Towards a complete description of the reaction mechanisms between nitrenium ions and water.

Context: Nitrenium ions are intermediates in the metabolic routes producing the highly carcinogenic nitrosamines and binding to DNA molecules. The reaction mechanism of nitrenium molecules with explicit water molecules is sensibly dependent on the number of waters: when a second molecule is involved, it acts as a catalyst for the reaction, lowering intrinsic activation barriers regardless of the substituent. For all cases, the reaction force constants and reaction electron flux indicate highly synchronous reactions for .

Natural resonance-theoretic conceptions of extreme electronic delocalization in soft materials.

In the broad context of Dalton's atomic hypothesis and subsequent classical quantum understanding of macroscopic materials, we show how Pauling's resonance-type conceptions, as quantified in natural resonance theory (NRT) analysis of modern wavefunctions, can be modified to unify description of interatomic interactions from the Lewis-like limit of localized e-pair covalency in molecules to the extreme delocalized limit of supramolecular "soft matter" aggregation. Such "NRT-centric" integration of NRT bond orders for hard- and soft-matter interactions is illustrated with application to a long-predicted and recently synthesized organometallic sandwich-type complex ("diberyllocene") that exhibits bond orders ranging from the soft limit ( ≈ 0.01) to the typical values ( ≈ 1.

"Noncovalent Interaction": A Chemical Misnomer That Inhibits Proper Understanding of Hydrogen Bonding, Rotation Barriers, and Other Topics.

We discuss the problematic terminology of "noncovalent interactions" as commonly applied to hydrogen bonds, rotation barriers, steric repulsions, and other stereoelectronic phenomena. Although categorization as "noncovalent" seems to justify classical-type pedagogical rationalizations, we show that these phenomena are irreducible corollaries of the orbital-level conceptions of electronic covalency and resonance that govern chemical bonding phenomena. Retention of such nomenclature is pedagogically misleading in supporting superficial dipole-dipole and related "simple, neat, and wrong" conceptions as well as perpetuating inappropriate bifurcation of the introductory chemistry curriculum into distinct "covalent" vs.

High-Density "Windowpane" Coordination Patterns of Water Clusters and Their NBO/NRT Characterization.

Cluster mixture models for liquid water at higher pressures suggest the need for water clusters of higher coordination and density than those commonly based on tetrahedral H-bonding motifs. We show here how proton-ordered water clusters of increased coordination and density can assemble from a starting cyclic tetramer or twisted bicyclic (Möbius-like) heptamer to form extended sequences of stable two-, three-, and four-coordinate "windowpane" motifs. Such windowpane clusters exhibit sharply reduced (~90°) bond angles that differ appreciably from the tetrahedral angles of idealized crystalline ice I.

Can a Wanzlick-like equilibrium exist between dicoordinate borylenes and diborenes?

Boron chemistry has experienced tremendous progress in the last few decades, resulting in the isolation of a variety of compounds with remarkable electronic structures and properties. Some examples are the singly Lewis-base-stabilised borylenes, wherein boron has a formal oxidation state of +I, and their dimers featuring a boron-boron double bond, namely diborenes. However, no evidence of a Wanzlick-type equilibrium between borylenes and diborenes, which would open a valuable route to the latter compounds, has been found.

Chlorine dioxide: An exception that proves the rules of localized chemical bonding.

We employ natural bond orbital and natural resonance theory tools to analyze the enigmatic properties of the C-symmetric isomer of chlorine dioxide radical (ClO), whose many challenges to Pauling-type localized bonding concepts were recognized by Linus Pauling himself. Although spin-contamination is minimal in this species, ClO exhibits an unusually strong form of "different Lewis structures for different spins" bonding pattern, intrinsically outside the framework of "maximal pairing" concepts. We show how the novel spin-unpaired donor-acceptor interactions lead to weakened bonding in the supramolecular domain of polyradical (ClO) homoclusters and aqueous ClO(HO) heteroclusters.

Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics.

Intermolecular bonding attraction at π-bonded centers is often described as "electrostatically driven" and given quasi-classical rationalization in terms of a "pi hole" depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO, CN) with simple atomic anions (H, F) or with one another.

