Photoionization and ion cyclotron resonance studies of the ion chemistry of ethylene oxide

Abstract

Time-resolved photoionozation mass spectrometry (PIMS), ion cyclotron resonance spectroscopy (ICR), and photoelectron spectroscopy have been employed to study the formation of the ethylene oxide molecular ion and its subsequent ion–molecule reactions which lead to the products C2H5O+ and C3H5O+. Earlier observations that a structurally and energetically modified species (C2H4O+) * is an intermediate in the production of C3H5O+ are confirmed. The PIMS data detail the effects of internal energy on reactivity, with the ratio of C3H5O+ to C2H5O+ increasing by an order of magnitude with a single quantum of vibrational energy. Evidence is presented for the formation of (C2H4O+) * in a collision-induced isomerization which yields a ring-opened structure by C–C bond cleavage. This species contains considerable internal excitation which is relaxed in collisions with ethylene oxide or bath gases such as SF6 prior to reaction. The relaxed ring-opened C2H4O+ ion reacts with neutral ethylene oxide by CH₂⁺ transfer to yield an intermediate product ion C3H6O+ which gives C3H5O+ by loss of H. Isotopic product distributions observed in a mixture of ethylene oxide and ethylene oxide-d4 are consistent with this mechanism. The effects of ion kinetic energy on reactivity are explored using ICR techniques. Increased reactant ion kinetic energy leads to collision-induced dissociation of C2H4O+ rather than isomerization to the open form.