Massimo Olivucci

The existence and stability of singlet tetramethylene biradicals: an ab initio MCSCF/MP2 study

Abstract The formation and evolution of singlet tetramethylene biradicals that occur in the dimerization of ethylene to cyclobutane has been investigated using ab initio MCSCF and MCSCF/MP2 methods with double-zeta and 6–31G* basis sets. Two minima, corresponding to a trans and gauche structure, have been located. In contrast with earlier studies, when the contribution of dynamic correlation is computed at the MP2 level, the fragmentation barriers associated with these two minima do not disappear. However when the zero-point vibrational energy corrections are taken into account, the region of the surface corresponding to the closure to cyclobutane becomes extremely flat and the gauche minimum disappears. These results seem to indicate that the tetramethylene biradical exists as a single species with a trans geometry. The MP2 results yield an activation barrier of 44.70 kcal/mol with the 6–31G* basis and 43.21 kcal/mol at the double-zeta level; the experimental value is 43.8 kcal/mol.

Applications of computational chemistry to the modelling of organic reactivity in ground and excited states

A review of methods for modelling chemical reactivity in ground and excited states is presented. The central problem is related to the correct representation of the changes in the spin‐coupling at a transition state or in an excited state. Both MC‐SCF and a hybrid method MM‐VB are discussed. The application of these techniques to photochemistry is one of the newest challenges to quantum chemistry. The applicability of modelling methods to this type of application is discussed.

A Strategy for Modelling of Chemical Reactivity using MC-SCF and MM-VB Methods

The problem of modelling chemical reactivity for larger molecular systems is discussed. A strategy involving VB theory implemented via a Heisenberg Hamiltonian is described. The central features of such an approach are a) it is possible to combine the quantum mechanics with force field methods for the study of reactivity and b) it is compatable with being parametrized using the best state of the art ab-initio methods that are available. The utility of the method is illustrated on the photochemistry of a steroid.

The Analysis of Potential Energy Surfaces in Terms of the Diabatic Surface Model

In this paper, we summarize the salient features of the Diabatic Surface model, which represents a theoretical instrument for analysing a Potential Energy Surface (PES). In particular this model provides information about the global properties of a PES and gives a rationalization of the main features of a critical point (e.g. intermediates, transition states, etc.), such as its origin, existence and index.