On the isomerization of β-pinene: a theoretical study

Abstract

The β-pinene → α-pinene isomerization reaction was investigated at the HF, MP2 and MP4(SDQ) levels of theory, using the 6-31G, 6-31G(d), 6-31+G(d) and 6-311++G(d,p) basis set. The analysis of the intrinsic reaction coordinate has shown that this reaction takes place through a progressive intramolecular proton transfer. The use of different basis set and the inclusion of the electron correlation effects has little influence on the relative stability of the pinene isomers. Contrary to this, the effects of basis set and electron correlation, affects drastically, the energy barrier for the process, varying by 32.5 kcal/mol on going from HF/6-31G to MP2/6-311++G(d,p) levels of theory. The 6-31G basis set gave a completely different result at both HF and MP2 levels for the relative stability of the isomers. The analysis of the nature of the bonds and charge distribution in the transition state species, is shown in the formation of a π-allylic system with a three-center bond. A mechanism to the isomerization reaction assisted by transition metal compounds is given.