contraction at interatomic regions in chemical reactions.

The negative of the Shannon entropy derivative is proposed to account for electron density contraction as the chemical bonds are breaking and forming during a chemical reaction. We called this property the electron density contraction index, EDC, which allows identifying stages in a reaction that are dominated by electron contraction or expansion. Four different reactions were analyzed to show how the EDC index changes along the reaction coordinate. The results indicate that the rate of change of Shannon entropy is directly related to the rate of change of the electron density at the bond critical points between all the atomic pairs in the molecular systems. It is expected that EDC will complement the detailed analysis of reaction mechanisms that can be performed with the theoretical tools available to date. Density functional theory calculations at the B3LYP/6-31G(d,p) level of theory were carried out using Gaussian 16 to analyze the reaction mechanisms of the four reactions studied. The reaction paths were obtained via the intrinsic reaction coordinate method, which served as the reaction coordinate to obtain the reaction force and the EDC profiles in each case. Shannon entropy and electron density at the bond critical points were calculated using the Multiwfn 3.7 package.

Synthetic, Electrochemical, DFT, and Synchrotron X-ray Charge-Density Studies on Oxo-centered Triruthenium Clusters Supported by Electron-Withdrawing Carboxylates.

We herein report the synthesis, characterizations, and synchrotron X-ray charge-density studies of oxo-centered triruthenium(II,III,III) clusters [Ru3O(CHCl2COO)6(py)3] (1) and [Ru3O(CHCl2COO)6(CO)(py)2] (2) (py = pyridine). Dichloroacetate was chosen for its large scattering factor of the Cl atom, and its electron-withdrawing nature results in significant stabilization of the targeted lower-valent Ru3II,III,III state in the cluster framework. Multipole analysis revealed that the difference in electron populations between two crystallographically independent Ru centers is small for 1 (Δ = 0.30 e) but large for 2 (Δ = 1.46 e). Remarkable differences between 1 and 2 are also found in their static deformation density maps; substantial local charge depletion was found around the central μ3O atom for 1, which is less pronounced for 2. According to the topological characterization of Ru-μ3O bonds associated with the bond critical point, bcp, the electron density, ρbcp, is in the range of 0.79-0.89 e Å-3, and the total energy density, Hbcp, is in the range of -0.21 to -0.05 hartree Å-3. These findings represent the first charge-density distribution analysis of mixed-valence multinuclear Ru complexes including comparison between 3d and 4d transition-metal systems.

Quantitative Characterization of Fluorine-Centered Noncovalent Interactions in Crystalline Benzanilides.

Six isomeric molecules, featuring a minimum of three fluorine atoms on either the benzoyl or aniline side, have been synthesized, crystallized and characterized through single crystal X-ray diffraction (SCXRD). In addition, two other compounds, containing six fluorine atoms, three on each of the benzoyl and aniline side of the benzanilide scaffold have also been characterized through SCXRD. This current study aims to augment the capacity for hydrogen bond formation, specifically involving organic fluorine, by elevating the acidity of the involved hydrogens through the incorporation of highly electronegative fluorine atoms, in the presence of strong N-H⋅⋅⋅O=C H-bonds. Lattice energy calculations and assessment of intermolecular interaction energies elucidate the contributions of electrostatics and dispersion forces in crystal packing. The topological analysis of the electron density is characterized by the presence of bond critical points (BCPs) involving C-H⋅⋅⋅F and F⋅⋅⋅F contacts, thus establishing the bonding nature of these interactions which play a crucial role in the crystal packing in addition to the presence of traditional N-H⋅⋅⋅O=C H-bonds.

Cuprophilic Interactions in Polymeric [Cu10O2(Mes)6]n.

