Directional O...F halogen bonds

Rational Computational Design of Systems Exhibiting Strong Halogen Bonding Involving Fluorine in Bicyclic Diamine Derivatives

Perhaps the most controversial and rare aspect of the halogen bonding interaction is the potential of fluorine in compounds to serve as a halogen bond donor. In this note, we provide clear and convincing examples of hypothetical molecules in which fluorine is strongly halogen bonding in a metastable state. Of particular note is a polycyclic system inspired by Selectfluor, which has been controversially proposed to engage in halogen bonding.

On the Ability of Nitrogen to Serve as an Electron Acceptor in a Pnicogen Bond

Whereas pnicogen atoms like P and As have been shown repeatedly to act as electron acceptors in pnicogen bonds, the same is not true of the more electronegative first-row N atom. Quantum calculations assess whether N can serve in this capacity in such bonds and under what conditions. There is a positive π-hole belt that surrounds the central N atom in the linear arrangement of NNNF, NNN-CN, and NNO, which can engage a NH₃ base to form a pnicogen bond with binding energy between 3 and 5 kcal/mol. Within the context of a planar arrangement, the π-hole above the N in NO₂OF, N(CN)₃, and CF₃NO₂ is also capable of forming a pnicogen bond, the strongest of which amounts to 11 kcal/mol with NMe₃ as base. In their pyramidal geometry, NF₃ and N(NO₂)₃ engage with a base through the σ-hole on the central N, with variable binding energies between 2 and 9 kcal/mol. AIM and NBO provide somewhat different interpretations of the secondary interactions that occur in some of these complexes.

Unravelling the electronic nature of C-FO-C non-covalent interaction in proteins and small molecules in the solid state

The participation of organic fluorine as a halogen bond donor is rare and is sensitive to the electronic environment in the vicinity of the fluorine atom. The enhancement in the electropositive character (the σ-hole formalism) in fluorine is established by the presence of electron withdrawing groups and this has been examined in the solid-state structures in small molecules and proteins. Short, directional FO contacts have been observed and physical insights obtained, from quantum mechanical calculations, via the molecular electrostatic potential, an analysis of their topological features from atoms-in-molecules, and donor-acceptor characteristics from natural bond orbital analyses. It was observed that such contacts, cooperatively act in the presence of other interactions, and the formed aggregates are stabilizing in nature. In addition, the FO has a bonding character and is attractive in nature. The halogen bonding character of fluorine is relevant in supramolecular chemistry.

Understanding noncovalent bonds and their controlling forces

The fundamental underpinnings of noncovalent bonds are presented, focusing on the σ-hole interactions that are closely related to the H-bond. Different means of assessing their strength and the factors that control it are discussed. The establishment of a noncovalent bond is monitored as the two subunits are brought together, allowing the electrostatic, charge redistribution, and other effects to slowly take hold. Methods are discussed that permit prediction as to which site an approaching nucleophile will be drawn, and the maximum number of bonds around a central atom in its normal or hypervalent states is assessed. The manner in which a pair of anions can be held together despite an overall Coulombic repulsion is explained. The possibility that first-row atoms can participate in such bonds is discussed, along with the introduction of a tetrel analog of the dihydrogen bond.

The X-ray constrained wavefunction of the [Mn(CO)4{(C6H5)2P-S-C(Br2)-P(C6H5)2}]Br complex: a theoretical and experimental study of dihalogen bonds and other noncovalent interactions

The synthesis and X-ray structure determination of the [Mn(CO)₄{(C₆H₅)₂P-S-C(Br₂)-P(C₆H₅)₂}]Br complex (1) are described. The C-Br...Br dihalogen bond present in 1 has been characterized by means of topological studies of the electron density. Both the quantum theory of atoms in molecules and the electron localization function approaches have been applied to several theoretically calculated wavefunctions as well as to an X-ray constrained wavefunction. In addition, a number of theoretical techniques, such as the source function, the reduced density gradient method and the interacting quantum atoms approach, among others, have been used to analyse the dihalogen bond as well as several intramolecular interactions of the type C-H...Br-C which have also been detected in 1. The results show clearly that while bonding in the latter interactions are dominated by electrostatic components, the former has a high degree of covalency.

