Physical Nature of Interactions in ZnII Complexes with 2,2′-Bipyridyl: Quantum Theory of Atoms in Molecules (QTAIM), Interacting Quantum Atoms (IQA), Noncovalent Interactions (NCI), and Extended Transition State Coupled with Natural Orbitals for Chemical Valence (ETS-NOCV) Comparative Studies

Comparative Analysis of Nickel-Phosphine Complexes with Cumulated Double Bond Ligands: Structural Insights and Electronic Interactions via ETS-NOCV and QTAIM Approaches

This study presents a comprehensive analysis of nickel-phosphine complexes, specifically Ni(PH3)2(OCCH2), Ni(PH3)2(H2CCO), Ni(PH3)2(H2CCCH2), Ni(PH3)2(NNCH2), and Ni(PH3)2(η1-H2CNN). Utilizing ETS-NOCV analysis, we explored orbital energy decomposition and the Hirshfeld charges of the ligands, providing insights into the electronic structures and donor-acceptor interactions within these complexes. The interactions in the ketene and allene complexes exhibit similar deformation densities and NOCV orbital shapes to those calculated for Ni(PH3)2(NNCH2), indicating consistent interaction characteristics across these complexes. The total interaction energy for all η2 complexes is observed to be over 60 kcal/mol, slightly exceeding that of the analogous carbon dioxide complex reported earlier. Furthermore, the study highlights the stronger back-donation as compared to donor interactions across all η2 complexes. This is further corroborated by Hirshfeld analysis, revealing the charge distribution dynamics within the ligand fragments. The research offers new perspectives on the electron distribution and interaction energies in nickel-phosphine complexes, contributing to a deeper understanding of their catalytic and reactive behaviors.

The effect of {O,N}=X⋯M={Ti,Zr,Hf} interactions on the sensitivity of CNO2 trigger bonds in FOX-7: Approach based on the QTAIM/EDA-NOCV analysis

The local chemical reactivity of FOX-7 (1,1-diamino-2,2-nitroethylene, also known as DADNE from DiAminoDiNitroEthylene) was elucidated through a quantitative study of the electrostatic potential on the molecular surface, topological analysis based on Bader's theory, and the EDA-NOCV method. Unlike (O2N)2CC(NH2)H2N⋯Cp2MCH3+ complexes, which exhibit both σ-donor and π-acceptor features, the situation is different concerning the (H2N)2CC(NO2)(O)NO⋯Cp2MCH3+ complexes, where both charge transfers correspond to the σ-donation. The two charge transfers reinforce each other, resulting in increased stability for (H2N)2CC(NO2)(O)NO⋯Cp2MCH3+. This seems to strengthen the (H2N)2CC(NO2)(O)NO⋯M={Ti,Zr,Hf} bond, which may explain the high stability of (H2N)2CC(NO2)(O)NO⋯Cp2MCH3+ compared to (O2N)2CC(NH2)-H2N⋯Cp2MCH3+. Results from topological analysis revealed that the decreased sensitivity to decomposition of CNO2 bonds depends on the chemical nature of the interacting metal, and the best achievements are obtained for the Hf-based complex. Our results demonstrate that the interaction of M={Ti,Zr,Hf} with CNO2 is more favourable than that with CNH2, this specific action on the trigger bond may support the use of Metallocene Methyl Cations (MMC) as possible neutralisers.

The ferryl generation by fenton reaction driven by catechol

The Fenton and Fenton-like reactions are based on the decomposition of hydrogen peroxide catalyzed by Fe(II), primarily producing highly oxidizing hydroxyl radicals (HO∙). While HO∙ is the main oxidizing species in these reactions, Fe(IV) (FeO2+) generation has been reported as one of the primary oxidants. FeO2+ has a longer lifetime than HO∙ and can remove two electrons from a substrate, making it a critical oxidant that may be more efficient than HO∙. It is widely accepted that the preferential generation of HO∙ or FeO2+ in the Fenton reaction depends on factors such as pH and Fe: H2O2 ratio. Reaction mechanisms have been proposed to generate FeO2+, which mainly depend on the radicals generated in the coordination sphere and the HO∙ radicals that diffuse out of the coordination sphere and react with Fe(III). As a result, some mechanisms are dependent on prior HO∙ radical production. Catechol-type ligands can induce and amplify the Fenton reaction by increasing the generation of oxidizing species. Previous studies have focused on the generation of HO∙ radicals in these systems, whereas this study investigates the generation of FeO2+ (using xylidine as a selective substrate). The findings revealed that FeO2+ production is increased compared to the classical Fenton reaction and that FeO2+ generation is mainly due to the reactivity of Fe(III) with HO∙ from outside the coordination sphere. It is proposed that the inhibition of FeO2+ generation via HO∙ generated from inside the coordination sphere is caused by the preferential reaction of HO∙ with semiquinone in the coordination sphere, favoring the formation of quinone and Fe(III) and inhibiting the generation of FeO2+ through this pathway.

