Products of the OH radical-initiated reaction of 3-hexene-2,5-dione

3-Hexene-2,5-dione [CH3C(O)CH=CHC(O)CH3] and other unsaturated 1,4-dicarbonyls are formed from the atmospheric photooxidations of aromatic hydrocarbons. We have investigated the products of the gas-phase reaction of OH radicals with 3-hexene-2,5-dione in the presence of NO at room temperature and atmospheric pressure of air using in situ atmospheric pressure ionization tandem mass spectrometry. Products of molecular weight 102 and 191 were identified and from consideration of the potential reactions these are attributed to CH3C(O)CH(OH)CHO and CH3C(O)CH(OH)CH(ONO2)C(O)CH3, respectively. After initial addition of the OH radical, the CH3C(O)CH(OH)CHC(O)CH3 radical adds O-2 to form the peroxy radical which reacts with NO to yield the molecular weight 191 hydroxynitrate CH3C(O)CH(OH)CH(ONO2)C(O)CH3 or the hydroxyalkoxy radical CH3C(O)CH(OH)CH(O)C(O)CH3 plus NO2. Our observation of a molecular weight 102 species indicates that the CH3C(O)CH(OH)CH(O)C(O)CH3 radical dominantly decomposes to CH3C(O)CH(OH)CHO plus CH3CO, rather than decomposing to CH3C(O)CHO plus CH3C(O)CHOH, isomerizing, or reacting with O-2. Thermochemical calculations and an empirical estimation method for calculating alkoxy radical reaction rates are consistent with our experimental observations.