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Despite tremendous efforts by instructors and textbook authors, students find it difficult to develop useful chemical intuitions about preferred structures, structural trends, and properties of even the most common d-block element organometallic species, that is d6, d8, and d10 systems. A full molecular orbital analysis of a transition metal species is not always feasible or desirable, and crystal field theory, while generally useful, is often too simplistic and limited. It would be helpful to give students of organometallic chemistry an additional toolkit that helps them to understand d-block compounds, in particular highly covalent ones. It is well known in the research literature in organometallic chemistry that hybridization arguments involving s and d orbitals (such as sd and sd2 hybridization for d8 and d6 systems, respectively) provides useful insight. However, this knowledge is much underused in undergraduate teaching and not taught in undergraduate textbooks. The purpose of this article is to make descriptions of bonding that are based on s,d-hybridized orbitals more accessible in a way that is directly useful for undergraduate teaching. Geometries of unusual low-coordinate structures can be successfully predicted. An in-depth physical explanation for the trans-influence, the weakening of a bond due to a strong bond trans to it, is provided. A clear explanation is given for why the cis isomer normally more stable than the trans isomer in square-planar d8 complexes of the type MR2L2 (R = alkyl/aryl, L = relatively weakly bonded neutral ligand). Similarly, the relative stability of fac versus mer isomers in octahedral d6 complexes of the type MR3L3 is explained. Relevant to catalysis, the method explains why strongly donating ligands do not always facilitate oxidative addition and why 12-electron and 14-electron Pd(0) species are thermodynamically much more accessible than one might expect. The method capitalizes on 1st year knowledge such as the ability to write Lewis structures and to use hybridization arguments. It also ties into the upper-year experience, including graduate school, where covalent d-block complexes may be encountered in research and where the hybridization schemes described here naturally emerge from using the NBO formalism. It is discussed where the method might fit into the inorganic curriculum.

Chemical formulas are represented with the help of Lewis structures. To illustrate reaction mechanisms, these structures are provided with arrows and the new structural formula is recorded. This procedure presents blind pupils with several hurdles: - How do these structures come about? - How can students spontaneously create tactile formulas in class? - How can the movements shown by arrows be traced? For the representation of atomic symbols, for which the element symbols from the periodic table are normally used, magnets were developed that differ in shape, color, and size. Since only a handful of symbols occur regularly and in large numbers in school, only a few symbols have been permanently assigned, some are variable and can be used appropriately defined for the respective teaching situation. With this symbol set, it is possible to quickly and individually manufacture almost all molecules relevant in school. These magnets allow the independent determination of reaction mechanisms in the further course of the lesson. Due to the mobility of the elements on the magnetic base, electrons and atoms can be moved and thus result in a new bonding situation that is reinterpreted by the students. The advantage of this procedure is that the pupils do not just paint arrows, but consciously think about all processes and can also make mistakes. When analyzing intermediate products, you can determine that your actions were either not compliant or compliant but not effective. This enables individual active learning in the classroom.

The prototype Diels-Alder (DA) reaction between butadiene and ethene (system 1) and the DA reaction involving 1-methoxy-butadiene and cyano-ethylene (system 2) are investigated with an explicit-time-dependent Density Functional Theory approach. Bond orders and atomic net charges obtained in the dynamics at the transition state geometry and along the reaction coordinate toward reactants are used to provide a picture of the process in terms of VB/Lewis resonance structures that contribute to a resonance hybrid. The entire dynamics can be divided into different domains (reactant-like, product-like, and transition state domains) where different Lewis resonance structures contribute with different weights. The relative importance of these three domains varies along the reaction coordinate. In addition to the usual reactant-like and product-like covalent Lewis structures, ionic Lewis structures have non-negligible weights. In system 2, the electron-donor OCH3 on the diene and the electron-acceptor CN on the dienophile make more important the contributions of ionic Lewis structures that stabilize the transition state and determine the decrease of the reaction barrier with respect to system 1.

Dative Bond (IUPAC): "The coordination bond formed upon interaction between molecular species, one of which serves as a donor and the other as an acceptor of the electron pair to be shared in the complex formed… The distinctive feature of dative bonds is that their minimum-energy rupture in the gas phase or in inert solvent follows the heterolytic bond cleavage path." This definition encompasses an immense number of molecules such as Lewis adducts, transition-metal complexes, supposedly hypervalent or hypovalent systems, and many molecules with multifaceted Lewis structures. Still, there is a large reticence to include dative bonds in the regular depiction of molecules, and even a larger unawareness of the dative bond arrow in many chemical circles. Herein we will discuss in simple chemical terms the past, present and future of such bonds. In addition, we will try to provide cleaner options to represent intricate molecules without sacrificing physical accuracy.

Oxidation number (ON) is taught as an electron-counting concept for redox reactions in chemistry curriculum. The molecular formula method, the Lewis formula method, and the structural formula method have all been used to determine ON. However, the task of assigning ON still poses problems for some teachers and students. This paper explores a new method, the fragmentation method, which is a visual approach for counting the individual ON of any atom according to its structural formula. The critical step is to break the carbon-heteroatom bond into organic fragments and inorganic fragments. The individual ON of carbon atoms and heteroatoms can be determined by the bond cleavages in the organic and biological compounds. The mean ON of carbons can be calculated by the arithmetic mean of all individual ON of carbons in a molecule or molecular ion. The step-by-step operating procedures and examples are provided. When comparing corresponding molecules in organic conversions, the change of individual ON of atoms can be used as a tool for determining the number of transferred electrons. Furthermore, a reaction site can be identified by their changes of individual ON, chemical composition, and bond order in metabolic redox reactions.

