Kinetics of the reaction of vinyl radicals with NO: Ab initio theory, master equation predictions, and laser absorption measurements

Abstract

The pulsed laser photolysis/cw laser absorption technique is used to investigate the reaction of vinyl (C₂H₃) with NO in the temperature range from 295 to 700 K and pressures from 10 to 320 Torr (1.33 to 42.6 kPa). Vinyl radicals are generated by photolysis of vinyl iodide at 266 nm and detected by visible laser absorption in a vibronic band of the (Ã ← ) transition near 403 nm. The potential energy surface is explored with both quadratic configuration interaction and multi-reference configuration interaction ab initio calculations. These ab initio predictions are employed in RRKM theory based master equation simulations of the temperature and pressure dependent kinetics. At room temperature, the overall rate constant for removal of vinyl radical by NO is measured to be 1.6 ± 0.4 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, with negligible pressure dependence from 10 Torr (1.33 kPa) to 160 Torr (21.3 kPa) of helium. At constant pressure the rate constant decreases rapidly with temperature. At higher temperatures, a falloff of the rate constant to lower pressure is observed. The ab initio characterizations suggest a significant contribution from HCN + CH₂O formation, with both isomerization transition states for the pathway leading to this product lying ∼15 kcal mol⁻¹ (63 kJ mol⁻¹) below the entrance channel. The master equation analysis provides a reasonably satisfactory reproduction of the observed kinetic data. The HCN + CH₂O bimolecular channel, which proceeds from the addition complex through tight ring forming and opening transition states, has a negative temperature dependence and is the dominant channel for pressures of about 50 Torr (6.7 kPa) and lower. The theoretically predicted zero pressure rate coefficient is reproduced by the modified Arrhenius expression 5.02 × 10⁻¹¹(T/298)^−3.382exp(−516.3/T) cm³ molecule⁻¹ s⁻¹ (with T in K).