Combined crossed molecular beam and theoretical studies of the N(2D) + benzene and toluene reactions: primary products, branching ratios and implications for the atmospheric chemistry of Titan

Abstract:

Gas-phase reactions of atomic nitrogen in its first electronically excited state N(²D) with small hydrocarbons lead to the formation of N-containing organic molecules (nitriles, imines, aziridine). These reactions are active now in the upper atmosphere of Saturn’s moon Titan, while they might have played a role in the formation of N-containing prebiotic molecules in the primitive terrestrial atmosphere.

Given the interest for the atmospheric chemistry of Titan and the potential implications in prebiotic chemistry, in our laboratory we have started a systematic investigation of bimolecular reactions involving N(²D) and those hydrocarbons which have been detected in the atmosphere of Titan. By employing the crossed molecular beam (CMB) technique with mass-spectrometric detection, we have been able to identify the reaction primary products and determine their branching ratios (BRs). These data are needed to set-up realistic photochemical models. A first series of experiments involved CH4 and other aliphatic hydrocarbons.

In this contribution, we present an extension to the case of the reactions of N(²D) with simple aromatic hydrocarbons, namely benzene and toluene, which have already identified in the atmosphere of Titan. The experimental study has been complemented by dedicated electronic structure calculations of the C₆H₆N and C₇H₈N ground state potential energy surfaces and statistical estimates of product BRs.

Somewhat surprisingly, the outcome of the two reactions is quite different as far as the preservation of the aromatic ring is concerned. In the case of the reaction with benzene, the main reaction channel is associated to the aromatic ring cleavage with the production of CN and C₅H₆ (BR ≈ 0.90), with only a small contribution coming from an H-displacement reaction mechanism (BR ≈ 0.10). In the case of the reaction with toluene, the presence of the methyl group seems to protect the aromatic ring and only 30% of the reaction proceeds through its cleavage. The other important channels are those associated to the H-displacement (BR ≈ 0.40) and CH₃-loss (BR ≈ 0.30) channels. The electronic structure and statistical calculations support the experimental results.
The implications for the chemistry of the atmosphere of Titan will be noted.