DETERMINATION OF THE ISOMERIZATION RATE-CONSTANT ENTER-DOT)CH3-](HOCHCH2CH2CH)-H-CENTER-DOT(OOH)CH3 - IMPORTANCE OF INTRAMOLECULAR HYDROPEROXY ISOMERIZATION IN TROPOSPHERIC CHEMISTRY

The rate constant of the title reaction is determined during thermal d ecomposition of di-n-pentyl peroxide C5H11O-OC5H11 in oxygen over the temperature range 463-523 K. The pyrolysis of di-n-pentyl peroxide in O-2/N-2 mixtures is studied at atmospheric pressure in passivated quar tz vessels. The reaction products are sampled through a micro-probe, c ollected on a liquid-nitrogen trap and solubilized in liquid acetonitr ile. Analysis of the main compound, peroxide C5H10O3, was carried out by GC/MS, GC/MS/MS [electron impact EI and NH3 chemical ionization CI conditions]. After micro-preparative GC separation of this peroxide, t he structure of two cyclic isomers (3S,6S*)3 alpha-hydroxy-6-methyl-1 ,2-dioxane and (3R,6S*)3 alpha-hydroxy-6-methyl-1,2-dioxane was deter mined from H-1 NMR spectra. The hydroperoxy-pentanal OHC -(CH2)(2)-CH( OOH)-CH3 is formed in the gas phase and is in equilibrium with these t wo cyclic epimers, which are predominant in the liquid phase at room t emperature. This peroxide is produced by successive reactions of the n -pentoxy radical: a first one generates the (CH3CH)-H-.(CH2)(3)OH radi cal which reacts with O-2 to form CH3CH(OO.)(CH2)(3)OH; this hydroxy-p eroxy radical isomerizes and forms the hydroperoxy (HOCH)-H-.(CH2)(2)C H(OOH)CH3 radical. This last species leads to the pentanal-hydroperoxi de (also called oxo-hydroperoxide, or carbonyl-hydroperoxide, or hydro peroxypentanal), by the reaction (HOCH)-H-.(CH2)(2)CH(OOH)CH3 + O-2 -- > O=CH(CH2)(2)CH(OOH)CH3 + HO2. The isomerization rate constant HOCH2C H2CH2CH(OO.)CH3 --> HOC. HCH2CH2CH(OOH)CH3 (k(3)) has been determined by comparison to the competing well-known reaction RO2 + NO --> RO + N O2 (k(7)). By adding small amounts of NO (0-1.6 X 10(15) molecules cm( -3)) to the di-n-pentyl peroxide/O-2/N-2 mixtures, the pentanal-hydrop eroxide concentration was decreased, due to the consumption of RO, rad icals by reaction (7). The pentanal-hydroperoxide concentration was me asured vs. NO concentration at ten temperatures (463-523 K). The isome rization rate constant involving the H atoms of the CH2-OH group was d educed: k(3) = (6.4 +/- 0.6) X 10(10)exp{- (16,900 +/- 700)cal mol(-1) /RT}s(-1) or per H atom: k(3(H)) = (3.2 +/- 0.3) x 10(10)exp{ - (16,90 0 +/- 700)cal mol(-1)/RT}s(-1) The comparison of this rate constant to thermokinetics estimations leads to the conclusion that the strain en ergy barrier of a seven-member ring transition state is low and near t hat of a six-member ring. Intramolecular hydroperoxy isomerization rea ctions produce carbonyl-hydroperoxides which (through atmospheric deco mposition) increase concentration of radicals and consequently increas e atmospheric pollution, especially tropospheric ozone, during summer anticyclonic periods. Therefore, hydrocarbons used in summer should co ntain only short chains (<C-4) hydrocarbons or totally branched hydroc arbons, for which isomerization reactions are unlikely. (C) 1998 John Wiley & Sons, Inc.