Topological analysis of electron localisation function (ELF) has been used to study the nature of the B X (X = F, Cl) bonds in 26 molecules. Twelve small molecules with formal B X, B X, and B X bonds and 14 organoboron molecules with the B Cl bond from the Cambridge Structural Database have been studied using CCSD, DFT(M062x) methods. As the V(B,F) basin population for the investigated molecules is smaller than 4 and 6e, the existence of B X, B X bonds is not confirmed. The results show higher polarity of B F bonds as compared to the B Cl bonds. Electronic structure of selected molecules has been discussed from the ELF-topological perspective. K E Y W O R D S B Cl, B F, chemical bond, double bond, electron localisation function, Lewis structure, QCT, quantum chemical topology, triple bond 1 | INTRODUCTION Nature of a localized, dicenter-two-electron (2c 2e) chemical bond, A-B, assumed to exist between a pair of atoms in a molecule, is one of the most important considerations of theoretical chemistry. Such bond can broadly be characterized as covalent or ionic. The concept of covalentbond is based on sharing electron densities, while ionic bond stems from electrostatic interactions. A contemporary paradigm, derived from quantum theory, states that chemical bonding can be described using the molecular orbital (MO) theory [1] or valence bond (VB) theory, [2] where covalent chemical bond between A and B is a consequence of atomic orbitals overlapping. Molecular orbitals, fully characterizing electronic structure of molecules are vectors in the Hilbert space. Notably, chemical bonding can also be described using the graph theory [3] or the information theory. [4] Another way of analyzing the nature of a chemical bond, developed in addition to the MO and VB theories, constitute of methods operating in a real (physical) space. They focus on topological properties of the scalar fields: electron density, ρ(r), [5] electron localisation function, ELF, η(r), [6,7] electron localisability indicator, ELI-D, [8] electrostatic potential, [9] and others. [10,11] All those methods have been gathered by Popelier [12] under the umbrella of quantum chemical topology (QCT).This article concentrates only on topological analysis of ELF from the whole range of the QCT, therefore, a nonexpert reader is encouraged to familiarize with the subject. [6,7,[13][14][15][16][17] In short, the two-center two-electron (2c 2e) covalent bond, A-B, is represented by an attractor (local maximum) of the η(r) field, V (A,B), found in the region of expected chemical bond. The attractor V(A,B) belongs to valence and disynaptic types. [13,18] Futher application of the gradient vector theory of dynamical systems enables calculation of its basin (localisation basin) for ELF, denoted also V (A,B). Projection of all points of η(r) field, defining attractor's basin in the electron density field, ρ(r) and integrating the electron density with respect to the localisation basin leads to the definition of its basin population, N. Further analysis includes d...
