Accurate pKa determination for a heterogeneous group of organic molecules.

Single-molecule studies that allow to compute pKa values, proton affinities (gas-phase acidity/basicity) and the electrostatic energy of solvation have been performed for a heterogeneous set of 26 organic compounds. Quantum mechanical density functional theory (DFT) using the Becke-half&half and B3LYP functionals on optimized molecular geometries have been carried out to investigate the energetics of gas-phase protonation. The electrostatic contribution to the solvation energies of protonated and deprotonated compounds were calculated by solving the Poisson equation using atomic charges generated by fitting the electrostatic potential derived from the molecular wave functions in vacuum. The combination of gas-phase and electrostatic solvation energies by means of the thermodynamic cycle enabled us to compute pKa values for the 26 compounds, which cover six distinct chemical groups (carboxylic acids, benzoic acids, phenols, imides, pyridines and imidazoles). The computational procedure for determining pKa values is accurate and transferable with a root-mean-square deviation of 0.53 and 0.57 pKa units and a maximum error of 1.0 pKa and 1.3 pKa units for Becke-half&half and B3LYP DFT functionals, respectively.     

Chemischer Transport von Cr2O3 mit CrI3/I2 – Experimente und Modellrechnungen zur Beteiligung von CrOI2,g

On the Chemical Transport of Cr₂O₃ with CrI₃/I₂ – Experiments and Model-Calculations for Participation of CrOI_2,g Gaseous chromium oxyiodides that were unknown up to now cause the migration of Cr₂O₃ in the temperature gradient 1 000°C→900°C when iodine (e. g. 0.1 mmol/ml) and CrI₃ is added (eq. (1)). Transport agent for Cr₂O₃ is gaseous CrI₄. With a smaller concentration of iodine (D(I₂) ? 0.016 mmol/ml) and lower temperatures (e.g. T? = 850°C) the influence of H₂O (from the wall of the silica ampoule) becomes more important. Under these conditions the transport of Cr₂O₃ is a result from the endothermic reactions (2), (3) and (4). H_2,g has on the basis of the decomposition of HI_g a positive difference of the solubility and H_2,g should not to be considered as a transport agent. Because of the range of equilibrium-values the reaction 4 has to be taken into consideration. Estimated value of the enthalpie for CrOI_2,g is fixed more precisely by thermodynamic model calculation to Δ_fH°₂₉₈(CrOI_2,g) = ?51.4 kcal/mol. The estimated limit of error for the enthalpie of formation is smaller than ± 5 kcal/mol. Without an addition of CrI₃ is in the system Cr₂O₃/I₂ a migration of Cr₂O₃ not observable.     

Ab initio study of the ground state and conformational stability of peroxyacetyl nitrate in internal rotation about the peroxide bond

The molecular and electronic structure of the ground state of peroxyacetyl nitrate (PAN) was calculated by the unrestricted Hartree-Fock-Roothaan method with the use of the standard 3–21G and 6–31G basis set. The potential curve of the internal rotation about the peroxide bond of PAN was calculated with the 6–31G basis set. The curve contains two maxima. The ground state of PAN is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in planes that are perpendicular to each other (the dihedral angle ϱ(COON) is 89.9°). The calculated barriers to rotation are 19.6 and 66.8 kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 600–604, April, 1998.     

Perfluoropoly-ether/peroxide compounds: spectroscopic studies and quantum chemical calculations

Perfluoropolyethers (PFPEs) are a class of high performance materials used in a wide range of applications (refrigeration, lubrication, semiconductor industry, etc.). PFPEs containing peroxidic units are intermediate materials for the preparation of commercial end products. In this work we study the spectroscopic properties of ether and peroxides linkages in this class of compounds; nuclear magnetic resonance (NMR) spectra are discussed, FT-Raman data presented. Quantum chemical calculations on model molecules were used as a tool for the interpretation of the Raman experimental data and physical-chemical properties.     

A Green's function approach to the photoelectron spectrum of bis(π-allyl)nickel[1]

The vertical ionization potentials of bis(-allyl)nickel (see (1) in Fig. 1) are calculated by means of the Green's function approach within a semiempirical INDO extension to the first transition metal series. The computed ionization potentials are in good agreement with an experimentally deduced assignment. In contrast to earlier theoretical and experimental studies, the 7a _u () level is predicted on top of the levels corresponding to the Ni 3d orbitals. Our approach leads to a complete assignment of the PE spectrum of (1) in the outer valence region.     

Lithium bromide in acetonitrile: Thermodynamics,theory, and simulation

A variety of methods has been used for the study of lithium bromide solutions in acetonitrile yielding by their combination reliable information on different levels of approximation. Osmotic coefficients based on precise vapor pressure measurements are reproduced by CM (chemical model) and HNC (hypernetted chain) calculations and by BD (brownian dynamics) simulations. The results of neutron scattering experiments are treated with the help of HNC and BD methods. Hartree-Fock calculations on isolated LiBr pairs and solvated lithium ions yield reliable particle distances and reveal the geometry of the lithium solvation sphere.     

MSINDO study of water adsorption on NiO surfaces

The dissociation of water adsorbed on the surface of NiO was investigated by using the semi-empirical SCF MO method MSINDO. Simulations were based on embedded cluster models representing the (100) surface, with and without a monatomic step. The angle formed between the metal adsorption site and the O–H bond associated with water has been found to be critical to the energetics of the dissociation process. Based on this criterion, it was shown that water dissociation is favorable on the stepped surface, but highly unlikely on the planar surface. In addition, the activation energy required for water dissociation in a monatomic NiO step was considerably lower than for dissociation at the planar surface. The high activation energy associated with water dissociation on the planar surface is attributed to the rigidity of the NiO lattice. Dedicated to Prof. K. Jug in honor of his 65th birthday     

A density functional study on a biomimetic non-heme iron catalyst: insights into alkane hydroxylation by a formally HO-FeV=O oxidant

The reactivity of [HO-(tpa)Fe(V)=O] (TPA=tris(2-pyridylmethyl)amine), derived from O-O bond heterolysis of its [H(2)O-(tpa)Fe(III)-OOH] precursor, was explored by means of hybrid density functional theory. The mechanism for alkane hydroxylation by the high-valent iron-oxo species invoked as an intermediate in Fe(tpa)/H(2)O(2) catalysis was investigated. Hydroxylation of methane and propane by HO-Fe(V)=O was studied by following the rebound mechanism associated with the heme center of cytochrome P450, and it is demonstrated that this species is capable of stereospecific alkane hydroxylation. The mechanism proposed for alkane hydroxylation by HO-Fe(V)=O accounts for the experimentally observed incorporation of solvent water into the products. An investigation of the possible hydroxylation of acetonitrile (i.e., the solvent used in the experiments) shows that the activation energy for hydrogen-atom abstraction by HO-Fe(V)=O is rather high and, in fact, rather similar to that of methane, despite the similarity of the H-CH(2)CN bond strength to that of the secondary C-H bond in propane. This result indicates that the kinetics of hydrogen-atom abstraction are strongly affected by the cyano group and rationalizes the lack of experimental evidence for solvent hydroxylation in competition with that of substrates such as cyclohexane.