The properties of cuprophilic compounds and the underlying fundamental principles responsible for the Cu(I)···Cu(I) interactions have been the subject of intense research as their diverse structural and physical attributes are being explored. In this light, we performed a new study of the compound [Cu10O2(Mes)6] reported by Haakansson et al. using state of the art experimental and theoretical analysis techniques. Doing this, we found the compound to be a polymer in the solid state, best written as [Cu10O2(Mes)6]n, with unsupported Cu(I)···Cu(I) contacts linking the monomers (2.776 Å). The monomeric unit also exhibits various cuprophilic contacts bridged by mesityl and/or oxo ligands. The compound was analyzed in its solid state, revealing luminescent properties resulting from two distinct fluorescent emissions, as well as in solution, in which its polymeric structure reversibly decomposes. A quantum theory of atoms in molecules (QTAIM) analysis based on density functional theory (DFT) calculations allows to characterize the various Cu(I)···Cu(I) contacts, in which only a few, and not necessarily the shortest, are associated with a bond critical point. Additionally, an energy decomposition analysis of the bonding between monomers indicates that it is dominated by dispersion forces in which the ligands play a dominant role, resulting in bonding energies significantly larger than found in previous DFT investigations based on less bulky models.

All-body concept and quantified limits of cooperativity and related effects in homodromic cyclic water clusters from a molecular-wide and electron density-based approach.

We strongly advocate distinguishing cooperativity from cooperativity-induced effects. From the MOWeD-based approach, the origin of all-body cooperativity is synonymous with physics- and quantum-based processes of electron (e) delocalization throughout water clusters. To this effect, over 10 atom-pairs contribute to the total e-density at a BCP(H,O) between water molecules in a tetramer. Intermolecular all-body e-delocalization, that is, cooperativity, is an energy-minimizing process that fully explains non-additive increase in stability of a water molecule in clusters with an increase in their size. A non-linear change in cooperativity and cooperativity-induced effects, such as (i) structural (e.g., a change in d(O,O)) or topological intra- and intermolecular properties in water clusters (e.g., electron density or potential energy density at bond critical points) is theoretically reproduced by the proposed expression. It predicted the limiting value of delocalized electrons by a H2O molecule in homodromic cyclic clusters to be 1.58e. O-atoms provide the vast majority of electrons that "travel throughout a cluster predominantly on a privileged exchange quantum density highway" (⋅⋅⋅O-H⋅⋅⋅O-H⋅⋅⋅O-H⋅⋅⋅) using Bader's classical bond paths as density bridges linking water molecules. There are, however, additional electron exchange channels that are not seen on molecular graphs as bond paths. A 3D visual representation of the "privileged" and "additional" exchange channels as well as detailed intra- and inter-molecular patterns of e-sharing and (de)localizing is presented. The energy stabilizing contribution made by three O-atoms of neighboring water molecules was found to be large (-597 kcal/mol in cyclic hexamer) and 5 times more significant than that of a classical O-H⋅⋅⋅O intermolecular H-bond.

A refreshing approach to understanding the action on DNA of vanadium (IV) and (V) complexes derived from the anticancer VCp2Cl2

A computational study based on derivatives of the anticancer VCp2Cl2 compound and their interaction with representative models of deoxyribonucleic acid (DNA) is presented. The derivatives were obtained by substituting the cyclopentadienes of VCp2Cl2 with H2O, NH3, OH−, Cl−, O2− and C2O42− ligands. The oxidation states IV and V of vanadium were considered, so a total of 20 derivative complexes are included. The complexes interactions with DNA were studied using two different models, the first model considers the interactions of the complexes with the pair Guanine-Cytosine (G-C) and the second involves the interaction of the complexes with adjacent pairs, that is, d(GG). This study compares methodologies based on density functional theory with coupled cluster like calculations (DLPNO-CCSD(T)), the gold standard of electronic structure methods. Furthermore, the change in the electron density of the hydrogen bonds that keep bonded the G-C pair and d(GG) pairs, due to the presence of vanadium (IV) and (V) complexes is rationalize. To this aim, quantities obtained from the topology of the electron densities are inspected, particularly the value of the electron density at the hydrogen bond critical points. The approach allowed to identify vanadium complexes that lead to significant changes in the hydrogen bonds indicated above, a key aspect in the understanding, development, and proposal of mechanisms of action between metal complexes and DNA.