Pauling's Conceptions of Hybridization and Resonance in Modern Quantum Chemistry.

We employ the tools of natural bond orbital (NBO) and natural resonance theory (NRT) analysis to demonstrate the robustness, consistency, and accuracy with which Linus Pauling's qualitative conceptions of directional hybridization and resonance delocalization are manifested in all known variants of modern computational quantum chemistry methodology.

Comment on "Superposition of Waves or Densities: Which Is the Nature of Chemical Resonance?" [J. Comput. Chem. 2021, 42, 412-417].

We reply to specific criticisms and misrepresentations of natural resonance theory (NRT) in a recent article [Y. Wang, J. Comput.

Sulfur Tetrahydride and Allied Superhydride Clusters: When Resonance Takes Precedence.

Sulfur offers a variety of bonding surprises compared to the parent oxygen atom of the chalcogen family. In the present work, we employ standard quantum chemistry methods to characterize formation of previously unrecognized sulfur tetrahydride (C -symmetric SH ) from hydrogen sulfide (H S) and molecular hydrogen (H ) on the ground state potential energy surface. The unusual intramolecular interactions of SH defy Lewis-like bonding conceptions, exhibiting the dominance of resonance-type donor-acceptor delocalizations well beyond those of SF (C sawhorse geometry) and other known tetrahalides.

NBO/NRT Two-State Theory of Bond-Shift Spectral Excitation.

We show that natural bond orbital (NBO) and natural resonance theory (NRT) analysis methods provide both optimized Lewis-structural bonding descriptors for ground-state electronic properties as well as suitable building blocks for idealized "diabatic" two-state models of the associated spectroscopic excitations. Specifically, in the framework of single-determinant Hartree-Fock or density functional methods for a resonance-stabilized molecule or supramolecular complex, we employ NBO/NRT descriptors of the ground-state determinant to develop a qualitative picture of the associated charge-transfer excitation that dominates the valence region of the electronic spectrum. We illustrate the procedure for the elementary bond shifts of S2-type halide exchange reaction as well as the more complex bond shifts in a series of conjugated cyanine dyes.

Substituted -Benzynes: Properties of the Triple Bond.

-benzyne has been well studied by both experiment and theory. Its substituted variants, however, have been less carefully examined. Benchmark data are computed for unsubstituted -benzyne using several density functional theory functionals and basis sets, up to cc-pVQZ.

Comment on "Observation of alkaline earth complexes M(CO) (M = Ca, Sr, or Ba) that mimic transition metals".

Wu (Reports, 31 August 2018, p. 912) claim that recently characterized octacarbonyls of Ca, Sr, and Ba mimic the classical Dewar-Chatt-Duncanson bonding motif of transition metals. This claim, which contradicts known chemistry and computed electron density distributions, originates in the assumption of a flawed reference state for energy decomposition analyses.

Nine questions on energy decomposition analysis.

The paper collects the answers of the authors to the following questions: Is the lack of precision in the definition of many chemical concepts one of the reasons for the coexistence of many partition schemes? Does the adoption of a given partition scheme imply a set of more precise definitions of the underlying chemical concepts? How can one use the results of a partition scheme to improve the clarity of definitions of concepts? Are partition schemes subject to scientific Darwinism? If so, what is the influence of a community's sociological pressure in the "natural selection" process? To what extent does/can/should investigated systems influence the choice of a particular partition scheme? Do we need more focused chemical validation of Energy Decomposition Analysis (EDA) methodology and descriptors/terms in general? Is there any interest in developing common benchmarks and test sets for cross-validation of methods? Is it possible to contemplate a unified partition scheme (let us call it the "standard model" of partitioning), that is proper for all applications in chemistry, in the foreseeable future or even in principle? In the end, science is about experiments and the real world. Can one, therefore, use any experiment or experimental data be used to favor one partition scheme over another? © 2019 Wiley Periodicals, Inc.

NBO 7.0: New vistas in localized and delocalized chemical bonding theory.