Many compounds can be represented well in terms of the two-center–two-electron (2c–2e) bond model. However, it is well-known that this approach has limitations; for example, certain compounds require the use of three-center–two-electron (3c–2e) bonds to provide an adequate description of the bonding. Although a classic example of a compound that features a 3c–2e bond is provided by diborane, B2H6, 3c–2e interactions also feature prominently in transition metal chemistry, as exemplified by bridging hydride compounds, agostic compounds, dihydrogen complexes, and hydrocarbon and silane σ-complexes. In addition to being able to identify the different types of bonds (2c–2e and 3c–2e) present in a molecule, it is essential to be able to utilize these models to evaluate the chemical reasonableness of a molecule by applying the octet and 18-electron rules; however, to do so requires determination of the electron counts of atoms in molecules. Although this is easily achieved for molecules that possess only 2c–2e bonds, the situation is more complex for those that possess 3c–2e bonds. Therefore, this article describes a convenient approach for representing 3c–2e interactions in a manner that facilitates the electron counting procedure for such compounds. In particular, specific attention is devoted to the use of the half-arrow formalism to represent 3c–2e interactions in compounds with bridging hydrogen atoms.

Using computational approaches, we qualitatively and quantitatively assess the bonding components of a series of experimentally characterized Au(I) diarylallenylidene complexes (N.Kim, R.A.Widenhoefer, Angew. Chem. Int. Ed. 2018, 57, 4722-4726). Our results clearly demonstrate that Au(I) engages only weakly in pi-backbonding, which is, however, a tunable bonding component. Computationally identified trends in bonding are clearly correlated with the substitution patterns of the aryl substituents in the Au(I) diarylallenylidene complexes and good agreement is found with the previously reported experimental data, such as IR spectra, C-13 NMR chemical shifts and rates of decomposition together with their corresponding barrier heights, further substantiating the computational findings. The description of the bonding patterns in these complexes allow predictions of their spectroscopic features, their reactivity and stability.

Understanding the mechanism of catalytic reactions is crucial for the future development of catalysts. In this computational study, dispersion-corrected Density Functional Theory (DFT) theory was used to calculate the various mechanistic pathways for ammonia and hydrazine synthesis on η-Mn3N2-(100) surfaces. A simple Lewis structure representation algorithm was used in order to locate various possible NxHy intermediates. Hydrogenation of dinitrogen results in significant activation of the inert triple bond and these intermediates have a significant role in the ammonia and hydrazine synthesis reaction on manganese nitrides via a Langmuir-Hinschelwood mechanism. It is anticipated that these findings are significant in developing new catalysts for hydrazine synthesis using η-Mn3N2 (100) catalysts.

Chemists explaining a molecule’s stability and reactivity often refer to the concepts of delocalization, resonance, and aromaticity. Resonance is commonly discussed within valence bond theory as the stabilizing effect of mixing different Lewis structures. Yet, most computational chemists work with delocalized molecular orbitals, which are also usually employed to explain the concept of aromaticity, a ring delocalization in cyclic planar systems which abide certain number rules. However, all three concepts lack a real space definition, that is not reliant on orbitals or specific wave function expansions. Here, we outline a redefinition from first principles: delocalization means that likely electron arrangements are connected via paths of high probability density in the many-electron real space. In this picture, resonance is the consideration of additional electron arrangements, which offer alternative paths. Most notably, the famous 4n + 2 Hückel rule is generalized and derived from nothing but the antisymmetry of fermionic wave functions., The concept of delocalization, resonance and aromaticity are commonly discussed within electronic structure frameworks relying on specific wave function expansions. Here the authors propose a redefinition of these concepts from first-principles by investigating saddle points of the all-electron probability density.

Students in a second semester general chemistry course used quantum chemical calculations to investigate and reinforce general chemistry concepts. Students explored the isomers of hypochlorous acid, made predictions of miscibility via dipole moments calculated from ab-initio means, experimentally validated/disqualified their miscibility predictions, and used molecular models to visualize intermolecular attraction forces between various compounds. Student responses in pre-/post-exercise assessments show evidence of student learning. Responses in pre-/post-exercise surveys showed an increase in student understanding of basic concepts and of the importance of quantum mechanics in common general chemistry topics.

There is a great variety of bond analysis tools that aim to extract information on the bonding situation from the molecular wavefunction. Because none of these can fully describe bonding in all of its complexity, it is necessary to regard a balanced selection of complementary analysis methods to obtain a reliable chemical conclusion. This is, however, not a feasible approach in most studies because it is a time-consuming procedure. Therefore, we provide the first comprehensive comparison of modern bonding analysis methods to reveal their informative value. The element-oxygen bond of neutral Hn XOH model compounds (X=Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl) is investigated with a selection of different bond analysis tools, which may be assigned into three different categories: i) real space bonding indicators (quantum theory of atoms in molecules (QTAIM), the electron localizability indicator (ELI-D), and the Raub-Jansen index), ii) orbital-based descriptors (natural bond orbitals (NBO), natural resonance theory (NRT), and valence bond (VB) calculations), and iii) energy analysis methods (energy decomposition analysis (EDA) and the Q-analysis). Besides gaining a deep insight into the nature of the element-oxygen bond across the periodic table, this systematic investigation allows us to get an impression on how well these tools complement each other. Ionic, highly polarized, polarized covalent, and charge-shift bonds are discerned from each other.

The nature of the electronic-structure of polyenes, their delocalization features, and potential diradicaloid characters constitute a fundamental problem in chemistry. To address this problem, we used valence bond self-consistent field (VBSCF) calculations and modeling of polyenes, C2nH2n+2 (n = 2–10). The theoretical treatment shows that starting with n = 5, the polyene’s wave function is mainly a shifting 1,4-diradicaloid, a character that increases as the chain length increases, while the contribution of the fundamental Lewis structure with alternating double and single bonds (1) decays quite fast and becomes minor relative to the diradicaloid pack. We show how, nevertheless, it is this wave function that predicts that polyenes will still exhibit alternating short/long CC bonds like the fundamental structure 1. Furthermore, despite the decay of the VB contribution of 1, it remains the single structure with the largest weight among all the individual structures. The mixing of all the 1,4-diradicaloid st...