F-Halogen Bond: Conditions for Its Existence

The question as to whether the F atom can engage in a halogen bond (XB) remains unsettled. This issue is addressed via density functional theory calculations which pair a wide range of organic and inorganic F-acids with various sorts of Lewis bases. From an energetic perspective, perfluorinated hydrocarbons with sp, sp², or sp³ C-hybridization are unable to form an XB with an N-base, but a very weak bond can be formed if electron-withdrawing C≡N substituents are added to the acid. There is little improvement for inorganic acids O₂NF, FOF, ClF, BrF, SiF₄, or GeF₄, but F₂ is capable of a stronger XB of up to 5 kcal/mol. These results are consistent with a geometric criterion, which compares the intermolecular equilibrium distance with the sum of atomic van der Waals radii. The intensity of the σ-hole on the F atom has predictive value in that a V_s,max of at least 10-15 kcal/mol is required for XB formation. Adding a positive charge to the Lewis acid enhances the strength of any XB and even more so if the base is anionic. The acid-base interaction induces a contraction of the r(AF) covalent bond in the acid in most cases and a deshielding of the NMR signal of the F nucleus.

Is the Fluorine in Molecules Dispersive? Is Molecular Electrostatic Potential a Valid Property to Explore Fluorine-Centered Non-Covalent Interactions?

Can two sites of positive electrostatic potential localized on the outer surfaces of two halogen atoms (and especially fluorine) in different molecular domains attract each other to form a non-covalent engagement? The answer, perhaps counterintuitive, is yes as shown here using the electronic structures and binding energies of the interactions for a series of 22 binary complexes formed between identical or different atomic domains in similar or related halogen-substituted molecules containing fluorine. These were obtained using various computational approaches, including density functional and ab initio first-principles theories with M06-2X, RHF, MP2 and CCSD(T). The physical chemistry of non-covalent bonding interactions in these complexes was explored using both Quantum Theory of Atoms in Molecules and Symmetry Adapted Perturbation Theories. The surface reactivity of the 17 monomers was examined using the Molecular Electrostatic Surface Potential approach. We have demonstrated inter alia that the dispersion term, the significance of which is not always appreciated, which emerges either from an energy decomposition analysis, or from a correlated calculation, plays a structure-determining role, although other contributions arising from electrostatic, exchange-repulsion and polarization effects are also important. The 0.0010 a.u. isodensity envelope, often used for mapping the electrostatic potential is found to provide incorrect information about the complete nature of the surface reactive sites on some of the isolated monomers, and can lead to a misinterpretation of the results obtained.

Weak inter-actions in crystals: old concepts, new developments

Andrei Batsanov presents a general review of weak interactions in crystals, which highlights the history, state-of-the art and future developments of this field.

Revealing Factors Influencing the Fluorine-Centered Non-Covalent Interactions in Some Fluorine-Substituted Molecular Complexes: Insights from First-Principles Studies

We examine the equilibrium structure and properties of six fully or partially fluorinated hydrocarbons and several of their binary complexes using computational methods. In the monomers, the electrostatic surface of the fluorine is predicted to be either entirely negative or weakly positive. However, its lateral sites are always negative. This enables the fluorine to display an anisotropic distribution of charge density on its electrostatic surface. While this is the electrostatic surface scenario of the fluorine atom, its negative sites in some of these monomers are shown to have the potential to engage in attractive engagements with the negative site(s) on the same atom in another molecule of the same type, or a molecule of a different type, to form bimolecular complexes. This is revealed by analyzing the results of current state-of-the-art computational approaches such as DFT, together with those obtained from the quantum theory of atoms in molecules, molecular electrostatic surface potential and symmetry adapted perturbation theories. We demonstrate that the intermolecular interaction energy arising in part from the universal London dispersion, which has been underappreciated for decades, is an essential factor in explaining the attraction between the negative sites, although energy arising from polarization strengthens the extent of the intermolecular interactions in these complexes.