# New analysis of line positions of the ν3 bands of 35ClNO2 and 37ClNO2 around 370 cm⁻¹

A. Anantharajah, F. Kwabia Tchana, L. Manceron, J. Orphal, J.-M. Flaud

► To cite this version:

A. Anantharajah, F. Kwabia Tchana, L. Manceron, J. Orphal, J.-M. Flaud. New analysis of line positions of the ν3 bands of 35ClNO2 and 37ClNO2 around 370 cm⁻¹. Journal of Quantitative Spectroscopy and Radiative Transfer, 2020, 253, pp.107078. 10.1016/j.jqsrt.2020.107078. hal-03583422

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# New **analysis of** line positions **of** the v₃ bands of ³⁵ClNO₂ and ³⁷ClNO₂ around 370 cm⁻¹

A. Anantharajah,ª F. Kwabia Tchana,ª,\* L. Manceron,ᵇ,ᶜ J. Orphal,ᵈ J.-M. Flaudª

Nb of Figures: 11

Nb of Tables: 01

Corresponding author: Fridolin Kwabia Tchana (fridolin.kwabia@lisa.u-pec.fr)

<footnote>ª Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Université de Paris, Université Paris-Est Créteil, Institut Pierre-Simon Laplace, 61 avenue du Général de Gaulle, 94010 Créteil Cedex, France</footnote>

<footnote>ᵇ Ligne AILES, Synchrotron SOLEIL, L'Orme des Merisiers, St-Aubin BP 48, F-91192 Gif-sur-Yvette Cedex, France</footnote>

<footnote>ᶜ Sorbonne Université, MONARIS, UMR CNRS 8233, 4 place Jussieu, F-75005 Paris, France</footnote>

<footnote>ᵈ Institute of Meteorology and Climate Research (IMK), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany</footnote>
**Abstract**

A new investigation of the $v_3$ bands of $^{35}$ClNO$_2$ and $^{37}$ClNO$_2$, located around 370 cm$^{-1}$ has been performed using a high resolution (0.00102 cm$^{-1}$) Fourier transform spectrum recorded at SOLEIL with highly improved experimental conditions as compared to previous work [Orphal J, Morillon-Chapey M, Klee S, Mellau GC, Winnewisser M. *J Mol Spectrosc* 1998;190:101-6]: (i) the use of synchrotron radiation which resulted in a *better signal-to-noise ratio*; (ii) a *resolution twice better*; (iii) a low temperature (221 K) with an optical path length of 8.16 m, allowing low pressure (0.025 hPa) in the sample leading to a *well resolved* *spectrum*. As a consequence, significantly better results than previously were obtained. Thanks to the new experimental conditions, the line assignments were pursued up to higher *J* and *K*$_a$ quantum number values, *J* = 83 and *K*$_a$ = 44. For both isotopomers, a total of 6331 transitions were reproduced with a root-mean-square deviation of 2×10$^{-4}$ cm$^{-1}$ using a Watson-type A-reduced Hamiltonian. Improved rotational and centrifugal distortion constants for the $v_3$ fundamental bands of $^{35}$ClNO$_2$ and $^{37}$ClNO$_2$ have been determined. The band centers are 370.1510773(92) cm$^{-1}$ for the $v_3$ fundamental band of $^{35}$ClNO$_2$ and 364.5218094(96) cm$^{-1}$ for the $v_3$ fundamental band of $^{37}$ClNO$_2$. The synthetic line list obtained in this study could be interesting for future measurements of ClNO$_2$ in the atmosphere, e.g. using the new space mission FORUM which is one of the concepts chosen by ESA to be developed further and which opens up a new window (150-1400 cm$^{-1}$) for understanding and quantifying the radiative processes, as well as air quality and pollution effects.

