Direct Kinetics and Product Measurement of Phenyl Radical + Ethylene

The phenyl + ethylene (C6H5 + C2H4) reaction network was
explored experimentally and theoretically
to understand the temperature dependence of the reaction kinetics
and product distribution under various temperature and pressure conditions.
The flash photolysis apparatus combining laser absorbance spectroscopy
(LAS) and time-resolved molecular beam mass spectrometry (MBMS) was
used to study reactions on the C8H9 potential
energy surface (PES). In LAS experiments, 505.3 nm laser light selectively
probed C6H5 decay, and we measured the total
C6H5 consumption rate coefficients in the intermediate
temperature region (400–800 K), which connects previous experiments
performed in high-temperature (pyrolysis) and low-temperature (cavity-ring-down
methods) regions. From the quantum chemistry calculations by Tokmakov
and Lin using the G2M­(RCC5)//B3LYP method, we constructed a kinetic
model and estimated phenomenological pressure-dependent rate coefficients, k(T, P), with the Arkane
package in the reaction mechanism generator. The MBMS experiments,
performed at 600–800 K and 10–50 Torr, revealed three
major product peaks: m/z = 105 (adducts,
mostly 2-phenylethyl radical, but also 1-phenylethyl radical, ortho-ethyl phenyl radical, and a spiro-fused ring radical),
104 (styrene, co-product with a H atom), and 78 (benzene, co-product
with C2H3 radical). Product branching ratios
were predicted by the model and validated by experiments for the first
time. At 600 K and 10 Torr, the yield ratio of the H-abstraction reaction
(forming benzene + C2H3) is measured to be 1.1%
and the H-loss channel (styrene + H) has a 2.5% yield ratio. The model
predicts 1.0% for H-abstraction and 2.3% for H-loss, which is within
the experimental error bars. The branching ratio and formation of
styrene increase at high temperature due to the favored formally direct
channel (1.0% at 600 K and 10 Torr, 5.8% at 800 K and 10 Torr in the
model prediction) and the faster β-scission reactions of C8H9 isomers. The importance of pressure dependence
in kinetics is verified by the increase in the yield of the stabilized
adduct from radical addition from 80.2% (800 K, 10 Torr) to 88.9%
(800 K, 50 Torr), at the expense of styrene + H. The pressure-dependent
model developed in this work is well validated by the LAS and MBMS
measurements and gives a complete picture of the C6H5 + C2H4 reaction.