Natural bond-bond polarizability: a Hückel-like electronic delocalization index

Computational study of the interaction between NO, NO+, and NO- with H2O

In this computational study the interaction of NO^., NO⁺, and NO^- with H₂O: [NO--H₂O]^., 1 ^., [NO--H₂O]⁺, 1 ⁺ , and [NO--H₂O]^-, 1 ^- was analysed. The optimized geometries indicate that the relative position of NO and H₂O depends on the total charge: (ON^.--H-OH), (NO^---H-OH), and (ON⁺--OH₂). Moreover, atomic spin density along with frontier molecular orbitals help to identify the preferred reduction or oxidation sites on the nitric oxide. Thus, quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), and natural bond-bond polarizability (NBBP) methods aid to quantify the electron delocalization level between NO and H₂O, 1 ⁺ > 1 ^. > 1 ^- , and show the predominantly ionic, and covalent character to inter-molecular, and intra-molecular chemical bonds, respectively. Furthermore, the natural bond orbital (NBO) and localized molecular orbital energy decomposition analysis (LMO-EDA) methods enable energy analyses of the interaction between NO and H₂O in the complexes 1 ^., 1 ⁺ , and 1 ^- . Where, the first method showed that the interaction between the natural bond orbitals in 1 ^- is more favorable, than in 1 ⁺ , and less in 1 ^., however, the second method designates that the total interaction energy is lower for 1 ⁺ in relation to 1 ^- and 1 ^., due mainly to the electrostatic component. As a final point, analysis of the electrostatic potential surfaces provides a clear and direct explanation for the relative position of the monomers. It also shows that the predominant Coulombic attraction between H₂O and the charged NO⁺, and NO^- compounds will be stronger in relation to the neutral NO^.. Graphical abstract ᅟ.

H-Bond: Τhe Chemistry-Biology H-Bridge

H-bonding, as a non covalent stabilizing interaction of diverse nature, has a central role in the structure, function and dynamics of chemical and biological processes, pivotal to molecular recognition and eventually to drug design. Types of conventional and non conventional (H-H, dihydrogen, H- π, CH- π, anti- , proton coordination and H-S) H-bonding interactions are discussed as well as features emerging from their interplay, such as cooperativity (σ- and π-) effects and allostery. Its utility in many applications is described. Catalysis, proton and electron transfer processes in various materials or supramolecular architectures of preorganized hosts for guest binding, are front-line technology. The H-bond-related concept of proton transfer (PT) addresses energy issues or deciphering the mechanism of many natural and synthetic processes. PT is also of paramount importance in the functions of cells and is assisted by large complex proteins embedded in membranes. Both intermolecular and intramolecular PT in H-bonded systems has received attention, theoretically and experimentally, using prototype molecules. It is found in rearrangement reactions, protein functions, and enzyme reactions or across proton channels and pumps. Investigations on the competition between intra- and intermolecular H bonding are discussed. Of particular interest is the H-bond furcation, a common phenomenon in protein-ligand binding. Multiple H-bonding (H-bond furcation) is observed in supramolecular structures.

Hyperconjugation in Carbocations, a BLW Study with DFT approximation

The geometry of ethyl cation is discussed, and the hyperconjugation effect in carbocations is evaluated at the B3LYP/6-311G(d) level. The Block Localized Wavefunction (BLW) method is used for all evaluations of the hyperconjugation, considered as the energy gained by the delocalization onto the C⁺ atom. This energy is defined as the energy difference between the delocalized (standard) calculation, where the electrons are freely delocalized, and a localized form where the positive charge sits on the carbon center. It is evaluated for 18 carbocations, including conjugated systems. In these cases we were particularly interested in the additional stabilization brought by hyperconjugative effects. Among other effects, the β-silicon effect is computed. Hyperconjugation amounts in several cases to an energy similar to conjugation effects.