Examining a Transition from Supramolecular Halogen Bonding to Covalent Bonds: Topological Analysis of Electron Densities and Energies in the Complexes of Bromosubstituted Electrophiles

Abstract

The transition from weak (noncovalent) interactions to fully developed covalent bonds is examined using the quantum theory of atoms in molecules in a series of halogen-bonded (XB) complexes of bromosubstituted electrophiles, RBr, with 1,4-diazabicyclo[2.2.2]octane (DABCO) and Cl – and Br – anions. The gradual decrease in the XB lengths in these associations, d Br···Y (where Y = Cl – , Br – , or N), was accompanied by the exponential increase in the binding energies and charge transfer, as well as electron densities and magnitudes of the kinetic and potential energy densities at the bond critical points (BCPs) on the Br···Y bond path. These indices, as well as characteristics of the adjacent bonds in the XB donor, followed remarkably close trend lines when plotted against the normalized XB length R BrY = d Br···Y /( r Br + r Y ) (where r Br and r Y are the van der Waals radii) regardless of the methods [MP2/6-311+G(d,p) or M062X/6-311+G(d,p)], media (gas phase or dichloromethane), and nucleophiles (Cl – , Br – , or DABCO). In the systems with an R BrY higher than about 0.78, the energy densities H ( r ) at BCPs at the Br···Y bond path were small and positive, and XBs did not substantially affect the characteristics of the adjacent R–Br covalent bond in the XB donor. Accordingly, the XB can be identified as noncovalent in this range. In the complexes with R BrY values between about 0.67 and 0.78, energy densities H ( r ) at Br···Y BCPs were negative, and their magnitudes increased with the decrease in the Br···Y separation. In this range, formation of XBs was accompanied by the increase in the R–Br bond length in the XB donor and the decrease in the magnitude of the (negative) H ( r ) values at the BCPs of the R–Br bonds. XBs can be classified as partially covalent in this R BrY range. At an R BrY less than about 0.67, electron densities were larger, and energy densities were more negative at BCPs of the Br···Y bond than those at BCPs of the R–Br bond in the XB donor. This indicates that Br···Y bonds were stronger than R–Br bonds, and these (Br···Y) XBs can be regarded as essentially covalent. The synchronous change of a variety of (R–Br and Br···Y) bonding characteristics with R BrY suggests that the normalized XB bond length can be used as a basic parameter in the identification of the type of intermolecular i...

Structures, Multicenter π-Bonding, and Spin Equilibria in the Mixed-Valence Trimers of Tetramethyltetrathiafulvalene Cation-Radicals

“…The BCPs located on the bond paths along the shortest S–S separations are characterized by electron densities of about 0.009 au, energy densities of about 0.0012 au and Laplacians of electron density of about 0.027 au. All of these values are comparable to the characteristics found for moderately strong halogen and hydrogen bonds. − Furthermore, a noncovalent indices (NCI) analysis showed mostly green surfaces representing nonbonding interactions between TMTTF moieties with blue spots indicating bonding interactions around the areas adjacent to the (3, −1) BCPs.…”

From weak to strong interactions: structural and electron topology analysis of the continuum from the supramolecular chalcogen bonding to covalent bonds

“…Following the earlier QTAIM studies of the HB and HaB associations, [27][28][29][30][31][32] we focused our analysis on electron densities, r(r), the Laplacians, r 2 r(r), kinetic and potential energy densities G(r) and V(r), and energy density H(r) = G(r) + V(r). These characteristics evaluated in the solid-state and optimized structures at the BCPs along the SeÁ Á ÁBr and TeÁ Á ÁI bond paths are listed in Tables S3-S6 in the ESI.…”

“…[26][27][28] Previous applications of QTAIM to the series of hydrogen-bonded (HyB) and HaB complexes demonstrated continuous variations of the characteristics of these bond critical points (BCPs) with a decrease in the interatomic distances from the van der Waals separations to the covalent bonds. [28][29][30][31][32] Based on the topology of electron densities and energies at BCPs, the interactions in these systems were classified as non-covalent (closed-shell), covalent (shared-shell) and intermediate (partially-covalent) bonding. 29,31 Moreover, synchronous changes of the bonding characteristics with the normalized interatomic separations, R XY = d XY /(R X + R Y ) (where d XY is an interatomic X-Y distance, and R X and R Y are van der Waals radii of X and Y) in the HaB systems suggested that R XY could be used for the classification of the interactions.…”

“…[28][29][30][31][32] Based on the topology of electron densities and energies at BCPs, the interactions in these systems were classified as non-covalent (closed-shell), covalent (shared-shell) and intermediate (partially-covalent) bonding. 29,31 Moreover, synchronous changes of the bonding characteristics with the normalized interatomic separations, R XY = d XY /(R X + R Y ) (where d XY is an interatomic X-Y distance, and R X and R Y are van der Waals radii of X and Y) in the HaB systems suggested that R XY could be used for the classification of the interactions. 31 In the current work we examine changes in the topologies of the electron densities and energies, and therefore the nature of bonding, with the interatomic distances in the ChB systems.…”

“…29,31 Moreover, synchronous changes of the bonding characteristics with the normalized interatomic separations, R XY = d XY /(R X + R Y ) (where d XY is an interatomic X-Y distance, and R X and R Y are van der Waals radii of X and Y) in the HaB systems suggested that R XY could be used for the classification of the interactions. 31 In the current work we examine changes in the topologies of the electron densities and energies, and therefore the nature of bonding, with the interatomic distances in the ChB systems. For clarity, the scope of the current study is limited to the structures containing Te-I and Se-Br interactions.…”