AbstractThe associationof C6H5O with NO was studied with the cavity‐ring‐down method by directly monitoring the decay of C6H5O in the presence of varying, excess amounts of NO. The biomolecular rate constant determined in the temperature range 297‐373 K can be effectively rate constant determined in the temperature range 297–373 K can be effectively represented byk1= 10 − 12 · 12 ± 0.24 e(194±185)/rcm3molecule−1with a negative activation energy of 0.8 kcal mol−1(1 kcal = 4.184 kJ). In order to understand better the mechanism of the reaction,ab initiomolecular orbital calculations were also carried out at the MP4(SDQ)/6‐31G* level of theory using the HF optimized geometries. The molecular structues and energetics of five C6H5N1O2isomers were calculated. Among them, the most likely and stable association product, phenyl nitrite (C4H5ONO), was found to be 17 kal mol−1below the reactants, C6H5O + NO. Combining the measured rate constant and the calculated equilibrium constant for the association reaction, C6H5O + NO = C6H5ONO the rate constant for the unimolecular decomposition of C6H5ONO was obtained ask−1= 4.6 × 1015E−8580/Ts−1. The relatively large frequency factor suggests that a loose transition state was involved in the reaction, akin to those of its alkyl anal