**Keywords:** Nitryl chloride; ClNO$_2$; far infrared high-resolution Fourier transform spectroscopy; line positions
# I. Introduction

Nitryl chloride ($ClNO_2$) is a molecule of environmental importance since it participates in the chemistry of the Earth's atmosphere. This species is produced in the polluted marine troposphere from the heterogeneous uptake of nitrogen pentoxide ($N_2O_5$), which is a nocturnal $NO_x$ reservoir, on chloride-containing aerosols (HCl or NaCl particles) [1,2]. In addition, $ClNO_2$ can be formed by the heterogeneous chemistry of $N_2O_5$ on HCl in polar stratospheric clouds [3,4]. Through these atmospheric reactions, gaseous $ClNO_2$ remains unreactive at night since it is a fairly stable compound (lifetime > 30 hours [5]) that acts as a Cl and $NO_x$ reservoir. After sunrise, however, $ClNO_2$ becomes a major source of chlorine (Cl) radicals and nitrogen dioxide ($NO_2$) molecules due to its rapid photolysis [5], which is its primary fate in the early morning hours. The UV/visible photolysis of $ClNO_2$ will thereby initiate oxidation of volatile organic compounds by Cl radicals and thus affects air quality and the atmospheric oxidation capacity, with harmful consequences which are tropospheric ozone and secondary aerosol formation, as well as – possibly – further stratospheric ozone destruction.

The important role of $ClNO_2$, as a nocturnal $NO_x$ reservoir and a diurnal Cl source, triggered since 2006 close attention to the quantification of this species in the atmosphere by various *in situ* ground-based and aircraft observations using chemical ionization mass spectrometry [5]. These observations, carried out mostly in North America, Europe and China, indicate that nighttime $ClNO_2$ is a trace gas constituent measured up to around 8 parts-per-billion by volume (ppbv) in polluted coastal regions impacted by sea-salt aerosols during night [5-15] and to about 450 parts-per-trillion by volume (pptv) in continental air during night or early morning [16-21]. These observed high levels were correlated with continental pollution, such as biomass burning and power plant plumes. The highest $ClNO_2$ mixing ratio (8.3 ppbv) was measured during a night of a severe haze event in winter in the south of China and is due to the conversion of a larger fraction of $NO_x$ into $ClNO_2$ [14]. This is confirmed also in a recent study explaining that "25-60% of $NO_x$ is converted to $N_2O_5$ via multiphase reactions between gas phase nitrogen oxide reservoirs and aerosol particles, with ~93% reacting in the marine boundary layer to form > 2.5 ppbv of $ClNO_2$" [22].

Some observations have also been carried out in indoor environments during bleach cleaning, revealing a chemistry that occurs between chlorinated and nitrogenated compounds [23, 24]. $ClNO_2$ has been identified as one of the bleach-related compounds observed in indoor air [23] but, up to date, only one measurement exists for the indoor mixing ratio that
reaches up to 30 ppbv (!), a value that is ten thousand times higher than the outdoor mixing ratio measured during bleach cleaning events [24].

In addition to these *in situ* measurements, ClNO₂ needs an accurate quantification in the atmosphere using optical remote sensing techniques such as Fourier transform (FT) spectroscopy. However, to the best of our knowledge, there is no infrared detection of this trace gas species using FT spectroscopy until now. The ClNO₂ spectrum exhibits sharp *Q* branches, *in particular that of the v₂ band*, in the 12.6 µm region [25] which is an atmospheric “window” and could then be used for a possible atmospheric detection. In particular, the new space mission (IASI-NG, to be launched around 2023) with twice the resolution of IASI and a noise level reduced by a factor of 2 to 4 (depending of the spectral range) should allow the observation of new key atmospheric species such as ClNO₂. Also, one has to mention the future existence of the instrument FORUM (150-1400 cm⁻¹) which is one of the concepts chosen by ESA to be developed further as a ninth Earth Explorer mission, opening up a new window for understanding and quantifying the radiative processes, as well as the air quality and pollution effects. In addition, FORUM will fly in close formation with IASI-NG allowing synergistic observations in the far and mid-IR regions. However, this will be possible only if the necessary spectroscopic data exist. The 12.6 µm region contains not only the absorption of the v₂ band but also that of various hot bands (v₂+v₃–v₃, v₂+v₄–v₄ and v₂+v₆–v₆) involving low energy vibrations (i.e. the bands v₃ at 370 cm⁻¹, v₄ at 652 cm⁻¹ and v₆ at 408 cm⁻¹) of ClNO₂. In order to provide an accurate linelist for the remote sensing of ClNO₂, a complete and detailed analysis of the low energy vibrations is thus required.

