Hybrid DFT study on non-covalent interactions and their influence on pKa's of magnesium-carboxylate complexes.

The electronic structure, stability, acidity, topological properties and charge transfer of octahedral magnesium-carboxylate complexes have been investigated at M05-2X/6-311G** level of theory. The interaction energy studies confirm the stability of the pentahydrated magnesium compounds. The calculated pKa values using SMD solvation model through the deprotonation of metal-ion bound water molecule vary from 15.08 to 19.36, enunciate the role of Mg-O bonds on the acidity of the Mg+2 complexes. The closed shell ionic interactions of Mg-ligand bonds are assessed by the atoms in molecules (AIM) and the electron localization function (ELF) analyses. A reduced density gradient approach is utilized to explore the weak interactions including dihydrogen bonds.

Spectroscopic investigations and hydrogen bond interactions of 8-aza analogues of xanthine, theophylline and caffeine: a theoretical study.

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.

Microsolvation and hydrogen bond interactions in Glycine Dipeptide: molecular dynamics and density functional theory studies.

Molecular dynamics (MD) simulations were carried out to study the conformational characteristics of Glycine Dipeptide (GD) in the presence of explicit water molecules for over 10 ns with a MD time step of 2 fs. The density functional theory (DFT) methods with 6-311G** basis set have been employed to study the effects of microsolvation on the conformations of GD with 5-10 water molecules. The interaction energy with BSSE corrections and the strength of the intermolecular hydrogen bond interactions have been analyzed. The Bader's Atoms in Molecules (AIM) theory has been employed to investigate H-bonding patterns in water interacting complexes. The natural bond orbital (NBO) analysis has been carried out to analyze the charge transfer between proton acceptor to the antibonding orbital of the XH bond in the hydrated complexes. NMR calculations have been carried out at B3LYP/6-311G (2d, 2p) level of theory to analyse the changes in structure and hydrogen bonding environment that occur upon solvation.