We briefly outline some leading features of the newest version, NBO 7.0, of the natural bond orbital (NBO) wavefunction analysis program. Major extensions include: (1) a new NPEPA module implementing Karafiloglou's "polyelectron population analysis" in the NBO framework; (2) new RDM2 program infrastructure for describing electron correlation effects based on full evaluation of the second-order reduced density matrix; (3) improved convex-solver implementation of natural resonance theory (NRT), allowing a greatly expanded range of applications and associated "resonance NBO" (RNBO) visualization of chemical reactivity; (4) a variety of other improvements in well-established NBO algorithms.

What Is the Nature of Supramolecular Bonding? Comprehensive NBO/NRT Picture of Halogen and Pnicogen Bonding in RPH···IF/FI Complexes (R = CH, OH, CF, CN, NO).

We employ a variety of natural bond orbital (NBO) and natural resonance theory (NRT) tools to comprehensively investigate the nature of halogen and pnicogen bonding interactions in RPH···IF/FI binary complexes (R = CH, OH, CF, CN, and NO) and the tuning effects of R-substituents. Though such interactions are commonly attributed to "sigma-hole"-type electrostatic effects, we show that they exhibit profound similarities and analogies to the resonance-type 3-center, 4-electron (3c/4e) donor-acceptor interactions of hydrogen bonding, where classical-type "electrostatics" are known to play only a secondary modulating role. The general 3c/4e resonance perspective corresponds to a continuous range of interatomic A···B bond orders (), spanning both the stronger "covalent" interactions of the molecular domain (say, ≥ ½) and the weaker interactions ( ˂ ½, often misleadingly termed "noncovalent") that underlie supramolecular complexation phenomena.

Efficient optimization of natural resonance theory weightings and bond orders by gram-based convex programming.

We describe the formal algorithm and numerical applications of a novel convex quadratic programming (QP) strategy for performing the variational minimization that underlies natural resonance theory (NRT). The QP algorithm vastly improves the numerical efficiency, thoroughness, and accuracy of variational NRT description, which now allows uniform treatment of all reference structures at the high level of detail previously reserved only for leading "reference" structures, with little or no user guidance. We illustrate overall QPNRT search strategy, program I/O, and numerical results for a specific application to adenine, and we summarize more extended results for a data set of 338 species from throughout the organic, bioorganic, and inorganic domain.

To Be or Not to Be: Demystifying the 2nd-Quantized Picture of Complex Electronic Configuration Patterns in Chemistry with Natural Poly-Electron Population Analysis.

We provide a didactic introduction to 2nd-quantized representation of complex electron-hole (e/h) excitation patterns in general configuration interaction wave functions built from orthonormal local orbitals of natural atomic orbital or natural bond orbital (NBO) type. Such local excitation patterns of chemically oriented basis functions can be related to the resonance concepts of valence bond theory, and quantitative evaluation of the associated excitation probabilities then provides an alternative assessment of resonance "weighting" that may be compared with those of NBO-based natural resonance theory. We illustrate the usefulness of anticommutation relations in deriving Pauli-compliant expressions for allowed excitation patterns, showing how the exciton-like promotions φ  → φ (creating an e/h excitation with h in φ and e in φ ) impose strict constraints on associated e/h-probabilities (requiring, e.

Resonance Theory Reboot.

What is now called "resonance theory" has a long and conflicted history. We first sketch the early roots of resonance theory, its heritage of diverse physics and chemistry conceptions, and its subsequent rise to reigning chemical bonding paradigm of the mid-20th century. We then outline the alternative "natural" pathway to localized Lewis- and resonance-structural conceptions that was initiated in the 1950s, given semi-empirical formulation in the 1970s, recast in ab initio form in the 1980s, and successfully generalized to multi-structural "natural resonance theory" (NRT) form in the 1990s.

Quantitative Theoretical Predictions and Qualitative Bonding Analysis of the Divinylborinium System and Its Al, Ga, In, and Tl Congeners.

A substituted divinylborinium cation was synthesized recently and characterized crystallographically as a gauche structure with a 153° C1-C2-C3-C4 dihedral angle. A full theoretical geometrical optimization of the bis(2-mesityl-1,2-diphenylvinyl)-borane cation shows excellent agreement with the crystal structure. However, for the parent unsubstituted divinylborinium cation, we predict a nearly 90° C1-C2-C3-C4 dihedral angle using the CCSD(T)/cc-pVTZ coupled cluster method.

