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Density functional theory method and B3LYP/6‐311++G(d,p) level of theory were used to determine the acidity of α‐carbon in the hydroxyl derivatives of beta diketones in the gas phase. An investigation of acidity strength in the gas phase indicates that α‐carbon of hydroxyl derivatives of beta diketones become stronger acids than the α‐carbon of beta diketone itself as their conjugate bases gain more stability via both enolate and hydrogen bond formation. Natural bond orbital and quantum theory of atoms in molecules analyses also confirm the role of hydrogen bond interactions on increasing the acidity of α‐carbon of hydroxyl derivatives of beta diketones. [ABSTRACT FROM AUTHOR]

In our previous report (J. Phys. Org. Chem., 2017), we discussed the dual behavior of gold nanocluster (Au3 NC), where it scavenged reactive oxygen species (ROS) while promoted their generation to a lesser extent. Continuing this quest, we investigate the effects of Au3 NC on common reactive nitrogen species (RNS: ON˙ and ON˙O) and their precursors (ONH and ONOH, respectively), at B3LYP/LACVP+* level of theory. We compare the results with those of prevalent ROS (HO˙ and HOO˙) and their precursors (HOH and HOOH, respectively). To this end, various parameters are probed such as binding energy (Eb), bond dissociation energy (BDE), bond lengths, Mullikan spin density (MSD), critical point (CP) eigenvalues (atoms in molecules [AIM] analysis), and natural bond orbital (NBO) analysis including natural population analysis (NPA) and change of charge transfer energy (ΔECT). Our results show that in the presence of Au3 NC, BDEs of the NH and OH bonds increase in both ONHAu3 (1) and ONOHAu3 (2) compared with their free forms (ONH and ONOH, respectively). In other words, the tendency to dissociate these bonds and form their corresponding RNS (1′ and 2′) decreases in the presence of gold. This is exactly the opposite of what is observed for the OH bonds in HOHAu3 (3), HOOHAu3 (4), and their corresponding ROS (3′ and 4′). Therefore, we can conclude that gold shows a different behavior in the face of RNS compared with ROS where it is a better scavenger of ROS than RNS. [ABSTRACT FROM AUTHOR]

Abstract: Anticancer character of gold cluster has been indicated through its free radical scavenging properties. This is in contrast to its free radical promoting ability suggested by other workers. Here, we address this controversy by probing the stabilizing effects of Au3 cluster on RO• vs its impacts on RO–H bond dissociation enthalpy, at B3LYP/ LACVP+* level (R═H, methyl, ethyl, n‐propyl, i‐propyl, n‐butyl, t‐butyl, and phenyl). In the presence of Au3 cluster, bond dissociation enthalpy of O–H bond and the spin density at the RO• oxygen are reduced dramatically. These are clear evidences for both the Au3 facilitation of the RO–H bond breakage and its scavenging of RO• radical. Since O–Au anchoring bond is responsible for the interaction of Au3 cluster and ROH (or RO•), its nature was interpreted by means of the quantum theory of atoms in molecules and the natural bond orbital. The results indicate that O–Au bond is stronger and has more covalent character in RO•–Au3 than in ROH–Au3. The interaction of Au3 cluster with RO• is 1.5 to 3 times more than that with ROH. As a result, gold cluster scavenging property appears more prominent than its free radical initiation activity. [ABSTRACT FROM AUTHOR]

For the first time, the mechanism of Fischer esterification between acetic acid and ethanol over silica-functionalized propylsulfonic acid (silica-propyl-SO 3 H) catalyst was explored by means of computational modeling techniques. For this purpose, 6-edge-atom cage-like cluster comprising Si–O–Si sequences has been selected to represent the surface of the catalyst. The results indicate that the reaction goes through concerted transition states. In all optimized structures no proton (H + ) transfer occurs from catalyst to the substrates and the role of the catalyst is via the activation of the substrates through the formation of strong hydrogen bonds (H-bonds). Furthermore, the energetic diagram demonstrates that the activation energies of forward (esterification) and reverse (ester hydrolysis) reactions only differ by 0.3 kcal/mol. The quantum theory of atoms in molecules (QTAIM) analysis was performed to investigate the nature of H-bonds. Another mechanism in which the catalyst has been acted as the Brønsted acid/base was also provided for this reaction. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) was used to investigate the acidity of the various silica alkyl sulfonic acids. In this regard, cluster models with various alkyl spacer lengths were selected to mimic the surface of silica gel. The effects of distance from the surface and the role of hydrogen bond (H–bond) on the ΔHacidityvalues of these catalysts were investigated. DFT calculations revealed that a notable gap of ΔHacidityvalues exists between the structures considering lateral hydrogen bonding with the surface of the silicaHB structureand the structures with omitted surface interactions (non-HB structures). Natural bonding orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses were carried out to obtain detailed information about the nature of the H–bonds. [ABSTRACT FROM AUTHOR]