Explaining the High Catalytic Activity in Bis(indenyl)methyl Zirconium Cation Using Combined EDA-NOCV/QTAIM Approach

The main purpose of this study is to elucidate some discrepancies already observed in the catalytic activity values of some zirconocene methyl cations. The EDA-NOCV scheme was employed for a theoretical description of the interactions between an ethylene molecule and five catalysts of zirconocene methyl cation. The nature of the chemical interactions has been elucidated through the QTAIM topological analysis. The steric hindrance due to the ligands was evaluated qualitatively by means of an IRI-based analysis and quantitively through Fisher information. The findings prove that the indenyl ligand seems to favor the orbital interaction between the ethylene molecule and the metal centre of zirconocene methyl cation. Both electrostatic and orbital contributions play a crucial role in stabilising the studied complexes. Based on the NOCV deformation density contributions, the strongest orbital interaction is reached with the bis(indenyl)methyl zirconium cation, which is the only one exhibiting covalent interactions. Especially, the strong contribution of π-back donation (occurring from the occupied orbitals of the zirconium atom to the π* anti-bonding orbital of ethylene) may be a key to understand why this catalyst has a higher polymerisation yield than the other studied catalysts. This work suggests a perspective for predicting values of catalytic activity when theoretically designing novel catalysts of zirconocene type.

Insight into the dual action mechanism of 3V-PPh3 polymers as carriers and ligands in the Rh/3V-PPh3 heterogeneous catalytic hydroformylation of ethylene to propionaldehyde

An experimentally confirmed porous vinyl-functionalized PPh₃ (3V-PPh₃) polymer-supported Rh-based catalyst exhibits the significant advantages of high activity, high stability, and easy separation in the synthesis of propionaldehyde, which fundamentally solves the problem of Rh precious-metal loss. In this paper, the microscopic mechanism and electronic structure characteristics of two kinds of cross-linked 3V-PPh₃ polymer-supported Rh-based catalyst were studied by means of quantum chemistry (QC). With 3V-PPh₃ as the carrier, stable adsorption configurations of Rh and 3V-PPh₃ were investigated, and the results showed that Rh and P had the strongest effects, while the vinyl group enhanced the adsorption strength of Rh. Moreover, it was found that a high concentration of exposed P was beneficial to the dispersion of Rh. With 3V-PPh₃ as the ligand, the properties of the HRh(CO)(P-frame)₃ complex were investigated, and the results of structure analysis indicated that there were strong interactions between Rh and P, which contributed more to the non-loss of Rh. Among the four different configurations, the Rh-P coplanar configuration of cross-linking mode 2 had the highest Rh-P bond energy. The results of AIM analysis suggested that the Rh-P and Rh-C(CO) bonds involve closed-shell (donor-acceptor) interactions. The Mulliken charge and molecular electrostatic potential results revealed that the Rh activity of the Rh and P non-coplanar configuration was higher in the two cross-linking methods. Hopefully, this work will clarify the structure-activity relationship between 3V-PPh₃ polymer and Rh, and provide theoretical guidance for the design and development of high-efficiency heterogeneous catalysts for the hydroformylation of ethylene to propionaldehyde.

