A thermodynamical cycle is proposed to calculate absolute pK a values for a Brönsted acid in aqueous solution. The solvent (water) was represented by a dielectric using the polarizable continuum model (PCM), and the absolute pK a values of some aliphatic carboxylic acids were computed. The results indicate that the proposed methodology seems to be capable of predicting reasonably good absolute pK a values, although in some cases appreciable deviations are observed, which can be related to neglecting the molecular motion contributions (ΔG Mm) to the solvation energy (ΔG solv).

A thermodynamical cycle is proposed to calculate absolute pK a values for Brönsted acids in aqueous solution. The polarizable continuum model (PCM) was used to describe the solvent, and absolute pK a values were computed for different classes of organic compounds:  aliphatic alcohols, thiols, and halogenated derivatives of carboxylic aliphatic acids. The model furnishes pK a values in good agreement with the experimental results for some classes of compounds. For the cases where appreciable deviations are observed, we have tried to establish a correlation among the neglected components of Δ G solv resulting from the model adopted, the level of calculation employed, and the pK a deviations relative to the experimental results.

Ab initio calculations were performed for X ϩ (H 2) n clusters ͓XϭLi, Na (nϭ1-7) and K (n ϭ1-3)͔. For nϭ1-6, the equilibrium geometries correspond to spherically symmetrical distributions of H 2 units around the X ϩ. The binding energies and the geometric parameters indicate that the seventh H 2 unit opens a new shell of ligands for the cluster with XϭLi but not for XϭNa.

The main driving force for the formation of the covalent bond is the quantum-mechanical interference effect among one-electron states, as has been suggested in several works by the use of partition schemes to calculate the interference contributions to the energy. However, due to some difficulties associated with the original approaches, calculations were only carried out for a few, mostly diatomic molecules. In this work, we propose a general approach of partitioning based on generalized product functions with generalized valence bond at the perfect pairing approximation and spin-coupled groups, which should allow the investigation of a broader array of molecules, and hopefully, shed light on the nature of the chemical bond in molecules with unusual chemical features. Among other things, this approach lends itself naturally to the investigation of interference in individual bonds or groups of bonds in a molecule.

The dehydrogenation reaction of light alkanes in gallium-containing zeolites was studied by using density functional theory (DFT) and a nonframework gallium species in the dihydridegallium ion form. Two different mechanisms were considered: a 3-step mechanism and a 1-step concerted mechanism. The reactions occurring through the 3-step mechanism showed smaller activation barriers than the ones following the concerted mechanism. However, the energy barrier for the 3-step mechanism seems to be more influenced by the size and type (linear or branched) of the hydrocarbon chain and demands major conformational rearrangement, which could be hampered by the zeolite framework, especially for larger and/or branched hydrocarbons. On the other hand, the concerted mechanism seems to be much less dependent on the substrates geometry. Therefore, the concerted mechanism could be preferential when dealing with larger and/or branched alkanes.