Highlights: [•] Increasing methylene groups in aryl amines results in higher cation–π interactions. [•] There is good correlation between proton affinities and Hammett’s parameters. [•] Hydrogen atom of ammonium group is responsible for making cation–π interactions. [ABSTRACT FROM AUTHOR]

Silica gelâ€confined ionic liquid (IL) is a class of heterogeneous catalysts with broad catalytic applications. Leaching of the IL from the surface of the support is the major drawback of these catalysts, which reduce the catalyst efficiency during the chemical reactions. To investigate the effect of the hydrogen bonding on the leaching phenomena, the interaction between the 1â€ethylâ€3â€methylimidazoliumâ€based IL with various anions (Clâˆ', Brâˆ', HSO4âˆ', NO3âˆ', BF4âˆ', and PF6âˆ') and the surface of the silica gel were studied using density functional theory. Hence, a hydroxylated cageâ€like cluster of silica gel, Si4O6(OH), was selected to mimic the surface. The values of Î'Einteraction show that ILs with halogen counter ions have stronger interactions than that of the IL with BF4âˆ' and PF6âˆ' counter ions. We also carried out stirringâ€'filtration method for some prepared samples of IL@silica to experimentally explore the leaching phenomena. The results show good agreement with computational achievements. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights: [•] Good correlation exists between experimental and computed pK a values. [•] The average error was found to be less than 1.6pK a unit. [•] In the series of analogous structures, the PA values decrease as the number of phenyl groups increases. [ABSTRACT FROM AUTHOR]

In this study, the drastic influence of the boron atom on the acidity of alcohol has been con-sidered. The calculated ΔHacid (320.9--338.1 kcal/mol) and pKa range of boron containing alcohol (--0.1--9.4) indicate that the boronation of alcohol leads to considerable enhancement of its acidity. For instance, we have obtained the ΔHacid values 338.1, 335.2 kcal/mol and the pKa values 4.12, 2.81 for BH2CH2OH, BF2CH2OH alcohols, respectively, which are much smaller than that of CH3OH (with ΔHacid = 374.9kcal/mol and pKa = 15). The increase in the acidity of boronated alcohol can be related to the stabilization of alkoxy ion due to overlap of unoc-cupied orbital of boron atom with the electron pairs of negative oxygen. All gas phase computations were performed at MP2/6-311++G(d,p)//(B3LYP/6-31+G(d)) level. The primary results indicate that the presence of boron atom in an alcohol might make it as acidic as nitric acid. The geometry optimization of studied structures was performed with DFT computation and optimized structures were used to carry out natural bond orbital (NBO) analysis. NBO analysis revealed that the increase in the acidity of boron-containing alcohols is due to the charge transfer from the negative oxygen (in deprotonated structure) to the empty orbital of --BH2 and --BF2. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of bonds formed in the deprotonated structure. [ABSTRACT FROM AUTHOR]

Abstract: With the specific target of calculating the pK a values of boronic acids (RB(OH)2) in aqueous solution we inquired the solute–solvent interactions of these acids and their corresponding conjugate base. Relative pK a values were computed for each boronic acid using methylboronic acid (CH3B(OH)2) as a reference. All gas phase computations were performed at (MP2/6-311++G(d,p)//B3LYP/6-31+G(d)) level of theory. Solvation was included in the calculations using the polarized continuum model (PCM) at the HF/6-31G(d,p) level. The geometry optimization of studied structures was performed with DFT computation and the optimized structures were used to carry out Natural Bond Orbital (NBO) analysis. Natural Bond Orbital (NBO) analysis was examined as indicator for the variations observed in the calculated pK a. Natural resonance theory (NRT) was used to calculate the natural bond order and predict bond polarity. The NRT results indicate that the O–H bonds in all studied compounds have an electrovalency character. The results revealed that difference in acid strength is more relevant to the type of substituent. The electron withdrawing substituent will increase the acidic strength. However, the electron releasing substituent will have a reverse effect and decrease the acidic strength. There are good agreement between theoretical values and experimental results for this series of compounds, the average error was found to be less than 1.2 pK a unit. [Copyright &y& Elsevier]

