Maja Nendel

EXPLORATION OF PERICYCLIC REACTION TRANSITION STRUCTURES BY QUANTUM MECHANICAL METHODS : COMPETING CONCERTED AND STEPWISE MECHANISMS

Abstract Density functional theory and multiconfigurational SCF calculations have been applied to a number of pericyclic reactions, including cycloadditions, electrocyclizations and sigmatropic shifts. Emphasis is on the competition between concerted and stepwise mechanisms, comparisons of computed and experimental activation energies and isotope effects, and the performance of MP2, CASSCF and DFT calculations. Various diradical processes, such as the vinylcyclopropane rearrangement and cyclobutane isomerizations, were also studied to test the performance of DFT with diradical processes.

s-Indacene, a quasi-delocalized molecule with mixed aromatic and anti-aromatic character

Abstract The aromaticity of s -indacene has been evaluated by analysis of geometries, energies, and magnetic properties obtained by B3LYP/6-31G* calculations.

THEORETICAL STUDIES OF NOVEL AROMATIC MOLECULES AND TRANSITION STATES

Monocyclic conjugated molecules have stabilities, bond lengths, and magnetic properties following expectations from Huckels Rule. Two hydrocarbons which seem to deviate from these generalizations, s-indacene and cyclohepta(def)fluorene, were studied with density functional theory. The former has characteristics expected for both aromatic and antiaromatic molecules; the latter is a ground state triplet. Potentially aromatic transition states with 10 electrons were also investigated. The mechanism of the 5,5-sigmatropic shift is predicted to be stepwise involving diradical intermediates.

Retro-cycloadditions and sigmatropic shifts: the C 7 H 8 and C 7 H 10 potential energy surfaces

Retro-cycloadditions and thermal rearrangements of hydrocarbons have been investigated with density functional theory (B3LYP) and complete active space multiconfiguration self consistent field theory (CASSCF and CASPT2). We review recent results from our laboratories. Subjects are the laser-induced retro-Diels- Alder reactions of norbornene, thermal retro-Diels-Alder reactions of norbornenes and isopropylidenorbornene, 1,3-sigmatropic shifts of vinylcyclopropane and bicyclo(3.2.0)heptenes, and the 1,5-sigmatropic shifts of norcaradienes. The competition between concerted and stepwise processes and the nature of diradicals formed in these processes are described.