Exploring the nature of anion-π bonding by means of the Quantum Theory of Atoms in Molecules (QTAIM) and an energy decomposition scheme on the basis of Interacting Quantum Atoms (IQA) theory led us to conclude that these non-classical interactions benefit from "multi-center covalency" far more than from the electrostatics. Comparing anion-π systems to closely related covalent anion-σ complexes reveals that the anion-π systems benefit from an extensive degree of electron sharing between the anions and all atoms of the π-rings. Besides, decomposition of the binding energy into classical (electrostatics) and non-classical (exchange-correlation) components demonstrates that in contrast to previous reports, the anion-π complexes are local minima, if and only if the non-classical contribution to binding energy surpasses that of the electrostatics. This suggests that the stable anion-π complexes with the anions atop the π-rings might be prepared with π-systems that benefit more from the exchange-correlation term, such as extended π-systems, but not with strong electrostatic π-receptors. This conclusion is in line with the tendency of strong π-acids to form the σ-complexes with more covalent character instead of the π-complexes.

We present new insight into the nature of aromaticity in metal clusters. We give computational arguments in favor of using the ring-current model over local indices, such as nucleus independent chemical shifts, for the determination of the magnetic aromaticity. Two approaches for estimating magnetically induced ring currents are employed for this purpose, one based on the quantum theory of atoms in molecules (QTAIM) and the other where magnetically induced current densities (MICD) are explicitly calculated. We show that the two-zone aromaticity/antiaromaticity of a number of 3d metallic clusters (Sc3(-), Cu3(+), and Cu4(2-)) can be explained using the QTAIM-based magnetizabilities. The reliability of the calculated atomic and bond magnetizabilities of the metallic clusters are verified by comparison with MICD computed at the multiconfiguration self-consistent field (MCSCF) and density functional levels of theory. Integrated MCSCF current strength susceptibilities as well as a visual analysis of the calculated current densities confirm the interpretations based on the QTAIM magnetizabilities. In view of the new findings, we suggest a simple explanation based on classical electromagnetic theory to explain the anomalous magnetic shielding in different transition metal clusters. Our results suggest that the nature of magnetic aromaticity/antiaromaticity in transition-metal clusters should be assessed more carefully based on global indices.

In the present account we investigate a theoretical link between the bond length, electron sharing, and bond energy within the context of quantum chemical topology theories. The aromatic stabilization energy, ASE, was estimated from this theoretical link without using isodesmic reactions for the first time. The ASE values obtained from our method show a meaningful correlation with the number of electrons contributing to the aromaticity. This theoretical link demonstrates that structural, electronic, and energetic criteria of aromaticity - ground-state aromaticity - belong to the same class and guarantees that they assess the same property as aromaticity. Theory suggests that interatomic exchange-correlation potential, obtained from the theory of Interacting Quantum Atoms (IQA), is linearly connected to the delocalization index of Quantum Theory of Atoms in Molecules (QTAIM) and the bond length through a first order approximation. Our study shows that the relationship between energy, structure and electron sharing marginally deviates from the ideal linear form expected from the first order approximation. The observed deviation from linearity was attributed to a different contribution of exchange-correlation to the bond energy for the σ- and π-frameworks. Finally, we proposed two-dimensional energy-structure-based aromaticity indices in analogy to the electron sharing indices of aromaticity.

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In the present work the nature of lone-pair-π interactions between water molecules and a number of π-rings with different substituents/hetero-atoms in the light of quantum chemical topology approaches is studied. The Quantum Theory of Atoms in Molecules (QTAIM) and Interacting Quantum Atoms (IQA) were employed for distinguishing the role of heteroatoms and electron withdrawing substituents in the complex formation between water and π-rings. Our IQA study identified three classes of water-π complexes on the basis of the relative role of electrostatics (classical) and exchange-correlation (non-classical) factors in the interaction energy between the oxygen of water (the lone-pair donor) and the sp(2) atoms of the π-ring, i.e. the primary lp-π interaction. Considering both the primary and secondary (the rest of interatomic interactions except Owater-π-ring atoms) interactions demonstrates that the exchange-correlation is the dominant contributor to the binding energy. This proves a non-negligible contribution of non-classical factors in the stabilization of the lone-pair-π complexes. However, in spite of a relatively large contribution of the exchange-correlation, this part of the interaction energy is virtually counterbalanced by the deformation energy, i.e. the increase in atomic kinetic energy upon complexation. This finding clarifies why water-π interactions can be modelled by simple electrostatics without the need to invoke quantum effects.

1,1,3,3-N,N,N¢,N¢-Tetramethylguanidinium trifluoroacetate as an ionic liquid, efficiently promotes one-pot, three-component condensation of aldehydes, alkyl nitriles and a-hydroxy or a-amino activated C-H acid to afford the corresponding pyran annulated heterocyclic systems. Ionic liquid can be recycled for subsequent reactions without any appreciable loss of efficiency.

A recent study (Sci. Adv. 2017, 3, e1602833) has shown that FH⋅⋅⋅OH hydrogen bond in a HF⋅H O pair substantially shortens, and the H-F bond elongates upon encapsulation of the cluster in C fullerene. This has been attributed to compression of the HF⋅H O pair inside the cavity of C . Herein, we present theoretical evidence that the effect is not caused by a mere compression of the H O⋅HF pair, but it is related to a strong lone-pair-π (LP-π) bonding with the fullerene cage. To support this argument, a systematic electronic structure study of selected small molecules (HF, H O, and NH ) and their pairs enclosed in fullerene cages (C , C , and C ) has been performed. Bonding analysis revealed unique LP-π interactions with a charge-depletion character in the bonding region, unlike usual LP-π bonds. The LP-π interactions were found to be responsible for elongation of the H-F bond. Thus, the HF appears to be more acidic inside the cage. The shortening of the FH⋅⋅⋅OH contact in (HF⋅H O)@C originates from an increased acidity of the HF inside the fullerenes. Such trends were also observed in other studied systems.

The electron density versus NICS(zz) (the out-of-plane component of nucleus-independent chemical shifts (NICS)) scan for assessing magnetic aromaticity among similar molecules with different ring sizes is improved by scanning the Laplacian of electron density versus NICS(zz) to include molecules containing different types of atoms. It is demonstrated that the new approach not only reproduces the results of the previous method but also surpasses that in the case of species with different atom types. The relative positions of curves in the plots of the Laplacian of electron density versus NICS(zz) correlate well with the ring current intensities of these molecules both near and far from the ring planes of the considered molecules. Accordingly, relative magnetic aromaticity of a number of planar hydrocarbons and a group of double aromatic metallic/semimetallic species are studied and discussed.