Detection and identification of free radicals in the gaseous-phase ozonolysis of ethylene: microreactor-matrix ESR spectroscopy

Abstract

An investigation of the gas-phase ozonolysis of ethylene with the aid of a combination of a microreactor with matrix ESR spectroscopy is reported. Formation of radicals by the ESR signal is unequivocally proven. Concentration and type of radicals are shown to depend both on residence time and initial concentration, in particular on initial O 2 concentration. For millisecond residence times and O 2 initially present predominantly HOO (ROO) is formed: for reaction times ⩾ 40 s the radicals disappear. In absence of O 2 at short reaction time, H atoms and the CH 3(C 2H 5) radical are identified, in addition to HOO (ROO). Besides these radical species NO 2 and NO 3 were found to be present. UV irradiation of the reaction matrices leads to formation of H atoms. CH 3− C 2H 5 and HCO and destruction of peeroxy-type radicals. Kinetic calculations are in qualitative agreement with the observed dependence of radical concentration on reaction time. Reaction schemes explaining the observed behavior are proposed. For the two radicals HOO and CH 3OO extended ab initio SCF MCI calculations were carried out. Besides electronic energies for ground ( 2A″) and first excited ( 2A′) electronic states, predictions for isotropic coupling constants for all nuclei, molecular electric dipole and quadrupole values, spin densities and electric-field gradients at the oxygen nuclei are given. A short discussion is devoted to the discrepancies between observed and calculated coupling constants.