Molecular-beam experiments for photodissociation of propenal at 157 nm and quantum-chemical calculations for migration and elimination of hydrogen atoms in systems C3H4O and C3H3O.

We investigated the dynamics of photodissociation of propenal (acrolein, CH2CHCHO) at 157 nm in a molecular beam and of migration and elimination of hydrogen atoms in systems C3H4O and C3H3O using quantum-chemical calculations. Compared with the previous results of photodissociation of propenal at 193 nm, the major difference is that the C3H3O fragment present at the 193-nm photolysis disappears at the 157-nm photolysis whereas the C3H2O fragment absent at 193 nm appears at 157 nm. Optimized structures and harmonic vibrational frequencies of molecular species with gross formula C3H2-4O were computed at the level of B3LYP/6-311G(d,p) and total energies of those molecules at optimized structures were computed at the level of CCSD(T)/6-311+G(3df,2p). Based on the calculated potential-energy surfaces, we deduce that the C3H3O fragment observed in the photolysis of propenal at 193 nm is probably CHCCHOH (2A″) and/or CH2CCOH (2A″) produced from an intermediate hydroxyl propadiene (CH2CCHOH) following isomerization. Adiabatic and vertical ionization potentials of eight isomers of C3H3O and two isomers of C3H2O were calculated; CHCCHOH (2A″) and CH2CCOH (2A″) have ionization potentials in good agreement with the experimental value of ∼7.4 eV. We also deduce that all the nascent C3H3O fragments from the photolysis of propenal at 157 nm spontaneously decompose mainly to C2H3 + CO and C3H2O + H because of the large excitation energy. This work provides profound insight into the dynamics of migration and elimination of hydrogen atoms of propenal optically excited in the vacuum-ultraviolet region. [ABSTRACT FROM AUTHOR]