ClNO₂ has been the subject of a number of microwave and infrared high resolution studies [25-30 and refs herein]. The v₃ band of ClNO₂ has been previously studied in Ref. [30] but the spectrum was recorded at a resolution of 0.002 cm⁻¹, with a pressure of 3.06 hPa and at room temperature, resulting in a very dense rotational structure that was not fully resolved. In this paper we present a higher resolution Fourier transform spectrum of nitryl chloride, taken at 0.00102 cm⁻¹ resolution using the SOLEIL synchrotron radiation, an optical path length of 8.16 m at a pressure of 0.025 hPa and a temperature of 221 K. These much improved experimental conditions resulted in a well resolved spectrum, allowing a new and extensive line positions analysis of the v₃ bands of ³⁵ClNO₂ and ³⁷ClNO₂.

## II. Experimental details
Not being commercially available, ClNO₂ must be synthesized in the laboratory and kept at low temperature. Chemical syntheses were carried out under vacuum using a glass apparatus equipped with PTFE-sealed stopcocks (shown in Fig. 1(a)). Glass tubes, also with PTFE-sealed stopcocks, were connected to the apparatus via PTFE fittings and were used for trapping, evaporating, and storing samples at liquid nitrogen temperature. Pressure measurements were monitored with membrane pressure gauges (Thyracont, Germany).

ClNO₂ was synthesized following the reaction [31 and refs herein]:

$$HNO_3 + ClHSO_3 \rightarrow ClNO_2 + H_2SO_4,$$

where nitric acid (HNO₃) was obtained as follows [32]:

$$KNO_3 + H_2SO_4 \rightarrow HNO_3 + KHSO_4.$$

Approximately 30 cm³ of highly concentrated sulfuric acid (H₂SO₄, 99.99% purity purchased from Sigma-Aldrich) were added dropwise to 20 g of anhydrous potassium nitrate (KNO₃) held in a 0°C bath. The mixture was then stirred and warmed slowly to complete the process, allowing the production of nitric acid, a colorless liquid, into a trap at -196°C. The vapor pressure of HNO₃ was checked at 22°C, similar to Ref. [33], yielding 68 mbar, and the sample purity was assessed spectroscopically in the mid IR.

A stoichiometric amount of that pure HNO₃ sample was added to 15 cm³ of commercial chlorosulfonic acid (ClHSO₃) cooled at -196°C. The mixture was then warmed and volatile components, among them the target ClNO₂, were distilled into another trap. While impurities of ClHSO₃ and NO₂ were sublimated off at -60°C and -70°C, respectively, it was very difficult to separate the remaining Cl₂O from ClNO₂, since their vapor pressures are closer. The sample was distilled at -85°C to remove Cl₂O without losing too much ClNO₂ by monitoring the latter's vapor pressure. Then, purified ClNO₂, a deep orange liquid (see Fig. 1(b)), has a vapor pressure of 10 mbar at -85°C, and its purity was checked using low resolution FTIR spectroscopy before use.

A high resolution spectrum was recorded at Synchrotron SOLEIL (Saint-Aubin, France) on the AILES Beamline [34]. As ClNO₂ quickly decomposes in contact with metal or oxidizable surfaces, a special long path gas cell made of glass for the cell cylindrical body, Teflon™ bellows for mirror steering or PTFE-coated aluminum alloy for the flanges [35] was used. The cell was fitted with wedged diamond windows (Advanced Diamond Materials, USA) mounted with Teflon™-coated silicone or Kalrez gaskets (Eriks, France). The cell uses a simple White-type optical multi-pass configuration with gold coated mirrors having a special Al₂O₃ solid protective layer. A triple-envelope glass cell body has been manufactured
(Verrerie Soufflée Normalisée, France) with an inner volume of about 14 liters (15 cm inner diameter × 80 cm length) surrounded by an annular cylindrical space for circulating a cooling ethanol bath and a third envelope for the insulating vacuum (5×10⁻⁵ mbar). The metal flanges supporting the windows, mirror mounts and viewports were covered with a thick PTFE coating (STIM, France) and the mirrors were mounted on Teflon™ bellows (Elkinger, Germany) allowing for travel and tilt adjustments. The optical path length was set to 8.16 m. The spectrum was recorded at a temperature of 221 K. In order to prevent any decomposition of ClNO₂, the cell was passivated several times with N₂O₅, before recording the spectrum.

The Bruker IFS125HR interferometer was using synchrotron radiation [36] as the light source (500 mA ring current in 416 equal electron bunches). It was equipped with a My/Si composite beamsplitter, a 4 K Si-Bolometer detector (Infrared Laboratories, USA) fitted with a 0-700 cm⁻¹ mesh filter (QMC, UK) and a wedged diamond window (Advanced Diamond Materials, USA). Interferograms were acquired (without apodization) at 60 kHz scanner fringe frequency and a maximum optical path difference (MOPD) of 882 cm. According to the Bruker definition (resolution = 0.9/MOPD), this corresponds to a resolution of 0.00102 cm⁻¹.

