Photochemical water decomposition in the troposphere: DFT study with a symmetrized Kohn-Sham formalism.

Photochemical reaction of the electronically excited NO(2)* species with the water molecule is studied in terms of a new version of density functional theory by selecting the specific (2)A'' symmetry of the whole system, which is different from the ground-state pattern. The excited C(2)A(2) state of the NO(2) molecule is found to be distorted to the equilibrium structure O=N-O(.), which poses the (2)A'' symmetry in the C(s) point group. With the B3LYP functional it is shown that such an electronically excited NO(2)* molecule, generated by visible light (λ=420 nm), can react with water vapor to produce OH+HONO species, an important source of tropospheric hydroxyl radicals. This photochemical process can be considered as a possible mechanism of atmosphere self-cleaning.