Description: This article offers comments on the competitive preferential solvation theory. Abstract: Simple additive relationships for the physico-chemical properties of a solute dissolved in binary solvent mixtures are developed from the competitive preferential solvation model. Additive expressions for solute mole fraction solubility and logarithm of solute solubility are mathematically identical to equations derived previously from the microscopic partition and basic nearly ideal binary solvent models. Calculated values based on the various additive relationships are compared to carbazole solubilities in ten binary solvent mixtures containing dibutyl ether with n-hexane, n-heptane, n-octane, cyclohexane, cyclo-octane, methylcyclohexane, iso-octane, n-hexadecane, squalane and t-butylcyclohexane.

Description: This article discusses principal resonance contributors to high-valent, transition-metal alkylidene complexes. The results of ab initio calculations are reported for prototypical high-valent, alkylidene complexes. Stationary points on each potential energy surface are characterized and compared to experimental information where available; as long as a suitably flexible valence basis set is used, good agreement between theoretically calculated and experimentally determined geometries is obtained. The complexes of interest include group IVB (Ti, Zr and Hf) and group VB (Nb and Ta) alkylidenes with hydride ligands as well as models for the four-coordinate, olefin metathesis catalysts (Mo-, W-, and Re-alkylidenes) which have been recently synthesized and characterized. In light of the fact that much of the discussion concerning the reactivity of transition-metal carbene complexes has been presented in terms of the resonance contributors derived from rearranging the electrons in the M-C σ and π orbitals, the minima obtained from the portion of the study are then subjected to a further procedure to calculate these contributions. Resonance structures in which the carbon is the negative end of the M-C bond (i.e., nucleophilic resonance structures) contribute 50% to the ground-state wave function of these complexes. Those in which the carbon is formally neutral account for ...

Description: This article discusses K-shell x-ray-production cross sections. Abstract: K-shell x-ray-production cross sections are reported for elements with K-shell x-ray energies between 277 eV (C) and 1487 eV (Al). The x-ray measurements were made with a windowless Si(Li) detector that was calibrated for efficiency by comparing bremsstrahlung spectra from electron bombardment of thin foils of aluminum, silver, and gold with theoretically determined bremsstrahlung spectral distributions. The x-ray-production cross-section measurements are compared to first-order Born and perturbed-stationary-state with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR) ionization theories using single-hole fluorescence yields. The ECPSSR and first-order Born theoretical predictions are, in general, in close agreement with each other and both generally fit the data quite well.

Description: This article discusses methane activation by group IVB imido complexes. An ab initio study of methane activation by group IVB imido complexes, when coupled with available experimental data, reveals an interesting picture of this important reaction. Initial interaction of methane and (H)₂M=NH leads to the formation of alkane complexes bound by ≈9 kcal mol⁻¹. Experiment indicates that the polarity of the metal-ligand bond upon which the C-H is activated plays an important role in facilitating subsequent scission. Calculations support this hypothesis and suggest that formation of the alkane complex acts to increase Cδ-Hδ polarization, setting the stage for C-H cleavage. Calculated methane elimination barriers for (H)₂M(CH₃)(NH₂) (M=Ti, Zr, Hf) are in good agreement with experimental models in terms of absolute numbers and trends as a function of metal. Calculated methane activation barriers follow the order Ti > Zr > Hf, in line with calculated exothermicities. Calculated geometries indicate a late transition state for methane elimination, in agreement with experimentally determined activation parameters. The TSs have a kite-shaped geometry with an obtuse angle about the H of the C-H bond being activated (Ht) and a short MHt distance, 1-2% greater than normal. The short MHt distance suggests a stabilizing interaction, supported ...

Description: This articles discusses transition metal imido complexes. A wide range of transition metal imido (TMI) complexes is studied using ab initio molecular orbital (MO) calculations. The main computational point of interest is the further testing of effective core potentials (ECPs) and valence basis sets to allow for the accurate calculation of properties for reasonably sized transition metal complexes. On the chemical side, several results from the study are to be noted. The agreement between geometries for calculated models and their experimental counterparts ranges from very good to excellent, as found in previous studies of multiply bonded transition metal-group IVA complexes. Taken together, these data suggest that the accuracy one has come to expect for the prediction of structural properties for main-group compounds may yet become a reality for transition metal complexes. The correct prediction of molecular structure also leads one to infer that the bonding in these complexes is also accurately described. The MC/LMO/CI (multiconfigurational/localized MO/configuration interaction) technique shows that eight resonance structures are most significant in the description of the metal-imido linkage. Three of these account for roughly two-thirds of the total contributions; two resonance structures, both possessing a dative σ bond, correspond to novel bonding descriptions for TMI ...