J. Raúl Alvarez-Idaboy

Gas phase reactions of C1–C4 alcohols with the OH radical: A quantum mechanical approach

CCSD(T)//BHandHLYP/6-311G(d,p) calculations have been performed to study the OH hydrogen abstraction reaction from C1–C4 aliphatic alcohols. A complex mechanism involving the formation of a stable pre-reactive complex is proposed and the temperature dependence of the rate coefficients is studied over the temperature range of 290–500 K, using conventional transition state theory (CTST). Excellent agreement between calculated and experimental k at 298 K has been obtained. Arrhenius expressions are proposed for 1-propanol and 1-butanol, k1-Prop = 3.06 × 10−12exp(140/T) and k1-But = 2.14 × 10−12exp(440/T) cm3 molecule−1·s−1, respectively. The rate coefficient for the formation of the alpha radical is found significantly larger than that of the competing channels for C1–C3 alcohols. The finding that at room temperature the rate constant of 1-butanolγ is the largest one supports some of the previous experimental results.

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

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 Moller–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−1. 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. Atkinson, S. M. Aschmann, E. C. Tuazon, J. Arey and B. Zielinska, Int. J. Chem. Kinet., 1989, 21, 594 (). Calculated overall rate constants are obtained, in excellent agreement with experimental values. The two-parameter equation for the calculated overall rate coefficient was found to be (2.12 ± 0.42) × 10−11 exp[(384 ± 55)/T] cm3 molecule−1 s−1, while the best fit for the four channels studied here correspond to the following expressions: k1 = (2.25 ± 0.51) × 10−11 exp[(253 ± 62)/T], k2 = (9.60 ± 1.18) × 10−13 exp[(−2871 ± 35)/T], k3 = (1.81 ± 0.22) × 10−12 exp[(−1567 ± 33)/T], and k4 = (2.39 ± 0.27) × 10−12 exp[(676 ± 33)/T] cm3 molecule−1 s−1.

On the role of s-cis conformers in the reaction of dienes with OH radicals

The reactions of OH radicals with s-cis and s-trans-butadiene and s-cis-isoprene have been modeled by ab initio Molecular Orbital Theory. Density Functional Theory (BHandHLYP) calculations have been performed for both butadiene and isoprene, and Moller–Plesset Perturbation Theory to the second-order (MP2) has also been used for s-cis-isoprene in order to compare with previous work. Pre-reactive complexes are identified in all cases, with the OH radical placed over either one of the double bonds at a distance of about 2 A and the H atom pointing towards the C–C bond. The geometries of the transition states corresponding to OH addition at all positions have been fully optimized. The calculated apparent activation energies are negative for addition at the terminal carbon atoms and in excellent agreement with the experimental measurements. The possible role of direct additions at the internal carbon atoms in the formation of furan-like compounds is discussed. Energy barriers for the s-cis conformers are found to be smaller than those for the s-trans conformers, especially for addition at the internal carbons, suggesting that the s-cis conformers could play a role in the tropospheric oxidation of dienes. Calculated overall rate constants are in good agreement with experimental values. Partial rate coefficients corresponding to the different channels are reported. The temperature dependence is studied in the 290–500 K range and two-parameter equations are reported for each rate coefficient. The calculated partial rate coefficients of addition to internal carbon atoms are not large enough to account for the observed yield of 3-methylfuran.