Modulation of CO2 adsorption in novel pillar-layered MOFs based on carboxylate-pyrazole flexible linker

Metal-organic frameworks (MOFs) have attracted significant attention as sorbents due to their high surface area, tunable pore volume and pore size, coordinatively unsaturated metal sites, and ability to install desired functional groups by post-synthetic modification. Herein, we report three new MOFs with pillar-paddlewheel structures that have been synthesized solvothermally from the mixture of the carboxylate-pyrazole flexible linker (H₂L), 4,4-bipyridine (BPY)/triethylenediamine (DABCO), and Zn(ii)/Cu(ii) ions. The MOFs obtained, namely [Zn^II(L)BPY], [Cu^II(L)BPY], and [Cu^II(L)DABCO], exhibit two-fold interpenetration and dinuclear paddle-wheel nodes. The Zn(ii)/Cu(ii) cations are coordinated by two equatorial L linkers that result in two-dimensional sheets which in turn are pillared by BPY or DABCO in the perpendicular direction to obtain a neutral three-dimensional framework that shows one-dimensional square channels. The three pillar-layered MOFs were characterized as microporous materials showing high crystalline stability after activation at 120 °C and CO₂ adsorption. All MOFs contain uncoordinated Lewis basic pyrazole nitrogen atoms in the framework which have an affinity toward CO₂ and hence could potentially serve as CO₂ adsorption material. The CO₂ uptake capacity was initially enhanced by replacing Zn with Cu and then replacing the pillar, going from BPY to DABCO. Overall, all the MOFs exhibit low isosteric heat (Q_st) of adsorption which signifies an advantage due to the energy required for the adsorption and regeneration processes.

Interacting Quantum Atoms-A Review

The aim of this review is threefold. On the one hand, we intend it to serve as a gentle introduction to the Interacting Quantum Atoms (IQA) methodology for those unfamiliar with it. Second, we expect it to act as an up-to-date reference of recent developments related to IQA. Finally, we want it to highlight a non-exhaustive, yet representative set of showcase examples about how to use IQA to shed light in different chemical problems. To accomplish this, we start by providing a brief context to justify the development of IQA as a real space alternative to other existent energy partition schemes of the non-relativistic energy of molecules. We then introduce a self-contained algebraic derivation of the methodological IQA ecosystem as well as an overview of how these formulations vary with the level of theory employed to obtain the molecular wavefunction upon which the IQA procedure relies. Finally, we review the several applications of IQA as examined by different research groups worldwide to investigate a wide variety of chemical problems.

New Sensitive and Selective Chemical Sensors for Ni2+ and Cu2+ Ions: Insights into the Sensing Mechanism through DFT Methods

We report the synthesis and theoretical study of two new colorimetric chemosensors with special selectivity and sensitivity to Ni²⁺ and Cu²⁺ ions over other metal cations in the CH₃CN/H₂O solution. Compounds (E)-4-((2-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (A) and (E)-4-((3-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (B) exhibited a drastic color change from yellow to colorless, which allows the detection of the mentioned metal cations through different techniques. The interaction of sensors with these metal ions induced a new absorption band with a hypsochromic shift to the characteristic signal of the free sensors. A theoretical study via time-dependent density functional theory (TD-DFT) was performed. This method has enabled us to reproduce the hypsochromic shift in the maximum UV-vis absorption band and explain the selective sensing of the ions. For all of the systems studied, the absorption band is characterized by a π → π* transition centered in the ligand. Instead of Ni²⁺ and Cu²⁺ ions, the transition is set toward the σ* molecular orbital with a strong contribution of the 3d_x²-_y² transition (π → 3d_x²-_y²). These absorptions imply a ligand-to-metal charge transfer (LMCT) mechanism that results in the hypsochromic shift in the absorption band of these systems.

Structural Characterization, DFT Calculation, NCI, Scan-Rate Analysis and Antifungal Activity against Botrytis cinerea of (E)-2-{[(2-Aminopyridin-2-yl)imino]-methyl}-4,6-di-tert-butylphenol (Pyridine Schiff Base)

Botrytis cinerea is a ubiquitous necrotrophic filamentous fungal phytopathogen that lacks host specificity and can affect more than 1000 different plant species. In this work, we explored L1 [(E)-2-{[(2-aminopyridin-2-yl)imino]-methyl}-4,6-di-tert-butylphenol], a pyridine Schiff base harboring an intramolecular bond (IHB), regarding their antifungal activity against Botrytis cinerea. Moreover, we present a full characterization of the L1 by NMR and powder diffraction, as well as UV–vis, in the presence of previously untested different organic solvents. Complementary time-dependent density functional theory (TD-DFT) calculations were performed, and the noncovalent interaction (NCI) index was determined. Moreover, we obtained a scan-rate study on cyclic voltammetry of L1. Finally, we tested the antifungal activity of L1 against two strains of Botrytis cinerea (B05.10, a standard laboratory strain; and A1, a wild type strains isolated from Chilean blueberries). We found that L1 acts as an efficient antifungal agent against Botrytis cinerea at 26 °C, even better than the commercial antifungal agent fenhexamid. Although the antifungal activity was also observed at 4 °C, the effect was less pronounced. These results show the high versatility of this kind of pyridine Schiff bases in biological applications.

