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, NTIa and NTIb, forming larger and smaller alkyl radicals, respectively. For a data set of 20 small, atmospherically relevant carbonyls we predict NTIa and NTIb thresholds on the S₀, S₁, and T₁ electronic states. The calculated NTIa T₁ 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 S₀, S₁, and T₁ electronic states are predicted. We find RI-B2GP-PLYP/def2-TZVP calculations on S₀ and unrestricted RI-B2GP-PLYP/def2-TZVP calculations on T₁ give MADs of 6.1 kJ/mol for S₀ asymptotic energies and 6.3 kJ/mol for S₀ → T₁ 0-0 excitation energies, with respect to available experimental data. A composite method is used to determine S₁ thresholds, with bt-STEOM-CCSD/cc-pVQZ calculation of vertical excitation energies and TD-RI-B3LYP/def2-TZVP calculations on S₁, 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 S₁ are unlikely to be important at tropospherically relevant photolysis energies (<400 kJ/mol). In contrast, at these energies almost all possible NTI channels on T₁ are open, and all barrierless S₀ 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.