Effects of Collision and Vibrational Energy on the Reaction of CH₃CHO⁺(ν) with C₂D₄

Abstract

The reaction of acetaldehyde cations with ethene has been studied as a function of collision energy and acetaldehyde vibrational state. REMPI through different vibrational levels of the B̃ electronic state is used to produce CH3CHO+ with controlled excitation in different vibrational modes. Reactions are studied in a guided ion beam instrument, including measurements of product ion recoil velocity distributions. In addition, we calculated the structures and energetics of 13 different complexes that potentially could serve as intermediates to reaction. Three reactions are observed. Hydrogen atom transfer (HT) dominates at low collision energies and is suppressed by collision energy and, to a lesser extent, vibration. The HT reaction is clearly direct at high collision energies but appears to be mediated by a reactant-like precursor complex at low energies. The most energetically favorable product channel corresponds to elimination of CH3 from an intermediate complex. Nonetheless, this channel accounts for only ∼0.5% of the total product signal. The cross section for endoergic charge transfer (CT) is strongly enhanced by collision energy in the threshold region. Over a wide range of collision and vibrational energy, CH3CHO+ vibrational excitation enhances CT, but only 18% as much as for the equivalent amount of collision energy. This effect is interpreted in terms of competition between the CT and other product channels. The expected proton-transfer channel is not observed, an absence also attributed to competition.

Dynamical control of ‘statistical’ ion–molecule reactions

“…These are not isolated examples. Similar effects have been observed for other ion-molecule systems ranging in size from six atoms (e.g., OCS + + OCS [63]) to much larger systems such as reaction of C 6 H 5 OH + with NH 3 [64] or of acetaldehyde with a number of small molecules [65][66][67][68]. As in the examples presented, strong dynamical effects are observed even in reactions that are exoergic, with no activation barriers in excess of the reactant energy.…”

The mechanisms of collisional activation of ions in mass spectrometry

“…The latter case seems to be the most common at the higher T com values they studied, however in some cases it was shown that increasing the vibrational energy of the ion decreased the efficiency of T ! V energy transfer in the collisions (Kim, Liu, & Anderson, 2002), a result seemingly in contradiction to the idea that energy transfer should be easier the more anharmonic the vibration (Process 2). They have even shown that for CID processes that seemingly occur statistically (i.e., are mediated by long-lived complexes that have lifetimes consistent with statistical theories), one cannot guarantee that all internal energy is statistically distributed, since vibrationally selected ions still show dynamical effects.…”

“…Anderson and co-workers have explored the dynamics of ion-molecule reactions as a function of collision energy and vibrational energy in the reacting ion, and in doing so have explored the efficiency of translational-to-internal energy conversion in the 0-$10 eV T com range (Yang et al, 1991;Kim, Liu, & Anderson, 2002;Liu, Devener, & Anderson, 2002Liu, Devener, & Anderson, 2005;Liu et al, 2006a,b). They use supersonic expansion to create molecules that are internally cold, followed by resonance-enhanced multiphoton ionization (REMPI) to generate projectile ions with particular vibrational modes excited by one or two quanta of energy.…”

“…Velocity maps can be created which give information on the degree of forward and backward scattering of the product ions. Several systems have been studied including CH 2 CO þ /Ne, Xe (Liu, Devener, & Anderson, 2002); CH 3 CHO þ / C 2 D 4 (Kim, Liu, & Anderson, 2002); CH 2 CO þ /C 2 D 4 (Liu, Devener, & Anderson, 2004); CH 2 CO þ /CH 4 ; C 2 H þ 2 =CH 4 ; CH 2 CO þ /C 2 H 2 (Liu, Devener, & Anderson, 2005); NO þ 2 =Ne, Ar, Kr, Xe (Liu et al, 2006a); CH 2 CO þ /CO 2 (Liu et al, 2006b). They have developed a theoretical model for the velocity maps that appears to capture the basics of the physics of the interaction between the ion and neutral in these systems.…”

Reaction of formaldehyde cation with molecular hydrogen: Effects of collision energy and H2CO+ vibrations

“…33 In contrast, for CT of CH 3 CHO ϩ with C 2 D 4 (⌬ r Hϭ0.28 eV), the E vib ͑fit)/E vib ͑actual) ratio was ϳ18%, irrespective of the vibrational state. 34 Finally, for CT of OCS ϩ with C 2 H 2 (⌬ r H ϭ0.23 eV) the vibrational efficiency is quite mode-specific. E vib ͑fit)/E vib ͑actual) is ϳ100% for 3 ϩ ͑C-S stretch, 87 meV͒, and essentially 0% for both 1 ϩ ͑C-O stretch, 256 meV͒ and 2 ϩ ͑bend, 62 meV͒ excitation.…”