Water binding to FeIII hemes studied in a cooled ion trap: characterization of a strong 'weak' ligand

The interaction of a water molecule with ferric heme-iron protoporphyrin ([PP Fe^III]⁺) has been investigated in the gas phase in an ion trap and studied theoretically by density functional theory. It is found that the interaction of water with ferric heme leads to a stable [PP-Fe^III-H₂O]⁺ complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-Fe^III]⁺ homologue, without spin crossing during water attachment. Using the Van't Hoff equation, the reaction enthalpy for the formation of a Fe-OH₂ bond has been determined for [PP-Fe^III-H₂O]⁺ and [PP-Fe^III-(H₂O)₂]⁺. The corrected binding energy for a single Fe-H₂O bond is -12.2 ± 0.6 kcal mol^-1, while DFT calculations at the OPBE level yield -11.7 kcal mol^-1. The binding energy of the second ligation yielding a six coordinated Fe^III atom is decreased with a bond energy of -9 ± 0.9 kcal mol^-1, well reproduced by calculations as -7.1 kcal mol^-1. However, calculations reveal features of a weaker bond type, such as a rather long Fe-O bond with 2.28 Å for the [PP-Fe^III-H₂O]⁺ complex and the absence of a spin change by complexation. Thus despite a strong bond with H₂O, the Fe^III atom does not show, through theoretical modelling, a strong acceptor character in its half filled 3d_z² orbital. It is also observed that the binding properties of H₂O to hemes seem strikingly specific to ferric heme and we have shown, experimentally and theoretically, that the affinity of H₂O for protonated heme [H PP-Fe]⁺, an intermediate between Fe^III and Fe^II, is strongly reduced compared to that for ferric heme.

Catalytic activity of a new Ru(ii) complex for the hydrogen transfer reaction of acetophenone and N-benzylideneaniline: synthesis, characterization and relativistic DFT approaches

The synthesis and characterization of a new ruthenium(ii) complex containing a hemilabile P^N-ligand are reported.

Structural versatility of the quasi-aromatic Möbius type zinc(ii)-pseudohalide complexes – experimental and theoretical investigations

In this contribution we report for the first time fabrication, isolation, structural and theoretical characterization of the quasi-aromatic Möbius complexes [Zn(NCS)₂L^I] (1), [Zn₂(μ_1,1-N₃)₂(L^I)₂][ZnCl₃(MeOH)]₂·6MeOH (2) and [Zn(NCS)L^II]₂[Zn(NCS)₄]·MeOH (3), constructed from 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane (L^I) or benzilbis(acetylpyridin-2-yl)methylidenehydrazone (L^II), respectively, and ZnCl₂ mixed with NH₄NCS or NaN₃. Structures 1–3 are dictated by both the bulkiness of the organic ligand and the nature of the inorganic counter ion. As evidenced from single crystal X-ray diffraction data species 1 has a neutral discrete heteroleptic mononuclear structure, whereas, complexes 2 and 3 exhibit a salt-like structure. Each structure contains a Zn^II atom chelated by one tetradentate twisted ligand L^I creating the unusual Möbius type topology. Theoretical investigations based on the EDDB method allowed us to determine that it constitutes the quasi-aromatic Möbius motif where a metal only induces the π-delocalization solely within the ligand part: 2.44|e| in 3, 3.14|e| in 2 and 3.44|e| in 1. It is found, that the degree of quasi-aromatic π-delocalization in the case of zinc species is significantly weaker (by ∼50%) than the corresponding estimations for cadmium systems – it is associated with the Zn–N bonds being more polar than the related Cd–N connections. The ETS-NOCV showed, that the monomers in 1 are bonded primarily through London dispersion forces, whereas long-range electrostatic stabilization is crucial in 2 and 3. A number of non-covalent interactions are additionally identified in the lattices of 1–3.

Interplay between various types of non-covalent interactions allowed for isolation of rare examples of Zn(ii) based quasi-aromatic Möbius type species.