The polyazoles have five-membered heterocyclic rings with N/C ratios of at least 2/3. Their high nitrogen contents result in relatively high crystal densities and large positive heats of formation, features that make them attractive as frameworks for energetic materials. However the presence of linked nitrogens (catenation) is accompanied by reduced stability (reflected in the large heats of formation). Several related factors may be involved in this, including the weakness of N-N bonds, the possibility of decomposing through release of the very stable N2(g) and the repulsion between lone pairs of neighboring nitrogens. We show that in the polyazoles a particularly important source of reduced stability is the presence of adjacent doubly-coordinated nitrogens, especially if connected by a formal double bond. Our computed polyazole electrostatic potentials are consistent with significant repulsion between the lone pairs of these nitrogens. Introducing an N-oxide linkage on one of them leads to some stabilization; the oxygen withdraws electronic charge from the heterocyclic ring, reducing the electronic repulsion within it. However the N-oxide derivatives having more nitrogen catenation are still the less stable ones. Comparing the polyazoles to the polyazines (six-membered N/C heterocyclic rings), the π electrons in the former are less delocalized and the computed bond lengths in the polyazole rings are consistent with the formal single and double bonds in their Lewis structures. An interesting consequence is that if a second N-oxide is introduced next to the original one, the bond between the two nitrogens remains intact in polyazoles if it is formally double but it is considerably weakened or broken in polyazines, and in polyazoles if formally single.

The papers mentioned in the title of the present comment article contain some calculations/results that are in disagreement with some basic concepts of chemistry. The calculations include (i) the calculation of an energy difference, more than 1000 J mol−1, between one pair of enantiomers, (ii) the calculation of an energy difference between the Lewis structures (resonance forms), and (iii) the calculation of bond orders in different pathways of resonance process.

The properties and reactivities of transition metal complexes are rooted in the stereoelectronic properties of their ligands. While the bulk of a ligand can be easily evaluated and compared by the drawing of its Lewis structure, prediction on the electronic contributions is often less straightforward. Thus, several electronic parameters have been developed for the experimental evaluation of ligands throughout the years. This article accounts for the most recent one developed by the Huynh group, which employs 13C NMR spectroscopy to determine ligand donor strengths using N-heterocyclic carbene complexes. This parameter not only proves to be safer, more convenient and accurate in comparison to existing methodologies, but it also provides, in certain cases, more intuitive and reliable results. Furthermore, it is currently the only one that allows the direct comparison of various Werner-type and organometallic ligands on a unified scale.

In this work we report on the progress that has been made towards gaining an understanding of the molecular mechanism of 1,3-dipolar cycloadditions using the bonding evolution theory (BET). A detailed analysis of the flow of electron density along the reaction pathway of the formal 1,3-dipolar cycloaddition reaction between cyclic nitrones (pyrroline-1-oxide and 2,3,4,5-tetrahydropyridine-1-oxide) and ethyl acrylate, as a case study, allowed the nature of the molecular mechanisms to be characterized. The present study provides a deep insight into the reaction mechanism, based on the electron density rearrangements given by the structural stability domains, and their connection with the bond breaking/forming processes along the reaction pathway. The electron pushing formalism is a powerful tool to describe chemical reactivity. Here, we show how the Lewis structures can be recovered and how curly arrows describe electron density transfers in chemical reaction mechanisms based on the BET results. The reaction mechanism is described by four consecutive events taking place as the reaction progresses: (1) the population of the initial N–C double bond is transferred to the N and C atoms; (2) the population of the initial double C–C bond is transferred to the C atoms. Along the ortho pathway the next steps are: (3) the C–C bond-forming process, and (4) the O–C bond-forming process. The order of (3) and (4) is inverted in the meta channel. Based on the sequence of the structural stability domains along the intrinsic reaction coordinate, a new synchronicity index is proposed, allowing us to classify and quantify the (a)synchronicity of the 1,3-DC reactions and, therefore, the nature of the reaction mechanism.

It is well known that closo-borane dianions BnHn2− are stable aromatic cages and possess n + 1 valence electron pairs, in accordance with Wade's rules. However, the electronic structures of closo-BnHn2− cannot be explained by a Lewis structure because of their electron-deficient character, while recent theoretical and experimental studies reveal that multicenter bonding is a key part in their electronic structures. In this work, flexible multicenter bonding of BnHn2− (n = 2–17) is studied using the adaptive natural density partitioning (AdNDP) method in order to get insight into their stability and aromaticity. The large HOMO–LUMO gaps and negative NICS values indicate their close-shell electronic structure. Further chemical bonding analysis shows that all 2n + 2 delocalized valence electrons in closo-BnHn2− are involved in various delocalized σ and π multicenter bonding systems on their cage surface, well matched with different symmetric configurations. There are five types of multicenter bonds, including an open 3c–2e BBB bond, closed 3c–2e BBB bond, 4c–2e bond, 8c–2e bond and a fully delocalized bond, which are delocalized on a B3 broken-line, B3 triangle, B4 rhombus, B8 double-ring and the whole surface of the boron cage, respectively. Our work reveals the flexibility of multicenter bonding in diversified closo-BnHn2− clusters, giving new insights into the bonding nature of BnHn2−.

The interaction of Lewis acids and bases in both classical Lewis adducts and frustrated Lewis pairs (FLPs) is investigated to elucidate the role that London dispersion plays in different situations. The analysis comprises 14 different adducts between tris(pentafluorophenyl)borane and a series of phosphines, carbenes, and amines with various substituents, differing in both steric and electronic properties. The domain-based local pair natural orbital coupled-cluster (DLPNO-CCSD(T)) method is used in conjunction with the recently introduced local energy decomposition (LED) analysis to obtain state-of-the-art dissociation energies and, at the same time, a clear-cut definition of the London dispersion component of the interaction, with the ultimate goal of aiding in the development of designing principles for acid/base pairs with well-defined bonding features and reactivity. In agreement with previous DFT investigations, it is found that the London dispersion dominates the interaction energy in FLPs, and is also remarkably strong in Lewis adducts. In these latter systems, its magnitude can be easily modulated by modifying the polarizability of the substituents on the basic center, which is consistent with the recently introduced concept of dispersion energy donors. By counteracting the destabilizing energy contribution associated with the deformation of the monomers, the London dispersion drives the stability of many Lewis adducts.

