Kinetics of the reaction of propargyl radical with nitric oxide

The reaction between the propargyl radical (C3H3) and NO has been investigated as a function of temperature (195-473 K) and pressure (3-100 Torr) by using color center laser infrared kinetic spectroscopy. At room temperature and below, the reaction rate was found to depend strongly on the helium buffer gas pressure and, at any fixed buffer gas density, to decrease with increasing temperature. This behavior is consistent with the reaction occurring by termolecular addition to produce C3H3NO. Data collected over a wide pressure range at 195 and 296 K were fitted to a semiempirical model developed by Tree for reactions of this type. The structure and energetics of the adduct were explored by performing both B3LYP 6-311++G(2df,2pd) and G2 calculations. The enthalpy change, Delta H, for addition of NO to the CPT end of propargyl was determined as -123 and -138 kJ/mol, respectively, by using these two methods. The calculations also showed that NO can add at the CH end of propargyl but with a smaller binding energy. Estimates of the equilibrium constant for adduct formation, made using data obtained from these calculations, revealed that the addition reaction should shift from an equilibrium position strongly favoring the adduct to one strongly favoring free propargyl as the temperature is raised from 550 to 650 K, This temperature regime is higher than any of the temperatures reached experimentally.