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 S 0 , S 1 , and T 1 electronic states. The calculated NTIa 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.

Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls

“…The computational protocols used in this paper to study S 1 , T 1 , and S 0 NTII photolysis thresholds are largely the same as reported by the authors in an earlier publication on the NTI reaction in carbonyls. 49 Photolysis thresholds are calculated by the addition of the intrinsic reaction barrier (relative to the relevant electronic state minimum) to the 0-0 excitation energy (from the S 0 minimum to the excited state minimum). The 0-0 excitation energies used here are calculated using protocol detailed by the authors in an earlier paper studying carbonyl excitation energies.…”

“…48 Butyrophenone has a phenyl group conjugated with the C O moiety and as an aromatic carbonyls it has similarities to the α, β-unsaturated carbonyls discussed here. This includes a T 2 state that is energetically accessible at tropospheric photon energies, 49 as well as a S 1 /T 2 /T 1 three-state intersection. This intersection is not at configurations accessed by the triplet biradical, and does not intersect with S 0 , and so does not decrease the triplet biradical lifetime.…”

“…In our earlier study on the NTI reaction, photolysis thresholds and intrinsic reaction barriers determined using TD-B3LYP were found to reasonable agreement with respect to experimental data. 49 Here, however, the presence of an S 1 /S 0 CI near S 1 biradicals and/or TS structures means that linear response theories, like TD-DFT, may not be appropriate and multiconfigurational methods must be used. 58 This, and other potential limitations of the theoretical methods used, are discussed below.…”

Predicting Carbonyl Excitation Energies Efficiently Using EOM-CC Trends

“…The error spread can be significantly reduced by using more accurate methods for E vert . A simple composite scheme was used in previous work by the authors, 1 where E vert is replaced with an inexpensive EOM-CC method (STEOM) and all other quantities were calculated with (TD-)DFT. This procedure (columns 3 and 4 in Figure 3) significantly decreases the error distribution, regardless of whether CAM-B3LYP or B3LYP is used.…”

Predicting Carbonyl Excitation Energies Efficiently Using EOM-CC Trends

“…We have previously identified 20 small carbonyl species that are relevant to tropospheric chemistry and for which experimental data is available. 1 Here, we add an additional 18 larger carbonyl species that require an affordable protocol for theoretical calculation of their excitation energies to be viable. The states of interest here for carbonyls are the ground and excited singlet states (S 0 and S 1 respectively), since direct transition from the ground state to the triplet state (S 0 → T 1 ) is spin-forbidden in the n → π * excitation.…”