Insight into the Bonding Mechanism and the Bonding Covalency in Noble Gas–Noble Metal Halides: An NBO/NRT Investigation

Equivalent and Complement of the ω-Bonding Model and Charge-Shift Bonding Model: A Natural Bond Orbital/Natural Resonance Theory/Atoms in Molecules Investigation via Cu/Ag/Au Bonding in BMX (B = H2O, H2S, NH3, and PH3; M = Cu, Ag, and Au; and X = F, Cl, Br, and I)

Chemical bonding is the language of logic for chemists. Two new resonance bonding models, ω-bonding and charge-shift (CS) bonding, are gaining popularity among chemists. This study investigated the Cu/Ag/Au bonding in BMX (B = H₂O, H₂S, NH₃, and PH₃; M = Cu, Ag, and Au; and X = F, Cl, Br, and I) complexes using natural bond orbital (NBO) analysis, natural resonance theory (NRT), and atoms in molecules (AIM) method. The main aim is to reveal the relation between ω-bonding model and CS bonding model via Cu/Ag/Au bonding. Our studies demonstrate that the Cu/Ag/Au bonds exhibit the characteristics of both ω-bonding and CS bonding. Further studies found that ω-bonding and CS bonding models are equivalent and complement each other in understanding the studied Cu/Ag/Au bonding. Another interesting finding is the implication of the omega symbol in the ω-bonding model. These findings could help to promote the communication and CS bonding understanding of many more similar ω-bonded complexes.

Charge-Shift Bonding in Xenon Hydrides: An NBO/NRT Investigation on HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CCH, CN) via H-Xe Blue-Shift Phenomena

Noble-gas bonding represents curiosity. Some xenon hydrides, such as HXeY (Y = Cl, Br, I) and their hydrogen-bonded complexes HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CN, CCH), have been identified in matrixes by observing H-Xe frequencies or its monomer-to-complex blue shifts. However, the H-Xe bonding in HXeY is not yet completely understood. Previous theoretical studies provide two answers. The first one holds that it is a classical covalent bond, based on a single ionic structure H-Xe+ Y-. The second one holds that it is resonance bonding between H-Xe+ Y- and H- Xe+-Y. This study investigates the H-Xe bonding, via unusual blue-shifted phenomena, combined with some NBO/NRT calculations for chosen hydrogen-bonded complexes HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CN, CCH). This study provides new insights into the H-Xe bonding in HXeY. The H-Xe bond in HXeY is not a classical covalent bond. It is a charge-shift (CS) bond, a new class of electron-pair bonds, which is proposed by Shaik and Hiberty et al. The unusual blue shift in studied hydrogen-bonded complexes is its H-Xe CS bonding character in IR spectroscopy. It is expected that these studies on the H-Xe bonding and its IR spectroscopic property might assist the chemical community in accepting this new-class electron-pair bond concept.

A theoretical investigation on Cu/Ag/Au bonding in XH2P⋯MY(X = H, CH3, F, CN, NO2; M = Cu, Ag, Au; Y = F, Cl, Br, I) complexes

Intermolecular interaction of XH₂P···MY (X = H, CH₃, F, CN, NO₂; M = Cu, Ag, Au; Y = F, Cl, Br, I) complexes was investigated by means of an ab initio method. The molecular interaction energies are in the order Ag < Cu < Au and increased with the decrease of R_P···M. Interaction energies are strengthened when electron-donating substituents X connected to XH₂P, while electron-withdrawing substituents produce the opposite effect. The strongest P···M bond was found in CH₃H₂P···AuF with -70.95 kcal/mol, while the weakest one was found in NO₂H₂P···AgI with -20.45 kcal/mol. The three-center/four-electron (3c/4e) resonance-type of P:-M-:Y hyperbond was recognized by the natural resonance theory and the natural bond orbital analysis. The competition of P:M-Y ↔ P-M:Y resonance structures mainly arises from hyperconjugation interactions; the bond order of b_P-M and b_M-Y is in line with the conservation of the idealized relationship b_P-M + b_M-Y ≈ 1. In all MF-containing complexes, P-M:F resonance accounted for a larger proportion which leads to the covalent characters for partial ionicity of MF. The interaction energies of these Cu/Ag/Au complexes are basically above the characteristic values of the halogen-bond complexes and close to the observed strong hydrogen bonds in ionic hydrogen-bonded species.

