Rubicelia Vargas

Strength of the NH···OC and CH···OC Bonds in Formamide andN-Methylacetamide Dimers

The structures of the dimers of formamide and N-methylacetamide have been calculated at the ab initio electronic structure theory level, 2nd order Miller-Plesset perturbation theory (MP2) with augmented correlation consistent basis sets. Five unique structures were optimized for the formamide dimers at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels. At the optimized geometries obtained with the aug-cc-pVTZ basis set, MP2 energies were evaluated with the aug-cc-pVQZ basis set allowing an extrapolation of the energies to the complete basis set limit. Four structures were found for the N-methylacetamide dimer at the MP2/aug-cc-pVDZ level and single point energies were calculated at the MP2/aug-cc-pVTZ level. In both systems, the basis set superposition error (BSSE) was estimated with the counterpoise method. The strength of the N-HiiiO=C bond has a mean value of 7.1 kcal/mol in the formamide dimers and a mean value of 8.6 kcal/mol in the N-methylacetamide dimers. The difference in hydrogen bond strengths is attributed to differences in basicity at the carbonyl oxygen receptor site. In several dimers C-HiiiO=C hydrogen bonds play an important role in stabilizing these intermolecular complexes increasing the interaction energy by 1.1 to 2.6 kcal/mol per interaction.

What is Important to Prevent Oxidative Stress? A Theoretical Study on Electron-Transfer Reactions between Carotenoids and Free Radicals

Oxidative stress is related to the development of a large number of health disorders. Therefore, the study of molecular systems capable of preventing its onset by fighting free radicals is a crucial area of research. Carotenoids are one of the most efficient families of compounds fulfilling this purpose. In the present work, the free-radical-scavenger efficiency, expressed as the one-electron-donating capability, of different carotenoids has been studied using density functional theory. A large number of free radicals were considered, as well as environments of different polarity. A new donor-acceptor map is proposed that allows a rapid evaluation of full electron-transfer processes. Its efficiency for predicting the feasibility of electron transfer (ET) between carotenoids and free radicals was tested and validated through comparison with the corresponding Gibbs free energies of reaction. Our results demonstrate that ET reactions between carotenoids and free radicals are strongly influenced by the nature of the latter. Moreover, it is proposed that the electron affinity (EA) of the reacting free radical has an important effect on the viability of these reactions. The reactions were found to become exergonic when the EA of the free radical involved reaches a value of approximately 5 eV.

Free Radical Scavenger Properties of α-Mangostin: Thermodynamics and Kinetics of HAT and RAF Mechanisms

Mangosteen is a tropical fruit that presents beneficial effects on human health since it is rich in anthocyanins and xanthones, which are considered bioactive compounds that have been described as good free radical scavengers. One of its most active compounds is α-mangostin. In this report, a theoretical study on the free radical scavenger capacity of α-mangostin and its monoanion is analyzed using the density functional theory approximation. Two well-known reaction mechanisms are investigated: the hydrogen atom transfer (HAT) and the radical adduct formation (RAF). Two other mechanisms are also considered: sequential electron proton Transfer (SEPT) and proton coupled electron transfer (PCET). According to thermodynamics and kinetics, α-mangostin and its deprotonated form are good free radical scavenger through the HAT mechanism, with the anionic (deprotonated) form being more reactive than the neutral one. Their capacity to scavenge OOH free radical is similar to that of carotenes, higher than that of allicin, much higher than that of melatonin and N-acetylcysteine amide, and about 15 times lower than that of 2-propenesulfenic acid.

Mean excitation energy, static polarizability, and hyperpolarizability of the spherically confined hydrogen atom

Calculations of mean excitation energy, Im, static polarizability, α, and hyperpolarizability, γ, using the variation perturbation procedure are reported for the spherically confined hydrogen atom. The electric response properties α and γ have been found to strongly depend upon the radius of confinement. The hyperpolarizabilty changes sign and becomes negative under strong confinement.

