Collision-induced dissociation of formaldehyde cations: The effects of vibrational mode, collision energy, and impact parameter

Abstract

We report a study of collision-induced dissociation (CID) of H2CO+, including measurement of the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the CID integral and differential cross sections. CID was studied for collision with both Xe and Ne, and the Ne results provide a very detailed probe of energy transfer collisions leading to CID. The CID appearance threshold is found to depend only on total energy, but for all energies above threshold, vibrational energy is far more effective at driving CID than Ecol, with some mode-specificity. Results are fit with an impact parameter-based mechanism, and considerable insight is obtained into the origins of the Ecol and vibrational effects. A series of ab initio and RRKM calculations were also performed to help interpret the results.

The effects of vibrational mode and collision energy on the reaction of formaldehyde cation with carbonyl sulfide

“…As discussed previously, 15 the TS for this isomerization is Ͼ1.9 eV above the H 2 CO ϩ energy-far too high to be driven by reactant internal energy. It is certainly possible that isomerization might occur in the course of reaction, while in one of the OCS-H 2 CO ϩ complexes.…”

“…The guided ion beam tandem mass spectrometer used in this study has been described previously, 14,15 along with the operation, calibration and data analysis procedures. The H 2 CO precursor was generated by heating a mixture of solid paraformaldehyde ͑Aldrich 95%͒ and anhydrous MgSO 4 ͑Merck͒, held in a stainless steel tube connected directly to a pulsed molecular beam valve.…”

“…This assumption is supported by our recent CID experiments, showing that a CID threshold shift for v 2 ϩ ϭ1 equal to the v 2 ϩ vibrational energy, as expected. 15 The ion vibrational levels studied are summarized in Table I. Under even low intensity REMPI conditions, H 2 CO ϩ produced in the ground electronic state is readily photodissociated to HCO ϩ and CO ϩ by absorbing additional photons. To remove these fragment ions from the beam, the ions were generated inside a short quadrupole ion guide which focused the ions through a pair of ion lenses into a conventional mass-selecting quadrupole.…”

Quasiclassical Trajectory Study of the Collision-Induced Dissociation Dynamics of Ar + CH3SH+ Using an Ab Initio Interpolated Potential Energy Surface

“…First, presumably the experimental data only takes into account those ions dissociating within the timeof-flight in their triple-quadrupole double-octopole photoionization ion-molecule apparatus. Second, as pointed out elsewhere, 21,22 it is known that the experimental cross sections are possibly somehow too small because some strongly sidewaysscattered product ions are not detected. However, these possible sources of discrepancy probably cannot fully account for the large differences between theory and experiment.…”

Dynamical control of ‘statistical’ ion–molecule reactions

“…TOF is used to measure the actual reactant ion beam velocity distribution (and thus the E col distribution) at each nominal E col . Two types of differential cross-sections can be measured [20,[34][35][36]. TOF is always used to measure the product ion axial velocity distribution, i.e., the projection of the full velocity distribution on the ion guide axis.…”