Semi-empirical valence bond models of conjugated alkenes with donor and acceptor substituents

Abstract

The Heisenberg model is used to describe low-lying states of conjugated alkene ‘bridges’ with donor and/or acceptor substituents. Using a compact set of transferable exchange parameters, each fitted to the ground state ab initio energy of just one molecule, useful semi-empirical energies can be generated for different donor/acceptor combinations and for different chain lengths. A new Hamiltonian is introduced which augments the covalent description with additional parameters for ionic configurations, and for the mixing between covalent and ionic spaces. This leads to useful energies also for higher states. The general approach should be applicable to a wide range of different donor/acceptor/bridge combinations.