Quantum Interference Contribution to the Dipole Moment of Diatomic Molecules

Effects of Carbon Spacer Length on Conformational Transitions and Protein Adsorption of Polyzwitterions

The properties of polyzwitterions are closely linked to their carbon spacer length (CSL) between oppositely charged groups. A thorough understanding of the effect of CSL on the properties of polyzwitterion-functionalized membranes is important for their fouling resistance and separation performances. In this work, polyzwitterion-functionalized membranes with different CSLs are prepared by coupling selective swelling-induced pore generation with zwitterionization, and the investigation is focused on comprehending the molecular mechanisms underlying protein resistance and conformational transitions within polyzwitterions under varying CSLs. The zwitterionized films show an enhancement in the surface negative potential with the increase of CSL, attributed to the negatively charged groups distanced from the positively charged groups. Quartz crystal microbalance with dissipation (QCM-D) demonstrates that zwitterionized films with different CSLs display distinct levels of resistance to protein adsorption. The trimethylamine N-oxide-derived polymer (PTMAO, CSL = 0) zwitterionized film shows the highest resistance compared to the poly(3-[dimethyl(2'-methacryloyloxyethyl] ammonio) ethanesulfonate (PMAES, CSL = 2) zwitterionized film and the poly(sulfobetaine methacrylate) (PSBMA, CSL = 3) zwitterionized film, owing to its electrical neutrality and pronounced hydrophilicity. Moreover, analysis of the anti-polyelectrolyte behaviors reveals that PTMAO does not undergo a significant conformation transition in deionized water and salt solutions, while the conformations of PMAES and PSBMA display to be more salt-dependent as the CSL increases, attributed to their increased polarization and dipole moment. As a result, the permeability of zwitterionized membranes exhibits enhanced salt responsiveness with the increase in CSL. The findings of this study are expected to facilitate the design of adsorption-resistant surfaces desired in diverse fields.

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.

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.