title: Unusually fast 1,6-h shifts of enolic hydrogens in peroxy radicals: formation of the first-generation C2 and C3 carbonyls in the oxidation of isoprene.
abstract: In a theoretical investigation using the CBS-QB3//UB3LYP/6-31+G** method supported by higher-level computations such as CBS-QB3//UQCISD/6-31+G**, the 1,6-H shifts of the enolic hydrogen in peroxy radicals of the type Z-HO-CH═CH-CH(2)-OO(•) were found to face exceptionally low energy barriers of only about 11 kcal mol(-1)--i.e., 6-9 kcal mol(-1) lower than the barriers for similar shifts of alkane hydrogens--such that they can proceed at unequaled rates of order 10(5) to 10(6) s(-1) at ambient temperatures. The unusually low barriers for enolic 1,6-H shifts in peroxy radicals, characterized here for the first time to our knowledge, are rationalized. As cases in point, the secondary peroxy radicals Z-HO-CH═C(CH(3))-CH(OO(•))-CH(2)OH (case A) and Z-HO-CH═CH-C(CH(3))(OO(•))-CH(2)OH (case B) derived from the primary Z-δ-hydroxy-peroxy radicals in the oxidation of isoprene, are predicted to undergo 1,6-H shifts of their enolic hydrogens at TST-calculated rates in the range 270-320 K of k(T)(A) = 5.4 × 10(-4) × T(5.04) × exp(-1990/T) s(-1) and k(T)(B) = 109 × T(3.13) × exp(-3420/T) s(-1), respectively, i.e., 2.0 × 10(6) and 6.2 × 10(4) s(-1), respectively, at 298 K, far outrunning in all relevant atmospheric and laboratory conditions their reactions with NO proposed earlier as their dominant pathways (Dibble J. Phys. Chem. A 2004, 108, 2199). These fast enolic-H shifts are shown to provide the explanation for the first-generation formation of methylglyoxal + glycolaldehyde, and glyoxal + hydroxyacetone in the oxidation of isoprene under high-NO conditions, recently determined by several groups. However, under moderate- and low-NO atmospheric conditions, the fast interconversion and equilibration of the various thermally labile, initial peroxy conformers/isomers from isoprene and the isomerization of the initial Z-δ-hydroxy-peroxy radicals, both recently proposed by us (Peeters et al. Phys. Chem. Chem. Phys. 2009, 11, 5935), are expected to substantially reduce the yields of the small carbonyls at issue.
authors.affiliation: Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium. jozef.peeters@chem.kuleuven.be
authors.forename: Jozef
authors.initials: J
authors.lastname: Peeters
authors.forename: Thanh Lam
authors.initials: TL
authors.lastname: Nguyen
mesh_terms: D002070:Butadienes / Q000737:chemistry*; D005609:Free Radicals / Q000138:chemical synthesis / Q000737:chemistry; D045782:Hemiterpenes / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010420:Pentanes / Q000737:chemistry*; D010545:Peroxides / Q000138:chemical synthesis* / Q000737:chemistry; D011789:Quantum Theory
chemical_list: D002070:Butadienes; D005609:Free Radicals; D045782:Hemiterpenes; D010420:Pentanes; D010545:Peroxides; C005059:isoprene; D006859:Hydrogen