Experimental Fourier Transform Infra-Red spectrum and Fourier Transform Raman spectrum with Micro Raman spectrum of the N-Benzyloxycarbonyl-L-Aspartic acid 1-Benzyl Ester have been analyzed for molecular structural determination. A hybrid Becke's three-parameter technique with the Lee-Yang-Par (B3LYP) correlation method of Density functional theory (DFT) with 6–311++G(d,p) basic set was used for the investigations on quantum computational spectroscopic calculations. Chemical stabilization energy associated with hyper-conjugative interactions with polarization coefficients and natural Lewis structures were determined by Natural bond orbital analysis. Molecular dynamic simulations with global reactive descriptors were obtained by calculated Frontier molecular orbital energies. The charge density distribution has been visualized by electrostatic potential surfaces. Local reactive behaviors of the chemical compound with the graphical representation of the Fukui functions were obtained. The UltraViolet-Visible light absorptions with different electronic excitations were constructed by the method of Time-dependent DFT. Molecular docking was introduced for N-Benzyloxycarbonyl-L-Aspartic acid 1-Benzyl Ester into different proteins (1ITO, 2ZCO, 1J1J, and COVID-19 proteins).

Consideration is given to (electronically) hypervalent increased-valence structures, which possess 2c–1e bonds, fractional 2c–2e bonds, and usually normal 2c–2e bonds., Consideration is given to (electronically) hypervalent increased-valence structures, which possess 2c–1e bonds, fractional 2c–2e bonds, and usually normal 2c–2e bonds. For singlet-spin electron-rich systems, increased-valence structures, with Heitler–London 2c–2e bond wavefunctions, are equivalent to resonance between non-hypervalent Kekulé and Dewar (or singlet diradical) type Lewis structures. Dewar structures are not considered in the Chem. Sci. 2015, 6, 6614 Edge article on hypervalency. Using one-electron delocalizations from lone-pair atomic orbitals into separate bonding molecular orbitals, increased-valence structures for PCl5, O3, SO42–, NO3–, N2O4 and SN2 reactions are derived from the Edge-article's Kekulé-type Lewis structures, and compared with the Edge article's hypervalent structures with 2c–2e bonds. It is also shown that Durrant's method to determine the γ parameter for XAY-type systems that possess a symmetrical 3c–4e bonding unit is related to the A-atom charge density.

A thermally robust triple-decker complex [bis(η5-pentamethylcyclopentadienyltitanium)-μ-(η4:η4-1,2,4,5-tetrakis(trimethylsilyl)cyclohexa-1-4-diene-3,6-diyl)] (3) was obtained in 4% yield by thermolysing Cp*TiMe3 in the presence bis(trimethylsilyl)acetylene (BTMSA). The solid-state structure of centrosymmetric 3 features rather long all C–C bonds in the nearly planar bridging ligand (1.4720(14)–1.4896(15) A) and a short distance of its least-square plane to the titanium atoms (1.7381(5) A). Computational results revealed the bonding of the central ligand to be accomplished through back-bonding of its two CC bonds and through the simultaneous generation of two σ-Ti–C(H) bonds. Based on CASSCF and CASPT2 results, the molecule acquires several electronic configurations simultaneously, which hinders its representation by one single Lewis structure. Apart from being coordinated to the central ligand, the metal atoms are involved in a direct Ti–Ti bonding by the formation of one σ- and two δ-bonds between them. The bond order of this Ti–Ti overlap shows only a slight decrease upon electronic excitation. The presence of ionic contribution to the bonding of the central ligand is manifested by the charge −1.4e summed on the carbon atoms of the bridging ring. Based on computational results, the spin multiplicity of the ground state is singlet, while the first low lying excitation state is triplet. This is in agreement with the absence of EPR signal in either toluene solution and glass, and with slight downfield shifts of broadened 1H NMR signals of SiMe3 and Cp* methyl groups observed with increasing temperature.

An embedding method for a one-electron reduced density matrix (1-RDM) is proposed. It is based on partitioning of 1-RDM into domains and describing each domain in the effective potential of the other ones. To assure N-representability of the total 1-RDM N-representability and strong-orthogonality conditions are imposed on the domains. The total energy is given as a sum of single-domain energies and domain-domain electron interaction contributions. Higher than two-body inter-domain interaction terms are neglected. The two-body correlation terms are approximated by deriving inter-domain correlation from couplings of density fluctuations of two domains at a time. Unlike in most density embedding methods kinetic energy is treated exactly and it is not required that densities pertaining to the domains are only weakly overlapping. We propose to treat each domain by a corrected perfect-pairing functional. On a few examples it is shown that the embedding reduced density matrix functional method (ERDMF) yields excellent results for molecules that are well described by a single Lewis structure even if strong static intra-domain or dynamic inter-domain correlation effects must be accounted for.