The stability, kinetics and inter-fragment electron communication of the tautomers of twelve selected β-diketone molecules: A computational study

Keto-enol equilibrium is known to depend on the difference in the free energy between the keto and enol tautomers and is greatly influenced by the nature of the substituents, temperature, and the polarity of the solvents. New insight was gained into the series of twelve differently substituted β-diketones (A-L), showing that the keto form of each β-diketone has a lesser tendency towards resonance formation, compared to their enol forms. For molecules G and H (which contain an electron-withdrawing CF₃ substituent), the experimentally reported high tendency towards the enol tautomer, was computationally traced to a high level of the alternative enolic resonance weight also in their keto structure, as well as to the highly favourable enolic inter-fragment stability of the energy of interaction. Computational results further showed that the polar solvent dioxane enhances the enol form of these twelve molecules more effectively than water and chloroform media. The abundance of either the enol or keto tautomer, was also found to be dependent on the competitive ratio of both the forward and reverse reaction rate constants (namely the computed values for K_forward/K_reverse). High similarity was observed between the experimental and computed UV spectra of the selected molecules in their enol forms, which provides further evidence supporting predictions for the most favourable position of the enolization (for unsymmetrical molecules E and G-L with two possible enol positions), as well as confirming the previously reported trend of their experimental equilibrium K_e/k values.

Resonance bonding in XNgY (X = F, Cl, Br, I; Ng = Kr or Xe; Y = CN or NC) molecules: an NBO/NRT investigation

Several noble-gas-containing molecules XNgY were observed experimentally. However, the bonding in such systems is still not understood. Using natural bond orbital and natural resonance theory (NBO/NRT) methods, the present work investigated bonding of the title molecules. The results show that each of the studied XNgY molecules should be better described as a resonance hybrid of ω-bonding and [Formula: see text]-type long-bonding structures: X:^- Ng⁺ - Y, X - Ng⁺: Y^-, and X^{^}Y. The ω-bonding and long-bonding make competing contributions to the composite resonance hybrid due to the accurately preserved bond order conservation principle. We find that the resonance bonding is highly tunable for these noble-gas-containing molecules due to its dependence on the nature of the halogen X or the central noble-gas atoms Ng. When the molecule XNgY consists of a relatively lighter Ng atom, a relatively low-electronegative X atom, and the CN fragment rather than NC, the long-bonding structure X^{^}Y tends to be highlighted. In contrast, the heavy Ng atom and high-electronegative X atom will enhance the ω-bonding structure. Overall, the present work provides electronic principles and chemical insights that help understand the bonding in these XNgY species.

Understanding and modulating the high-energy properties of noble-gas hydrides from their long-bonding: an NBO/NRT investigation on HNgCO+/CS+/OSi+ and HNgCN/NC (Ng = He, Ar, Kr, Xe, Rn) molecules

The noble-gas hydrides, HNgX (X is an electronegative atom or fragment), represent potential high-energy materials because their two-body decomposition process, HNgX → Ng + HX, is strongly exoergic. Our previous studies have shown that each member of the HNgX (X = halogen atom or CN/NC fragment) molecules is composed of three leading resonance structures: two ω-bonding structures (H-Ng+ :X- and H:- Ng+-X) and one long-bonding structure (H∧X). The last one paints a novel [small sigma, Greek, circumflex]-type long-bonding picture. The present study focuses on the relationship between this novel bonding motif and the unusual energetic properties. We chose HNgCO+/CS+/OSi+/CN/NC, with the formula HNgAB (Ng = He, Ar, Kr, Xe, Rn; AB = CO+/CS+/OSi+/CN/NC) as the research system. We first investigated the bonding of HNgCO+ and its analogous HNgCS+/OSi+ species using NBO/NRT methods, and quantitatively compared the bonding with that in HNgCN/NC molecules. NBO/NRT results showed that each of the HNgCO+/CS+/OSi+ molecules could be better represented as a resonance hybrid of ω-bonding and long-bonding structures, but the long-bonding is much weaker than that in HNgCN/NC molecules. Furthermore, we introduced the long-bonding concept into the rationalization of the high-energy properties, and found a good correlation between the highly exothermic two-body dissociation channel and the long-bond order, bH-A. We also found that the long-bond order is highly tunable for these noble-gas hydrides due to its dependence on the nature of the electronegative AB fragments or the central noble-gas atoms, Ng. On the basis of these results, we could optimize the energetic properties by changing the long-bonding motif of our studied molecules. Overall, this study shows that the long-bonding model provides an easy way to rationalize and modulate the unusual energy properties of noble-gas hydrides, and that it is helpful to predict some noble-gas hydrides as potential energetic materials.