Natural Bond Orbital Theory of Pseudo-Jahn-Teller Effects.

We describe a unified picture of symmetry-breaking electronic interactions that are usually described as "pseudo-Jahn-Teller (PJT) effects" and attributed to vibronic coupling but can also be associated with hyperconjugative donor-acceptor interactions in the framework of natural bond orbital (NBO) and natural resonance theory (NRT) analysis. We show how NBO/NRT descriptors offer a simplified alternative to the vibronic coupling picture of PJT effects that yields both improved cause-effect specificity and chemically enriched understanding of symmetry-breaking phenomena but with no necessary input from ground-state vibrational or excited-state electronic properties. Comparative NBO/NRT vs vibronic coupling analyses of PJT effects are illustrated for two well-known cases: trans-bending in SiH and higher Group-14 homologues of ethylene and chain-kinking in cyclopentadienylideneketene (CHCCO) and related cumulene ketones.

Theoretical Prediction of Robust Second-Row Oxyanion Clusters in the Metastable Domain of Antielectrostatic Hydrogen Bonding.

We provide ab initio and density functional theory evidence for a family of surprisingly robust like-charged clusters of common HSO and HPO oxyanions, ranging up to tetramers of net charge 4-. Our results support other recent theoretical and experimental evidence for "antielectrostatic" hydrogen-bonded (AEHB) species that challenge conventional electrostatic conceptions and force-field modeling of closed-shell ion interactions. We provide structural and energetic descriptors of the predicted kinetic well-depths (in the range 3-10 kcal/mol) and barrier widths (in the range 2-4 Å) for simple AEHB dimers, including evidence of extremely strong hydrogen bonding in the fluoride-bisulfate dianion.

Why Do Cumulene Ketones Kink?

We employ ab initio and density functional methods to investigate the equilibrium structure and vibrational frequencies of extended cumulene monoketones [CH═(C═)O] and diketones [O═(C═)O], in order to elucidate the electronic origin of the curious "kinked'" spine geometries that are common in such species. The dominant role of symmetry-breaking n-σ* interactions between the p-type lone pair of the terminal oxygen and adjacent unfilled CC antibonding orbital is demonstrated by NBO second-order delocalization energies, Fock matrix deletions, and natural resonance theory (NRT) descriptors, showing the general connection between cumulene kinking and CC bond-breaking reactions that split off CO. Our results provide simple rationalizations for (i) pronounced even/odd alternation patterns in the magnitude or direction of kinking, (ii) the nonexistence of O = C═C═O, (iii) the clear preference for trans-like over cis-like kinks, and (iv) the extreme sensitivity of kinking with respect to weak perturbations, such as cage or solvent effects, remote chemical substituents, improved treatments of electron correlation, and the like.

Polyion Covalency: Exotic Species from the Unexplored World of Electrostatically Shielded Molecular Ion Chemistry.

Standard quantum chemical methods have been employed to describe a variety of kinetically stable polyionic molecular species that are trapped in appreciable potential wells by chemical bonding forces, despite powerful electrostatic opposition that challenges conventional chemical detection and characterization. The studied species are covalent or dative analogs of "anti-electrostatic" hydrogen-bonded (AEHB) species, all illustrating how short-range quantum covalency can overcome the powerful "shielding" opposition of long-range electrostatic forces to form highly charged molecular species, analogous to known neutral or singly ionic counterparts. Computational predictions of representative structural, spectroscopic, and NBO-based electronic signatures of multiply charged analogs of common neutral species (CH CH , CO , FeCO) are provided to suggest the unique material properties characteristic of this shielded domain of polyionic chemical phenomena.

Predicting the Ionic Product of Water.

We present a first-principles calculation and mechanistic characterization of the ion product of liquid water (K ), based on Quantum Cluster Equilibrium (QCE) theory with a variety of ab initio and density functional methods. The QCE method is based on T-dependent Boltzmann weighting of different-sized clusters and consequently enables the observation of thermodynamically less favored and therefore low populated species such as hydronium and hydroxide ions in water. We find that common quantum chemical methods achieve semi-quantitative accuracy in predicting K and its T-dependence.