Dibble Group Isoprene Project


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Dibble Group Results on the Atmospheric Chemistry of Isoprene

Isoprene (2-methyl-1,3-butadiene) is the single most important volatile carbon contributing to formation of ground-level ozone. It constitutes about 40% by mass of non-methane organic compound emissions to the atmosphere, and it is emitted almost exclusively by natural sources. Isoprene contributes significantly to ozone production in polluted air and, perhaps, to production of organic aerosols. It is thought to have a large influence on the oxidative capacity of the atmosphere regionally and even globally.

The complexity of isoprene oxidation in the atmosphere makes it hard to unravel the identity and yields of some oxidation products. The initial steps in isoprene oxidation by OH are shown below:

We found that that chemically activated allylic radicals formed after addition of OH radical to C1 or C4 undergo E/Z isomerization in picoseconds. Therefore, the allylic radicals are stabilized as both E and Z isomers. 1 Hydrogen bonding complicates efforts to compute the spectra and reactivity of the peroxy radicals (identified by Roman numerals above). 2

Reactions of these peroxy radicals form β- and δ-hydroxy alkoxy radicals (ROĽ). Our studies indicate that the β-hydroxy alkoxy radicals decompose via C-C bond scission (see Figure below) on a picosecond to nanosecond time scale (see Figure below), while decomposition of the δ-hydroxy alkoxy radicals would take at least milliseconds- far longer than the time required for the radicals to isomerize or react with O2. 3 The isomerization reactions that abstract H-atoms from HOCH2- groups are very fast, while abstraction from the methyl group is not so fast as to rule out reaction with O2. 4

In later stages of this chemistry, an alkoxy radicals is formed with two intramolecular hydrogen bonds (see thumbnail image below) This changes the chemistry, leading to the double H-atom transfer. 4, 5 See an overview of this chemistry in Powerpoint, or click on the thumbnail below to see an animation.

Image of a molecule

Click to animate




REFERENCES

[1] Isomerization of OH-isoprene adducts and hydroxyalkoxy isoprene radicals, T. S. Dibble, J. Phys. Chem. A, 106, 6643-6650 (2002).

[2] Computations on the Transition of Isoprene-OH-O2 Peroxy Radicals, T. S. Dibble. J. Comput. Chem. A, 26, 836-845 (2005).

[3] Quantum Chemical Study of the C-C Bond Fission Pathways of Alkoxy Radicals Formed Following OH Addition to Isoprene, T. S. Dibble, J. Phys. Chem. A, 103, 8559-65 (1999).

[4] Intramolecular hydrogen bonding and double H-atom transfer in peroxy and alkoxy radicals from isoprene, T. S. Dibble, J. Phys. Chem. A, 108, 2199-2207 (2004).

[5] Prompt chemistry of alkenoxy radical products of the double H-atom transfer of alkoxy radicals from isoprene, T. S. Dibble, J. Phys. Chem. A, 108, 2208-2215 (2004).

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