Theoretical studies on Lewis acid-base behavior of hypervalent halogen fluorides, F3X and F5X (X = Cl, Br, I) have been instrumental in guiding this work. We have also examined whether the hole-lump concept explains the formation of the F5XCO complexes. Calculations of proton affinities (PA) and gas-phase basicity (GB) on hypervalent halogen fluorides show that F3X and F5X molecules can act as Lewis bases in gas phase. Moreover, theoretical calculations indicate that F3X and F5X molecules can act as Lewis acids forming stable complexes with a Lewis base as CO. The quantum theory of atoms in molecules (QTAIM) shows that the electrostatic interaction between the lone pair of the Lewis base (CO) and nucleus of the hypervalent halogen atom (X) plays a key role in stabilizing and determining the optimal geometry of the F5XCO complexes, as in conventional XBs. The localized molecular orbital energy decomposition analysis (LMOEDA) reveals that electrostatic component plays an important role in the stability of the FnXCO complexes. Fil: Buralli, Gabriel Jesús. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Química Basica y Aplicada del Nordeste Argentino. Universidad Nacional del Nordeste. Facultad de Cs.exactas Naturales y Agrimensura. Instituto de Química Basica y Aplicada del Nordeste Argentino; Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Química. Laboratorio de Estructura Molecular y Propiedades; Argentina Fil: Duarte, Darío Jorge Roberto. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Química. Laboratorio de Estructura Molecular y Propiedades; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Química Basica y Aplicada del Nordeste Argentino. Universidad Nacional del Nordeste. Facultad de Cs.exactas Naturales y Agrimensura. Instituto de Química Basica y Aplicada del Nordeste Argentino; Argentina Fil: Sosa, Gladis Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Química Basica y Aplicada del Nordeste Argentino. Universidad Nacional del Nordeste. Facultad de Cs.exactas Naturales y Agrimensura. Instituto de Química Basica y Aplicada del Nordeste Argentino; Argentina. Universidad Tecnológica Nacional. Facultad Regional Resistencia. Departamento de Ingeniería Química. Laboratorio de Química Teórica y Experimental; Argentina Fil: Peruchena, Nelida Maria. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Química. Laboratorio de Estructura Molecular y Propiedades; Argentina

A recent analysis of the bonding in transition metal (TM) complexes with cyclic aminoalkyl carbene (cAAC) ligands, TM(cAAC)2 (TM = Cu, Ag, and Au), purports to show that metal–ligand bonding involves the TM in the excited 2P state and that TM(pπ) → (cAAC)2 backdonation is not properly recognized in NBO analysis because of biases against participation of np functions in transition metal bonding. The questions of TM np orbital involvement in bonding and the possible biases in the NBO occupancy-weighted symmetric orthogonalization procedure have been examined by performing NBO analyses in two ways: (1) single Lewis structure (loc) analysis with TM np orbitals treated as valence (NBOs) or nonvalence (NBOx) and (2) direct comparison of a two-configuration resonance model (res/NBOs) treatment with a single configuration model using the expanded valency (loc/NBOx) treatment. The principal bonding picture that emerges from NBO analysis features a TM cation with two “non-innocent” cAAC ligands that are each reduce...

Light-driven chemical reactions on semiconductor surfaces have potential for addressing energy and pollution needs through efficient chemical synthesis; however, little is known about the time evolution of excited states that determine reaction pathways. Here, we study the photo-oxidation of methoxy (CH3O) derived from methanol on the rutile TiO2(110) surface using ab initio simulations to create a molecular movie of the process. The movie sequence reveals a wealth of information on the reaction intermediates, time scales, and energetics. The reaction is broken in three stages, described by Lewis structures directly derived from the "hole" wave functions that lead to the concept of "photoinduced C-H acidity". The insights gained from this work can be generalized to a set of simple rules that can predict the efficiency of photo-oxidation reactions in reactant-catalyst pairs.

A comparative analysis of molecular graphs of the electronic density and the molecular electrostatic potential (MEP) for lithium, sodium and copper clusters with up to nine atoms is presented. Whereas the density molecular graphs are qualitatively different for these clusters the corresponding MEP molecular graphs are surprisingly similar. Only for copper clusters MEP and density molecular graphs are similar. Our analysis indicates that MEP molecular graphs for these metal clusters show connectivities very much in the spirit of Lewis structures.

Hydrogen molecule was chosen, for simplicity, to analyze its valence bond (VB) wave function with the Quantum Theory of Atoms in Molecules (QTAIM). We find that the usual association of its covalent and ionic contributions to individual Lewis structures does not agree with the localization and delocalization indices between both hydrogens defined within the QTAIM framework. Concretely, we analyzed the dependence of both indices with the internuclear distance and with different ionic/covalent relative weightings. The results indicate that QTAIM localization and delocalization indices should not be estimated from interpretations of the VB wave function in terms of the Lewis model.

Comprehensive structural, spectroscopic, and quantum chemical analyses of new donor-acceptor complexes between N-heterocyclic carbenes and 1,2,5-telluradiazoles and a comparison with previously known complexes involving tellurenyl cations showed that the dative C-Te bonds cannot be solitarily described with only one Lewis formula. Canonical Lewis formulas that denote covalency and arrows emphasizing ionicity complement each other in varying extents. Evaluation of the relative weights of these resonance forms requires proper bonding description with a well-balanced toolbox of analytical methods. If for conciseness only, one resonance form is used, it must be the most significant one according to the analytical evaluation. If unclear, all significant resonance forms should be displayed.

High¿level quantum chemical calculations are performed to investigate C=Se¿¿¿Se=C interactions. Bounded structures are found with binding energies between ¿4 and ¿7¿kJ¿mol¿1. An energy decomposition analysis shows that dispersion is the more attractive term, and in all cases save one, the electrostatic interaction is attractive despite each selenium atom having a positive ¿¿hole at the extension of the C=Se bond. The topological analysis of the molecular electrostatic potential and L(r)=¿¿2¿(r) function, and natural bond orbital analysis reveal that these particular Se¿¿¿Se contacts can be considered to be quadruple Lewis acid¿base interactions., G.J.B., D.J.R.D., and N.M.P. acknowledge SEGCYT-UNNE and CONICET for financial support. I.A. thanks the financial support from the Spanish Ministerio de Econom&a y Competitividad (Project No. CTQ2015-63997-C2-2-P) and Comunidad Autónoma de Madrid (Project FOTOCARBON, ref. S2013/MIT-2841). The authors also acknowledge the use of CPUs from the CECONEA. N.M.P. is a career research of CONICET, Argentina.

