Kinetics of the reaction of propargyl radical with nitric oxide

The reaction between the propargyl radical (C3H3) and NO has been investiga ted as a function of temperature (195-473 K) and pressure (3-100 Torr) by u sing color center laser infrared kinetic spectroscopy. At room temperature and below, the reaction rate was found to depend strongly on the helium buf fer gas pressure and, at any fixed buffer gas density, to decrease with inc reasing temperature. This behavior is consistent with the reaction occurrin g by termolecular addition to produce C3H3NO. Data collected over a wide pr essure range at 195 and 296 K were fitted to a semiempirical model develope d by Tree for reactions of this type. The structure and energetics of the a dduct were explored by performing both B3LYP 6-311++G(2df,2pd) and G2 calcu lations. 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 t hese two methods. The calculations also showed that NO can add at the CH en d of propargyl but with a smaller binding energy. Estimates of the equilibr ium constant for adduct formation, made using data obtained from these calc ulations, revealed that the addition reaction should shift from an equilibr ium position strongly favoring the adduct to one strongly favoring free pro pargyl as the temperature is raised from 550 to 650 K, This temperature reg ime is higher than any of the temperatures reached experimentally.