Interacting Quantum Atoms Approach and Electrostatic Solvation Energy: Assessing Atomic and Group Solvation Contributions

The interacting quantum atoms (IQA) method decomposes the total energy of a molecular system in terms of one- and two-center (atomic) contributions within the context of the quantum theory of atoms in molecules. Here we incorporate electrostatic continuum solvent effects into the IQA energy decomposition. To this end, the interaction between the solute electrostatic potential and the solvent screening charges as defined within the COSMO solvation model is now included in a new version of the PROMOLDEN code, allowing thus to apply IQA in combination with COSMO-quantum chemical methods as well as to partition the electrostatic solvation energy into effective atomic and group contributions. To test the robustness of this approach, we carry out COSMO-HF/aug-cc-pVTZ calculations followed by IQA calculations on more than 400 neutral and ionic solutes extracted from the MNSol database. The computational results reveal a detailed atomic mapping of the electrostatic solvation energy that is useful to assess to what extent the solvation energy can be decomposed into atomic and group contributions of various parts of a solute molecule, as generally assumed by empirical methodologies that estimate solvation energy and/or logP values.

Unveiling the role of intra and interatomic interactions in the energetics of reaction schemes: a quantum chemical topology analysis

In this work we present a detailed analysis of selected reaction schemes in terms of the atomic components of the electronic energy defined by the quantum theory of atoms in molecules and the interacting quantum atoms method. The aim is to provide an interpretation tool for the energy change involved in a chemical reaction by means of the atomic and interaction contributions to the energies of the molecules involved. Ring strain in cyclic alkanes, the resonance energy of aromatic and antiaromatic molecules, local aromaticity in polycyclic aromatic hydrocarbons, intermolecular bonding in hydrogen fluoride clusters, and hydration of d-block metal dications were selected for the study. It was found that in addition to the changes in the strong C-C interactions in the carbon skeleton of the organic molecular rings, other contributions not usually considered to be important such as those between C and H atoms (either bonded or not) need to be considered in order to account for the net energy changes. The analysis unveils the role of the ionic and covalent contributions to the hydrogen bonding in HF clusters and the energetic origin and extent of cooperative effects involved. Moreover, the "double-hump" behavior observed for the hydration energy trend of [M(H2O)6]2+ complexes is explained in terms of the deformation energy of the metal cation and the increasingly covalent metal-water interactions. In addition, proper comparisons with the description provided by other methodologies are briefly discussed. The topological approach proposed in this contribution proves to be useful for the description of energy changes of apposite reaction schemes in chemically meaningful terms.

A fully analytical integration of properties over the 3D volume of the β sphere in topological atoms

Atomic multipole moments associated with a spherical volume fully residing within a topological atom (i.e., the β sphere) can be obtained analytically. Such an integration is thus free of quadrature grids. A general formula for an arbitrary rank spherical harmonic multipole moment is derived, for an electron density comprising Gaussian primitives of arbitrary angular momentum. The closed expressions derived here are also sufficient to calculate the electrostatic potential, the two types of kinetic energy, as well as the potential energy between atoms. Some integrals have not been solved explicitly before but through recursion and substitution are broken down to more elementary listed integrals. The proposed method is based on a central formula that shifts Gaussian primitives from one center to another, which can be derived from the well-known plane-wave expansion (or Rayleigh equation). © 2018 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

An interacting quantum atom study of model SN 2 reactions (X- ···CH3 X, X = F, Cl, Br, and I)

The quantum chemical topology method has been used to analyze the energetic profiles in the X-  + CH3 X → XCH3  + X- SN 2 reactions, with X = F, Cl, Br, and I. The evolution of the electron density properties at the BCPs along the reaction coordinate has been analysed. The interacting quantum atoms (IQA) method has been used to evaluate the intra-atomic and interatomic energy variations along the reaction path. The different energetic terms have been examined by the relative energy gradient method and the ANANKE program, which enables automatic and unbiased IQA analysis. Four of the six most important IQA energy contributions were needed to reproduce the reaction barrier common to all reactions. The four reactions considered share many common characteristics but when X = F a number of particularities occur. © 2017 Wiley Periodicals, Inc.

