Excited States of Butadiene to Chemical Accuracy: Reconciling Theory and Experiment.

Abstract

We obtain the vertical excitation energies of the notoriously challenging lowest-lying dark and bright excitations of trans-butadiene to chemical accuracy using high-order equation-of-motion coupled-cluster theory. Convergence is demonstrated in both the one-particle basis set (up to augmented quintuple-zeta quality) and the coupled-cluster expansion (including up to connected quadruple excitations) within an incremental scheme. Our best estimates for the bright 11Bu+ and dark 21Ag– vertical transitions are 6.21 ± 0.02 eV and 6.39 ± 0.07 eV, respectively, establishing definitively that the vertical 11Bu+ transition lies below the 21Ag– transition. Our 11Bu+ excitation energy remains significantly higher than the generally cited experimental value of 5.92 eV. To rationalize this difference, we have computed the zero-point vibrational energy corrections, which reduce the theoretical 11Bu+ excitation energy to 6.11 eV. We also correct for nonverticality in the experimental value by recomputing the transition as the weighted intensity average of the electron impact energy loss spectra, which gives the range 5.96–6.05 eV. The corrected best theoretical and experimental 11Bu+ excitation energies are then in good agreement, resolving a long-standing discrepancy.