Showing papers by "Ernest R. Davidson published in 2000"

Is the Hydrogen Bond in Water Dimer and Ice Covalent

Abstract: The changes in charge and momentum distributions upon forming a hydrogen bond in the water dimer are examined. The computed Compton profile anisotropies show the same oscillations as were observed for solid ice. These oscillations are already found when the unperturbed orbitals of the water monomers are used to construct a Slater determinant for the dimer. Hence we conclude that the oscillations are irrelevant to the discussion of the covalent character of the bond. Rather they just reflect the result of antisymmetrizing the product of monomer wave functions. In fact, at the oxygen−oxygen distance in ice, the calculations indicate a net antibonding contribution to energy from overlap effects.

Coordinated carbenes from electron-rich olefins on RuHCl(PPr3i)2

Abstract: Dehydrohalogenation of RuH2Cl2L2 (L=PPr3i) gives (RuHClL2)2, shown to be a halide-bridged dimer by X-ray crystallography; the fluoride analog is also a dimer. (RuHClL2)2 reacts with N2, pyridine and C2H4 (L′) to give RuHClL′L2, but with vinyl ether and vinyl amides, H2CCH(E) [E=OR, NRC(O)R′] such olefin binding is followed by isomerization to the heteroatom-substituted carbene complex L2HClRuC(CH3)(E). The reaction mechanism for such rearrangement is established by DFT(B3PW91) computations, for C2H4 as olefin (where it is found to be endothermic), and the structures of intermediates are calculated for H2CC(H)(OCH3) and for cyclic and acyclic amide-substituted olefins. It is found, both experimentally and computationally, that the amide oxygen is bonded to Ru, with a calculated bond energy of approximately 9 kcal mol−1 for an acyclic model. Less electron-rich vinyl amides or amines form η2-olefin complexes, but do not isomerize to carbene complexes. Calculated ΔE values for selected ‘‘ competition’’ reactions reveal that donation by both Ru and the heteroatom-substituted X are necessary to make the carbene complex L2HClRuC(X)(CH3) more stable than the olefin complex L2HClRu(η2-H2CCHX). This originates in part from a diminished endothermicity of the olefin→carbene transformation when the sp2 carbon bears a π-donor substituent. The importance of a hydride on Ru in furnishing a mechanism for this isomerization is discussed. The compositional characteristics of Schrock and Fischer carbenes are detailed, it is suggested that reactivity will not be uniquely determined by these characteristics, and these new carbenes RuHCl[C(X)CH3]L2 are contrasted to Schrock and Fischer carbenes.

Transition Regions in the Cope Rearrangement of 1,5-Hexadiene and Its Cyano Derivatives

Abstract: Substituent effects on the mechanism of the Cope rearrangement were studied by computing C2h (Cs) cuts through the potential energy surface (PES) for the reaction of 1,5-hexadiene and its di-, tri-, and tetracyano derivatives at the (U)B3LYP/6-31G* and (U)BPW91/6-31G* levels The stabilization of substituted structures along the cuts is discussed in terms of the energies of isodesmic formation from allyl radicals and acetonitrile molecules Cyano groups at C1, C3, and C5 provide a nearly additive stabilization of each point along the Cs cut even though their influence on the geometry is competitive Evaluation of the density of effectively unpaired electrons at various geometries indicates that the radical character of a transition state (TS) is not altered by radical stabilizing substituents as such but depends solely on the interallylic bond length Although the UB3LYP diyl intermediate for the parent compound is plausible when compared to the lowest triplet PES, neither MRPT2 at the UB3LYP geometries n

Kinetic and potential energy of isoelectronic atomic ions from density functional theory compared with exact values

Abstract: The performance of the Fock exchange and the two most common Becke exchange functionals, B88 and B3, in combination with Perdew and Wang's correlation functional (PW91) has been studied for highly charged atomic ions. The study focuses on the accuracy of the components of total energy: kinetic and potential energy. It is found that Becke's exchange functionals introduce a significant error in the kinetic and nuclear attraction energy for most ions studied, and the use of these exchange functionals might lead to inaccurate momentum distributions when their Kohn-Sham orbitals are transformed to momentum space.