Quantum Information Patterns Between Atoms in a Molecule.

Quantum information theory provides a powerful toolbox of descriptors that characterize many-electron systems based on quantum information patterns between open quantum systems. Despite the wealth of insights gained in the condensed matter community, the use of these descriptors to study interactions between atoms in a molecule remains limited. In this study, we develop a quantum information framework for molecules that characterizes the quantum information patterns between quantum atoms as defined in the Quantum Theory of Atoms in Molecules. We show that quantum information analyses capture key properties of quantum atoms and how they interact with their molecular environment. Additionally, we show that the presence of bond critical points can remain invariant despite large changes in the quantum information patterns between the quantum atoms. Our findings indicate that quantum information theory can shed a new light on molecular electronic structure.

An Energetic and Topological Approach to Understanding the Interplay of Noncovalent Interactions in a Series of Crystalline Spiropyrrolizine Compounds.

Synthesis of quinoline-containing spiropyrrolizine was achieved via a 1,3-dipolar cycloaddition reaction of azomethine ylide (generated in situ from ninhydrin and l-proline) and (E)-2-styrylquinoline. The synthesized compounds were characterized by 1H NMR, 13C NMR, HRMS, and single-crystal XRD analysis. The XRD data revealed that the solid-state structures of the compounds belong to the monoclinic system of the space group P21/c and are stabilized through various weak noncovalent interactions such as C-H···O, C-H···π, and π···π interactions. The noncovalent interactions are characterized and quantified through Hirshfeld surface analysis. Moreover, the interaction energies of the intermolecular noncovalent interactions are calculated through PIXEL calculation. The PIXEL calculation provides precise interaction energy with an energy decomposition scheme. Energy Framework calculations have also been performed to delve deeper into understanding the intermolecular interactions. The intermolecular interactions are further characterized using Bader's theory of "atoms in molecules" (QTAIM) and the "noncovalent" (NCI) interaction plot index. The nature and strength of noncovalent interactions are analyzed from the topological parameters at (3, -1) bond critical points (BCPs).

β-d-Glucopyranose – Silver+ (1:1) complex assisted aromaticity involving electron deficient X3 (X=Be, Mg) core

Seeing the frontier molecular orbitals (FMO) picture of β-d-glucopyranose–silver+ (1:1) complex ([Ag(C6H12O6)]+) reported in a recent article (Mondal, 2024) [53], we used the FMO based approach to obtain inverted sandwich like organometallic complex comprising X3 (X=Be, Mg) core, in silico. Thermodynamic feasibility of obtaining [(C6H12O6)Ag-X3-Ag(C6H12O6)] (X=Be, Mg) complexes are explored. Conceptual density functional theory (CDFT) based reactivity descriptors guide to ascertain the stability order of such complexes. By the help of natural bond orbital analysis, Wiberg bond indices (WBI), bond critical point analysis, electron density descriptors, and gradient isosurface between selected contacts, the nature of interaction between the X3 core and Ag metals are studied. FMO of the complexes further help in ascertaining the nature of binding. Energy decomposition analysis marks the interaction between X3 and Ag metals in [(C6H12O6)Ag-X3-Ag(C6H12O6)] (X=Be, Mg) as partially covalent. Negative values of nucleus independent chemical shift (NICS) at the triangular ring (X3) centres [NICS(0)] indicate aromatic property.

The Halogen Bond to Ethers - Prototypic Molecules and Experimental Electron Density.