Density functional theory (DFT) calculations were performed at the B3LYP/6-311++G(d, p) level to determine coordination geometries, absolute metal ion affinities, and free energies for all possible complexation stable products formed by monovalent metal cations including Li+, Na+, K+ with the nucleoside 2′-Deoxyguanosine. All computations indicate that the metal ion affinity (MIA) decreases on going from Li+ to Na+ and K+ for 2′-Deoxyguanosine. For example, the affinities for the metal ions described above are 75.2, 57.3, 43.4 kcal/mol, respectively. Furthermore, the influence of metal cationization and O6 protonation on the strength of the N-glycosidic bond, torsion angles and angle of pseudorotation (P) have been studied. With respect to the results, it has been found that metal binding significantly change the values of the phase angle of pseudorotation (P) in the sugar unit of these nucleosides but, O6-protonation do not significantly change the values of the torsion angles and angle of pseudorotation (P). In all modified forms, the length of the C1'–N9 bond increases. The Mulliken population analysis and natural bond orbital (NBO) analysis on atomic charges have been carried out on the optimized geometries. [ABSTRACT FROM AUTHOR]

The effects of selected metal ions on the gas-phase acidity of weak organic acids have been explored using the DFT and Moller–Plesset Perturbation Theory (MP2) calculations. The three organic acids selected for this study were acetic acid (aliphatic), benzoic acid (aromatic), and glycine (amino acid). The acidities of these compounds are compared with the acidity of their Li+-, Na+-, and K+-complexed species. The results indicate that upon complexation with Li+, Na+, and K+ at 298 K, the gas-phase acidity of acetic acid, for example, varies from 345.3 to 218.8, 230.2, and 240.1 kcal/mol, respectively (i.e., its dissociation becomes much less endothermic). These values indicate that a weak organic acid can be converted to a super acid when it is complexed with an ionic metal. Copyright © 2007 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The derivatives of glucose such as glucosamine (β-D-GlcN) and N-acetyl-D-β-glucosamine (GlcNAc) are significant in several biological systems. D-GlcN has been used widely to treat osteoarthritis in humans and animal models as well as GlcNAc has been proposed as a treatment for autoimmune diseases. The DFT/B3LYP/6–311++G (d,p) method as well as QTAIM and NBO analyses were used to the acidity values of D-GlcN and GlcNAc sugars and their complexes with alkali ions in the gas phase. The Li+, Na+ and K+ prefer bi-dentate chelate in these complexes. The computed results indicate that metal ion affinity (MIA) in GlcNAc is higher than that in D-GlcN. There are direct correlations between the MIA values of D-GlcN and GlcNAc sugars and the atomic numbers of Li, Na, and K. The calculated acidity values for GlcNAc at C 2 –NH and C 6 –HO 6 sites are 331 and 333 kcal mol−1, respectively. Whereas the calculated acidity values for D-GlcN at C 2 –NH and C 6 –HO 6 sites are 365 and 372 kcal mol−1, respectively. The AIM and NBO analyses indicate the presence of intramolecular H-bonds in GlcNAc sugar in both its neutral form and conjugate base; whereas D-GlcN indicates intramolecular H-bonds in only its conjugate base. Image 1 • Hydrogen bonding effect on Acidity of d -Glucosamine and N-Acetyl- d -Glucosamine. • Metal Ion Affinity of d -Glucosamine and N-Acetyl- d -Glucosamine. • D-Glucosamine and N-acetyl-D-glucosamine as ideal cation exchangers models. [ABSTRACT FROM AUTHOR]

Abstract: Histidine as a natural amino acid is found to be biologically important and is known to function as a nucleophile or enzyme co-factor, or in proton transfer process. The properties of gaseous aromatic amino acid histidine depend on the structural forms it may take in gas-phase. Ab initio method has been used to characterize the gas-phase conformer/tautomers of histidine. Wide range of possible structures for histidine was surveyed at the MM level, and then the geometries of the unique conformers were refined at the B3LYP/6-311++G (d,p) levels. At this theoretical level, 25 conformers were located for both tautomers of histidine i.e., His [NπH] and His [NτH]. The MM level provides a poor description of the relative energies. Calculations at the B3LYP/6-311++G (d,p) level represent a significant improvement. Ab initio dipole moments are reported for all the conformers/tautomers. The results are compared to previous studies of amino acids and are analyzed in terms of intramolecular hydrogen-bonding interactions and Newman projections. In addition, we calculated the infrared frequencies and intensities of the most stable structures in order to assist in the assignment of hydrogen-bonding. Furthermore, Ramachandran backbone potential energy surfaces (PES) of 25 conformers of His [NπH] and His [NτH] tautomers of histidine were considered. The stationary points characterizing the potential energy profile of the various conformers/tautomers of histidine were investigated by the same density functional theory B3LYP/6-311++G (d,p). Minima and transition states characterizing the energetic paths for the interconversion of various structures of histidine were explored in detail. [ABSTRACT FROM AUTHOR]