Propargyl + O₂ Reaction in Helium Droplets: Entrance Channel Barrier or Not?

Abstract

A combination of liquid He droplet experiments and multireference electronic structure calculations is used to probe the potential energy surface for the reaction between the propargyl radical and O2. Infrared laser spectroscopy is used to probe the outcome of the low temperature, liquid He-mediated reaction. Bands in the spectrum are assigned to the acetylenic CH stretch (ν1), the symmetric CH2 stretch (ν2), and the antisymmetric CH2 stretch (ν13) of the trans-acetylenic propargyl peroxy radical ((•)OO-CH2-C≡CH). The observed band origins are in excellent agreement with previously reported anharmonic frequency computations for this species [Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Flowers, B. A.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2010, 114, 1498]. The Stark spectrum of the ν1 band provides further evidence that the reaction leads only to the trans-acetylenic species. There are no other bands in the CH2 stretching region that can be attributed to any of the other three propargyl peroxy isomers/conformers that are predicted to be minimum energy structures (gauche-acetylenic, cis-allenic, and trans-allenic). There is also no evidence for the kinetic stabilization of a van der Waals complex between propargyl and O2. A combination of multireference and coupled-cluster electronic structure calculations is used to probe the potential energy surface in the neighborhood of the transition state connecting reactants with the acetylenic adduct. The multireference based evaluation of the doublet-quartet splitting added to the coupled-cluster calculated quartet state energies yields what are likely the most accurate predictions for the doublet potential curve. This calculation suggests that there is no saddle point for the addition process, in agreement with the experimental observations. Other calculations suggest the possible presence of a small submerged barrier.

Infrared laser spectroscopy of the helium-solvated allyl and allyl peroxy radicals

“…18,19 The skimmed droplet beam passes in front of an effusive pyrolysis source, 20,21 which generates the allyl radical, formaldehyde (CH 2 O) and nitric oxide (NO) via the thermal dissociation of 3-butenyl nitrite (H 2 C=CHCH 2 CH 2 ONO). The pyrolysis source consists of a quartz tube heated to ≈800 K by a tantalum filament connected to water-cooled copper electrodes.…”

Decomposition kinetics for HONO and HNO2

“…The CASPT2 and MRCI+Q calculations were performed with aug-cc-pVQZ and aug-cc-PV5Z basis sets and extrapolated to the complete basis set limit. To further improve the accuracy of the multi-reference calculations, a separate set of potentials was computed on a higher spin surface [20][21][22][23]. The potential energy of the triplet surface was computed using both the ANL1 method of Equation…”

Recombination of aromatic radicals with molecular oxygen

“…The VaReCoF program allows for the evaluation of these interactions on-the-fly, with direct calls to ab initio electronic structure methods. The complete active space self-consistent field theory with second-order perturbation (CASPT2) [21] method combined with the cc-pVDZ basis set efficiently yields interaction energies of suitable accuracy [15,16] . Here, a 7-electron 5-orbital (7e,5o) active space was chosen to optimize the MEP for all four R + O 2 reactions.…”

“…The radical characteristics of triplet O 2 result in a barrierless minimum energy path (MEP) for its addition to radicals [15,16] . Prior applications of the VRC-TST approach, such as the recent application to the closely related C 2 H 3 + O 2 addition reaction [15] , have amply demonstrated its ability to accurately predict the rate coefficients for such barrierless radical-radical reactions.…”