Excitation energies of the n → π* 3A″ and π → π* 3A′ states of acrolein

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The vertical and adiabatic excitation energies between the ground and lowest two triplet states of acrolein were studied using ab initio correlated wavefunctions. A double zeta basis set was used in the configuration interaction calculations and each configuration expansion included about 6000 determinants selected from the list of singly and doubly substituted determinants, relative to the SCF reference wavefunction. The calculated vertical and adiabatic excitation energies are respectively, 27 200 cm −1 and 21 700 cm −1 for the n → π* 3A″ state, and 32 000 cm −1 and 23 100 cm −1 for the π → π* 3A′ state. For the 3A″ state, an upward correction of ≈ 1000 cm −1 for the effects of unlinked clusters yields reasonable agreement with the experimental T e = 24 300 cm −1.