Substituent effects on the spectroscopic properties of Criegee intermediates

Restricted coupled cluster RCCSD(T) and explicitly correlated coupled cluster RCCSD(T)-F12 methods have been applied to investigate the electronic structures, rotational constants, and harmonic vibrational frequencies of the simplest Criegee intermediate (CH₂OO) and its methyl and vinyl substituted analogs. The vinyl substitution of hydrogen in CH₂OO slightly increases the C=O bond length but does not affect the OO bond, whereas the methyl substitution of hydrogen increases the OO bond but does not affect the CO bond. Vertical excitation energy for the four low-lying singlet electronic states for each Criegee intermediate has been calculated at the equation-of-motion coupled-cluster method inclusive of single and double excitations and two different basis sets, aug-cc-pVXZ (X = T and Q). The results suggest that the 2¹A′ electronic state of C₂H₃CHOO is stabilized relative to the 2¹A′ electronic state of CH₂OO and CH₃CHOO, whereas the 2¹A′ state of syn-CH₃CHOO is destabilized relative to the 2¹A′ state of CH₂OO. The simulated UV spectra of these species show that there are bathochromic and hyperchromic effects for C₂H₃CHOO. Finally, the absorption spectra of syn-CH₃CHOO, anti-C₂H₃CHOO, and syn-C₂H₃CHOO has been red-shifted relative to CH₂OO whereas anti-CH₃CHOO was blue-shifted.