Proton Tunneling in the Loss of Hydrogen Bromide from Energy-Selected Gas-Phase 2-Bromobutane Cations

Abstract

The dissociation dynamics of energy-selected 2-bromobutane ions has been investigated by photoelectron photoion coincidence (PEPICO). The data were collected with a conventional gas discharge light source at the University of North Carolina and at the SuperACO synchrotron storage ring of LURE in Orsay, France. The 2-bromobutane ion dissociates via Br and HBr loss within 0.3 eV of the ionization limit. The Br loss channel produces the sec-butyl ion C4H9 + via a simple bond cleavage and is found to proceed at a fast rate. At lower ion energies, the HBr loss channel produces the 2-butene ion via a concerted mechanism with rate constants in the microsecond range. A model that involves tunneling associated with the proton transfer is proposed. Asymmetric time-of-flight distributions and breakdown diagrams are fitted to the statistical RRKM theory with the aid of GAUSSIAN ab initio molecular orbital calculations at the MP2/6-31G* level. The fitting of the data was optimized for a barrier height of 0.438 eV and an imaginary frequency associated with the H atom transfer step of 1245 cm-1. This compares with the ab initio results of 0.796 eV and 1265 cm-1. Heats of formation at 0 K for 2-bromobutane cation (880.1 ± 2.7 kJ mol-1) and sec-butyl cation (794 ± 3 kJ mol-1) were derived from this work.

Threshold photoelectron photoion coincidence studies of parallel and sequential dissociation reactions

“…In that case, the rates are often dominated by tunnelling through these barriers, 26,27 which offer opportunities to model tunnelling steps. Because of the large ion neutral attractive potentials, ionic reactions provide excellent examples of dissociation events with no barriers.…”

Photodissociation of the Ethyl Bromide Cation at 355 nm by Means of TOF-MS and Ion Velocity Imaging Techniques

“…More recently, several groups published a collaborative work on the dissociation of 2-Bromobutane ions [18]. They found that Br loss channel proceeds at a fast rate, while the HBr loss is slow and tunneling is again involved in the reaction.…”

Dissociation dynamics of propargyl chloride molecular ion near the reaction threshold: manifestation of quantum mechanical tunneling

“…Underestimation of the barrier by a few tenth of eV is possible. 2,[7][8][9] We have taken 0.64 eV as the best estimate of the classical barrier for the ratedetermining step compatible with the PEPICO data (see below). The tunneling-corrected RRKM rate-energy relation calculated with this critical energy is shown in Fig 5. According to the calculated rate-energy relation, observation of the dissociation on microsecond time scales both in the previous PEPICO and current MIKES experiments means that the reaction occurring at 0.1-0.2 eV below the threshold via quantum mechanical tunneling was detected.…”

“…This has often been invoked to explain some of the unusual kinetic phenomena observed experimentally. [1][2][3][4][5][6][7][8][9] As is well known, the tunneling effect may appear prominently for reactions involving H-atom transfer. Miller reported an algorithm to evaluate the tunneling-corrected transition state rate constant using the curvature at the transition state obtained from the quantum chemical calculation.…”