A Simple and Efficient Method for Simulating the Electronic Absorption Spectra of Criegee Intermediates: Benchmarking on CHOO and CHCHOO.

Criegee intermediates (CIs) play a vital role in the atmosphere-known most prominently for enhancing the oxidizing capacity of the troposphere. Knowledge of their electronic absorption spectra is of vital importance for two reasons: (1) to aid experimentalists in detecting CIs and (2) in deciding if their removal is affected by solar photolysis. In this article we report a simple and efficient method based on the nuclear ensemble method that may be effectively used to compute the electronic absorption spectra of Criegee intermediates without the need for extensive computation of preparing the initial configurations of the starting geometry. We use this method to benchmark several excited-state electronic structure methods and their efficacy in reproducing the electronic absorption spectra of two well-known cases of CI: CHOO and CHCHOO. The success and computational feasibility of the methodology are crucial for its applicability to CIs of increasing molecular complexity, which have no known experimentally measured electronic absorption spectra, allowing a guide for experimentalists. Application of the methodology to more complex CIs (e.g., those with extended conjugation or those derived from endocyclic alkenes) will also reveal if solar photolysis becomes a competitive removal process when compared to unimolecular decay or bimolecular chemistry.