Trans->cis isomerization in long linear polyenes as beta-carotene models: a comparative CAS-PT2 and DFT study

This computational density functional theory (DFT) study provides information about the ground state trans --> cis isomerization process for a set of all-trans linear conjugated polyenes including the undecaene term which can be considered a model of beta-carotene. To obtain information about the accuracy of the isomerization barriers computed with the DFT approach we have compared, for a series of shorter all-trans polyenes, the DFT results with those obtained using the CAS-PT2 approach. This comparison has shown that the DFT barriers agree with the CAS-PT2 ones within 1 kcal mol(-1). We found for the undecaene term that the singlet ground state central double bond isomerization process occurs through a flat twisted transition state with small rotational barrier (22.9 kcal mol(-1)) and no special stabilizing effects. We have also investigated, at the DFT level, the trans --> cis isomerization process in the triplet state of undecaene and the DFT results show again two planar low-lying minima (a more stable all-trans minimum and a central bond-cis minimum) connected by a twisted transition state (having the same geometry and energy of the twisted singlet ground state transition structure) with an unfavourable intersystem crossing between the two states. These results suggest that the deactivating ability of beta-carotene is associated with the attack of the reactive species upon the two singlet closed shell minima.