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

Abstract

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.