Insights into mechanistic photochemistry of urea

In this work, we have studied the mechanistic photochemistry of urea (NH2CONH2) by means of ab initio calculations on the stationary and intersection structures in the lowest three electronic states (S-1, T-1, and S-0). Two dominant decay channels were found for NH2CONH2 to decay from the S-1 state, internal conversion to the S-0 state via the S-1/S-0 minimum-energy conical intersection, and intersystem crossing to the T-1 state in the vicinity of the S-1/T-1 minimum-energy crossing point. Once in the S-0 state, the isomerization and deamination reactions prevail due to their low barrier heights. This is different from the mechanism proposed experimentally, in which the deamination and dehydration reactions proceed in the S-1 and T-1 states. The C-N bond cleavage was determined to occur mainly along the T-1 pathway as a result of the S-1/T-1 intersystem crossing. (C) 2010 American Institute of Physics. [doi:10.1063/1.3397067]