Structural Effects on the Norrish Type I α-Bond Cleavage of Tropospherically Important Carbonyls.

Norrish Type I (NTI) α-bond cleavage is the dominant photolysis mechanism in small carbonyls and is an important source of radicals in the troposphere. In nonsymmetric species two cleavages are possible, NTI a and NTI b , forming larger and smaller alkyl radicals, respectively. For a data set of 20 small, atmospherically relevant carbonyls we predict NTI a and NTI b thresholds on the S ₀ , S ₁ , and T ₁ electronic states. The calculated NTI a T ₁ thresholds give a mean absolute deviation (MAD) of 5.8 kJ/mol with respect to the available experimental thresholds of five carbonyls. In addition, the intrinsic barrier heights to dissociation on the S ₀ , S ₁ , and T ₁ electronic states are predicted. We find RI-B2GP-PLYP/def2-TZVP calculations on S ₀ and unrestricted RI-B2GP-PLYP/def2-TZVP calculations on T ₁ give MADs of 6.1 kJ/mol for S ₀ asymptotic energies and 6.3 kJ/mol for S ₀ → T ₁ 0-0 excitation energies, with respect to available experimental data. A composite method is used to determine S ₁ thresholds, with bt -STEOM-CCSD/cc-pVQZ calculation of vertical excitation energies and TD-RI-B3LYP/def2-TZVP calculations on S ₁ , which achieves a MAD of 7.2 kJ/mol, with respect to experimental 0-0 excitation energies. Our calculations suggest, with the exception of bifunctional carbonyls and enones, NTI reactions on S ₁ are unlikely to be important at tropospherically relevant photolysis energies (<400 kJ/mol). In contrast, at these energies almost all possible NTI channels on T ₁ are open, and all barrierless S ₀ NTI dissociations are accessible. Our calculations allow a number of structural effects on both 0-0 excitation energies and intrinsic reaction barriers, on a given electronic state, to be elucidated and rationalized.