Reaction of NO3 radicals with 1,3-cyclohexadiene alpha-terpinene, and alpha-phellandrene: Kinetics and products

The gas-phase reaction of NO3 radicals with 1,3-cyclohexadiene alpha-terpinene, and alpha-phellandrene has been studied in a flow system at 298 K in the pressure range 8 < P (mbar) < 250 using GC-MS/FID MS, and long-path FT-IR as detection techniques. At a total pressure of 15 mbar N-2 rate constants for the primary attack of NO3 were determined with the relative rate method to be (1.08 +/- 0.02) 10(-11), (1.03 +/- 0.06) 10(-10), and (5.98 +/- 0.20) 10(-11), respectively (unit: cm(3) molecule(-1) s(-1)). As buffer gas in the product studies served N-2 or He as well as O-2/N-2 mixtures. In the case of 1,3-cyclohexadiene the formation of the corresponding oxirane (7-oxa-bicyclo[4.1.0]hept-2-ene) up to a yield of 90% at P < 35 mbar and the formation of benzene (about 6%) was observed. The oxirane yield was independent on the amount of added O-2 and decreased with increasing total pressure. In the case of alpha-terpinene and alpha-phellandrene no indication of oxirane formation was found. However, the corresponding aromatic compound (p-cymene) was detected for both terpenes, maximum yield 6% for alpha-terpinene and 22% for alpha-phellandrene. For all reaction systems investigated the formation of organic nitrate-compounds was proved by means of long-path FT-IR measurements. In the case of the 1,3-cyclohexadiene degradation with [O-2] > 5 . 10(15) molecule cm(-3) an absorption band was detected in addition assigned to a carbonylic group. There were no indications for a reaction of O-2 with the adduct radical (NO3/diene).