Halogen bonds to dialkyl ether molecules have remained largely unexplored. We here address the synthesis and the structural chemistry of the first halogen-bonded noncyclic alkyl ethers, combining 1,4-diiodotetrafluorobenzene and the prototypic or commonly used ethers dimethyl ether, tetrahydrofuran, and methyl-tert-butyl ether as halogen acceptors. Two different structural motifs based on moderately strong halogen bonds were obtained: Discrete trimolecular aggregates are formed, and unexpected halogen-bonded supramolecular chain adducts feature oxygen-bifurcated halogen bonds with 1:1 donor:acceptor ratio. Both structure types may be selectively obtained even for the same ether by adjusting the stoichiometry in the crystallization experiments. The geometric features of the etheric oxygen center were found to be flexible, in contrast to the almost linear geometry about the halogen donor atom. A high-resolution X-ray diffraction experiment on the extended adduct of dimethyl ether allowed us to study the electronic details of the acceptor-bifurcated I···O···I halogen bonds. The electron density in the bond critical points and derived properties such as the Laplacian indicate essentially electrostatic interactions and explain the geometrical flexibility of ethers in halogen bonds. Our studies demonstrate the great versatility of ethers as halogen bond acceptors, that can occur in many geometrical arrangements and whose contribution to nature's structural designs should not be underestimated.

Metronidazole-Picric acid salt formation: Synthesis, characterization, Hirshfeld surface analysis, single crystal X-ray diffraction, computational studies, and molecular docking studies against Clostridioides difficile

The salt formation of metronidazole (MET) and picric acid (PA) is obtained via slow evaporation of their mixture in methanol which afforded a novel organic salt, C6H10N3O3+⋅C6H2N3O7- (METPA). The structure of METPA salt was analyzed using single crystal X-ray diffraction. METPA crystallizes in the triclinic space group P-1 with cell dimensions: a = 8.2446(1), b = 9.1843(1), c = 11.5832(2) Å, α = 106.190(1)°, β = 99.483(1)°, γ = 106.635(1)°, Volume (Å3) = 778.02(2) and Z = 2. The formation of METPA salt was analyzed by comparing the IR spectrum of MET, PA and METPA salt. In order to visualize the intermolecular interactions in the crystal of METPA salt, Hirshfeld surface analysis was employed. QTAIM calculations were performed using the AIMALL program package to estimate the strength of hydrogen bonding interactions in terms of bond critical points (BCP) and ring critical points (RCP). Molecular docking of the MET and METPA was conducted against three toxins of Clostridioides difficile such as C. difficile toxin A (TcdA), C. difficile toxin B (TcdB), C. difficile transferase (CDT). The results showed that the docking score calculated based on the knowledge-based iterative scoring function ITScore-PP indicates that METPA with higher negative value of docking score in case of all three toxins when compared to MET. This shows that the binding model of METPA salt is more favorable than MET.

Charge Density Study of Two-Electron Four-Center Bonding in a Dimer of Tetracyanoethylene Radical Anions as a Benchmark for Two-Electron Multicenter Bonding.

The dimer of the tetracyanoethylene (TCNE) radical anions represents the simplest and the best studied case of two-electron multicenter covalent bonding (2e/mc or pancake bonding). The model compound, N-methylpyridinium salt of TCNE•-, is diamagnetic, meaning that the electrons in two contiguous radicals are paired and occupy a HOMO orbital which spans two TCNE•- radicals. Charge density in this system is studied as a benchmark for comparison of charge densities in other pancake-bonded radical systems. Two electrons from two contiguous radicals indeed form a bonding electron pair, which is distributed between two central ethylene groups in the dimer, i.e., between four carbon atoms. The topology of electron density reveals two bond critical points between the central ethylene groups in the dimer, with maximum electron density of 0.185 e Å-3; the corresponding theoretical value is 0.118 e Å-3.