We call iodabenzene a cyclic (CH)5I molecule. A planar iodabenzene would have 8 π electrons, a situation best avoided by an out-of-plane distortion to a bird-like geometry. The electronic structure and charge distribution of this molecule resemble those of Meisenheimer complexes, derivatives of (CH)5CH2–. A similar substitution strategy, of π-acceptors in ortho and para positions, works in both cases to planarize and stabilize such derivatives. Some 40 kcal/mol (73 kcal/mol for the unsubstituted case) below the bird, a classical 5-iodocyclopentadiene structure awaits, reached through a bicyclic transition state. The calculated activation barrier for the highly exothermic reaction to a classical Lewis structure nevertheless make us optimistic about the chances of detecting and even isolating the bird isomer.

Hydroboration of allyldimethylamine, diallylmethylamine, triallylamine, and diallylmethylphosphane with dimeric 9-BBN yielded the corresponding singly, doubly, and triply Lewis-acid-functionalized intramolecular Lewis pairs. For the singly Lewis-acid-functionalized derivative Me2N(CH2)(3)-9-BBN no evidence for the existence of an equilibrium involving an open-chain form was found in solution. For the doubly and triply Lewis-acid-functionalized compounds Me(3)xE[(CH2)(3)-9-BBN](x) [E = N (x = 2, 3), P (x = 2)] a dynamic exchange of the free Lewis-acid functions with an intramolecular Lewis acid base complex was observed and investigated by variable-temperature NMR spectroscopy. The free energies of activation of the exchange processes were determined by the coalescence method and found to be lower for the Lewis-base component nitrogen than for phosphorus. To further understand the exchange process of the Lewis acids at the central Lewis base, transition states for two different exchange mechanisms were considered and searched for.

We design theoretically small molecule candidates for singlet fission chromophores, aiming to achieve a balance between sufficient diradical character and kinetic persistence. We develop a perturbation strategy based on the captodative effect to introduce diradical character into small π-systems. Specifically, this can be accomplished by replacing pairs of not necessarily adjacent C atoms with isoelectronic and isosteric pairs of B and N atoms. Three rules of thumb emerge from our studies to aid further design: (i) Lewis structures provide insight into likely diradical character; (ii) formal radical centers of the diradical must be well-separated; (iii) stabilization of radical centers by a donor (N) and an acceptor (B) is essential. Following the rules, we propose candidate molecules. Employing reliable multireference calculations for excited states, we identify three likely candidate molecules for SF chromophores. These include a benzene, a napthalene, and an azulene, where four C atoms are replaced by a pair of B and a pair of N atoms.

Quantum calculations at the DFT-D3/def2-TZVPD level of theory have been used to examine complexes between O2YBr (Y═N, P, and As) molecules and several Lewis bases, that is, NH3, H2O, and HF. The interactions of the lone pair of the ammonia, water, and hydrogen fluoride with the σ-hole and π-hole of O2YBr molecules have been considered. In general, the complexes where the Lewis base lone pair interacts with the π-hole are more favorable than those with σ-hole. The nature of the interactions has been characterized with the Bader theory of atoms in molecules (AIM). We have also studied the ability of trifluoronitromethane and nitromethane to interact with anions using their π-hole along with an analysis the Cambridge Structural Database. We have found a large number of hits that provide strong experimental support for ability of the nitryl (-NO2) group to interact with anions and Lewis bases. In some X-ray structures, the π-hole interaction is crucial in the crystal packing and has a strong influence in the solid state architecture of the complexes. Finally, due to the relevance in atmospheric chemistry, we have studied noncovalent σ/π-hole complexes of nitryl bromide with ozone.

This work aims to contribute to the discussion about the suitability of bond paths and bond-critical points as indicators of chemical bonding defined within the theoretical framework of the quantum theory of atoms in molecules. For this purpose, we consider the temporal evolution of the molecular structure of [Fe{C(CH2 )3 }(CO)3 ] throughout Born-Oppenheimer molecular dynamics (BOMD), which illustrates the changing behaviour of the molecular graph (MG) of an electronic system. Several MGs with significant lifespans are observed across the BOMD simulations. The bond paths between the trimethylenemethane and the metallic core are uninterruptedly formed and broken. This situation is reminiscent of a "hopping" ligand over the iron atom. The molecular graph wherein the bonding between trimethylenemethane and the iron atom takes place only by means of the tertiary carbon atom has the longest lifespan of all the considered structures, which is consistent with the MG found by X-ray diffraction experiments and quantum chemical calculations. In contrast, the η(4) complex predicted by molecular-orbital theory has an extremely brief lifetime. The lifespan of different molecular structures is related to bond descriptors on the basis of the topology of the electron density such as the ellipticities at the FeCH2 bond-critical points and electron delocalisation indices. This work also proposes the concept of a dynamic molecular graph composed of the different structures found throughout the BOMD trajectories in analogy to a resonance hybrid of Lewis structures. It is our hope that the notion of dynamic molecular graphs will prove useful in the discussion of electronic systems, in particular for those in which analysis on the basis of static structures leads to controversial conclusions.

In organic chemistry, as defined by Abegg, Kossel, Lewis and Langmuir, compounds are normally represented using structural formulas called Lewis structures. In these structures, the octet rule is used to define the number of covalent bonds that each atom forms with its neighbors and multiple bonds are frequent. Lewis’ octet rule has unfortunately shown limitations very early when applied to non-organic compounds: most of them remain incompatible with the “rule of eight” and location of charges is uncertain. In an attempt to unify structural formulas of octet and non-octet molecules or single-charge ions, an even-odd rule was recently proposed, together with a procedure to locate charge precisely. This even-odd rule has introduced a charge-dependent effective-valence number calculated for each atom. With this number and the number of covalent bonds of each element, two even numbers are calculated. These numbers are both used to understand and draw structuralformulas of single-covalent-bonded compounds. In the present paper, a procedure is proposed to adjust structural formulas of compounds that are commonly represented with multiple bonds. In order to keep them compatible with the even-odd rule, they will be represented using only single covalent bonds. The procedure will then describe the consequences of bond simplification on charges locations. The newly obtained representations are compared to their conventional structural formulas, i.e. single-bond representation vs. multiple-bond structures. Throughout the comparison process, charges are precisely located and assigned to specific atoms. After discussion of particular cases of compounds, the paper finally concludes that a rule limiting representations of multiplecovalent bonds to single covalent bonds, seems to be suitable for numerous known compounds.

