Theoretical study of the initial reaction between OH and isoprene in tropospheric conditions

Abstract

The reaction of isoprene with OH radicals has been investigated by ab initio molecular orbital theory. We report the energetics of four different pathways, involving the direct addition of OH to four of the carbon atoms. Calculations have been performed using both density functional theory (BHandHLYP) and Møller-Plesset perturbation theory to the second-order (MP2). Two pre-reactive complexes have been identified, whose stabilization energy with respect to the separated reactants is about 12 kJ mol À1 . Their structure is similar to the ones previously reported for OH-ethene and OH-propene adducts: the OH radical is placed over either one of the double bonds at a distance of about 2.1 A ˚, with the H atom pointing towards the C-C bond. The geometries of the transition states corresponding to OH addition at the four different positions have been optimized. The calculated apparent activation energies are negative for addition at the terminal carbon atoms and in excellent agreement with the experimental measurements. Direct addition at the internal carbon atoms involves much higher energy barriers, and these pathways are expected to be negligible at normal temperatures. Thus, the observed formation of 3-methylfuran must occur after radical addition to the terminal carbon atoms, following a pathway such as the one proposed by R.

Thermodynamics of the Hydroxyl Radical Addition to Isoprene

“…Francisco-Márquez and coworkers predicted four pre-reactive complexes for isoprene, two for each isoprene conformation, using BHandHLYP and MP2 theories with the 6-311G** basis set 39,56. Their pre-reactive complexes, when isoprene is in the s-trans conformation, have a E zpv of −2.80 and −2.99 kcal·mol −1 (BHandHLYP) and −3.30 and −3.49 kcal·mol −1 (MP2) relative to the reactants, and lie below the transition state.…”

“…Adduct formation is favored along the four C 1 and C 4 pathways, by a minimum of 6.1 kcal·mol −1 with respect to the C 2 and C 3 pathways. This is consistent with twenty previous calculations whose ΔH 298 ° and ΔE reaction energies for pathway C 1 180 range from −31.5 to −47.7 kcal·mol −1 (average = −39.6), for C 2 180 range from −13.4 to −32.1 (average = −26.2), for C 3 180 range from −14.6 to −31.7 (average = −25.6), and for C 4 180 range from −29.6 to −45.0 kcal·mol −1 (average = −37.1) 24,25,39,51,56. In regards to isoprene’s s-gauche conformation, Francisco-Marquez and coworkers determined PMP2/cc-pVTZ//MP2/6-311G** and BHandHLYP/6-311G(d,p)//BHandHLYP/6-311G(d,p) reaction energies along the C 1 41:C 2 41:C 3 41:C 4 41 pathways, with values of −44.7:−27.3:−26.6:−42.8 and −38.0:−21.0:−20.7:−36.1 kcal·mol −1 , respectively 56.…”

“…There are numerous articles that discuss the possibility of hydroxyl radical addition to all four unsaturated carbons of isoprene,11,23–25,29,34,39,44,49,51,55–57 and experimentalists have found supportive evidence for formation of all four adducts 24. A subject of debate has been the relative importance of the four possible adducts 11,24,25,28,39,41,51.…”

OH+ Isoprene: A Direct Dynamics Study

“…The energies of AC1 and AC4 are about 5–18 kcal/mol lower than the energies of AC2 and AC3. This is due to the fact that AC1 and AC4 are stabilized by allylic resonance, while AC2 and AC3 are not (Figure ) …”

“…This is due to the fact that AC1 and AC4 are stabilized by allylic resonance, while AC2 and AC3 are not ( Figure 7). 4,5,11,13,31,36,37,40 Isomers AC2R and AC3R are produced by isomerization of the vibrationally excited hydroxyacyl radicals AC2 and AC3. It was found that the rearrangement after addition is observed at sites C2 and C3, but not at sites C1 and C4, probably because hydroxyacyl radicals AC1 and AC4 are more stable than AC2 and AC3.…”