The following procedure was used for the measurements. A background spectrum was first recorded at a resolution of 0.02 cm⁻¹ while the cell was being continuously evacuated. The infrared gas cell was then filled and passivated with ClNO₂ vapor and, for the final measurement, a sample pressure of 0.025 hPa was used. 260 interferograms were averaged in blocks of ten over 17 hours duration for the averaged spectrum. For the Fourier transformation, a Mertz-phase correction with a 2 cm⁻¹ phase resolution, a zero-filling factor of 2 and no apodization (boxcar option) were applied to the averaged interferograms. In order to improve the measurement of line positions, a post zero-filling factor of 4 was applied to the averaged spectrum. The spectrum was calibrated by matching the measured positions of 30 lines of residual CO₂ and H₂O observed therein to reference wavenumbers available in HITRAN [37] with a root mean square (RMS) deviation of 0.00009 cm⁻¹. The absolute accuracy of the measured ClNO₂ line positions was estimated as the square root of the sum of squares of the accuracy of the reference CO₂ and H₂O line positions in HITRAN (better than 0.0001 cm⁻¹) plus twice the RMS deviation between the observed frequencies and values listed in HITRAN for CO₂ and H₂O, yielding in total 0.0002 cm⁻¹. Fig. 2 gives an overview of the ClNO₂ spectrum, observed in this work. The distinctive shape of the A-type v₃ band of ³⁵ClNO₂ with its narrow Q-branch is clearly visible. Fig. 3 shows a portion of the spectrum
observed in this work and a comparison with that observed by Orphal *et al.* [30]. As can be seen, the new experimental conditions allow a fully resolved spectrum and thus more complete and accurate assignments.

### III. Analysis and results

ClNO₂ is a planar asymmetric-top molecule with C₂v symmetry, the Cl-N bond being the C₂ axis. **Fig. 4** provides the corresponding C₂v point group, together with the symmetry types of the six vibrational modes, as well as the components of an axial and a polar vector and the symmetry types of the rotational levels. Since the Bose-Einstein statistics applies to the two symmetric ¹⁶O nuclei with zero nuclear spin, all vibration-rotation wavefunctions must have even parity with respect to the axis of the molecule. As a consequence, only rotational levels with even Kₐ values exist for states of A₁ or A₂ symmetry (the ground and 3¹ states of the molecule in the present study). Conversely, only rotational levels with odd Kₐ values exist for states of B₁ or B₂ symmetry. The v₃ band of ClNO₂ is an A-type band extending from about 337 to 385 cm⁻¹.

The analysis of the v₃ band of ³⁵ClNO₂ was started using the rotational and centrifugal distortion constants of Orphal *et al.* [30] for a first prediction of the line positions. Line assignments were started by picking out series of transitions belonging to the P-, Q-, and R-branches and checking the assignments with ground state combination differences. Once a few lines were firmly assigned, an initial fit was performed yielding preliminary values for the band center and the rotational constants; the centrifugal distortion constants were fixed to their ground state values [30]. Once these constants were obtained, a synthetic spectrum was computed and compared to the experimental spectrum permitting the extension of the assignment to new *J* and Kₐ transitions, *varying* first the upper state centrifugal distortion constants Δ*J* and Δ*JK* for values of Kₐ ≥ 24 and *J* ≥ 50. Then δ*J* was *varied* for Kₐ ≥ 30 and *J* ≥ 64 and finally Δ*K* and δ*K* for Kₐ ≥ 34 and *J* ≥ 64. This process was repeated until it was not possible to make new line assignments. For the ³⁵ClNO₂ isotopomer a total of 3416 transitions with *J* and Kₐ up to 83 and 44 respectively, were included in the fit as well as the 7 microwave transitions in the v₃=1 vibrational state from Morino and Tanaka [38]. The calculations were performed in the *I*'' representation of the A-reduced Watson-type Hamiltonian [39] (*see equation (1)*), using a program written by Maki [40].
$$
\begin{align}
H_W ={}& E_v + \left[ A_v - \frac{1}{2}(B_v + C_v) \right] J_z^2 + \frac{1}{2}(B_v + C_v) \mathbf{J}^2 + \frac{1}{2}(B_v - C_v) J_{xy}^2 \nonumber \\
& - \Delta_K^\nu J_z^4 - \Delta_{JK}^\nu J_z^2 \mathbf{J}^2 - \Delta_j^\nu (\mathbf{J}^2)^2 - \delta_K^\nu \{J_z^2, J_{xy}^2\} - 2 \delta_j^\nu J_{xy}^2 \mathbf{J}^2 \nonumber \\
& + H_K^\nu J_z^6 + H_{KJ}^\nu J_z^4 \mathbf{J}^2 + H_{JK}^\nu J_z^2 (\mathbf{J}^2)^2 + H_j^\nu (\mathbf{J}^2)^3 \nonumber \\
& + h_K^\nu \{J_z^4, J_{xy}^2\} + h_{KJ}^\nu \{J_z^2, J_{xy}^2\} \mathbf{J}^2 + 2 h_j^\nu J_{xy}^2 (\mathbf{J}^2)^2 \tag{1} \\
\text{with } \{A, B\} = AB + BA \text{ and } J_{xy}^2 = J_x^2 - J_y^2. \nonumber
\end{align}
$$