The concept of covalent bonding is characterized by an interconnected knowledge framework based on Lewis and quantum models of atoms and molecules. Several research studies have shown that students at all levels of chemistry learning find the quantum model to be one of the most difficult subjects to understand. We have tried in this paper to analyze the extent to which Algerian students, at the end of their training, have integrated the covalent bonding theories based on the quantum model of atom theory and are able to interpret Lewis structures using the quantum model. The analysis of the responses to a written questionnaire showed that this integration was not achieved by our students and that they are not able to correctly describe covalent bonds in a Lewis structure using the concepts of the quantum model. They have a “quantum box” conception of atomic or hybrid orbitals. This conception acts as a “pedagogical learning impediment” to the integration of the geometrical representations of atomic and hybrid orbitals, the conditions of their overlapping to give bonds and consequently the description of covalent bonds using the quantum model. So, the students use an alternative conceptual framework based on the use of Lewis model paired valence electrons to form covalent bonds that we have named the: “electrons pair framework”. Furthermore, the students restricted the denomination of a covalent bond to the sharing of one electron (either s or p but not spn) from each atom to give one “electron pair σ”, and thought that σ bonds are only formed in single bonds.

For the Fe-O2(S = 0) linkages of oxyhemes, valence bond (VB) structures are re-presented for the McClure [Fe(II)(S = 1) + O2(S = 1)], Pauling-Coryell [Fe(II)(S = 0) + O2*(S = 0)], and Weiss [Fe(III)(S = ½) + O2 (-)(S = ½)] models of bonding. The VB structures for the McClure and Weiss models are of the increased-valence type, with more electrons participating in bonding than occur in their component Lewis structures. The Fe-O bond number and O-O bond order for the McClure structure are correlated with measured Fe-O and O-O bond lengths for oxymyoglobin. Back-bonding from O2(-) to Fe(III) of the Weiss structure gives a restricted form of the McClure structure. The McClure and Weiss increased-valence structures are used to provide VB formulations of mechanisms for the oxyhemoglobin + NO reaction. The products of these two formulations are Hb(+) and NO3(-) (where Hb is hemoglobin) and Hb(+) and OONO(-), respectively. Because Hb(+) and NO3(-) are the observed products, they provide an experimental procedure for distinguishing the McClure and Weiss models. It is also shown that the same type of agreement between McClure-type theory and experiment occurs for oxycoboglobin + NO, cytochrome P450 monooxygenases, and related hydrogen atom transfer reactions. In the appendices, the results of density functional theory and multireference molecular orbital calculations for oxyhemes are related to one formulation of the increased-valence wavefunction for the McClure model, and theory is presented for the calculation of approximate weights for the Lewis structures that are components of the McClure increased-valence structure.

Numerous studies have underlined the putative diradical character of π-conjugated molecules that can be described by closed-shell Lewis structures, for instance, p-dimethylene p-n phenylenes, or long polyacenes. In the latter compounds, the only way to save the aromaticity of the six-membered rings is to give up the Lewis electron pairing in the singlet biradical ground state. The present work considers the possibility of doing the same by using the basic C2 units of carbo-meric architectures. A series of acyclic and cyclic carbo-meric architectures is studied by using UB3LYP DFT broken-symmetry calculations, including spin decontaminations and subsequent geometry optimization of the singlet diradical. The C2 units are shown to stabilize the singlet biradical by spin delocalization, two of them playing approximately the same role as one radical-insulating 1,4 phenylene moiety. The results are generalized to the investigation of open-shell polyradical singlet states of rigid hydrocarbon structures, the symmetry and rigidity of which can assist cooperativity and self spin polarization effect. Several synthesis targets with challenging magnetic/spin properties are suggested in the carbo-mer series.

The intermolecular association of twelve combinations of six different Lewis acids and Lewis bases (i.e., R 3 A–BR′3 where A = B and Al; B = N and P; R = H, F, and C6F5; R′ = H, CH3, and C(CH3)3) was theoretically described by means of DFT calculations using the dispersion-corrected ωB97x-D and B97D functionals in conjunction with the 6-311++G(2d,2p) basis set including toluene as solvent through the PCM-SMD implicit solvent scheme. All the studied Lewis pairs appeared to be stable on the basis of computed BSSE-corrected interaction energies; however, the free energies of formation computed in solution (ΔG solv) indicate that three Lewis acid–base combinations can be considered frustrated Lewis pairs (FLPs). Besides, the four features that characterize FLPs are: (1) large distances between the acid and base centers, (2) negligible changes in the geometry of the acid, (3) weak interaction energies, and (4) non-covalent dispersion energy contributing to almost the entire interaction energy. In the present work, we introduce two ad hoc indexes intended to quantify separately the electronic and steric factors, which have a direct effect in the intermolecular association of Lewis acids and Lewis bases and can be used to distinguished FLPs from classical Lewis adducts. Based on the aforementioned ad hoc indexes, the existence of a new kind of complexes that are “intermediate” between classical complexes and FLPs is proposed.

Oxidation state (OS) is defined using ionic approximation of bonds. Two principal algorithms are outlined for OS determination in a chemical compound described by a Lewis formula or bond graph. Typical origins of ambiguous OS values are pointed out, and the relationship between OS and the d n electron configuration of transition metals is commented on.

The electron-pair bonding model that was introduced by Gilbert Lewis 100 years ago is discussed in the light of modern quantum chemical methods for analysing the electronic structures of some simple molecules. It is argued that Lewis structures in conjunction with accurate quantum chemical calculations are still very useful for the description of chemical bonding. The emphasis lies on the difference between electron-sharing bonds A–B and dative bonds A → B which were suggested by Lewis as a general definition for acids and bases. The electron-pair model, if combined with quantum chemical calculations, remains a powerful guide for the search of new molecules and for understanding molecular structures.