Adsorption and dissociation of NO on gold–palladium alloy clusters: A density functional theory study

Alloy nanoclusters often exhibit distinctive and exceptional performance, making them especially appealing as catalysts. In this study, the adsorption and dissociation of NO on AumPdn (m + n = 8) clusters was studied using density functional theory (DFT). The most stable structures of AumPdn (m + n = 8) clusters were reported by the CALYPSO search screening program and DFT geometric optimization. The binding energy and bond critical point (BCP) analysis show that alloying could lead to a more compact structure and relatively stronger stability. Molecular electrostatic potential (MESP) analysis revealed that the optimal site for the adsorption of the NO molecule on AumPdn (m + n = 8) clusters was at the top site, with Pd atoms being the preferred choice. Projected density of states (PDOS) calculations demonstrate that the d-band center of the Au5Pd3 cluster was closest to the Fermi level, enhancing the interaction between the cluster surface and adsorbed molecules. The activation energy of NO dissociation on the Au5Pd3 surface is the lowest at 0.48 eV. These findings indicate that the Au5Pd3 nano-alloy cluster shows promise as an effective catalyst in the direct transformation of NO into non-toxic substances.

Prebiotic Chemistry and Sepiolite: A Density Functional Theory Approach

In this study, the molecular interactions of sepiolite, a biocompatibility clay mineral with known as biomaterial, and purine and pyrimidine molecules forming the bases of DNA and RNA molecules were modeled by Density Functional Theory. In addition to the geometry optimization, interaction energy, bond critical point, and electrostatic potential calculations revealed that essential molecules for our source of life interact with the basal surface of the clay. For example, the best interaction energies between bases / sepiolite were found to be -127.47, -121.35 kJ / mol for guanine and cytosine, respectively. Looking at the modeling results, one of the most important factors affecting the interaction energies is H-bond. In order to reveal this, bond critical points analyze were performed and it was computed that a large amount of intermolecular interaction energies came from H-bonds. For example, it has been calculated that close to 70% of the total energy in the guanine/TOT model is from H-bonds. Besides, this value of the cytosine/TOT model was found to be around 72%. The most effective indexes in these two models are 145 and 135, and the H-bond energies were recorded as -22.41 and 31.41 kJ/mol, respectively. Considering all the analyzes run, it can be concluded that the basal surfaces of the sepiolite are a suitable host for the nitrogenous bases, which are the main sources of life.

QTAIM analysis of the bonding in anionic group 6 carbonyl selenide clusters: [Se2M3(CO)10]2- (M=Cr, Mo, W).

This research aims to offer a deeper understanding of the bonding interactions between M-Se and M-CO and how these interactions change across the group 6 transition metal series: [Se2M3(CO)10]2- (M = Cr, Mo, W). It also seeks to explore the impact of carbonyl groups on M-M interactions within the clusters. Seven criteria, which are based on QTAIM properties, have been considered and compared with the corresponding criteria in other transition metal clusters. The results confirm that no such bond critical points or bond baths occur between transition metals, which instead have 5c-7e bonding interactions delocalized over their five-membered M3(μ-Se)2 ring, as evidenced by the non-negligible nonbonding delocalization indices. The topological properties of three bond clusters, Cr-Se, Mo-Se, and W-Se, resemble those of "intermediate closed shell characters," which combine covalent and electrostatic properties. Source function calculations indicated that the bonded Se atom contributed the most to each Cr-Se and Mo-Se bcp. The OCO atoms and nonbonded Se atoms also contributed to some extent. However, metal atoms act as sinks rather than as sources of electron density. In contrast, the majority of the metal atoms, both bonded and nonbonded, contribute to Cr-W bcps. Analysis of the delocalization indices δ(M…O) in the three clusters indicates that CO significantly contributes to Cr π-back donation in cluster 1. In contrast, no π-back donation occurs from CO to Mo or W in clusters 2 or 3, respectively. The B3P86 hybrid functional was used for computations in the Gaussian 09 software. The LanL2DZ basis set was employed for Cr, Mo, and W, while the 6-31G (d, p) basis set was used for C, O, and Se atoms. We performed QTAIM analysis using the AIM2000 and Multiwfn packages, incorporating B3P86/WTBS for Cr, Mo, and W atoms. The 6-311++G(3df,3pd) basis set was used for C, O, and Se atoms. Additionally, we utilized the ELF and SF.

