Chemical Aristocracy: He3 Dication and Analogous Noble-Gas-Exclusive Covalent Compounds

Herein, we predict the first set of covalently bonded triatomic molecular compounds composed exclusively of noble gases. Using a combination of double-hybrid DFT, CCSD(T), and MRCI+Q calculations and a range of bonding analyses, we explored a set of 270 doubly charged triatomics, which included various combinations of noble gases and main group elements. This extensive exploration uncovered nine noble-gas-exclusive covalent compounds incorporating helium, neon, argon, or combinations thereof, exemplified by cases such as He32+ and related systems. This work brings to light a previously uncharted domain of noble gas chemistry, demonstrating the potential of noble gases in forming covalent molecular clusters.

The Nature of the Polar Covalent Bond

Quantum chemical calculations using density functional theory are reported for the diatomic molecules LiF, BeO, and BN. The nature of the interatomic interactions is analyzed with the EDA-NOCV method, and the results are critically discussed and compared with data from QTAIM, NBO and Mayer approaches. Polar bonds, like nonpolar bonds, are caused by the interference of wave functions, which lead to an accumulation of electronic charge in the bonding region. Polar bonds generally have a larger percentage of electrostatic bonding to the total attraction, but nonpolar bonds may also possess large contributions from Coulombic interaction. The term "ionic contribution" refers to VB structures and is misleading because it refers to separate fragments with negligible overlap that occur only in the solid state and in solution, not in a molecule. The EDA-NOCV method gives detailed information about the individual orbital contributions, which can nicely be identified by visual inspection of the associated deformation densities. It is very important, particularly for polar bonds to distinguish between the interatomic interactions of the final dissociation products after bond rupture and the interactions between the fragments in the eventually formed bond.

Three-center two-electron bonds from the quantum interference perspective

The nature of the three-center two-electron (3c2e) chemical bond is investigated by the Interference Energy Analysis (IEA) method and using of a SC(2, 3) (spin coupled wave function with two...

The Valence-Bond (VB) Model and Its Intimate Relationship to the Symmetric or Permutation Group

VB and molecular orbital (MO) models are normally distinguished by the fact the first looks at molecules as a collection of atoms held together by chemical bonds while the latter adopts the view that each molecule should be regarded as an independent entity built up of electrons and nuclei and characterized by its molecular structure. Nevertheless, there is a much more fundamental difference between these two models which is only revealed when the symmetries of the many-electron Hamiltonian are fully taken into account: while the VB and MO wave functions exhibit the point-group symmetry, whenever present in the many-electron Hamiltonian, only VB wave functions exhibit the permutation symmetry, which is always present in the many-electron Hamiltonian. Practically all the conflicts among the practitioners of the two models can be traced down to the lack of permutation symmetry in the MO wave functions. Moreover, when examined from the permutation group perspective, it becomes clear that the concepts introduced by Pauling to deal with molecules can be equally applied to the study of the atomic structure. In other words, as strange as it may sound, VB can be extended to the study of atoms and, therefore, is a much more general model than MO.

Substituent Effects on the Quantum Interference of Two-Center One-Electron Bonds: [B2X6]- (X = H, F, Cl, CN, OH, CH3, and OCH3)

The interference energy analysis (IEA) provided by the generalized product function energy partitioning (GPF-EP) method was applied to investigate the influence of the neighboring atoms on the nature of the two-center one-electron (2c1e) bonds in the anion dimers of BX₃ species (X = H, F, Cl, CN, OH, CH₃, and OCH₃). The species were studied at the GVB-PP(6/12).SC(1,2)/6-31**G++ level of calculation. The IEA has revealed that there is a balance between two main factors determining the chemical stability of the species. Quantum interference acts as the sole stabilizing effect in the formation of the chemical bonds, particularly as the result of the drop in kinetic energy, and the electronegativity of the substituent has a direct influence on the magnitude of this effect. The quasi-classical energy is responsible for the destabilizing factors, mainly the group bulkiness, and the "electron-withdrawing" effect in the case of the cyano group.

