ABSTRACT:The RAHB systems in malonaldehyde and its derivatives at MP2/ 6-311þþG(d,p) level of theory were studied and their intramolecular hydrogen bond energies by using the related rotamers method was obtained. The topological properties of electron density distribution in OAHÁÁÁO intramolecular hydrogen bond have been analyzed in term of quantum theory of atoms in molecules (QTAIM). Correlations between the H-bond strength and topological parameters are probed. The results of QTAIM clearly showed that the linear correlation between the electron density distribution at HB critical point and RAHB ring critical point with the corresponding hydrogen bond energies was obtained. Moreover, it was found a linear correlation between the electronic potential energy density, V(r cp ), and hydrogen bond energy which can be used as a simple equation for evaluation of HB energy in complex RAHB systems. Finally, the similar linear treatment between the geometrical parameters, such as OÁÁÁO or OAH distance, and Lp(O)!r* OH charge transfer energy with the intramolecular hydrogen bond energy is observed.

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This study delves into the interaction between benzodiazepine (BZD) drugs and 2-hydroxypropyl-β-cyclodextrin (2HPβCD), a cyclodextrin (CD) known to improve drug delivery and enhance therapeutic outcomes. We find that the 2HPβCD’s atoms become more rigid in the presence of chlordiazepoxide (CDP), clonazepam (CLZ), and diazepam (DZM), whereas they become more flexible in the presence of nordazepam (NDM) and nitrazepam (NZP). We also investigated the structure of 2HPβCD and found that loading these drugs increases both the area and volume of the 2HPβCD cavity, making it more suitable for drug delivery. Moreover, this research found that all drugs exhibited negative values for the binding free energy, indicating thermodynamic favorability and improved solubility. The binding free energy order of the BZDs was consistent in both molecular dynamics and Monte Carlo methods, with CDP and DZM having the highest affinity for binding. We also analyzed the contribution of different interaction energies in binding between the carrier and the drugs and found that Van der Waals energy is the primary component. Our results indicate that the number of hydrogen bonds between 2HPβCD/water slightly decreases in the presence of BZDs, but the hydrogen bond’s quality remains constant.

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ABSTRACT:The conformational study of b-thioaminoacrolein was performed at various theoretical levels, HF, B3LYP, and MP2 with 6-311þþG(d,p) basis set, and the equilibrium conformations were determined. To have more reliable energies, the total energies of all conformers were recomputed at high-level ab initio methods, G2MP2, G3, and CBS-QB3. According to these calculations, the intramolecular hydrogen bond is accepted as the origin of conformational preference in thialamine (TAA) and thiolimine groups. The hydrogen bond strength in various resonance-assisted hydrogen bond systems was evaluated by HB energy, geometrical parameters, topological parameters, and charge transfers corresponding to orbital interactions. Furthermore, our results reveal that the TAA tautomer has extra stability with respect to the other tautomers. The population analyses of the possible conformations by NBO predict that the origin of this preference is mainly due to the p-electron delocalization in framework of TAA forms, especially usual p C¼ ¼C ! p* C¼ ¼S and Lp (N) ! p* C¼ ¼C charge transfers.