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Electrostatic and stereoelectronic effects associated with fluorine atoms can be exploited as conformational tools for the design of shape-controlled functional molecules. To gain further insight into the nature and strength of these effects, we use the Interacting Quantum Atoms (IQA) method augmented with the semiclassical pairwise dispersion potential to decompose the conformational energies of fluoro-substituted molecules into fragment-based energy contributions, which include deformation/distortion terms and the electrostatic, exchange-correlation and dispersion interactions. The studied molecules comprise various F-CH2-CH2-X and F-CH2-CO-X systems, as well as selected conformers of an α,β-difluoro-γ-amino-acid derivative that is potentially useful for the design of shape-controlled bioactive amino acids and peptides. We identify the most relevant exchange-correlation and/or electrostatic interaction terms contributing to the stability of the various conformers, and we show that IQA can be used to assess the gauche/anti or trans/cis preferences in molecules with two or more rotatable bonds as well as to study the roles played by other concomitant effects (e.g., CH/OH/NHF contacts). For the α,β-difluoro-γ-amino acid derivatives, our theoretical analysis indicates that the gauche/anti and trans/cis effects associated with fluorine bonds can be significantly attenuated by other specific intra-molecular contacts.

Based on the Interacting Quantum Atoms approach, we present herein a conceptual and theoretical framework of short-range electrostatic interactions, whose accurate description is still a challenging problem in molecular modeling. For all the noncovalent complexes in the S66 database, the fragment-based and atomic decomposition of the electrostatic binding energies is performed using both the charge density of the dimers and the unrelaxed densities of the monomers. This energy decomposition together with dispersion corrections gives rise to a pairwise approximation to the total binding energy. It also provides energetic descriptors at varying distance that directly address the atomic and molecular electrostatic interactions as described by point-charge or multipole-based potentials. Additionally, we propose a consistent definition of the charge penetration energy within quantum chemical topology, which is mainly characterized in terms of the intramolecular electrostatic energy. Finally, we discuss some practical implications of our results for the design and validation of electrostatic potentials., F.J.-G. and D.S. acknowledge the Spanish MICINN (Grant PGC2018-095953-B-I00), the FICyT (Grant FC-GRUPIN- IDI/2018/000177), and the European Union FEDER for financial support. F.J.-G. specially acknowledges the Spanish MICINN for a predoctoral grant (BES-2016-076986).

Among the different noncovalent interactions, halogen bonds have captured wide attention in the last years. Their stability has been rationalized in electrostatic terms by appealing to the σ-hole concept, a charge-depleted region that is able to interact favorably with electron rich moieties. This interpretation has been questioned, and in this work a set of anionic halogen model systems are used to shed some light on this issue. We use the interacting quantum atoms method, which provides an orbital invariant energy decomposition in which pure electrostatic terms are well isolated, and we complement our insights with the analysis of electrostatic potentials (ESPs) as well as with traditional descriptors of charge accumulation like the Laplacian of the electron density. The total electrostatic interaction between the interacting species is surprisingly destabilizing in many of the systems examined, demonstrating that although σ-holes might be qualitatively helpful, much care has to be taken in ascribing the stability of these systems to electrostatics. It is clearly shown that electron delocalization is essential to understand the stability of the complexes. The evolution of atomic charges as the aggregates forms reveals a charge transfer picture in which the central, σ-hole bearing halogen acts as a mere spectator. These systems may then be not far from engaging in a classical 3c-4e interaction. Since the presence of a σ-hole as characterized by the ESP mapped on a suitable molecular envelope isosurface does not guarantee attractive electrostatic interactions, we encourage to employ a wider perspective that takes into account the full charge distribution.

This paper addresses an assessment of the performance of a large set of exchange-correlation functionals in the description of hydrogen bonding within the interacting quantum atoms (IQA) energy partition. Specifically, we performed IQA analyses over a series of small water clusters $$(\hbox {H}_{2}\hbox {O})_{{n}}$$ with $$n \le 6$$. Apart from LDA-like approximations, all the considered families of exchange-correlation functionals (GGA, meta-GGA, and hybrid) reproduce the trends associated with hydrogen bond non-additive effects computed with reference Moller–Plesset and coupled cluster wave functions. In other words, the IQA energy partition together with most of the functionals addressed herein produce good results concerning the study of non-additivity in hydrogen bonds at a reduced cost as compared with correlated wave functions approximations. These conditions might be further exploited in the examination of larger hydrogen-bonded complexes.

Charge-shift bonding (CSB) has been introduced as a distinct third family of electron-pair links that adds to the covalent and ionic tradition. However, the full battery of orbital invariant tools provided by modern real space artillery shows that it is difficult to find CSB signatures outside the original valence-bond framework in which CSB was developed. The CSB concept should therefore be further investigated.

The interacting quantum atoms (IQA) method decomposes the total energy of a molecular system in terms of one- and two-center (atomic) contributions within the context of the quantum theory of atoms in molecules. Here we incorporate electrostatic continuum solvent effects into the IQA energy decomposition. To this end, the interaction between the solute electrostatic potential and the solvent screening charges as defined within the COSMO solvation model is now included in a new version of the PROMOLDEN code, allowing thus to apply IQA in combination with COSMO-quantum chemical methods as well as to partition the electrostatic solvation energy into effective atomic and group contributions. To test the robustness of this approach, we carry out COSMO-HF/aug-cc-pVTZ calculations followed by IQA calculations on more than 400 neutral and ionic solutes extracted from the MNSol database. The computational results reveal a detailed atomic mapping of the electrostatic solvation energy that is useful to assess to what extent the solvation energy can be decomposed into atomic and group contributions of various parts of a solute molecule, as generally assumed by empirical methodologies that estimate solvation energy and/or logP values.

The spin transition of Fe(II) complexes is the subject of intensive synthetic and computational efforts. In this manuscript, we analyze the spin crossover (SCO) of [Fe(Edpsp)(2)](2+) (1), which features a spin transition depending on the cocrystallizing solvent molecules. Whereas the use of acetone results in a hysteretic spin transition at similar to 170 K, the use of propylene carbonate (PC) results in a permanent diamagnetic signal up to 300 K. By means of DFT+U+D2 calculations in the solid state of the material, we unravel the reasons for such different behavior. Our results allow us to ascribe the relatively low transition temperature of 1(BF4)(2.) acetone to the distorted arrangement of the SCO molecules in the low-spin state of the material. In turn, intermolecular interactions play the primary role in the case of 1(BF4)(2).2PC. In particular, we found that solvent-solvent interactions actively promote the stability of the low-spin state due to the formation of PC dimers. These dimers would appear at larger distances in the high-spin phase, with the subsequent loss of phase stability. This is yet another proof of how subtle is the spin transition phenomenon in Fe(II)-based architectures.