The conical intersection dominates the generation of tropospheric hydroxyl radicals from NO2 and H2O.

In the present work, we report a quantitative understanding on how to generate hydroxyl radicals from NO(2) and H(2)O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory and density functional theory. The conical intersections dominate the nonadiabatic relaxation processes after NO(2) irradiated at approximately 410 nm in the troposphere and further control the generation of OH radical by means of hydrogen abstraction. In agreement with two-component fluorescence observed by laser techniques, there are two different photophysical relaxation channels along decreasing and increasing O-N-O angle of NO(2). In the former case, the conical intersection between B(2)B(1) and A(2)B(2) (CI ((2)B(2)/(2)B(1)) first funnels NO(2) out of the Franck-Condon region of B(2)B(1) and relaxes to the A(2)B(2) surface. Following the primary relaxation, the conical intersection between A(2)B(2) and X(2)A(1) (CI((2)B(2)/(2)A(1))) drives NO(2) to decay into highly vibrationally excited X(2)A(1) state that is more than 20,000 cm(-1) above zeroth-order |n(1),n(2),n(3) = 0 vibrational level. In the latter case, increasing the O-N-O angle leads NO(2) to relax to a minimum of B(2)B(1) with a linear O-N-O arrangement. This minimum point is also funnel region between B(2)B(1) and X(2)A(1) (CI((2)B(1)/(2)A(1))) and leads NO(2) to relax into a highly vibrationally excited X(2)A(1) state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40-50 kcal/mol) from water vapor, producing OH ((2)Pi(3/2)) radicals. The collision between NO(2) and H(2)O molecules not only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersections. The faster internal conversion favors more energy transfer from electronically excited states into highly vibrationally excited X(2)A(1) states. The collision (i.e., the heat motion of molecules) functions as the trigger and accelerator in the generation of OH radicals from NO(2) and H(2)O in the troposphere.