O−H Stretch Overtone Excitation in Methyl and Ethyl Hydroperoxides

Abstract

We use laser photoacoustic spectroscopy to obtain vibrational overtone spectra in the regions of four and five quanta of O−H stretch (4νOH and 5νOH) for gas-phase methyl (MeOOH), ethyl (EtOOH), and tert-butyl hydroperoxide (t-BuOOH). Integrated cross sections for the main peak due to O−H stretch excitation to 4νOH are similar for all three hydroperoxides (∼2 × 10-21 cm2 molecule-1 cm-1); cross sections for excitation to and 5νOH are an order of magnitude smaller. Interpretation of spectral features using a previously reported vibration−torsion model for ROOH molecules, where excitation of torsional motion about the O−O bond accompanies vibrational excitation, yields plausible fits to the spectra. Simulations with the vibration−torsion model and ab initio calculations suggest barriers for torsional motion about the O−O bond to be higher in t-BuOOH than in MeOOH and EtOOH, with a trend of increasing trans torsional barrier height with increasing size of the R group.