Results Of Valence Bond Calculations Research Articles

Valence Bond Formulations of the Identity Hydrogen Abstraction Reaction, X• + H−X‘ → X−H + X‘•, with Reactantlike and Productlike Complexes

Connections are made between two valence bond studies of hydrogen atom abstraction reactions: (a) J. Phys. Chem. A 2001, 105, 8226 and (b) J. Phys. Chem. 1993, 97, 12210; corrections 1994, 98, 3226. At each stage along the reaction coordinate, the variational-best valence bond formulation for the reacting system of (b) can be described in terms of resonance between valence bond structures for a reactantlike complex and a productlike complex. It is demonstrated that the (a) and (b) descriptions of the transition states are equivalent. Further development for (b) is presented, to compare expressions for the promotion gap (G) and the resonance energy at the avoided crossing (B) with those that arise in (a). By use of a triple-ζ 1s atomic orbital (AO) basis set to formulate the wave functions for the reactantlike and productlike complexes, it is demonstrated how these wave functions are constructed as linear combinations of localized molecular orbital (MO) configurations, each of which involves one AO per atomic center. Similar procedures that include 2p AOs as polarization functions in the treatment are also described. The results of valence bond calculations that use these two types of AO basis sets provide illustrative examples of how the formulation of (b) provides a compact valence bond representation for the electronic reorganization that is involved in the conversion of reactants into products.

Location of OH groups and oxidation processes in triclinic chloritoid

Triclinic Fe2+ and Fe3+-chloritoid end members, as well as Fe2+-Fe3+-chloritoid solid solutions were synthesized in order to determine the location of the OH groups in the structure and to investigate the possibility of Fe3+ incorporation on the M1B site and the corresponding oxidation mechanism. The samples were analyzed using transmission electron microscopy, electron energy-loss and 57Fe Mossbauer spectroscopy, polarized single-crystal, KBr powder and in situ high-pressure infrared spectroscopy. The structure of a natural triclinic chloritoid was refined by single-crystal X-ray diffraction in order to locate the proton-accepting oxygens in the triclinic structure. The results of valence bond calculations, polarized single-crystal and in situ high-pressure IR spectroscopy revealed that the orientations of the OH dipoles are the same as those in monoclinic chloritoid. The annealing experiments in air show that the incorporation of Fe3+ on M1B increases with temperature. After 3 h at 530 °C all Fe2+ was oxidized to Fe3+ without decomposition of the structure. This also holds true for a heating duration of 24 h. The incorporation of Fe3+ at M1B sites is coupled with a dehydration process starting at about 350 °C during which the H1A protons leave first.