Overlap properties of chemical bonds in generic systems including unusual bonding situations

Chemical bond is a ubiquitous and fundamental concept in chemistry, in which the overlap plays a defining role. By using a new approach based on localized molecular orbitals, the overlap properties, e.g., polarizability [Formula: see text], population p_OP, intra [Formula: see text], and inter [Formula: see text] repulsions, and density ρ_OP, of polyatomic systems were calculated, analyzed, and correlated. Several trends are shown for these properties, which are rationalized by the balance of some well-known effects, such as, electron donor/withdrawing character and electronegativity. The overlap properties of unusual bonds are also analyzed, revealing an OZn₄(OOCH)₆ structure with four equivalent Zn-O chemical bonds with overlap properties like the O-O bond in H₂O₂, while in protonated methane [Formula: see text], it is observed that a CH₃⋯[Formula: see text] bond pattern at the equilibrium structure changes to a [Formula: see text]⋯H₂ pattern upon dissociation. Charge-shift resonance energies, atom-in-molecule properties, and the lone-pair-bond-weakening effects are related to the overlap properties, which can provide alternative views and insights into chemical bonds. Graphical abstract A chemical bond analysis approach based on its overlap properties is presented for the first time. The model was applied directly to 25 diatomics and for 28 bonds in polytomics employing localized molecular orbitals. Correlations of the overlap properties with the charge-shift resonance energies and with atom-in-molecule (AIM) properties were uncovered. In addition, it provided insights into the Zn-O bonds in the unusual OZn₄(OOCH)₆ system as well as in the bonding patterns of [Formula: see text] at equilibrium and upon dissociation.

Unexpected reversal of stability in strained systems containing one-electron bonds

Ring strain energy is a very well documented feature of neutral cycloalkanes, and influences their structural, thermochemical and reactivity properties. In this work, we apply density functional theory and high-level coupled cluster calculations to describe the geometry and relative stability of C₆H₁₂⁺˙ radical cations, whose cyclic isomers are prototypes of singly-charged cycloalkanes. Molecular ions with the mentioned stoichiometry were produced via electron impact experiments using a gaseous cyclohexane sample (20-2000 eV). From our calculations, in addition to structures that resemble linear and branched alkenes as well as distinct conformers of cyclohexane, we have found low-lying species containing three-, four- and five-membered rings with the presence of an elongated C-C bond. Remarkably, the stability trend of these ring-bearing radical cations is anomalous, and the three-membered species are up to 11.3 kcal mol^-1 more stable than the six-membered chair structure. Generalized Valence Bond calculations and the Spin Coupled theory with N electrons and M orbitals were used in conjunction with the Generalized Product Function Energy Partitioning (GPF-EP) method and Interference Energy Analysis (IEA) to describe the chemical bonding in such moieties. Our results confirm that these elongated C-C motifs are one-electron sigma bonds. Our calculations also reveal the effects that drive thermochemical preference of strained systems over their strained-free isomers, and the origin of the unusual stability trend observed for cycloalkane radical cations.

One-electron bonds are not "half-bonds"

Despite the success of the molecular orbital (MO) and valence-bond (VB) models to describe the electronic structure and properties of molecules, neither MO nor VB provides an explanation for the nature of the chemical bond. The first to address this problem was Ruedenberg, who showed that chemical bonds result from quantum interference. He developed a method to calculate the interference contribution to the total electronic energy and density and applied it to molecules containing typical two-centre two-electron (2c-2e) covalent bonds. To test the generality of Ruedenberg's hypothesis, we developed a powerful Interference Energy Analysis (IEA) method to calculate the interference contributions of individual chemical bonds to the total energy of diatomic and polyatomic molecules, and showed that any two-electron bond, despite its polarity, results from quantum interference. Nevertheless, many stable molecules are experimentally known whose chemical structures clearly indicate the existence of two-centre one-electron bonds (2c-1e). Therefore, the question remains if quantum interference will be the dominant effect for these systems. This work describes the extension of the IEA for treating two-centre one-electron bonds, making use of a Generalised Product Function (GPF) built from spin coupled wave functions of N electrons in M orbitals, SC(N,M). Several diatomic and polyatomic molecules were analysed and whenever possible the results were compared with the analogous case of a two-electron bond. The results indicate that interference is the dominant effect for the one-electron bonds, which reinforces the role of quantum interference as the central element in chemical bonding theory.

