Theoretical study on mechanisms of the high-temperature reactions C2H3+H2O and C2H4+OH

The potential energy surface of the radical-molecule reaction C2H3 + H2O in the gas phase is explored at the 6-31G( d,p) and 6-311G(d,p) B3LYP and single-point QCISD(T)/6-311G(2df,p) levels. The most favorable channel is the direct H-abstraction from H2O to C2H3 leading to product P-1 C2H4 + OH, whereas the other channels leading to the products P-2 CH3 + CH2O, P-3 CH3CHO + H, P-4 cis-CH2CHOH+ H and P-4' trans-CH2 CHOH+ H are kinetically much less competitive. For the direct H-abstraction channel, high-level energetic calculations at the QCISD(T) /6-311G(2df, p), QCISD(T) /6-311+G(2df, 2p) and G2 levels using the B3LYP/6-31G(d, p) and QCISD/6-31G(d, p) optimized geometries are further performed to estimate the thermal rate constants over a wide temperature range 200 5000 K for comparison with future laboratory measurements. The calculated barrier heights at the QCISD(T)/6-311+G(2df, 2p) and G2 levels based on the QCISD/6-31G(d, p) geometries with zero-point vibrational energy (ZPVE) correction are 12.6 and 13.0 kcal mol(-1), respectively. The results indicate that the C2H3 + H2O reaction might play an important role at high temperatures ( T > 1800 K) in the presence of gaseous water and should be incorporated in the C2H3-modeling of hydrocarbon-fuel combustion processes. Discussions are also made in comparison with the analogous reactions C2H3 + H-2 and C2H + H2O. While the addition-elimination mechanism of another important radical-molecule reaction C2H4 + OH has been the subject of extensive theoretical and experimental studies, its H-abstraction process leading to C2H3 + H2O has received little attention. For the C2H4 + OH --> C2H3 + H2O channel, our calculations predict ZPVE-corrected barriers, 5.6 and 5.4 kcal mol(-1), respectively, at the QCISD(T) /6-311+ G(2df,p), QCISD/6-311+G(d,p) and G//QCISD/6-31G(d,p) levels, and reveal its importance at high temperatures (T > 560 K). In the range 720-1173 K, the calculated high- level rate constants are quantitatively in good agreement with the measured values. However, our calculated activation energy, 9.5 and 9.3 kcal mol(-1) at the QCISD( T) /6-311+G(2df, 2p) / /QCISD/6-31G(d, p) and G2/ / QCISD/6-31G(d, p) levels with ZPVE correction, respectively, suggests that the experimentally determined value 4-5 kcal mol(-1) may be underestimated and future rate constant measurements over a wide temperature range including T > 1200 K may be desirable.