Interpretation of Tolman electronic parameters in the light of natural orbitals for chemical valence

Understanding the nature and the strength of metal-ligand interactions in d- and f-block metal complexes has always been a central issue for both synthetic and theoretical chemists. These interactions are usually described according to the well accepted Dewar-Chatt-Duncanson model, and thus over the years numerous research groups directed their efforts to shed light on the role of σ- and π-contributions. Among others, the electronic parameter introduced by Tolman in the 1970s represents a milestone in this field. Herein we present a quantitative description of the nickel-phosphine bond in Tolman's nickel(0) carbonyl complexes. The combination of Natural Orbitals for Chemical Valence with Energy Decomposition Analysis resulted in the definition of a new parameter (T^phos) which comprises all the energetic contributions needed to describe the nickel-phosphine bond and thus stands as a reliable descriptor of the electronic properties of phosphines. Moreover, steric effects of phosphines (i.e. Tolman's cone angles) have been considered too, and a linear relation including Ni-P bond distances, T^phos and cone angle has been found.

Extension of the interacting quantum atoms (IQA) approach to B3LYP level density functional theory (DFT)

An interaction between two atoms, bonded or non-bonded, consists of interatomic contributions: electrostatic energy, exchange energy and electronic correlation energy. Together with the intra-atomic energy of an atom, these contributions are the basic components of the Interacting Quantum Atom (IQA) energy decomposition scheme. Here, we investigate IQA's proper use in conjunction with an explicit implementation of the B3LYP functional. The recovery of the total molecular energy from the IQA components is emphasised, for the first time. A systematic study of three model systems of biological relevance, N-methylacetamide (NMA), the doubly capped tripeptide GlyGlyGly and an alloxan dimer, shows the stabilization effect of B3LYP on most of the interatomic exchange energies (V) compared to their Hartree-Fock values. Diagrams of exchange energies versus interatomic distance show the clustering of interactions, one cluster for each 1,n (n = 1 to 6 where the atoms are separated by n - 1 bonds). The positioning of some V values outside their expected cluster marks interesting interactions.

Tuning the molecular antenna effect using donor and acceptor substituents on the optical properties of the [(C5F5)2ThMCp2]2+ and [(C5F5)2ThMCpL2]+ complexes, where M = Fe, Ru and Os and L = CO and C5H5N

In the present work, a theoretical methodology based on DFT is used to establish the effects of the electron rearrangements on the optical properties in a series of Th-Transtion metal complexes.

Formation of a bi-rhodium boron tube Rh2B18 and its great CO2 capture ability

The geometries, bonding and abilities for CO₂ capture of the doubly rhodium-doped boron cluster Rh₂B₁₈ are presented.

Electronic structure and bonding of the dinuclear metal M2(CO)10 decacarbonyls: applications of natural orbitals for chemical valence

The nature of the chemical metal-metal bond in M₂(CO)₁₀ (M = Mn, Re, Tc) dinuclear decacarbonyls complexes was investigated for the first time using the natural orbital chemical valence (NOCV) approach combined with the extended transition state (ETS) for energy decomposition analysis (EDA). The optimized geometries carried out at different levels of theory BP86, BLYP, BLYPD and BP86D, showed that the latter method, i.e., BP86D, led to the best agreement with X-ray experimental measurements. The BP86D/TZP results revealed that the computed covalent contribution to the metal-metal bond are 60.5%, 54.1% and 52.0% for Mn-Mn, Re-Re and Tc-Tc, respectively. The computed total interaction energies resulting from attractive terms (ΔE _orb and ΔE _eles), correspond well to experimental predictions, based on bond lengths and energy interaction analysis for the studied complexes.

Unfavorable regions in the ramachandran plot: Is it really steric hindrance? The interacting quantum atoms perspective

Accurate description of the intrinsic preferences of amino acids is important to consider when developing a biomolecular force field. In this study, we use a modern energy partitioning approach called Interacting Quantum Atoms to inspect the cause of the φ and ψ torsional preferences of three dipeptides (Gly, Val, and Ile). Repeating energy trends at each of the molecular, functional group, and atomic levels are observed across both (1) the three amino acids and (2) the φ/ψ scans in Ramachandran plots. At the molecular level, it is surprisingly electrostatic destabilization that causes the high-energy regions in the Ramachandran plot, not molecular steric hindrance (related to the intra-atomic energy). At the functional group and atomic levels, the importance of key peptide atoms (Oi-1 , Ci , Ni , Ni+1 ) and some sidechain hydrogen atoms (Hγ ) are identified as responsible for the destabilization seen in the energetically disfavored Ramachandran regions. Consistently, the Oi-1 atoms are particularly important for the explanation of dipeptide intrinsic behavior, where electrostatic and steric destabilization unusually complement one another. The findings suggest that, at least for these dipeptides, it is the peptide group atoms that dominate the intrinsic behavior, more so than the sidechain atoms. © 2017 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Theoretical insights into the nature of PtSn bond: Reevaluating the bonding/back-bonding properties of trichlorostannate with comparison to the cyano ligand

