Carbonyl compounds are a kind of the most important organic compounds. Because of the unique electronic structures of carbonyl compounds, their photochemistry has been extensively in the past decades. In comparison, thiocarbonyl compounds have received less attention, particularly their excited-state properties and photochemical reactions. In the present work. high-level quantum themical ab initio methods were used to investigate the structures and energies of H_2CS, CH_3CHS and CH_3CSCH_3 in the ground and excited states. It is found that the structural parameters by different computational methods and different basis sets are similar, which are in agreement with the experimental results. The present calculations predict that the S_1, T_1and T_2 states are close to each other in energy. However, there is a large different in energy between the S_2 and T_2 states. The similar conclutions were found for molecules of CH=CH_2, CH_2 CHO and CH_2 =CH=CH=CH_2. Upon photoexciation at 500 nm, the systems do not have enough internal energy to overcome the barrier on the T_1 pathway. Thus internal conversion to the ground state is the most probable pathway to deactivate. Photoexcitation at 210 nm results in the molecules in the S_2 states. From this state, the molecules that relax to the T_1 state are left with sufficient internal energy to undergo the C-H or C-C bond dissocition along the T_1 pathway.

Key words: EXCITED STATE, STRUCTURE, PHOTOCHEMICAL REACTION, AB INITIO CALCULATION, CARBONYL GROUP, COMPOUNDS, FORMALDEHYDE, ACETALDEHYDE, ACETONE