Theoretical studies for the detailed electronic structure of organotin(IV) derivatives of 4-fluoroanthranilic acid, X-ray structure of n-dibutyltin bis(4-fluoroanthranilate)

The present study describes the detailed DFT-based electronic structure calculations without any symmetry constraints being performed on di- and triorganotin derivatives of 4-fluoroanthranilic acid (AFA), viz. Me2SnL2 (1), n-Bu2SnL2 (2), Me3SnL (3), and Ph3SnL (4) (where L= monoanion of AFA). The structural and atomic charge analysis have confirmed the previously reported our experimental results, i.e. bicapped tetrahedral and distorted tetrahedral geometry for di- and triorganotin derivatives, respectively, and all the complexes 1–4 contain a positively charged central tin atom surrounded by a negatively charged system. The single crystal X-ray structure of complex 2 (n-Bu2SnL2) also suggests that AFA acts as a monoanion and bidentate ligand resulting a bicapped tetrahedral environment around tin. Various population analysis such as Mulliken (MPA), Hirshfeld (HPA), and natural population analysis (NPA) have been employed to calculate the charges at all the atoms. A finite difference approximation method has been used to explain the charge distribution within the studied complexes 1–4 based on the conceptual global and local reactivity DFT-based descriptions, frontier molecular-orbital analysis (FMOA), and molecular electrostatic potential maps (MEP). Further, the conceptual DFT-based global reactivity descriptors and frontier molecular orbital analysis show that the interactions between CT-DNA and complexes 1–4 may be groove binding or electrostatic. The integral equation formalism-polarizable continuum model (IEF-PCM) has been employed to calculate the UV–visible spectra of 1–4 in the solvent field, whereas the same in the gas phase has been obtained with the help of time-dependent DFT (TD-DFT) at the same level of theory. The intramolecular charge distribution in 1–4 has been investigated with the help of natural bond orbital (NBO) analysis. The topological and energetic parameters at the selected bond critical points in the coordination sphere of complexes 1–4 have been calculated using atoms-in-molecules (AIM) theory. The analysis of different kinds of non-covalent interactions (NCI) in complexes 1–4 has also been investigated.

Unraveling the role of non-planarity of free base porphyrins in intermolecular hydrogen bonding interactions with phenols

Three different types of meso‑substituted freebase porphyrins (H2TTP, H2TMP and H2DDMP) with varied number and nature of electron donating groups (-CH3 and –OCH3) were synthesized. The molecular interactions of the porphyrins with different phenol derivative were explored using single crystal X-ray, NMR and their affinity by varied spectroscopic techniques. All three freebase porphyrins undergo strong intermolecular interactions with 2,4,6-trinitrophenol (TNP) and crystallized in a triclinic crystal system with iso-structural behaviour. The crystal lattices are stabilized through intermolecular hydrogen bonding interactions (N–H∙∙∙O and C–H∙∙∙O) and electrostatic interactions. Increases in the non-planarity of freebase porphyrins enhance the reactivity which results in the formation of picrate anions with 1:2 adduct formation by proton transfer process. The role of weak intermolecular interactions was analysed and quantified through Hirshfeld surface and finger print analysis. DFT studies revealed that the electron-donating effect of the methoxy group plays a major role in decreasing HOMO-LUMO gap and in-plane co-operation through the decrease in the dihedral angles. The electron density at bond critical points follows the order H2TTP.2TNP>H2TMP.2TNP>H2DDMP.2TNP. Absorption and emission studies showed that dramatic changes in the Soret and Q-band pattern and the binding constants were altered with different phenols of varied pKa. The stabilization of molecular adducts were analyzed in correlation with 1H- NMR study to reveal the involvement of the hydrogen bonding.