This led to an accurate band center as well as rotational and centrifugal distortion constants which are given in the fourth column of **Table 1**. The fit is excellent as demonstrated by a standard deviation of 2.0×10⁻⁴ cm⁻¹.

The analysis of the v₃ band of ³⁷CINO₂ was started by using the same method as for the v₃ band of ³⁵CINO₂ using the upper state Hamiltonian constants of Ref. [30] and the ground state constants of Ref. [25]. It was possible to locate lines with medium *J* and *K*<sub>a</sub> values. These lines were used to improve the upper state constants, providing then better predictions and hence allowing new assignments. The process was repeated until no new assignment was possible. For this least abundant isotopomer, we have assigned three times as many lines than in the previous work [30]. Indeed, 2915 transitions with *J* and *K*<sub>a</sub> up to 83 and 44 respectively, compared to 968 transitions in the previous work, were included in the final fit. The improved upper state Hamiltonian constants derived from the final fit (standard deviation, 2.0×10⁻⁴ cm⁻¹) are given in the seventh column of **Table 1**. The list of measured and calculated line positions resulting from this work is provided as supplementary material.

## IV. Simulation of the experimental spectrum and comparison with previous work

Using constants derived from this work, synthetic spectra were generated. It should be mentioned that, in this study, no attempt was made to determine dipole moment operator parameters and only relative line intensities can be compared between experiment and simulation. The dipole moment was crudely estimated and used to calculate the synthetic spectra which were simply scaled to match the observed spectra. Fig. 5 gives a comparison of the observed and calculated spectrum over the entire region and shows that the v₃ bands of both ³⁵CINO₂ and ³⁷CINO₂ isotopologues are adequately simulated. In Figs. 6 and 7, we present a comparison between observation and calculation for the Q-branch regions of the v₃ bands of ³⁷CINO₂ and ³⁵CINO₂. In each case, the agreement between observed and calculated spectra is
quite good. Modeling properly such Q branches is the most difficult part of the work, given the piling of lines and the high J lines involved. As an additional illustration of the quality of the present fit, Fig. 8 presents residuals (Obs. – Calc.) values of the line positions for ³⁵CINO₂ and ³⁷CINO₂.

**Fig. 9** gives a portion of the ClNO₂ spectrum in the 366.12 cm⁻¹ spectral region. In this spectral region, the prediction of Orphal *et al.* [30] is in agreement with our new observed spectrum. This is not the case in the other two regions shown in Figs. 10 and 11 where one can clearly see the improvements brought by the present analysis.

## V. Discussion on the possible interaction between the v₃ = 1 and v₆ = 1 states

During our analysis, we noted a small shift between observed and calculated positions for lines with high J values of Kₐ = 24 series. The v₃ (A₁ symmetry) and v₆ (B₁ symmetry) bands being of different symmetry, a Coriolis interaction with ΔKₐ = ±1 or ΔKₐ = ±3 is therefore possible. In this case, the energies of the levels involved should be close. By limiting the value of the corresponding energies to the first two terms of Watson's Hamiltonian (equation (1)), the following equations can be written:

$$E_3 + \left(A_3 - \frac{B_3 + C_3}{2}\right) K_a^2 \approx E_6 + \left(A_6 - \frac{B_6 + C_6}{2}\right) (K_a - 1)^2 \quad (2)$$

or

$$E_3 + \left(A_3 - \frac{B_3 + C_3}{2}\right) K_a^2 \approx E_6 + \left(A_6 - \frac{B_6 + C_6}{2}\right) (K_a - 3)^2 \quad (3)$$

