Theoretical Chemistry Accounts

Die Eigenschaft des Tschitschibabinschen und des Wittigschen Kohlenwasserstoffs, Parawasserstoff in Orthowasserstoff umzuwandeln, wird durch eine quantenmechanische Behandlung von Stoßprozessen zwischen Wasserstoffmolekeln und Kohlenwasserstoffmolekeln erklärt. Als Wechselwirkung zwischen den Stoßpartnern wird die magnetische Elektronenspin-Protonenspin-Kopplung angenommen. Die Übereinstimmung der qualitativen Resultate der Theorie mit den experimentellen Daten ist befriedigend.

This work extends our Rung 3.5 exchange-correlation density functionals by incorporating empirical internuclear “-D” dispersion corrections. The resulting PBE+ $$\Uppi$$ (s)-D and $$\Uppi$$ 1PBE-D functionals are tested for a broad range of noncovalent interactions. The functionals provide reasonable accuracy approaching existing dispersion-corrected semilocal and hybrid functionals. Application to a molecule-surface reaction illustrates these methods’ potential.

Topologies are introduced into the nuclear configuration space R of molecular systems, based upon equipotential contour hypersurfaces on the otential energy hypersurface E. Critical level topologies T fc and T fc′, based upon the number and distribution of various critical points of E, are of particular importance, since they represent convenient yet rigorous mathematical models for relations between elementary reaction mechanisms, and for relations between open sets of nuclear geometries which are classically accessible at a given total energy.

Cluster calculations which model chemisorption on a surface are often composed of substrate atoms arranged in a periodic manner. This pseudo-lattice symmetry of a cluster is used to reduce the number of 2-electron integrals computed in a SCF calculation by evaluating only unique integrals identified by lattice displacement vectors. The method, without using any explicit symmetry, is shown to be competitive with calculations which utilize point group symmetry. It is also demonstrated that the pseudo-lattice method markedly reduces the number of 2-electron integrals in multi-layer clusters which have little or no symmetry.

The mechanism of the non-catalysed and the MgBr2-catalysed [3+2] cycloaddition (32CA) reactions between C-methoxycarbonyl nitrone and 2-propen-1-ol has been theoretically investigated within the molecular electron density theory using DFT methods at the B3LYP/6-31G(d) computational level. Analysis of DFT reactivity indices allows explaining the role of the MgBr2 Lewis acid in the catalysed 32CA reaction. The 32CA reaction between C-methoxycarbonyl nitrone and 2-propen-1-ol takes place with a relative high activation enthalpy, 13.5 kcal mol−1, as a consequence of the non-polar character of this zw-type 32CA reaction. Coordination of the MgBr2 LA to C-methoxycarbonyl nitrone accelerates the corresponding zw-type 32CA reaction by taking place through a polar mechanism and with lower activation enthalpy, 8.5 kcal mol−1. Both 32CA reactions, which take place through a one-step mechanism, are completely meta regioselective and present low exo stereoselectivity, which increases in the catalysed process. Energy and non-covalent interaction analyses at the transition-state structures indicate that the formation of an intramolecular H–Br hydrogen bond in the catalysed process could be responsible for the exo selectivity experimentally observed.