B. Vijaya Pandiyan

Studies on the σ–hole bonds (halogen, chalcogen, pnicogen and carbon bonds) based on the orientation of crystal structure

ABSTRACT The 4,7-dibromo-5,6-dinitro-2,1,3-benzothiadiazole (DDBT) crystal has two polymorphic forms [1], where the close contacts between two electronegative atoms are identified and studied through Hirshfeld surface analysis. Br---Br and O---O cut-off distances are addressed and analysed. The σ–hole of bromine, sulphur, oxygen atoms and π-hole of carbon and nitrogen atoms were subjected to study using molecular electrostatic potential map and 3D-deformation density map. Sixteen types of dimers from the two forms of crystal structure (6 for form I and 10 for form II) were studied using the charge transfer properties and interaction energies and made detailed analysis of halogen bond (Br---N), dihalogen bond (Br---Br), chalcogen bond (O---Br and S---Br), dichalcogen bond (S---O, O---S and O---O), pnicogen bond (N---O) and carbon bond (C---O and C---Br) interactions. The impact of orientations is discussed to define the type of interaction and its strength through charge transfer mechanism. The contribution of bond angle values for the σ-hole and π–hole bonds are discussed. Utilisation of σ–hole in smaller bond angle values (above 30°) of |θ1 − θ2| existing in type II halogen–halogen bond have been examined in the two forms.

A theoretical perspective of the nature of hydrogen-bond types – the atoms in molecules approach

Hydrogen bonds and their strength were analysed based on their X–H proton–donor bond properties and the parameters of the H–Y distance (Y proton acceptor). Strong, moderate and weak interactions in hydrogen-bond types were verified through the proton affinities of bases (PA), deprotanation enthalpies of acids (DPE) and the chemical shift (σ). The aromaticity and anti-aromaticity were analysed by means of the NICS (0) (nucleus-independent chemical shift), NICS (1) and ΔNICS (0), ΔNICS (1) of hydrogen-bonded molecules. The strength of a hydrogen bond depends on the capacity of hydrogen atom engrossing into the electronegative acceptor atom. The correlation between the above parameters and their relations were discussed through curve fitting. Baders theory of atoms in molecules has been applied to estimate the occurrence of hydrogen bonds through eight criteria reported by Popelier et al. The lengths and potential energy shifts have been found to have a strong negative linear correlation, whereas the lengths and Laplacian shifts have a strong positive linear correlation. This study illustrates the common factors responsible for strong, moderate and weak interactions in hydrogen-bond types.

Does the occurrence of resonance (by the delocalization of radical/cationic/anionic charges) induce the existence of intramolecular halogen–halogen contacts?

Exotic intramolecular homo/hetero dihalogen bonding (C–X⋯X–C: X = Br, Cl and F) in radical, cationic and anionic five-membered ring systems was analyzed using wave functional theory (MP2/aug-cc-pVTZ) analysis. The six types (Br–Br, Cl–Cl, F–F, Br–Cl, Cl–F and Br–F) of C–X⋯X–C interactions, stabilized by resonance, were created using delocalized radical/cationic/anionic carbon atoms in corresponding five-membered ring structures. The above interactions fall under the group of ‘resonance assisted noncovalent interactions’, where the impact of resonance is to induce the existence of intramolecular dihalogen bonding, even without electrostatic interaction. Further, the paper focuses on NCI bond length and stability, as these can be tuned through substitution effects. 3D-NCI plots and the presence of Bond Critical Points (BCP) clearly confirm the existence of dihalogen bonding. Natural bond orbital (NBO) analysis reveals that the dihalogen bonding in radical/cationic/anionic systems lacks charge transfer and orbital overlapping through non-interacting lobes. Specifically, σ- and π-holes exist not only for the electron depleted regions (positive regions) but also for the electron enriched regions (negative regions). The σ- and π-holes were not utilized for the C–X⋯X–C interactions because the interactions considered were not assisted by electrostatic interactions; instead, they were only assisted by resonance. Overall, this study clearly reveals that the impact of resonance formation (by delocalization of radical, cation, and anion charges) enhances the chance of occurrence of intramolecular halogen–halogen contacts.

Understanding the potency of fatty acids with the amino acid side chains of bovine β lactoglobulin—A quantum chemical approach

The present study aims to spotlight on the strength of interaction between different fatty acids with the Bovine-lactoglobulin (LGB) protein side chains along with the crystal water molecules at M062X/def2-QZVP level of theory. To analyse the role of carboxylic acid and its interaction with side chains and to reveal the significance of carboxylic acid, it was replaced by Acyl chloride (COCl), Acyl Bromide (COBr) and Acyl-Fluorine (COF) group and COS group. The ligands are linear with a straight and branched chain of carbon atoms, but extended methyl group make the ligand bend resulting in non-planar geometry. The least and highest band gap energy reveals the conductivity properties of ligands. 3UEW, the Palmitic acid is well-built owing to the interaction with the amino acid side chains Lys 69 and Glu 62 resulting in interaction energy of -124.98kcal/mol. 3D-NCIplot isosurface map and 2D-NCIplot graph plays a key role to confirm and analyse the occurrence of various non covalent interactions. The overall analysis of the fatty acids implies the fact that depending on the aliphatic chain length, the carboxyl group was capable of positioning favourable binding site.