The coordinating properties of the trichlorostannate ligand in hydrido platinum trichlorostannato and platinum bis-trichlorostannato complexes, containing various phosphine ligands, have been elucidated by means of charge decomposition analysis, the Ziegler-Rauk Energy Decomposition with the Natural Orbitals for Chemical Valence, Domain-Averaged Fermi Hole, and natural bond orbital methods. Trichlorostannate has been found to be a strong σ-donor and a weak π-acceptor ligand with coordination properties not far from those of the cyano ligand. For back-bonding, the tin-chlorine σ∗ orbitals are mostly responsible. In contrast to previous assumptions, the 5d orbitals of tin play no role at all in the interaction with the platinum center. QTAIM calculations suggest, that the platinum-tin interaction should be interpreted as donor-acceptor, rather than covalent type. Trichlorostannate has been found to have weaker trans influence in comparison to the cyano ligand. © 2017 Wiley Periodicals, Inc.

On the Stability of Cis- and Trans-2-Butene Isomers. An Insight Based on the FAMSEC, IQA, and ETS-NOCV Schemes

In the present account, the real space fragment attributed molecular system energy change (FAMSEC) approach, interacting quantum atoms energy decomposition scheme as well as molecular orbitals based the extended transition state scheme coupled with natural orbitals for chemical valence (ETS-NOCV) have been, for the first time, successfully used to delineate factors of importance for stability of the 2-butene conformers (cis-eq, cis-TS, trans-eq, trans-TS). Our results demonstrate that atoms of the controversial H-H contact in cis-eq (i) are involved in attractive interaction dominated by the exchange-correlation term, (ii) are weekly stabilized, (iii) show trends in several descriptors found in other typical H-bonds, and (iv) are part of most stabilized CH-HC fragment (loc-FAMSEC = -3.6 kcal/mol) with most favourably changed intrafragment interactions on trans-eq→cis-eq. Moreover, lower stability of cis-eq vs. trans-eq is linked with the entire HCCH (ethylenic) fragment which destabilized cis-eq (mol-FAMSEC, +3.9 kcal/mol) the most. Although the H-H contact can be linked with smaller, relative to trans-, rotational energy barrier in cis-2-butene, we have proven that to rationalize this phenomenon one must account for changes in interactions between various fragments that constitute the entire molecule. Importantly, we discovered a number of comparable trends in fundamental properties of equivalent molecular fragments on a methyl group rotation; for example, interaction between BP-free H-atoms in trans-eq (involving CH bonds of the methyl and ethylenic units) and BP-linked H-atoms in cis-eq. Clearly, rotational energy barrier cannot be entirely (i) rationalized by the properties of or (ii) attributed to the H-H contact in cis-eq. © 2016 Wiley Periodicals, Inc.

Theoretical insight into the interaction between SnX2 (X = H, F, Cl, Br, I) and benzene

For a series of five model complexes composed of a singlet SnX₂ molecule (X = H, F, Cl, Br, I) and a benzene molecule, the first-principles calculations of their energetics and the analysis of their electron density topology have been performed. The CCSD(T)/CBS interaction energy between SnX₂ and C₆H₆ fall into the range between −10.0 and −11.2 kcal/mol, which indicates that the complexes are rather weakly bound. The relevant role of electrostatic and dispersion contributions to the interaction energy between SnX₂ and C₆H₆ is highlighted in the results obtained from the symmetry-adapted perturbation theory (SAPT). The electron density topological analysis has been carried out using the quantum theory of atoms in molecules (QTAIM) and the noncovalent interactions (NCI) visualization index. Both QTAIM and NCI prove the closed-shell, noncovalent and attractive character of the interaction. A very small charge transfer from C₆H₆ to SnX₂ has been detected. The formation of the five complexes is accompanied by the electron density deformations that are spatially restricted mostly to the region around the Sn atom and its adjacent C atom. The results presented in this work shed some light on the nature of the interactions associated with crystalline structural motifs involving low-valent tin complexed with neutral aryl rings.