On the metastability of doubly charged homonuclear diatomics

Generalized valence bond (GVB) and spin-coupled (SC) calculations were used in conjunction with the generalized product function energy partitioning (GPF-EP) method to describe the origin of metastability in doubly charged homonuclear dications. A model to describe the formation of metastable potential wells based on interference and quasi-classical effects is presented. The GPF-EP picture of dications is the result of polarization-aided strong covalent bonding surpassing Coulomb electrostatic repulsion. Important differences in the quasi-classical density profiles of He₂²⁺ and Ne₂²⁺ reveal the underlying mechanism that could lead to bound or unbound states. Finally, the nature of the chemical bond of N₂²⁺, O₂²⁺, and F₂²⁺ is described. The results suggest that the ground states of the mentioned dications are bounded and that the depth of the potential wells of these exotic species is related to the interference effect, in the same way as in previously studied neutral molecules.

Quantum Interference Contribution to the Dipole Moment of Diatomic Molecules

The interference energy partitioning analysis method developed by our group and used to study the nature of the chemical bond was extended to partition the electric dipole moment in quasi-classical and interference contributions. Our results show that interference participates in charge displacement in polar molecules, providing, directly or indirectly, a relevant contribution for the total dipole moment. A linear correlation was found between the interference contribution of the dipole moment from the bond electron group, μ_INT(bond), and the difference of electronegativity of the atoms which form the bond, ΔX_AB. This interesting result reinforces the fact that electronegativity is not a property of an atom alone, but rather a property of the atom in the molecule and that ΔX_AB can only be associated with that part of the total charge displacement resulting from the formation of the chemical bond. The partitioning of the total dipole moment into quasi-classical and interference contributions provides new insights about the reasons for the failure of the ΔX_AB criterion in predicting the correct orientation of the dipole moment in several molecules. The results of the present work also bring additional evidence for the previously proposed mechanism of formation of polar bonds.

Are One-Electron Bonds Any Different from Standard Two-Electron Covalent Bonds?

The nature of the chemical bond is perhaps the central subject in theoretical chemistry. Our understanding of the behavior of molecules developed amazingly in the last century, mostly with the rise of quantum mechanics (QM) and QM-based theories such as valence bond theory and molecular orbital theory. Such theories are very successful in describing molecular properties, but they are not able to explain the origin of the chemical bond. This problem was first analyzed in the 1960s by Ruedenberg, who showed that covalent bonds are the direct result of quantum interference between one-electron states. The generality of this result and its quantification were made possible through the recent development of the generalized product function energy partitioning (GPF-EP) method by our group, which allows the partitioning of the electronic density and energy into their interference and quasi-classical (noninterference) contributions. Furthermore, with GPF wave functions these effects can be analyzed separately for each bond of a molecule. This interference energy analysis has been applied to a large variety of molecules, including diatomics and polyatomics, molecules with single, double, and triple bonds, molecules with different degrees of polarity, linear or branched molecules, cyclic or acyclic molecules, conjugated molecules, and aromatics, in order to verify the role played by quantum interference. In all cases the conclusion is exactly the same: for each bond in each of the molecules considered, the main contribution to its stability comes from the interference term. Two-center one-electron (2c1e) bonds are the simplest kind of chemical bonds. Yet they are often viewed as odd or unconventional cases of bonding. Are they any different from conventional (2c2e) bonds? If so, what differences can we expect in the nature of (2c1e) bonds relative to electron-pair bonds? In this Account, we extend the GPF-EP method to describe bonds involving N electrons in M orbitals (N < M) and show its application to (2c1e) bonds. As examples we chose the molecules H₂⁺, H₃C·CH₃⁺, B₂H₄^-, [Cu·BH₃(PH₃)₃], and an alkali-metal cation dimer, and we evaluated the components of the electronic energy and density, which account for the formation of the bond, and compared the results with those for the respective analogous molecules exhibiting the "conventional" two-electron bond. In all cases, it was verified that interference is the dominant effect for the one-electron bonds. The GPF-EP results clearly indicate that molecules exhibiting (2c1e) bonds should not be considered as special systems, since one- and two-electron bonds result from quantum interference and therefore there is no conceptual difference between them. Moreover, these results show that quantum interference provides a way to unify the chemical bond concept.