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.