Photochemical and Discharge-Driven Pathways to Aromatic Products from 1,3-Butadiene

A detailed study of the photochemical and discharge-driven pathways taken by gas-phase 1,3-butadiene has
been carried out. Photolysis or discharge excitation was initiated inside a short reaction tube attached to the
outlet of a pulsed valve. Bath gas temperatures near 100 K were achieved in the reaction tube by the constrained
expansion of the gas mixture into the tube, simulating temperatures of relevance in Titan's atmosphere.
Photolysis of 1,3-butadiene was initiated at 218 nm with a laser pulse that counter-propagated the reaction
tube. Discharge excitation was carried out using discharge electrodes imbedded in the reaction tube walls,
enabling the study of the photochemical and discharge products under similar conditions. Products were
detected using either single-photon VUV photoionization (118 nm = 10.5 eV) or resonant two-photon ionization
(R2PI) spectroscopy in a time-of-flight mass spectrometer. Emphasis was placed on characterization of the
aromatic products formed, since these may be of particular relevance to Titan's atmosphere, where benzene
has been positively identified and 1,3-butadiene is projected as the principle pathway to its formation. Consistent
with previous studies of the photodissociation of 1,3-butadiene, C3H3 + CH3 is the dominant primary product
formed. Under the temperature−pressure conditions present in the reaction tube (T ∼ 75−100 K, P = 50
mbar), C6H6 is the dominant secondary photochemical product formed. A 1:1 C4H6:C4D6 mixture was used
to prove that the C6H6 product was formed by recombination of two C3H3 radicals; however, a careful search
for benzene revealed none, indicating that less than 1% of the C6H6 formed in the reaction tube is benzene.
This is consistent with expectations for these temperatures and pressures based on previous modeling of
propargyl recombination. Two aromatic products were observed from the photochemistry:  ethylbenzene and
3-phenylpropyne. Plausible pathways leading to these products are proposed. In the discharge, C3H3 + CH3
are also identified as significant primary neutral products and C6H6 as a dominant higher-mass product. In
this case, the C6H6 was identified as benzene via its R2PI spectrum, appearing with intensity about 10 times
larger than any other aromatic formed in the discharge. R2PI spectra of a total of about 15 aromatic products
were recorded from the 1,3-butadiene discharge, among them toluene; styrene; phenylacetylene; o-, m-, and
p-xylene; ethylbenzene; indane; indene; β-methylstyrene; and naphthalene. Previously unidentified spectra in
the m/z 142 and 144 mass channels were positively identified as the 1,3- and 1,4-isomers of phenylcyclopentadiene and the analogous 1-phenylcyclopentene.