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

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

PHOTOIONIZATION MASS-SPECTROMETRIC STUDIES OF THE METHYLSILANES SI(CH3)NH4-N (N = 0-3)

Photoionization efficiency curves for the low energy fragment ions (M - H)+, (M - H2)+, (M - CH3)+, and (M - CH4)+ for the series of methyl substituted silanes Si(CH3)nH4-n (n = 0-3) are reported. The molecular ions were undetectable except SiH4+. (M - H2)+ and (M - CH4)+ ions show sharp appearance onsets compared with (M - H)+ ions, which have distinct threshold curvature. (M - H2)+ ions are ascribed to silylene positive ions, SiR2+. The alternative silaethylene positive ion structure, CH2SiHR+, is unlikely because the fragmentation process yielding CH2SiD2+ with loss of HD is not observed in the photoionization mass spectrum of CH3SiD3. Thresholds are interpreted in terms of the thermochemistry of the various ionic and neutral silicon species and afford accurate calculation of hydride affinities of the silylene positive ions. The calculated hydride affinities for silylene positive ions are 263.4, 244.3 and 230.6 kcal mol-1 for SiH2+, SiMeH+ and SiMe2+, respectively. Within experimental error, the hydride affinities of the silylene positive ions are identical to those of the silicenium ions with the same number of methyl groups. The present results, combined with other available thermochemical data, lead to the critical assessment of the Si-H and Si-CH3 bond energies of the neutral and ionic fragments of methylsilanes, as well as ionization potentials of the silyl radicals and silylenes.X113229sciescopu