The changes in the electronic structure of B2 FeAl alloy with a Fe antisite and absorbed hydrogen

The electronic structure and bonding in a B2 FeAl alloy with and without hydrogen interaction with a Fe antisite were computed using a density functional theoretical method. The hydrogen absorption turns out to be a favorable process. The hydrogen was found close to an octahedral site where one of its Al capped is replaced by a Fe antisite. The Fe–H distance is of 1.45 Å same as the Al–H distance.The density of states (DOS) curves show several peaks below the d metal band which is made up mostly of hydrogen based states (>50% H_1s) while the metal contribution in this region includes mainly s and p orbitals.An electron transfer of nearby 0.21e⁻ comes from the metal to the H. The overlap population values reveal metal–metal bond breaking, the intermetallic bond being the most affected. The H bond mainly with the Al atom and the reported Fe–H overlap population is much lower than that corresponding to FePd alloys and BCC Fe. The changes in the overlap population show the Fe–Al bond is weakened nearly 41.5% after H absorption, while the Fe–Fe bond is only weakened 34.5%. H also develops a stronger bond with the Al atoms. The main bond is developed with Al being twice stronger than Fe–H.     

Theoretical study of the mechanism of the Wittig reaction: Ab initio and MNDO-PM3 treatment of the reaction of unstabilized,semistabilized, and stabilized ylides with acetaldehyde

In this study, we describe the results of ab initio (HF and MP2) and MNDO-PM3 calculations on the model reactions of unstabilized (Me₃P=CH–CH₃), semi-stabilized (Me₃P=CH–C≡CH), and stabilized (Me₃P=CH–C≡N) ylides with acetaldehyde to form their respective Z and E olefins and trimethylphosphine oxide. These reactions occur in three stages: oxaphosphetane formation, oxaphosphetane pseudorotation, and oxaphosphetane decomposition. The calculated barriers for these processes vary considerably depending on the level of theory employed (ab initio vs. MNDO-PM3 or HF vs. MP2 at the ab initio level). However, self-consistent geometries of reactants, intermediates, transition states and products are obtained at all levels. Oxaphosphetane formation is best described as very asynchronous cycloaddition (borderline two-step mechanisms). The geometries of the transition states are near planar with respect to P, C, C, and O atoms. Analysis of the bond indices of these reactions shows that the C–C bonds are between 44% (unstabilized case) and 60% (stabilized case) formed, whereas the corresponding P–O bonds have not been formed to any significant degree. Oxaphosphetane decomposition can be described as a very asynchronous retrocycloaddition where P–C bond breakage runs ahead of C–O bond breakage. These results are compared with experimental findings for the Wittig reaction, and its relevance to the overall mechanism of the olefination is discussed. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 557–569, 1997