CaO/SiC alkaline fillers for High-Temperature reduction of mercury (II)

The reduction of Hg2+ to Hg0 through high temperature is a crucial technique for mercury monitoring. However, this application encounters two challenges: suboptimal thermal conditions in the reaction zone leading to incomplete decomposition of Hg2+ and the susceptibility of the decomposed Hg0 to re-oxidation by Cl species. This work proposes a novel alkaline filler with alkaline earth metal oxides (AEMOs) as the active component, which enhances the thermal conditions of the reaction while effectively removing Cl species. DFT reveals increased adsorption energies of AEMOs for HCl and Cl with the growing atomic period. Further comparison of the electronic structures and adsorption behaviors between AEMOs and Cl species indicates a similar trend in electron transfer and electron density at the bond critical points. The frontier molecular orbital analysis shows a positive correlation between orbital energy level difference and adsorption energy. Experimental validation of the DFT results, combined with a comparison of costs and de-HCl performance, identifies CaO as the optimal active component. Subsequently, the CaO is loaded onto SiC, and the impact of precursor and loading ratio on the composite performance is investigated through experiments and characterization. The optimal preparation method involves using calcium acetate monohydrate as the precursor with a loading rate of 50 %. Finally, the developed alkaline filler is tested in a Hg-CEMS for industrial application, it shows errors of below 7 % compared to the EPA Method 30B results, validating its industrial feasibility. This work offers a practical solution for Hg2+ high-temperature reduction in industrial settings.

Molecular Hydrogen Acts as a Hydrogen Bond Proton Acceptor: From Protonated Betaine Tagging to the Weakest Hydrogen Bond.

In an attempt to gain further insights into the intermolecular interactions implied by Rizzo's group's cautionary tale related to molecular tagging in infrared multiple photon dissociation (IRMPD) spectroscopy with molecular messengers [Masson, A. . J. Chem. Phys. 2015, 143, 104313], in the present study, we provide an in-depth analysis of the noncovalent interaction between the molecular hydrogen and protonated betaine molecule in the gas phase. We aim to shed some new light on the fundamental issues concerning the wide diapason of hydrogen-bonding-type intermolecular interactions, with a wide variety of proton acceptors. We demonstrate that in the course of tagging the protonated betaine with molecular hydrogen from the OH group side, it is the σ bond of molecular hydrogen that plays the role of hydrogen-bonding proton acceptor. The tagging thus induces a small yet significant red shift of the protonated betaine O-H stretching mode. We investigate the performance of a wide range of density functional theory (DFT) functionals for the calculation of anharmonic vibrational frequency shifts of the studied system, which are essential for the correct interpretation of the experimental IRMPD data. For an accurate prediction of the OH stretching frequency shifts, specifically designed functionals such as Handy's group HCTH/407 should be applied. The empirical dispersion correction enhances the systematic overestimation of the anharmonic frequency shift, characteristic of the most widely used DFT functionals. Combining the full-wave function approach with the charge field perturbation and natural bond orbital (NBO) deletion analyses, we demonstrate that the frequency shift in the OH-tagged structure is governed by the σHH → σ*OH intermolecular charge transfer. This interaction stabilizes the OH-tagged dimer as well, in contrast to the dipole-quadrupole electrostatic interaction energy term. Topological analysis of the electron density reveals the presence of an intermolecular bond critical point with a positive value of the density Laplacian very close to the lower limit for hydrogen bonds. NCI analyses demonstrate that the OH···H2 interaction is weaker than the intramolecular CH···O one within the protonated betaine molecule, with the through of reduced density gradient appearing at less negative sign(λ2)·ρ values. Analyzing the O-H stretching vibrational potential with the second-generation absolutely localized molecular orbitals energy decomposition analysis (ALMO-EDA 2) revealed that in the case of betaineH(+) tagged from the OH group side, the permanent electrostatics (ΔEelec), polarization (ΔEpol), and charge-transfer (ΔEct) contributions to the total intermolecular interaction energy contribute favorably to the weak hydrogen bond formation and to the red shift of the fundamental O-H stretching frequency, the ΔEct contribution being the most significant in the last context. The Pauli repulsion term, on the other hand, favors an O-H stretching frequency blue shift as a consequence of the vibrational confinement effects.