Radical–Radical Reaction Dynamics Probed Using Millimeterwave Spectroscopy: Propargyl + NH₂/ND₂

Abstract

We apply chirped-pulse uniform flow millimeterwave (CPUF-mmW) spectroscopy to study the complex multichannel reaction dynamics in the reaction between the propargyl and amino radicals (C3H3 + NH2/ND2), a radical–radical reaction of importance in the gas-phase chemistry of astrochemical environments and combustion systems. The photolytically generated radicals are allowed to react in a well-characterized quasi-uniform supersonic flow, and mmW rotational spectroscopy (70–93 GHz) is used for simultaneous detection of the reaction products: HCN, HNC, HC3N, DCN, DNC, and DC3N, while spectral intensities of the measured pure-rotational lines allow product branching to be quantified. High-level electronic structure calculations were used for theoretical prediction of the reaction pathways and branching. Experimentally deduced product branching fractions were compared with the results from statistical simulations based on the RRKM theory. Product branching was found to be strongly dependent on the excess internal energy of the C3H3 and NH2/ND2 reactants.

Multichannel Radical–Radical Reaction Dynamics of NO + Propargyl Probed by Broadband Rotational Spectroscopy

“…Therefore, because of this excess energy, the reactive landscape changes entirely and the reaction dynamics may yield various products with different branching ratios. This was shown in our recent investigation on the radical–radical reaction of propargyl + NH 2 /ND 2 . In that experiment, analogous to the study presented here, the radicals were produced by the 193 nm laser photolysis of propargyl bromide and NH 3 /ND 3 .…”

“…This was shown in our recent investigation on the radical−radical reaction of propargyl + NH 2 /ND 2 . 26 In that experiment, analogous to the study presented here, the radicals were produced by the 193 nm laser photolysis of propargyl bromide and NH 3 /ND 3 . We estimated that the radicals were produced with at least 200 kJ mol −1 of excess energy.…”

Direct D-atom Incorporation in Radicals: An Overlooked Pathway for Deuterium Fractionation

“…We note that other species are present in the flow as discussed in our previous work (Broderick et al 2018;Gurusinghe et al 2021b) and mentioned above. Aside from deuterium atoms, the primary deuterated species will be ND 2 .…”

“…Direct D-atom-exchange reactions were studied in a quasiuniform Laval flow of helium in which precursor molecules were seeded then exposed to 193 nm laser radiation as described above. In the past, we have used the variation of the density in the quasi-uniform flow to characterize the density-dependent chemistry observed in the experiment, as regions of different conditions arrive at the detector at welldefined times (Broderick et al 2018;Gurusinghe et al 2021b). This approach is employed here as well.…”

“…A single photon dissociates the propargyl bromide and ND 3 resulting mostly in propargyl (C 3 H 3 ), ND 2 , D atoms, and H atoms (Nakajima et al 1991;Lee & Lin 1998;Foley et al 2019). The C 3 H 3 can react with ND 2 to give a range of products; these are the subject of a separate study reported elsewhere (Gurusinghe et al 2021b). A second photon of the same wavelength that is absorbed within the 20 ns laser pulse may dissociate the propargyl radical giving mainly C 3 H 2 isomers and H atoms.…”