The destabilization of hydrogen bonds in an external E-field for improved switch performance

The Cis-Effect Explained Using Next-Generation QTAIM

We used next-generation QTAIM (NG-QTAIM) to explain the cis-effect for two families of molecules: C₂X₂ (X = H, F, Cl) and N₂X₂ (X = H, F, Cl). We explained why the cis-effect is the exception rather than the rule. This was undertaken by tracking the motion of the bond critical point (BCP) of the stress tensor trajectories T_σ(s) used to sample the U_σ-space cis- and trans-characteristics. The T_σ(s) were constructed by subjecting the C1-C2 BCP and N1-N2 BCP to torsions ± θ and summing all possible T_σ(s) from the bonding environment. During this process, care was taken to fully account for multi-reference effects. We associated bond-bending and bond-twisting components of the T_σ(s) with cis- and trans-characteristics, respectively, based on the relative ease of motion of the electronic charge density ρ(r_b). Qualitative agreement is found with existing experimental data and predictions are made where experimental data is not available.

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.

New insights of QTAIM and stress tensor to finding non-competitive/competitive torquoselectivity of cyclobutene

This study aims to investigate the phenomenon of torquoselectivity through three thermal cyclobutene ring-opening reactions (N1-N3). This research focuses on the nature of the chemical bond, electronic reorganization, predicting non-competitive or competitive reactions, and torquoselectivity preference within Quantum Theory of Atoms in Molecules (QTAIM) and stress tensor frameworks. Various theoretical analyses for these reactions, such as metallicity ξ(r_b), ellipticity ε, total local energy density H(r_b), stress tensor polarizability ℙ_σ, stress tensor eigenvalue λ_3σ, and bond-path length, display differently for non-competitive and competitive reactions as well as for the conrotatory preferences either it is the transition state outward conrotatory (TS_out) or transition state inward conrotatory (TS_in) directions by presenting degeneracy or non-degeneracy in their results. The ellipticity profile provides the motion of the bond critical point locations due to the different substituents of cyclobutene. In agreement with experimental results, examinations demonstrated that N1 is a competitive reaction and N2-N3 are non-competitive reactions with TS_out and TS_in preference directions, respectively. The concordant results of QTAIM and stress tensor scalar and vectors with experimental results provide a better understanding of reaction mechanisms.

Hydrogen and Halogen Bonding in Homogeneous External Electric Fields: Modulating the Bond Strengths

Recent years have witnessed various fascinating phenomena arising from the interactions of noncovalent bonds with homogeneous external electric fields (EEFs). Here we performed a computational study to interpret the sensitivity of intrinsic bond strengths to EEFs in terms of steric effect and orbital interactions. The block-localized wavefunction (BLW) method, which combines the advantages of both ab initio valence bond (VB) theory and molecular orbital (MO) theory, and the subsequent energy decomposition (BLW-ED) approach were adopted. The sensitivity was monitored and analyzed using the induced energy term, which is the variation in each energy component along the EEF strength. Systems with single or multiple hydrogen (H) or halogen (X) bond(s) were also examined. It was found that the X-bond strength change to EEFs mainly stems from the covalency change, while generally the steric effect rules the response of H-bonds to EEFs. Furthermore, X-bonds are more sensitive to EEFs, with the key difference between H- and X-bonds lying in the charge transfer interaction. Since phenylboronic acid has been experimentally used as a smart linker in EEFs, switchable sensitivity was scrutinized with the example of the phenylboronic acid dimer, which exhibits two conformations with either antiparallel or parallel H-bonds, thereby, opposite or consistent responses to EEFs. Among the studied systems, the quadruple X-bonds in molecular capsules exhibit remarkable sensitivity, with its interaction energy increased by -95.2 kJ mol^-1 at the EEF strength 0.005 a.u.

Manipulation of Diatomic Molecules with Oriented External Electric Fields: Linear Correlations in Atomic Properties Lead to Nonlinear Molecular Responses

Oriented external electric fields (OEEFs) have been shown to have great potential in being able to provide unprecedented control of chemical reactions, catalysis, and selectivity with applications ranging from H₂ storage to molecular machines. We report a theoretical study of the atomic origins of molecular changes because of OEEFs since understanding the characteristics of OEEF-induced couplings between atomic and molecular properties is an important step toward comprehensive understanding of the effects of strong external fields on the molecular structure, stability, and reactivity. We focus on the atomic and molecular (bond) properties of a set of homo- (H₂, N₂, O₂, F₂, and Cl₂) and heterodiatomic (HF, HCl, CO, and NO) molecules under intense external electric fields in the context of quantum theory of atoms in molecules (QTAIM). It is shown that the atomic properties (atomic charges, energies, and localization indices) correlate linearly with the field strengths, but molecular properties (bond length, electron density at the bond critical point, and electron delocalization index) exhibit nonlinear responses to the imposed fields. In particular, the changes in the electron density distribution alter the shapes and locations of the zero-flux surfaces, atomic volumes, atomic electron population, and localization/delocalization indices. The topography and topology of the molecular electrostatic potential undergo dramatic changes. External fields also perturb the covalent-polar-ionic characteristic of the studied chemical bonds, hallmarking the impact of electric fields on the stability and reactivity of chemical compounds. The findings are well-rationalized within the framework of the QTAIM and form a coherent conceptual understanding of these effects in prototypical diatomic molecules.

Next-generation QTAIM for scoring molecular wires in E-fields for molecular electronic devices

The effect of a varying, directional E _x , E _y , and E _z electric field on the ethene molecule was investigated using next-generation quantum theory of atoms in molecules (QTAIM). Despite using low E-field strengths that are within the realm of experiment and do not measurably alter the molecular geometry, significant changes to the QTAIM properties were observed. Using conventional QTAIM, the shifting of the C─C and C─H bond critical points (BCPs) demonstrates polarization through an interchange in the size of the atoms involved in a bond, since a BCP is located on the boundary between a pair of bonded atoms. Next-generation QTAIM, however, demonstrates the polarization effect more directly with a change in morphology of the 3-D envelope around the BCP. Modest increases of ≈ 2% in the ellipticity ε of the BCP were uncovered when the C─C bond was aligned parallel or anti-parallel to the applied E _x -field. Significant asymmetries were found in the response of the next-generation QTAIM 3-D paths of the C─H bonds to the applied E-field. When the E-field coincided with the C─C bond, the BCP moved in response and was accompanied by the envelope constructed from 3-D next-generation paths. The response displayed a polarization effect that increased with increasing magnitude of the E _x -field parallel and anti-parallel to the C─C bond. Our analysis demonstrates that next-generation QTAIM is a useful tool for understanding the response of molecules to E-fields, for example, for the screening of molecular wires for the design of molecular electronic devices. © 2019 Wiley Periodicals, Inc.