Low-Temperature Kinetics of the HCCN + O ₂ Reaction in an Extended Laval Nozzle with Detection by Chirped-Pulse Fourier-Transform Millimeter-Wave Spectroscopy

We report the first low-temperature kinetics measurements of an HCCN radical–radical reaction. We measured the bimolecular rate coefficients for the HCCN + O2 reaction under pseudo-first-order conditions inside an extended Laval nozzle with detection by chirped-pulse Fourier-transform millimeter-wave (CP-FT-mmW) spectroscopy. The extended nozzle in which the reaction is performed is followed by a shock-free secondary expansion to low density and low temperature optimal for detection by rotational spectroscopy. We determined the bimolecular rate as (1.67 ± 0.06) × 10–11 cm3 molecule–1 s–1 at 20 K and (7.59 ± 0.12) × 10–12 cm3 molecule–1 s–1 at 70 K, showing a pronounced negative temperature dependence. The results are discussed in light of recent theoretical calculations and a single previous room-temperature measurement of the reaction rate.