The lowest-lying excited singlet and triplet electronic states of propanal: an ab initio molecular orbital investigation of the potential energy surfaces

Abstract

This study explores the potential energy surfaces of the So, S₁ and T₁ states using ab initio theory to provide insight into the spectroscopy, photochemistry and reaction dynamics of propanal. Minima associated with the formyl potential energy coordinate in the S₁ and T₁ states are found to be ~60° out-of-phase with the So state. Furthermore, the excited states possess a pyramidal formyl carbon atom that leads to a double minimum at ±33° and ±49° for the S₁ and T₁ states, respectively. An exploration of the C–C dissociation coordinate on the T₁ surface, yielding the products CH₃CH₂ + CHO, shows that a three-fold potential due to the formyl torsion is still operational, resulting in three unique transition states that lead to dissociation. The lowest energy pathway to dissociation occurs at a barrier height of 4766 cm⁻¹ and the energy of the products is found to be 1017 cm⁻¹, relative to the T₁ global minimum. Consequently, a reverse barrier of 3749 cm⁻¹ is calculated. Parameters calculated for the lowest energy transition state geometry are distinctly different from structures inferred from experiment, which assumed an isotropic dissociation channel.