Reactions of the alkoxy radicals formed following OH-addition to alpha-pinene and beta-pinene. C-C bond scission reactions.
The atmospheric degradation pathways of the atmospherically important terpenes alpha-pinene and beta-pinene are studied using density functional theory. We employ the correlation functional of Lee, Yang, and Parr and the three-parameter HF exchange functional of Becke (B3LYP) together with the 6-31G(d) basis set. The C-C bond scission reactions of the beta-hydroxyalkoxy radicals that are formed after OH addition to alpha-pinene and beta-pinene are investigated. Both of the alkoxy radicals formed from the alpha-pinene-OH adduct possess a single favored C-C scission pathway with an extremely low barrier (approximately 3 kcal/mol) leading to the formation of pinonaldehyde. Neither of these pathways produces formaldehyde, and preliminary computational results offer some support for suggestions that 1,5 or 1,6 H-shift (isomerization) reactions of alkoxy radicals contribute to formaldehyde production. In the case of the alkoxy radical formed following OH addition to the methylene group of beta-pinene, there exists two C-C scission reactions with nearly identical barrier heights (approximately 7.5 kcal/mol); one leads to known products (nopinone and formaldehyde) but the ultimate products of the competing reaction are unknown. The single C-C scission pathway of the other alkoxy radical from beta-pinene possesses a very low (approximately 4 kcal/mol) barrier. The kinetically favored C-C scission reactions of all four alkoxy radicals appear to be far faster than expected rates of reaction with O2. The rearrangement of the alpha-pinene-OH adduct, a key step in the proposed mechanism of formation of acetone from alpha-pinene, is determined to possess a barrier of 11.6 kcal/mol. This value is consistent with another computational result and is broadly consistent with the modest acetone yields observed in product yield studies.
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Department of Chemistry, State University of New York-Environmental Science and Forestry, 1 Forestry Drive, Syracuse, New York 13210, USA. tsdibble@syr.edu
D000096:Acetone / Q000737:chemistry; D000393:Air Pollutants / Q000737:chemistry*; D000081008:Bicyclic Monoterpenes; D001643:Bridged Bicyclo Compounds / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005609:Free Radicals / Q000737:chemistry; D017665:Hydroxyl Radical; D007536:Isomerism; D039821:Monoterpenes; D013729:Terpenes / Q000737:chemistry*
D000393:Air Pollutants; D000081008:Bicyclic Monoterpenes; D001643:Bridged Bicyclo Compounds; D005609:Free Radicals; D039821:Monoterpenes; D013729:Terpenes; D000096:Acetone; D017665:Hydroxyl Radical; C010789:beta-pinene; C005451:alpha-pinene