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The gas-phase reaction of NO, radicals with 1,3-~yclohexadiene, a-terpinene, and a-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, rate constants for the primary attack of NO, were determined with the relative rate method to be (1.08t0.02) lo-'', (1.03k0.06) lo-", and (5.98t0.20) lo-", respectively (unit: cm3 molecule-' s-').As buffer gas in the product studies served N, or He as well as O,/N, mixtures. In the case of 1,3-~yclohexadiene the formation of the corresponding oxirane (7-0xa-bicyclo[4.1 .O]hept-2-ene) up to a yield of 90Vo at P < 35 mbar and the formation of benzene (about 6%) was observed. The oxirane yield was independent on the amount of added 0, and decreased with increasing total pressure. In the case of a-terpinene and aphellandrene no indication of oxirane formation was found. However, the corresponding aromatic compound (pcymene) was detected for both terpenes, maximum yield 6% for a-terpinene and 22Vo for a-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,] > 5lo" molecule cm-3 an absorption band was detected in addition assigned to a carbonylic group. There were no indications for a reaction of 0, with the adduct radical (NO,/diene).

Rate constants for the gas-phase reaction of NO3 radicals with 1 -butene, trans-butene, 2-methyl-2-butene and 2.3-dimethyl-2-butene have been obtained in a flow system at 298i2 K and a pressure of 3 mbar He using NO3 LIF measurements. NO3 radicals were produced via thermal decomposition of N205 at ca. 400 K, i.e.Applying pseudo-first-order conditions, the rate constants for 1 -butene and trans-butene were found to be ( I .04*0.1 I). I O-I4 and (3.74i0.45).respectively. For 2-methyl-2-butene and 2,3-dimethyl-2-butene the rate constants were obtained under second-order conditions to be (1.03+0.09)~10-" and (5.49a0.42).10-", respectively (unit: cm3 molecule-' s-').N20s+M + N03+N02+M.

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