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