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

Abstract

The direct formation of N-heterocycles from aromatic hydrocarbons has been observed in nitrogen-based lowtemperature plasmas; the mechanism of this unusual nitrogenfixation 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 (C 7 H 8 ) 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 (C 6 H 8 N + ) and a protonated nitrogen-addition (C 7 H 8 N + ) 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 + , N 2 + , and N 4 + react with toluene to form a small abundance of the N-addition product, while N( 4 S) 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 excitedstate nitrogen N( 2 P) and N( 2 D) with toluene ions can directly form protonated methylpyridine, ejecting a carbon atom from the aromatic ring. N( 2 P) 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( 2 P) with increasing voltage.

Triplet state harvesting and search for forbidden transition intensity in the nitrogen molecule

“…The discovery of new N 2 transitions forbidden by the spin-selection rule and induced by SOC perturbation is an important part of optical nitrogen monitoring at different altitudes. The intensity origin of the known emission bands that are forbidden by the electric dipole selection rules is also an important task of N 2 spectroscopy ( Deslandres, 1902 ; Wilkinson and Mulliken, 1959 ; Brown and Winkle, 1970 ; Lofthus and Krupenie, 1977 ; Werner et al, 1984 ; Partridge et al, 1988 ; Piper, 1993 ; Minaev et al, 1995 ; Lewis et al, 2008 ; Ndome et al, 2008 ; Hochlaf et al, 2010a ; Ni et al, 2017 ; Begley et al, 2022 ). This work presents multi-reference configuration interaction (MRCI) calculations of the highly excited states of the nitrogen molecule and an explanation of the intensity origin of several forbidden optical transitions.…”

“…In contrast, the ground state of the nitrogen molecule possesses zero spin and several high-energy triplet excited states with deep potential wells. The lowest of them, , can harvest a stock of 6.22 eV energy, being a strongly metastable triplet state with a relatively long radiative lifetime (τ r ) of 2 s ( Brown and Winkle, 1970 ; Minaev et al, 1995 ; Begley et al, 2022 ). Accounting for the short UV wavelength of the transition, this τ r value is indeed unusually large.…”

“…N 2 is a very stable and inert molecule in the ground state with high dissociation energy (D e = 9.76 eV). At the same time, N 2 possesses a variety of quite stable valence excitations of the π u –π g and 3σ g –π g types; these excited states have large D e values (around 4–6 eV) and are mostly metastable since their emission to the ground state is strictly forbidden by the electric dipole selection rules ( Wilkinson and Mulliken, 1959 ; Brown and Winkle, 1970 ; Begley et al, 2022 ; Minaev et al, 1995 ; Lewis et al, 2008 ; Lofthus. and Krupenie, 1977 ).…”

“…In gaseous electric discharges, when a molecule is irradiated by an electron flux, N 2 dissociates into the ground state N ( 4 S) atoms; they can recombine forming the lowest singlet (X 1 Σ g + ), triplet (A 3 Σ u + ), and quintet (A′ 5 Σ g – ) basic states. The last two, shown in Figure 1 , are involved in the so-called active nitrogen phenomenon detected by the characteristic “yellow afterglow” ( Brown and Winkle, 1970 ; Begley et al, 2022 ). Its study together with aurora borealis involves a large number of metastable states and forbidden transitions in the N 2 spectrum ( Figure 2 ).…”

Discriminating alkylbenzene isomers with tandem mass spectrometry using a dielectric barrier discharge ionization source

“…While these studies focus on generating intact molecular ions, N‐addition product ions [M + N] + of alkylbenzenes have been reported in APCI 24 and for volatile saturated hydrocarbons in electric fields 25 . Previously, we investigated the formation of these products for aromatic hydrocarbons in a DBDI and found that excited state N‐atoms N( 2 P) can react with aromatic hydrocarbon cations [M] •+ in the source to form either N‐addition or ‐replacement products 26 . We hypothesized that the reaction could be used to discriminate alkylbenzene isomers based on steric hindrance of the isomers, without the addition of special reagents.…”