Persistent collision complexes in the reaction of silyl ions with ethylene

Abstract

The reaction of SiH3+ with C2H4 has been studied in a tandem mass spectrometer at relative kinetic energies in the range of 0.47 to 2.4 eV and at C2H4 pressures in the range of 2-80 X Torr. At low relative kinetic energies persistent collision complexes SiC2H7+ are directly observed, in the absence of collisional stabilization, some 2 X s after formation. Collisional stabilization by C2H4 increases the yield of SiC2H7+ so that at 80 X Torr this ion accounts for -80% of the observed products. The other observed products are SiQHS+, SiCH3+, and SiC2H3+. Extrapolation of product yields to infinite pressure indicates that the collision complex accounts for -90% of total reaction with the other 10% of reaction being due to a direct process forming SiCH3+. The experimental data yield absolute values for the lifetime of energy-rich collision complexes, for the unimolecular rate constants for decomposition of the complex to SiC*H5+ and SiCH3+, and for the single-collision stabilization of SiC2H7+* by C2H4. The strong collision assumption breaks down at the higher relative kinetic energies and a value of 0.7-1.1 eV for the maximum energy transferred per collision from SiC2H7+* to C2H4 may be derived. RRKM calculations based on the chemically activated molecule being CH3CH2SiH2+ and in which angular momentum of reactants is conserved give order-of-magnitude agreement with experiment.

Ion-Molecule reactions in volatile silanes

“…As with the previous study [91], the effect of increasing the C2H 4 pressure + is to increase markedly the relative abundance of SiC2H 7, so that at 5 x lO "3…”

“…Collisional stabilization of complexes. The collisional stabilization mechanism is well accepted in chemical kinetics and when applied to ion-molecule reactions [91], may be written as follows: B. Monosilane-Ethylene As noted earlier, Mayer and Lampe [51] reported that a beam of SiH~ ions re-I acted with C2H 4 in a collision cell to form SiC2H ~ ~ intermediates, which were detected after a transit time of 10 -5 s. Since the C2H 4 pressure in the collision chamber was low (10 -3 torr) and the phenomenological cross-section for detection was extremely sensitive to collision energy, approaching zero at 1.5 eV (CM), it was concluded that the SiC2H ~ ions were being detected as unstabilized and therefore energy-rich complexes.…”

“…A few years later, Allen and Lampe [91] reported the direct detection of an complex resulting from the reaction of low energy SiH~ unstabil ized collision ions with C2H 4. The reaction was studied at relative kinetic energies in the range of 0.47-2.4 eV, but only at or below 1 eV were unstabilized collision complexes sufficiently long-lived to survive the transit time of the apparatus.…”

“…Although it is not a likely event at this pressure, collisional stabilization can occur. Allen and Lampe [91] Illustrated in Figure 16 [91] are such plots for the reaction at several primary ion energies (LAB). The plot for l eV primary ion energy (LAB), in Figure 16(a), when extrapolated to zero pressure, shows a positive intercept.…”

“…In this mechanism the complex SiC2H 7 is assumed to be stabilized to SiC2H; . as a result of the internal transfer of energy that occurs upon collision with C2H 4 [91].…”