Abstract
Decomposition and isomerization pathways on both singlet and triplet surfaces of crotonaldehyde are investigated using quantum chemical calculations. The geometries of intermediates, transition states, and dissociation products involved in these electronic surfaces are optimized and their energetics are obtained at B3LYP/6-311++G(d,p) level. The accurate energies are further calculated at level CCSD(T)/6-311G(d,p) base on the optimized geometries. Five primary dissociation pathways are found: (i) H-atom, (ii) HCO, (iii) CH3, (iv) CO, and (v) H2CO elimination and four isomerization channels are correlated: (i) 1,3-butadienol, (ii) 3-butenal, (iii) ethylketene, and (iv) CH3CHCH2CO. On the singlet surface, crotonaldehyde exhibits four conformers: E-s-trans, E-s-cis Z-s-trans, and Z-s-cis. The main dissociation channels are C3H6 + CO and C3H4 + H2CO. Direct isomerization to 3-butenal is obtained to have an energy threshold 344 kJ/mol or can be formed from isomer 1,3-butadienol first with a threshold 146 kJ/mol via a two-step process. The first electronic excited singlet state is an n-p* transition at about 357.9 kJ/mol above the ground state and the second excited state a p-p* transition at 579.2 kJ/mol above. When crotonaldehyde is excited to the S2 state it undergoes rapid internal conversion to the S1 surface. Then the molecule can either relax to the 3T (pp*) surface from intersystem crossing then to 3T (np*) via the conical intersection or internally convert to the S0 surface. From the results of calculations for the triplet and singlet surfaces isomerization processes have lower energy thresholds than those for the dissociation channels. Hence for photochemical reactions of crotonaldehyde, isomerization process should be the major channel.