Spectroscopy of the Simplest Criegee Intermediate CH2OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra

CH2OO, the simplest Criegee intermediate, and ozone are isoelectronic. They both play very important roles in atmospheric chemistry. Whilst extensive experimental studies have been made on ozone, there were no direct gas-phase studies on CH2OO until very recently when its photoionization spectrum was recorded and kinetics studies were made of some reactions of CH2OO with a number of molecules of atmospheric importance, using photoionization mass spectrometry to monitor CH2OO. In order to encourage more direct studies on CH2OO and other Criegee intermediates, the electronic and photoelectron spectra of CH2OO have been simulated using high level electronic structure calculations and FranckCondon factor calculations, and the results are presented here. Adiabatic and vertical excitation energies of CH2OO were calculated with TDDFT, EOM-CCSD, and CASSCF methods. Also, DFT, QCISD and CASSCF calculations were performed on neutral and low-lying ionic states, with single energy calculations being carried out at higher levels to obtain more reliable ionization energies. The results show that the most intense band in the electronic spectrum of CH2OO corresponds to the ${{\rm{\tilde B}}}$1A' ? ${{\rm{\tilde X}}}$1A' absorption. It is a broad band in the region 250450 nm showing extensive structure in vibrational modes involving OO stretching and C-O-O bending. Evidence is presented to show that the electronic absorption spectrum of CH2OO has probably been recorded in earlier work, albeit at low resolution. We suggest that CH2OO was prepared in this earlier work from the reaction of CH2I with O2 and that the assignment of the observed spectrum solely to CH2IOO is incorrect. The low ionization energy region of the photoelectron spectrum of CH2OO consists of two overlapping vibrationally structured bands corresponding to one-electron ionizations from the highest two occupied molecular orbitals of the neutral molecule. In each case, the adiabatic component is the most intense and the adiabatic ionization energies of these bands are expected to be very close, at 9.971 and 9.974 eV at the highest level of theory used.