Ab initio study of the reactivity and plausible polymerization process of a labdatriene monomer

The feasibility of the different initial reaction pathways in the gas-phase polymerization of labdatriene monomers, important constituents in amber resins, has been analyzed through ab initio electronic structure calculations. Based on the computed values of density functional theory local Fukui functions and softnesses, tendencies towards free radical additions were quantitatively derived. Results indicate those unsaturated carbons at the diene group 12 and 15 positions possess higher reactivity towards addition reactions. This implies that, in the gas phase, 1,4-addition is the most favored in the ground state. A simple model to account for intermolecular forces and their possible effects to modify the predicted reactivity does not suggest that these forces may be significant to indeed modify it. However, our frontier orbitals calculations suggest the initial formation of a triplet excited state in order to prepare the π-conjugated labdane monomer to undergo the experimentally observed 3,4-addition.