Isomerization and ionization of N ₂O ₄ on model ice and silica surfaces

Isomerization and ionization of N ₂O ₄ on model ice and silica surfaces, hypothesized as key steps in atmospheric HONO formation, were studied using B3LYP and MP2 methods. The models employed are (H ₂O) ₂₀ for ice and Si ₈O ₁₆H ₁₂ for silica. It is found that dangling surface -OH bonds play a key role in the isomerization to generate the 'active', asymmetric ONONO ₂. However, the computed barrier is high. Potential catalytic effects due to additional water molecules or local heating caused by photoabsorption are discussed. The results suggest that the isomerization of N ₂O ₄ into the active ONONO ₂ form on a variety of tropospheric surfaces having dangling -OH bonds should be considered in heterogeneous NO _x chemistry.