Ab initio study on the structures and properties of trans-p-coumaric acid in low-lying electronic states

The five lowest-lying electronic states of trans-p-coumaric acid have been studied by using the complete active space self-consistent field and multi-reference configuration interaction methods. The adiabatic excitation energies from the ground state to the S-1, T-1, S-2, and T-2 states have been determined in the present study. On the basis of the optimized structures, the calculated energies, and analysis of molecular orbitals, the S-1 and S-2 states were assigned to (1)n pi* and (1)pi pi* in nature, respectively, which are different from the assignment in previous work. The T-1 and T-2 states, which have not been investigated before, were characterized as (3)pi pi* localized in the C=C double bond and the aromatic ring regions, respectively. Photoisomerization dynamics of trans-p-CA should be reinvestieated. due to controversy over the character of the excited electronic states. (c) 2004 Elsevier B.V. All rights reserved.