By solving these equations with A₃, B₃ and C₃ the rotational constants of the v₃=1 excited state obtained in this work; A₆, B₆ and C₆ fixed to the rotational constants of the ground state; E₃ = 370 cm⁻¹ and E₆ = 411 cm⁻¹, we came to a value of Kₐ = 63 from equation (2) and Kₐ = 24 from equation (3). It could thus exist a Coriolis resonance with ΔKₐ = ±3 between the v₃ and v₆ bands, separated by about 40 cm⁻¹. This might be investigated in the future in more detail with a theoretical model that takes explicitly into account such resonances.

## VI. Conclusions

Using a high resolution Fourier transform spectrum recorded at SOLEIL synchrotron facility with improved experimental conditions, a new investigation of the v₃ absorption
bands of <sup>35</sup>CINO<sub>2</sub> and <sup>37</sup>CINO<sub>2</sub>, located around 370 cm<sup>-1</sup>, was performed, improving significantly the quality of the results achieved previously. Accurate sets of rotational and centrifugal distortion constants as well as band centers for the 3<sup>1</sup> states of <sup>35</sup>CINO<sub>2</sub> and <sup>37</sup>CINO<sub>2</sub> have been determined. Also, we noted during our analysis a very weak local perturbation. In order to clarify this, it would be necessary in the future to study the ν<sub>3</sub> and ν<sub>6</sub> (around 411 cm<sup>-1</sup>) bands together with a model that takes into account such resonances. The synthetic line list obtained in this study is potentially interesting for the atmospheric retrieval of nitryl chloride. As far as the intensities are concerned, more work is needed to better extract from the observed spectra, which are very dense, reliable line by line intensities. This will ultimately provide a line list including positions and intensities for the ν<sub>3</sub> band of CINO<sub>2</sub> that will be available in the HITRAN and GEISA databases.

## ACKNOWLEDGEMENTS

This work was supported by the French National program LEFE « Les Enveloppes Fluides et l'Environnement » through the « MEANSPECFORUM » project. It is also funded by the French National program ANR (ANR-19-CE29-0013) through the « QUASARS » project. Soleil Support (project 20180511) is also acknowledged.

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<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) View of the synthesis set-up at LISA. (b) View of a pure ClNO₂ sample (deep orange liquid at this temperature) in a Pyrex glass tube.</figcaption></figure>
**Figure 2.** Overview of the observed spectrum of the $v_3$ band of ClNO$_2$ recorded using synchrotron light source with 500 mA ring current, a resolution of 0.00102 cm$^{-1}$, an optical path length of 8.16 m, a nitryl chloride pressure of 0.025 hPa and a temperature of 221 K. The distinctive shape of the A-type $v_3$ band of $^{35}$ClNO$_2$ with its narrow Q-branch is clearly recognizable.

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Portion of the spectrum observed in this work, compared with that observed by Orphal *et al.* [30]. The new experimental conditions resulted in a fully resolved spectrum. The observed spectra are in transmittance and shifted vertically for clarity.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Structure and symmetry properties of the nitryl chloride (CINO₂) molecule.</figcaption></figure>