Recently, it has been shown that the superatom concept is intimately connected to relevant tools of great chemical significance, such as the Lewis structure model and the VSEPR theory, which has been employed to understand hybridized and dimeric-like molecules. This suggests a potential rational construction of superatomic clusters mimicking more complex structures. Here, we extend another well-employed concept to the superatomic clusters, to construct a novel Au42 isomer with resemblance to cyclic aromatic molecules. It is shown that the Huckel (4n+2)π rule is ready to be applied, predicting aromatic behavior latterly supported by the favorable evaluation of the induced shielding cone formation. The D6 h isomer of Au42 described here exhibits inherent characteristics mimicking aromatic hydrocarbon rings, displaying π-superatomic orbitals and related properties. This new cluster is the first member of the superatomic clusters family to exhibit an aromatic π-electron system.

A density functional theory computational chemistry exercise on the structure and vibrational spectrum of the carbene hydroxymethylene is presented. The potential energy curve for the decomposition reaction of the carbene to formaldehyde and the geometry of the transition state are explored. The results are in good agreement with recent experimental studies on the synthesis and trapping of hydroxymethylene in low-temperature, rare-gas matrices. The calculations are easy to perform and increase student understanding of a variety of important concepts in the undergraduate chemistry curriculum such as the octet rule, formal charge, VSEPR theory, bonding, vibrational spectroscopy, tunneling, transition states, and potential energy curves. The exercise nicely illustrates the power and versatility of computational chemistry methods.

Natural bond orbital (NBO) methods encompass a suite of algorithms that enable fundamental bonding concepts to be extracted from Hartree-Fock (HF), Density Functional Theory (DFT), and post-HF computations. NBO terminology and general mathematical formulations for atoms and polyatomic species are presented. NBO analyses of selected molecules that span the periodic table illustrate the deciphering of the molecular wavefunction in terms commonly understood by chemists: Lewis structures, charge, bond order, bond type, hybridization, resonance, donor–acceptor interactions, etc. Upcoming features in the NBO program address ongoing advances in ab initio computing technology and burgeoning demands of its user community by introducing major new methods, keywords, and electronic structure system/NBO communication enhancements. © 2011 John Wiley & Sons, Ltd.

Main factors responsible for the red to near infrared (NIR) absorption of the benzobis(1,2,5-thiadiazole) (BBT) derivatives have been investigated using high level computational studies. These molecules with NIR emission are of importance due to the recent reports of possible role in NIR organic light emitting diodes (OLED) development. The electronic structure has been determined using closed-shell/open-shell DFT methods (B3LYP and BHandHLYP). The wavefunction stabilities of these BBT derivatives have been tested. We notice that using the BHandHLYP functional, the wave function becomes instable though large HOMO–LUMO gaps (HLG) are obtained. On the other hand a stable wavefunction is obtained using the B3LYP functional but the HLG is small. The B3LYP HLG is in good agreement with the available experimental data. Nevertheless results from both functionals indicate a possible LUMO occupation/biradicaloid character (BRC). We calculated the BRC for all the molecules using different methods and observed that these molecules have a large BRC which correlates well with excitation energies. Larger the BRC smaller the excitation energy. Charge transfer based on Mulliken charges of both ground and excited state are obtained from high level SAC/SAC-CI studies. We carried out the VB studies of the unsubstituted BBT to predict the relative weights of the individual Lewis structures to the resonance picture. It is concluded that a more general description which include the zwitterionic and biradical structures are necessary for these molecules and not just simple donor–acceptor–donor (D–A–D) structures as suggested in the literature. Copyright © 2011 John Wiley & Sons, Ltd.

Theoretical study of several benzyl nitrites have been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6-31G∗∗ basis set. Geometries obtained from DFT calculations were used to perform NBO analysis. It is noted that weakness in the O 3 –N 2 sigma bond is due to n O 1 → σ O 3 – N 2 ∗ delocalisation and is responsible for the longer O 3 –N 2 bond lengths in the selected benzyl nitrites. It is also noted that decreased occupancy of the localized σ O 3 – N 2 orbital in the idealized Lewis structure, or increased occupancy of σ O 3 – N 2 ∗ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital (NHO) of σ O 3 – N 2 bond orbital. In addition, the charge transfer energy decreases with the increasing of the Hammett constants of substituent groups.

I demonstrate that a modification of the resonance color theory (in its form advocated by Brooker (Rev. Mod. Phys. 1942, 14, 275) and by Platt (J. Chem. Phys. 1956, 25, 80)) provides an accurate framework for rationalizing the ab initio excitation energies of the protonation states of the green fluorescent protein (GFP) chromophore (an asymmetric oxonol dye). I suggest that the original model space used in the resonance theory (specifically, a pair of Lewis structures) is formally inconsistent with a core aspect of the theory (specifically, a relationship between excitation energies and group-specific basicities (Brooker basicities) of the terminal rings). I argue that a more appropriate model space would consist of a complete active space ansatz based on group-localized orbitals. I then show that there is a solution to the state-averaged complete active space self consistent field (SA-CASSCF) problem with exactly this form. This family of SA-CASSCF solutions provides an objectively rigorous foundation fo...

Electronic structure of N-heterocyclic carbenes with five-member rings and two nitrogen atoms was studied using the natural resonance theory (NRT) in the framework of density functional theory. Also the role of the hydroxyl substituents on the carbon atoms was investigated. The analysis provides a consistent picture of chemical bonding, even in cases where no single Lewis structure can be drawn. It interprets the observed stabilities and aromaticities of the different isomers. A carbene lone pair on C 1 atom was identified in all cases, while the electron donation to the formally empty p z orbital of this atom was found to be a crucial effect in all cases.