With modern computers and standard software, it is now possible to make quite detailed investigations of organic reaction mechanisms (reaction intermediates, transition structures, reaction paths and so on) using ab initio methods. However, because of the computational cost, such investigations will always be limited to prototype examples of certain classes of reaction. In contrast, for structural problems, molecular mechanics (MM) provides an everyday laboratory tool that can be used before detailed experimental investigations are carried out. The difficulty with chemical reactivity problems is that the complicated electronic reorganizations that accompany bond breaking/making require a quantum mechanical description. We have developed a new hybrid scheme, MM-VB, for modelling covalent multi-bond reactivity problems using a combination of molecular mechanics (MM) and valence bond (VB) theory. The valence bond part of the model is in the form of a parametrized Heisenberg Hamiltonain, whilst the molecular mechanics part is a standard MM2 force-field.

Our method can reproduce the MC-SCF structures of minima, transition structures, reaction intermediates and surface-crossings (conical intersections) for a wide range of ground- and excited-state reactions. Furthermore, the structures generated in MM-VB can provide starting geometries for ab initio calculations that are sufficiently accurate that rapid convergence in the geometry optimization is usually achieved.

One of the main benefits of the MM-VB approach lies in the ability to study reactivity problems in large molecular systems, where ab initio computations are beyond the range of current computing technology. As an example we have studied the photochemistry of ergosterol (illustrated above), a steroid which is a precursor of vitamin D₂. Ergosterol undergoes an electrocyclic ring opening involving a cyclohexadiene ring, to form a hexatriene system. The six 'active' carbon atoms are treated quantum mechanically within the VB part of the model, whilst the remaining framework of the system is treated using molecular mechanics.

We have extended the MM-VB code by incorporating a semi-classical dynamics algorithm, allowing us to compute "Classical Wavepackets" of trajectories. Together, these techniques have provided a powerful tool for studying a whole series of organic photochemical reactivity problems.

MMVB has been Interfaced with a development version of the Gaussian computational chemistry program.
Recent developments are summarized in:

"Excited States of Conjugated Hydrocarbon Radicals using the Molecular Mechanics - Valence Bond (MMVB) method"
M. J. Bearpark, M. Boggio-Pasqua, Theor. Chem. Acc. 110, 105-114 (2003).
DOI:10.1007/s00214-003-0461-3
"Excited States of Conjugated Hydrocarbons using the Molecular Mechanics - Valence Bond (MMVB) Method: Conical Intersections and Dynamics"
M. J. Bearpark, M. Boggio-Pasqua, M. A. Robb, F. Ogliaro, Theor. Chem. Acc. (in press 2006).
DOI: 10.1007/s00214-006-0113-5

back :: next

Home - General Information - Group Members - Research - Publications - CASSCF - Available Positions