DFT reactivity indices in confined many-electron atoms

The density functional descriptors of chemical reactivity given by electronegativity, global hardness and softness are reported for a representative set of spherically confined atoms of IA, IIA, VA and VIIIA series in the periodic table. The atomic electrons are confined within the impenetrable spherical cavity defined by a given radius of confinement satisfying the Dirichlet boundary condition such that the electron density vanishes at the radius of confinement. With this boundary condition the non-relativistic spin-polarized Kohn-Sham equations were solved. The electronegativity in a confined atom is found to decrease as the radius of confinement is reduced suggesting that relative to the free state the atom loses its capacity to attract electrons under confined conditions. While the global hardness of a confined atom increases as the radius of confinement decreases, due to the accompanying orbital energy level crossing, it does not increase infinitely. At a certain confinement radius, the atomic global hardness is even reduced due to such crossover. General trends of the atomic softness parameter under spherically confined conditions are reported and discussed.

Static dipole polarizability of shell-confined hydrogen atom

Using the Sternheimer perturbation-numerical procedure, calculations of static dipole polarizability are reported for the shell-confined hydrogen atom as defined by two impenetrable concentric spherical walls. Unusually high polarizability states are predicted for the hydrogen atom as the inner sphere radius is increased to larger values inside the outer sphere of a constant radius. Implications of this model in mimicking internal compression leading to the metallic behaviour of the shell-confined hydrogen atoms are discussed.

Shell structure in free and confined atoms using the density functional theory

Abstract The average local electrostatic potential function, defined as the electrostatic potential divided by the electron density, is used to study the shell structure in free and confined atoms within Kohn–Sham density functional theory. Several exchange-correlation functionals have been used to calculate the average potential function. It was observed that the self-interaction correction significantly alters the shell structure along the large radial distances. Many electron atoms confined in a sphere exhibit a gradual loss of the shell structure as the confinement is increased. The loss of structure can be characterized by the sphere radius r c and in the limit r c →0, the electron gas behavior is obtained.

The association of neutral systems linked by hydrogen bond interactions: a quantitative electrochemical approach

The electrochemical characterization of the neutral–neutral association by hydrogen bonds was performed on the basis of voltammetric current measurements. The diffusion coefficient of the electroactive compound is modified by effect of association, this provoke important variations in the voltammetric current peak. As an example of the weak hydrogen bond between neutral complexes, it was determined that 1,4-benzoquinone (Q) and benzoic acid (HBz) can associate with a 1:1 stoichiometry with a conditional association constant between 10 and 15 M � 1 . The Q(HBz) complex geometry was optimized using density functional theory and Moller–Plesset perturbation theory. In both theories, the most stable geometry is flat and exhibits two hydrogen bond interactions: O–H ��� O and C–H ��� O interactions. The binding energy at our best level of theory was )7.7 kcal/mol, that supports the stability of the 1:1 Q–HBz complex and which is accord with the values of the conditional association constant obtained from the electrochemical method here described. 2002 Elsevier Science B.V. All rights reserved.

Atomic ionization radii using Janak's theorem

Abstract Accurate calculations of the atomic ionization radius defined as the critical radius at which a spherically confined atom undergoes ionization are reported for the atoms He–Ca using the Kohn–Sham variational method in conjunction with the Slater transition state approximation to Janaks theorem of the density functional theory.

Roothaan’s approach to solve the Hartree-Fock equations for atoms confined by soft walls: Basis set with correct asymptotic behavior

In this report, we use a new basis set for Hartree-Fock calculations related to many-electron atoms confined by soft walls. One- and two-electron integrals were programmed in a code based in parallel programming techniques. The results obtained with this proposal for hydrogen and helium atoms were contrasted with other proposals to study just one and two electron confined atoms, where we have reproduced or improved the results previously reported. Usually, an atom enclosed by hard walls has been used as a model to study confinement effects on orbital energies, the main conclusion reached by this model is that orbital energies always go up when the confinement radius is reduced. However, such an observation is not necessarily valid for atoms confined by penetrable walls. The main reason behind this result is that for atoms with large polarizability, like beryllium or potassium, external orbitals are delocalized when the confinement is imposed and consequently, the internal orbitals behave as if they were in an ionized atom. Naturally, the shell structure of these atoms is modified drastically when they are confined. The delocalization was an argument proposed for atoms confined by hard walls, but it was never verified. In this work, the confinement imposed by soft walls allows to analyze the delocalization concept in many-electron atoms.