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

摘要

This study explores the potential energy surfaces of the S_0,S_1 and T_1 states using ah initio theory to provide insight into the spectroscopy,photochemistry and reaction dynamics of propanal.Minima associated with the fonnyl potential energy coordinate in the S_1 and T_1 states are found to be approx 60 deg out-of-phase with the S_0 state.Furthermore,the excited states possess a pyramidal formyl carbon atom that leads to a double minimum at ±33 deg and ±49 deg for the S_1 and T_1 states,respectively.An exploration of the C-C dissociation coordinate on the T_1 surface,yielding the products CH_3CF_2+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 T_1 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.