Gladis L. Sosa

Halogen Bonding: A Study based on the Electronic Charge Density

Density functional theory (DFT) and atoms in molecules theory (AIM) were used to study the characteristic of the noncovalent interactions in complexes formed between Lewis bases (NH(3), H(2)O, and H(2)S) and Lewis acids (ClF, BrF, IF, BrCl, ICl, and IBr). In order to compare halogen and hydrogen bonds interactions, this study included hydrogen complexes formed by some Lewis bases and HF, HCl, and HBr Lewis acids. Ab initio, wave functions were generated at B3LYP/6-311++G(d,p) level with optimized structures at the same level. Criteria based on a topological analysis of the electron density were used in order to characterize the nature of halogen interactions in Lewis complexes. The main purpose of the present work is to provide an answer to the following questions: (a) why can electronegative atoms such as halogens act as bridges between two other electronegative atoms? Can a study based on the electron charge density answer this question? Considering this, we had performed a profound study of halogen complexes in the framework of the AIM theory. A good correlation between the density at the intermolecular bond critical point and the energy interaction was found. We had also explored the concentration and depletion of the charge density, displayed by the Laplacian topology, in the interaction zone and in the X-Y halogen donor bond. From the atomic properties, it was generally observed that the two halogen atoms gain electron population in response to its own intrinsic nature. Because of this fact, both atoms are energetically stabilized.

A search for C–H⋯O type hydrogen bonds in Lamivudine (3TC). An exploratory conformational and electronic analysis

Abstract A conformational study of the molecule Lamivudine (3TC), or cis-1-[2′-hydroxymethyl-5′-(1,3-oxathiolanyl)] cytosine, was carried out. Rotation about the C–N bond (ϕ1) and about the C–CH2(OH) bond (ϕ2), which connects the hydroxymethyl group to the five member ring, led to a conformational potential energy surface. The conformational potential energy 2D map, obtained at the HF/3-21G level of theory, had several minima. A topological analysis of the electron density was carried out on four selected ab initio minimum energy conformations, using judiciously constructed hartree–fock (RHF) wave functions. In order to see all possible hydrogen bonding, the DFT wave function was generated using a mixed basis set; a 6-311++G∗∗ basis was employed on atoms involved in hydrogen bonding interactions and a 3-21G basis on all other atoms. For this analysis the theory of atoms in molecules, developed by Bader, was used. The stability of the intramolecular hydrogen bonding interactions was analyzed in terms of the results obtained.

Multicenter (FX)n/NH3 Halogen Bonds (X = Cl, Br and n = 1–5). QTAIM Descriptors of the Strength of the X∙∙∙N Interaction

In the present work an in depth deep electronic study of multicenter XBs (FX)n/NH3 (X = Cl, Br and n = 1–5) is conducted. The ways in which X∙∙∙X lateral contacts affect the electrostatic or covalent nature of the X∙∙∙N interactions are explored at the CCSD(T)/aug-cc-pVTZ level and in the framework of the quantum theory of atoms in molecules (QTAIM). Calculations show that relatively strong XBs have been found with interaction energies lying between −41 and −90 kJ mol−1 for chlorine complexes, and between −56 and −113 kJ mol−1 for bromine complexes. QTAIM parameters reveal that in these complexes: (i) local (kinetics and potential) energy densities measure the ability that the system has to concentrate electron charge density at the intermolecular X∙∙∙N region; (ii) the delocalization indices [δ(A,B)] and the exchange contribution [VEX(X,N)] of the interacting quantum atoms (IQA) scheme, could constitute a quantitative measure of the covalence of these molecular interactions; (iii) both classical electrostatic and quantum exchange show high values, indicating that strong ionic and covalent contributions are not mutually exclusive.