Nitrogen incorporation in saturated aliphatic C6-C8 hydrocarbons and ethanol in low-pressure nitrogen plasma generated by a hollow cathode discharge ion source

Excited-State N Atoms Transform Aromatic Hydrocarbons into N-Heterocycles in Low-Temperature Plasmas

The direct formation of N-heterocycles from aromatic hydrocarbons has been observed in nitrogen-based low-temperature plasmas; the mechanism of this unusual nitrogen-fixation reaction is the topic of this paper. We used homologous aromatic compounds to study their reaction with reactive nitrogen species (RNS) in a dielectric barrier discharge ionization (DBDI) source. Toluene (C₇H₈) served as a model compound to study the reaction in detail, which leads to the formation of two major products at "high" plasma voltage: a nitrogen-replacement product yielding protonated methylpyridine (C₆H₈N⁺) and a protonated nitrogen-addition (C₇H₈N⁺) product. We complemented those studies by a series of experiments probing the potential mechanism. Using a series of selected-ion flow tube experiments, we found that N⁺, N₂⁺, and N₄⁺ react with toluene to form a small abundance of the N-addition product, while N(⁴S) reacted with toluene cations to form a fragment ion. We created a model for the RNS in the plasma using variable electron and neutral density attachment mass spectrometry in a flowing afterglow Langmuir probe apparatus. These experiments suggested that excited-state nitrogen atoms could be responsible for the N-replacement product. Density functional theory calculations confirmed that the reaction of excited-state nitrogen N(²P) and N(²D) with toluene ions can directly form protonated methylpyridine, ejecting a carbon atom from the aromatic ring. N(²P) is responsible for this reaction in our DBDI source as it has a sufficient lifetime in the plasma and was detected by optical emission spectroscopy measurements, showing an increasing intensity of N(²P) with increasing voltage.

Gas-phase amination of aromatic hydrocarbons by corona discharge-assisted nitrogen fixation

This paper reports on the gas-phase amination reaction of aromatic hydrocarbons occurring under corona discharge conditions with N₂ gas as the nitrogen source. The corona discharge device within an atmospheric pressure chemical ionization source was employed to achieve the plasma-assisted N₂ fixation, and the coupled ion trap mass spectrometer (IT-MS) was used to detect positively charged product ions. In the model case, under APCI conditions, unusual product ions, [M + 16]⁺ and [M + 14]⁺, were observed. Based on the high resolution MS data and tandem mass spectrometric information, [M + 16]⁺ was confirmed to be protonated p-toluidine and [M + 14]⁺ was confirmed to be p-methylphenylnitrenium ion. According to the experimental results of the isotopic labelling and substituent effect, one feasible mechanism is proposed as follows. Firstly, N₂ is activated by plasma caused via the corona discharge and then electrophilically attacks toluene, yielding the key intermediate, p-methylphenylnitrenium; secondly, the intermediate undergoes double-hydrogen transfer reaction to give rise to the final product ion, protonated p-toluidine. This study may provide a novel idea to explore new and green method for the synthesis of anilines.

Mass spectrometric monitoring of oxidation of aliphatic C6-C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas

Experimental and theoretical studies on the oxidation of saturated hydrocarbons (n-hexane, cyclohexane, n-heptane, n-octane and isooctane) and ethanol in 28 Torr O₂ or air plasma generated by a hollow cathode discharge ion source were made. Ions corresponding to [M + 15]⁺ and [M + 13]⁺ in addition to [M - H]⁺ and [M - 3H]⁺ were detected as major ions where M is the sample molecule. The ions [M + 15]⁺ and [M + 13]⁺ were assigned as oxidation products, [M - H + O]⁺ and [M - 3H + O]⁺ , respectively. By the tandem mass spectrometry analysis of [M - H + O]⁺ and [M - 3H + O]⁺ , H₂ O, olefins (and/or cycloalkanes) and oxygen-containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O₂ plasma was postulated as the oxidizing reagent. As an example, the reactions of C₆ H₁₄^+• with O₂ and of C₆ H₁₃⁺ (CH₃ CH₂ CH⁺ CH₂ CH₂ CH₃ ) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C₆ H₁₃⁺ leads to the formation of protonated ketone, CH₃ CH₂ C(=OH⁺ )CH₂ CH₂ CH₃ . In air plasma, [M - H + O]⁺ became predominant over carbocations, [M - H]⁺ and [M - 3H]⁺ . For ethanol, the protonated acetic acid CH₃ C(OH)₂⁺ (m/z 61.03) was formed as the oxidation product. The peaks at m/z 75.04 and 75.08 are assigned as protonated ethyl formate and protonated diethyl ether, respectively, and that at m/z 89.06 as protonated ethyl acetate. Copyright © 2016 John Wiley & Sons, Ltd.