Towards the Detection of Explosive Taggants: Microwave and Millimetre‐Wave Gas‐Phase Spectroscopies of 3‐Nitrotoluene

Abstract

The monitoring of gas-phase mononitrotoluenes is crucial for defence, civil security and environmental interests because they are used as taggant for TNT detection and in the manufacturing of industrial compounds such as dyestuffs. In this study, we have succeeded to measure and analyse at high-resolution a room temperature rotationally resolved millimetre-wave spectrum of meta-nitrotoluene (3-NT). Experimental and theoretical difficulties have been overcome, in particular, those related to the low vapour pressure of 3-NT and to the presence of a CH internal rotation in an almost free rotation regime (V =6.7659(24) cm ). Rotational spectra have been recorded in the microwave and millimetre-wave ranges using a supersonic jet Fourier Transform microwave spectrometer (T <10 K) and a millimetre-wave frequency multiplication chain (T=293 K), respectively. Spectral analysis of pure rotation lines in the vibrational ground state and in the first torsional excited state supported by quantum chemistry calculations permits the rotational energy of the molecule, the hyperfine structure due to the N nucleus, and the internal rotation of the methyl group to be characterised. A line list is provided for future in situ detection.

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

“…The analysis of 4-NT follows strictly the steps of the 3-NT analysis. [17] Based on quantum chemistry calculations described in computational methods section and presented in details in Table S1 of supporting information I, the jet-cooled MW spectrum (see section 5.1) has been firstly measured and analysed up to J 00 ¼ 12 and K 0 0 a ¼ 6 taking into account both the coupling with internal rotation of the methyl group, the NQC and the spin statistics. The A1/A2 and B1/B2 species were preliminary assigned using the SPFIT/SPCAT Pickett's programs [19] (semi-rigid rotor model in the I r representation of A-reduced Watson's Hamiltonian) and completed with the E1/ E2 species arising from internal rotation coupling using Ilyushin's code RAM36hf.…”

“…Although DFT calculations of NQC constants for nitrogen containing molecules are usually accurate, [31] none of the most studied species contains a nitro group, for which difficulties were pointed out in an early study [32] and in the case of 3-NT. [17] Adding to this the rather complex treatment of an internal rotor, NQC results for the present species should be considered carefully. However, based on relative considerations only (i.e.…”

“…In this study, a high complexity was encountered from the experimental and theoretical point of view. [17] From an experimental point of view, the mm-wave experimental instrument has been pushed to its limit of sensitivity and resolution resulting in a weak rotational spectrum with low signal-to-noise ratio (S/N) due to weak room temperature vapour pressure (P vap: 293K ð Þ = 8.55 Pa [18] ) and a high density of lines with many blended lines. Concerning the spectroscopic analysis, a specific effective Hamiltonian taking into account the nuclear quadrupole coupling (NQC) for the hyperfine structure observed in MW frequency range and the large amplitude motion of the methyl group presenting a very low internal rotation barrier (V 3 ¼ 6:7659 24 ð Þ cm À 1 ) were required to reproduce the spectra at the experimental accuracy.…”

Terahertz gas phase spectroscopy using a high-finesse Fabry–Pérot cavity

“…This is the case for the THz spectral region where the most intense pure rotational spectra of small and medium-sized molecules are typically found. Numerous applications such as, breath analysis [4], environmental surveillance [5,6], food spoilage monitoring [7], or detection of explosive taggants [8], should be feasible if sufficient instrument sensitivity and spectral resolution can be obtained. Despite numerous advances observed at THz frequencies including THz Quantum Cascade Lasers [9][10][11], solid-state electronic devices [12], photonic conversions [13], detectors [14,15], this spectral region remains hindered by its immature technology compared to the neighbouring microwave and infrared domains.…”

Recent Developments in Spectroscopic Techniques for the Detection of Explosives

“…Due to the importance of rapid, automatic, and non-contact detection of explosives for homeland security and environmental safety [8], a variety of spectroscopic technologies have been employed to detect trace quantities of explosives; for example, terahertz (THz) spectroscopy [9,10], laser induced breakdown spectroscopy (LIBS) [11,12,13,14,15,16], Raman spectroscopy [17,18,19,20,21,22], ion mobility spectrometry (IMS) [23,24,25,26], nuclear magnetic resonance (NMR) [27,28,29,30], nuclear quadrupole resonance (NQR) [31,32,33], laser-induced thermal emissions (LITE) [34,35], infrared (IR) spectroscopy [36,37,38], mass spectrometry [39,40,41,42,43,44,45,46], optical emission spectroscopy (OES) [47,48], photo-thermal infrared imaging spectroscopy (PT-IRIS) [49,50,51], photoacoustic techniques [52,53,54], FT-FIR spectroscopy [55], microwave [56], and millimeter-wave [57], etc. Various electromagnetic radiations such as X-ray [58] and γ rays [59] have also been employed in explosive detection.…”