The directional bonding of [1.1.1]propellane with next generation QTAIM

The Analysis of Electron Densities: From Basics to Emergent Applications

The electron density determines all properties of a system of nuclei and electrons. It is both computable and observable. Its topology allows gaining insight into the mechanisms of bonding and other phenomena in a way that is complementary to and beyond that available from the molecular orbital picture and the formal oxidation state (FOS) formalism. The ability to derive mechanistic insight from electron density is also important with methods where orbitals are not available, such as orbital-free density functional theory (OF-DFT). While density topology-based analyses such as QTAIM (quantum theory of atoms-in-molecules) have been widely used, novel, vector-based techniques recently emerged such as next-generation (NG) QTAIM. Density-dependent quantities are also actively used in machine learning (ML)-based methods, in particular, for ML DFT functional development, including machine-learnt kinetic energy functionals. We review QTAIM and its recent extensions such as NG-QTAIM and localization-delocalization matrices (LDM) and their uses in the analysis of bonding, conformations, mechanisms of redox reactions excitations, as well as ultrafast phenomena. We review recent research showing that direct density analysis can circumvent certain pitfalls of the FOS formalism, in particular in the description of anionic redox, and of the widely used (spherically) projected density of states analysis. We discuss uses of density-based quantities for the construction of DFT functionals and prospects of applications of analyses of density topology to get mechanistic insight with OF-DFT and recently developed time-dependent OF-DFT.

When, Where and Why Boron Prefers Boron to Nitrogen

Boron is the archetypal Lewis acid, and therefore it is only natural that it prefers to bind nitrogen, its usual Lewis base counterpart. To challenge this assumption, we present a computationally designed bicyclopentane molecule akin to [1.1.1]propellane, but with pyramidal B and N inner atoms bonded by an "inverted" dative bond. Unexpectedly, the dimer of this system prefers to interact via an atypical boron-boron bond over the supposedly obvious boron-nitrogen bond. A molecular orbital analysis shows that the boron in this peculiar entity acts both as an electron donor and an electron acceptor, making the dimerization an amphoteric-amphoteric interaction process.

Next generation quantum theory of atoms in molecules for the design of emitters exhibiting thermally activated delayed fluorescence with laser irradiation

The effect of a static electric (E)-field and an unchirped and chirped laser pulse field on the cycl[3.3.3]azine molecule was investigated using next-generation quantum theory of atoms in molecules (NG-QTAIM). Despite the magnitude of the E-field of the laser pulses being an order of magnitude lower than for the static E-field, the variation of the energy gap between the lowest lying singlet (S₁ ) and triplet (T₁ ) excited states was orders of magnitude greater for the laser pulse than for the static E-field. Insights into the response of the electronic structure were captured by NG-QTAIM, where differences in the inverted singlet-triplet gap due to the laser pulses were significant larger compared to those induced by the static E-field. The response of the S₁ and T₁ excited states, as determined by NG-QTAIM, switched discontinuously between weak and strong chemical character for the static E-field. In contrast, the response to the laser pulses, determined by NG-QTAIM, is to induce a continuous range of chemical character, indicating the unique ability of the laser pulses to induce polarization effects in the form of "mixed" bond types. Our analysis demonstrates that NG-QTAIM is a useful tool for understanding the response to laser irradiation of the lowest-lying singlet S₁ and triplet T₁ excited states of emitters exhibiting thermally activated delayed fluorescence. The chirped laser pulse led to more frequent instances of the desired outcome of an inverted singlet-triplet gap than the unchirped pulse, indicating its usefulness as a tool to design more efficient organic light-emitting diode devices.

1-Aza-2,4-disilabicyclo[1.1.0]butanes with Superelongated C-N σ-Bonds

Two silylene molecules oxidatively react under formal [1 + 2 + 1]-cycloaddition to the C≡N bond of nitriles to yield 1-aza-2,4-disilabicyclo[1.1.0]butanes (L)(Cl)Si[μ-η^1,2-NC(p-RC₆H₄)]Si(Cl)(L) (L = PhC(NtBu)₂, R = CF₃ (2), F (3), Cl (4), Br (5)). The strongly folded bicyclic SiCNSi cores in 2-5 feature inverted bridgehead carbon atoms and superelongated C-N bonds [1.745(12) to 1.801(2) Å], exceeding the lengths of C-N single bonds in known silaaziridines by up to 23%. Detailed bonding analysis discloses C-N bonding interactions, sharing far-reaching similarities with the central C-C bond in [1.1.1]propellane.

Comment on "The 'Inverted Bonds' Revisited. Analysis of 'in Silico' Models and of [1.1.1]Propellane Using Orbital Forces"

Inverted bonds: In this Correspondence, the authors comment on the recent paper on inverted bonds in [1.1.1]propellane by Chaquin et al.

Charge-Shift Bonding: A New and Unique Form of Bonding

Charge-shift bonds (CSBs) constitute a new class of bonds different than covalent/polar-covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence-bond pioneers and then demonstrates that the unique status of CSBs is not theory-dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis-à-vis covalent and ionic bonds. Furthermore, the covalent-ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g., benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity.