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Context: Herein, we compare and contrast the dual roles of Cun clusters (n = 3, 5, and 7 atoms) in scavenging or generating RO• free radicals from ROH at the theoretical levels (where R = H, methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, and phenyl). This investigation is performed in water media to mimic the actual environment in the biological system. In the presence of the Cun clusters, bond dissociation energy (BDE) of RO–H and R–OH is reduced. This is clear evidence for the increased possibility of both the RO–H and R–OH bonds breakage and scavenging of RO• radicals. The nature of anchoring bonds responsible for the interaction of Cun clusters with ROH and RO• are interpreted using the quantum theory of atoms in molecules (QTAIM) and the natural bond orbital (NBO) analysis. The DFT results indicate that the O•⋅⋅⋅•Cu bond is stronger and has more covalent character in RO•⋅⋅⋅•Cun radical complexes than in ROH⋅⋅⋅•Cun. Therefore, the interactions of Cun clusters with RO• radicals (antioxidant) are more pronounced than their interactions with ROH non-radicals (pro-oxidant). Methods: The GAMESS software package was utilized in this paper. The B3LYP and M06 functions with the 6–311 + + G(d,p), and LANL2DZ/SDD basis sets was used to perform the important geometrical parameters of RO•⋅⋅⋅•Cun and ROH⋅⋅⋅•Cun, binding energy (Eb), and bond dissociation energy (BDE). [ABSTRACT FROM AUTHOR]

A quantum chemical perspective of 31 structures contains electron acceptors: ASCl3 (arsenic trichloride), PCl3 (phosphorous trichloride) and NCl3 (nitrogen trichloride); forming non-covalent bond with various nitrogen-based electron donors that resulted in pnicogen bonds, AS...N, P...N and N...N were calculated at M062X/def2-QZVP level of theory. Besides the above method, MP2/def2-QZVP and CCSD(T)/def2-QZVP level of theories have also been analysed to have in depth knowledge about the bonds formed. The nature of the bonds was assumed from the electrostatic potential evaluated for all the monomers, where σ hole is positive for all the monomers. The strongest pnicogen bonds are ASCl3–NF2H, PCl3–NCH3CH3CH3 and NCl3–NCH3CH3CH3 having interaction energies as −4.15, −11.58 and −3.25 kcal/mol, respectively, at MP2/def2-QZVP level of theory. Further at CCSD(T)/def2-QZVP level of theory, ASCl3–NF2H and NCl3–NCH3CH3CH3 are found to be the most stable with interaction energies as −3.53 and −2.45 kcal/mol, respectively. The potential energy surface scan was performed for all the stable complexes in order to confirm the existences of energies are true minima. Moreover to confirm the halogen and pnicogen bonds, AIM analysis was carried out. The results from the above factors of pnicogen bond will help crystal growth, material science and engineering community to explore novel materials, which abide for modernization. [ABSTRACT FROM AUTHOR]

Ranitidine is a histamine H2-receptor antagonist that reduces gastric acid secretion. We studied the flexibility of the ranitidine molecule with the special focus on the network of diverse intramolecular hydrogen bonds: NH · · · O, N-H· · · N, C-H· · · O, C-H· · ·N and N-H· · · S. We performed static density functional theory calculations of global and local minima and analyzed their stability at finite temperature in the Car–Parrinello molecular dynamics simulations. We observed intramolecular H-bonds breaking/ formation crucial for the structural rearrangements leading to the folding process. The lifetimes of the closed structures of ranitidine were also estimated. The existence of hydrogen bonds and their strength were confirmed on the basis of topological parameters in the bond critical points utilizing Quantum Theory of Atoms in Molecules. [ABSTRACT FROM AUTHOR]

This work describes theoretical and experimental studies on glycerol esterification to obtain acetins focusing on the obtained isomers. The reaction of glycerol with acetic acid was carried out on Amberlyst 36 wet. Density functional theory calculations on the level of M06-2X functional and 6-311+G(d,p) basis set are carried out and the most stable structures of the reactants and products are located by considering a large number of conformers. The thermodynamics is discussed in terms of the calculated reaction Gibbs free energy. The AIM theory was used to characterize reactants and products. The glycerol esterification with acetic acid is found to be thermodynamically favored, with exothermal property. These agree well with experiments and allow us to explain the relative selectivity of products. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The structure, spectral properties and the hydrogen bond interactions of 8-aza analogues of xanthine, theophylline and caffeine have been studied by using quantum chemical methods. The time-dependent density functional theory (TD-DFT) and the singly excited configuration interaction (CIS) methods are employed to optimize the excited state geometries of isolated 8-azaxanthine, 8-azatheophylline tautomers and 8-azacaffeine in both the gas and solvent phases. The solvent phase calculations are performed using the polarizable continuum model (PCM). The absorption and emission spectra are calculated using the time-dependent density functional theory (TD-DFT) method. The results from the TD-DFT calculations reveal that the excitation spectra are red shifted relative to absorption in aqueous medium. These changes in the transition energies are qualitatively comparable to the experimental data. The examination of molecular orbital reveals that the molecules with a small H→L energy gap possess maximum absorption and emission wavelength. The relative stability and hydrogen bonded interactions of mono and heptahydrated 8-azaxanthine, 8-azatheophylline tautomers and 8-azacaffeine have been studied using the density functional theory (DFT) and Møller Plesset perturbation theory (MP2) implementing the 6-311++G(d,p) basis set. The formation of strong N-H...O bond has resulted in the highest interaction energy among the monohydrates. Hydration does not show any significant impact on the stability of heptahydrated complexes. The atoms in molecule (AIM) and natural bonding orbital (NBO) analyses have been performed to elucidate the nature of the hydrogen bond interactions in these complexes. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

A quantum chemical perspective of 31 structures contains electron acceptors: ASCl 3 (arsenic trichloride), PCl 3 (phosphorous trichloride) and NCl 3 (nitrogen trichloride); forming non-covalent bond with various nitrogen-based electron donors that resulted in pnicogen bonds, AS...N, P...N and N...N were calculated at M062X/def2-QZVP level of theory. Besides the above method, MP2/def2-QZVP and CCSD(T)/def2-QZVP level of theories have also been analysed to have in depth knowledge about the bonds formed. The nature of the bonds was assumed from the electrostatic potential evaluated for all the monomers, where σ hole is positive for all the monomers. The strongest pnicogen bonds are ASCl 3 -NF 2 H, PCl 3 -NCH 3 CH 3 CH 3 and NCl 3 -NCH 3 CH 3 CH 3 having interaction energies as -4.15, -11.58 and -3.25 kcal/mol, respectively, at MP2/def2-QZVP level of theory. Further at CCSD(T)/def2-QZVP level of theory, ASCl 3 -NF 2 H and NCl 3 -NCH 3 CH 3 CH 3 are found to be the most stable with interaction energies as -3.53 and -2.45 kcal/mol, respectively. The potential energy surface scan was performed for all the stable complexes in order to confirm the existences of energies are true minima. Moreover to confirm the halogen and pnicogen bonds, AIM analysis was carried out. The results from the above factors of pnicogen bond will help crystal growth, material science and engineering community to explore novel materials, which abide for modernization. Graphical abstract PCl 3 -NCH 3 CH 3 CH 3 complex with 2.61 Å and pnicogen angle of 178.54° is strong, and interaction energy is -11.58 kcal/mol. Electron donors - ASCl 3 , PCl 3 and NCl 3 and electron acceptors -NCH 3 CH 3 CH 3 , NH 3 C 2 and NHCO have strong electrostatic contribution. High and low values of (ρ) ∇ 2 (ρ) reveal the strong and weak pnicogen bond. Schematic representation of acceptors surrounded by its donors and Electrostatic Potential map.