High resolution FTIR study of the ν5, ν6, and ν9 fundamental bands of CH2D37Cl

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Highlights

  • Recording the first high-resolution infrared spectra of CH2D37Cl.

  • A study of the lowest fundamentals ν5, ν6 and ν9 has been performed.

  • The c-type Coriolis resonance between the v5 = 1 and v6 = 1 states is considered.

  • A set of spectroscopic constants for the excited vibrational states is determined.

  • The spectral simulations well reproduce the experimental data.

Abstract

The first high-resolution infrared spectra of CH2D37Cl have been investigated in the region 650–1100 cm−1 where the lowest fundamental bands ν5 (826.2626 cm−1), ν6 (708.4307 cm−1), and ν9 (986.3405 cm−1) occur. These vibrations perturb each other by different weak interactions and the v5 = 1 and v6 = 1 states were treated according to a model which accounts for a c-type Coriolis resonance. The spectral analysis resulted in the identification of 1664, 2550 and 2657 ro-vibrational transitions for ν5, ν6, and ν9 bands respectively, and to the determination of accurate spectroscopic parameters by using the Watson’s A-reduction Hamiltonian in the Ir representation. The simulations of the ro-vibrational structure of the ν5, ν6, and ν9 bands performed in different spectral regions adequately reproduce the experimental data.

Introduction

Among the different chlorine-bearing organic molecules present in the Earth’s atmosphere, chloromethane (CH3Cl) is one of the most abundant (around 553–559 pptv according to the 2016 data [1]). Natural sources of chloromethane are manifold, comprising not only the oceans but also plants [2], soils [3,4] and wildfires [5]. The anthropogenic emissions of CH3Cl are mainly related to chemical activities [6], and coal and biomass burning [5], but recently it has been found also in the human breath [7]. Following the first spectroscopic detection of chloromethane in the atmosphere through the strong ν1 feature around 3.4 µm [8], its global distribution (in both the upper troposphere and lower stratosphere) has been successfully measured by the ACE-FTS experiment [9,10].

Being so closely related to biological and anthropogenic activities, it is not surprising that CH3Cl has been proposed among the most promising biomarkers for the search of potentially habitable exoplanets [11]. Conversely, until a few years ago it was thought that interstellar chlorinated compounds were basically limited only to a few hydrides [12], thus excluding the presence of even simpler organohalides like chloromethane. The recent discovery of chloromethane (with both CH335Cl and CH337Cl isotopologues unambiguously detected) in the protostar IRAS 16,293–2422 [13] revealed that also this class of compounds must be properly considered and investigated in astrochemistry. Interestingly, many deuterated molecules have been discovered in the same protostar (including also the bi-deuterated isomers) like HDO and D2O [14], NHD and ND2 [15,16] or CH2DCN and CHD2CN [17]. These findings suggest that also CH3Cl might present a relevant deuterium fractionation in IRAS 16,293–2422, thus leading to potentially detectable amounts of the monodeuterated form (CH2DCl) provided that accurate spectroscopic predictions are available. When compared to the large amount of data available for the parent species CH3Cl, until quite recently only a few investigations were carried out on CH2DCl thus seriously hampering its spectroscopic detections as well as the accurate determination of the ratio CH2D35Cl/CH2D37Cl [18]. Therefore, we decided to support the search of mono-deuterated chloromethane in the interstellar medium by providing accurate spectroscopic data for both the CH2D35Cl and CH2D37Cl isotopologues. In our previous work [19] we measured the spectrum of these species in the millimeter region obtaining precise rest frequencies and an accurate set of ground state constants. The present study deals with the infrared spectrum of this molecule; to complement the data already available for CH2D35Cl [20], [21], [22] here we report on the ro-vibrational analysis of CH2D37Cl focusing on the intense absorptions falling in the region between 15.4 and 9.3 µm. This region is characterized by the very strong features of the ν6 band associated to the Csingle bondCl stretching centered at about 708 cm−1, and by the presence of two fundamentals related to the in-plane C-D bending (ν5, 826 cm−1) and the C-D/CH2 out-of-plane bending (ν9, 986 cm−1). An accurate set of spectroscopic parameters, also including the modeling of c-type Coriolis resonance between the v5 = 1 and v6 = 1 states, have been obtained for all the excited vibrational states presently investigated. These results, together with the previously available ones for the other isotopologue [20], [21], [22] provide a solid and reliable description of the spectroscopic properties of monodeuterated chloromethane in the infrared portion of the spectrum to support its possible observation and quantification.

Section snippets

Experimental details

The CH2D37Cl sample was prepared by reacting monodeuterated methanol (CDN Isotopes, 99.2% D-enriched) with Na37Cl (Cambridge Isotope Laboratory; 95%−37Cl enriched) in a solution of diluted sulphuric acid, following the method previously described for the 35Cl isotopologue [20]. Evidence of CH2D35Cl impurities were observed and identified in the infrared spectra of the sample.

The infrared spectra of CH2D37Cl were recorded at the University of Bologna (Italy) using a Bomem DA3.002 FTIR

General remarks

The CH2DCl molecule is a nearly-prolate asymmetric-top rotor (κ = −0.978) belonging to the symmetry point group Cs; the molecular symmetry plane contains the a- and b- axes, while the c-axis is perpendicular to it. Six of the nine vibrational modes are classified of symmetry species A′1 – ν6) and give rise a-/b-hybrid bands, while three (ν7 – ν9) are of A′′ symmetry and produce c-type absorptions. The a-type component presents a contour similar to that of parallel bands, while the b- and c

Description of the spectra

A low resolution (0.5 cm−1) survey spectrum of CH2D37Cl in the region analyzed is illustrated in Fig. 1, where the fundamentals ν5 (~826 cm−1), ν6 (~708 cm−1) and ν9 (~986 cm−1) are indicated.

The ν6 band is the lowest in wavenumber and the strongest fundamental of CH2D37Cl and is ascribed to Csingle bondCl stretching; the overview of the investigated region, illustrated in Fig. 1, shows that the a-type component is predominant. In the high-resolution spectra the qPK(J) manifolds present a rotational

Results and discussion

The analysis of ν6 of CH2D37Cl started with the identification of the resolved details of the qPK(J) and qRK(J) clusters using predicted values and considering the ro-vibrational structure of the same band of CH2D35Cl [20]. The preliminary calculations were performed by setting the band origin at 708.43 cm−1 [20] and employing the very recent ground state constants [19] for both ground and upper state. Then, new predictions with higher J and Ka quantum numbers were made, providing further

Conclusions

A detailed ro-vibrational study of the lowest fundamentals ν5, ν6, and ν9 of CH2D37Cl occuring in the region 650–1100 cm−1 has been performed for the first time. The analysis of the high-resolution infrared spectra led to identify of a large number of ro-vibrational transitions of the bands and to the determination of accurate spectroscopic parameters. The adequacy of the experimental data reproduction is confirmed by the good match between simulated and experimental spectra in the overall

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work has been supported by University Ca’ Foscari Venezia (ADiR funds) and by Bologna University (RFO funds). The authors gratefully remember Mr. A. Baldan for the preparation of the CH2D37Cl sample.

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