No abstract

The phenyl cation is known to have two lowenergy minima, corresponding to 1 A 1 and 3 B 1 states, the ®rst of which is more stable by ca. 25 kcal/mol. The minimum energy crossing point between these two surfaces, located at various levels including a hybrid method ®rst described here, lies just above the minimum of the triplet, 0.12 kcal/mol at the CCSD(T)/cc-pVDZ// B3LYP/SV level, and there is signi®cant spin-orbit coupling between the surfaces at this point. On the basis of these results, the lifetime of the triplet is expected to be very short.

No abstract

The cationic (C2H4)M+ complexes (M = Cu, Ag, and Au) have been examined by different ab initio molecular orbital, density functional (DFT), and density functional/Hartree−Fock (DFT/HF) hybrid methods using relativistic effective core potentials and a quasi-relativistic approach to account for relativistic effects. For (C2H4)Au+ a substantial relativistic stabilization is observed, such that the computed binding energies are almost twice as high than for (C2H4)Ag+ and still significantly higher than for (C2H4)Cu+. Structural features and energetics obtained at the various computational levels, although they differ significantly in their computational demands, are in satisfying agreement with each other, adding to the level of confidence that can be attributed to the computationally economic DFT and DFT/HF hybrid methods. In order to determine the nature of the bonding in these (C2H4)M+ complexes, an energy decomposition scheme is applied to the DFT results. For all three metal cations, the interaction with ethylene shows large covalent contributions. The major part of the covalent terms stems from σ-donor contribution from the ligand to the metal, whereas π-acceptor bonding (back-bonding) is less important. An atoms-in-molecules (AIM) analysis of the charge density distribution reveals cyclic structures for (C2H4)Au+ and (C2H4)Cu+, whereas (C2H4)Ag+ is T-shaped.

The potential energy surface (PES) corresponding to the reaction of the iron cation with ethane, which represents a prototype of the activation of C−C and C−H bonds in alkanes by transition metal cations, has been investigated employing the recently suggested hybrid density functional theory/Hartree−Fock method (B3LYP) combined with reasonably large one-particle basis sets. The performance of this computational approach has been calibrated against experimentally known Fe+−R binding energies of fragments R relevant to the [Fe,C2,H6]+ PES and against the relative energies of the possible exit channels. Both the C−C and C−H bond activation branches of the PES are characterized by a low barrier for the first step, the insertion of the iron cation into a C−C and C−H bond, respectively. Rate determining are the second steps which in the C−C bond activation branch corresponds to an [1,3]-H shift leading to a complex between FeCH2 + and methane. Along the C−H activation reaction coordinate, no transition state corresponding to a β-hydrogen shift resulting in a dihydrido species could be located, even though such a step has been often postulated. The decisive step is rather a concerted saddle point connecting the C−H inserted species directly with a complex of Fe+ with molecular hydrogen and ethylene. The mechanistic scenario provided by our calculations is in concert with all experimental information and allows for the first time a detailed and consistent view on the mechanistic details of this import reaction sequence. It further demonstrates the usefulness of the B3LYP approach for describing even complex electronic situations such as present in open-shell transition metal compounds.