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 (CHOO) and its methyl and vinyl substituted analogs. The vinyl substitution of hydrogen in CHOO 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 2A' electronic state of CHCHOO is stabilized relative to the 2A' electronic state of CHOO and CHCHOO, whereas the 2A' state of syn-CHCHOO is destabilized relative to the 2A' state of CHOO. The simulated UV spectra of these species show that there are bathochromic and hyperchromic effects for CHCHOO. Finally, the absorption spectra of syn-CHCHOO, anti-CHCHOO, and syn-CHCHOO has been red-shifted relative to CHOO whereas anti-CHCHOO was blue-shifted.