<table><caption>C<sub>2v</sub> group character table</caption><thead><tr><th rowspan="2"></th><th rowspan="2">I</th><th rowspan="2">C<sub>2z</sub></th><th rowspan="2">σ<sub>xz</sub></th><th rowspan="2">σ<sub>yz</sub></th><th rowspan="2">Polar vector</th><th rowspan="2">Axial vector</th><th rowspan="2">Vibrations</th><th colspan="2">Levels</th><th rowspan="2">Type</th></tr><tr><th>K<sub>a</sub></th><th>K<sub>c</sub></th></tr></thead><tbody><tr><td>A<sub>1</sub></td><td>1</td><td>1</td><td>1</td><td>1</td><td>T<sub>z</sub></td><td></td><td>ν<sub>1</sub>, ν<sub>2</sub>, ν<sub>3</sub></td><td>e</td><td>e</td><td>A</td></tr><tr><td>A<sub>2</sub></td><td>1</td><td>1</td><td>-1</td><td>-1</td><td></td><td>R<sub>z</sub></td><td></td><td>e</td><td>o</td><td></td></tr><tr><td>B<sub>1</sub></td><td>1</td><td>-1</td><td>1</td><td>-1</td><td>T<sub>x</sub></td><td>R<sub>y</sub></td><td>ν<sub>5</sub>, ν<sub>6</sub></td><td>o</td><td>e</td><td>B</td></tr><tr><td>B<sub>2</sub></td><td>1</td><td>-1</td><td>-1</td><td>1</td><td>T<sub>y</sub></td><td>R<sub>x</sub></td><td>ν<sub>4</sub></td><td>o</td><td>o</td><td>C</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Overview of the $v_3$ bands of $^{35}\text{CINO}_2$ and $^{37}\text{CINO}_2$. The observed spectrum is compared to the present calculation. The observed and synthetic spectra are in transmittance and shifted vertically for clarity.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Detailed view of the *Q*-branch region of the ν₃ band of ³⁷CINO₂ including strong *P*-branch transitions of the ν₃ band of ³⁵CINO₂. Note the excellent agreement between the observed and calculated spectra. The contributions of the ν₃ bands of ³⁵CINO₂ and ³⁷CINO₂ are also shown individually. Lines in the observed spectrum which are not reproduced in the calculated spectra are due to hot bands. **The observed and synthetic spectra are in transmittance and shifted vertically for clarity.**</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Detailed view of the *Q*-branch region of the *v*₃ band of ³⁵CINO₂ including strong *R*-branch transitions of the *v*₃ band of ³⁷CINO₂. Note the excellent agreement between the observed and calculated spectra. The contributions of the *v*₃ bands of ³⁵CINO₂ and ³⁷CINO₂ are also shown individually. Lines in the observed spectrum which are not reproduced in the calculated spectra are due to hot bands. **The observed and synthetic spectra are in transmittance and shifted vertically for clarity.**</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Residuals (Obs. – Calc.) values of the line positions of the *v*₃ bands of ³⁵CINO₂ and ³⁷CINO₂ plotted as a function of the rotational quantum number *J*.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 9.** Portion of the ClNO₂ spectrum in the 366.12 cm⁻¹ region. A comparison between the observed and calculated spectra is given, showing in this region an agreement between the present calculation and the previous work [30]. Lines in the observed spectrum which are not reproduced in the calculated spectrum are due to HNO₃ and hot bands.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 10.** Portion of the ClNO₂ spectrum in the 371.5 cm⁻¹ region. A comparison between the observed and calculated spectra is given, showing the progress achieved in the present calculation compared to the previous work [30]. The line in the observed spectrum which is not reproduced in the calculated spectrum is due to a hot band.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Portion of the ClNO₂ spectrum in the 363.15 cm⁻¹ region. A comparison between the observed and calculated spectra is given, showing the progress achieved in the present calculation compared to the previous work [30]. Lines in the observed spectrum which are not reproduced in the calculated spectrum are due to hot bands.</figcaption></figure>
<table><caption>**Table 1.** Vibrational band centers, rotational and centrifugal distortion constants for the 3¹ states of ³⁵CINO₂ and ³⁷CINO₂.</caption><thead><tr><th rowspan="3">Constant</th><th colspan="3"><sup>35</sup>CINO<sub>2</sub></th><th colspan="3"><sup>37</sup>CINO<sub>2</sub></th></tr><tr><th rowspan="2">Ground state [30]</th><th colspan="2">Ref. [30]</th><th rowspan="2">Ground state [25]</th><th colspan="2">Ref. [30]</th></tr><tr><th>ν<sub>3</sub> = 1</th><th>ν<sub>3</sub> = 1</th><th>ν<sub>3</sub> = 1</th><th>ν<sub>3</sub> = 1</th></tr></thead><tbody><tr><td>ν<sub>0</sub></td><td>0.0</td><td>370.151513(18)</td><td>370.1510773(92)</td><td>0.0</td><td>364.522184(32)</td><td>364.5218094(96)</td></tr><tr><td>A</td><td>0.443311168</td><td>0.443077816(81)</td><td>0.4430780551(360)</td><td>0.4433086255</td><td>0.44308139(31)</td><td>0.4430809996(380)</td></tr><tr><td>B</td><td>0.172636540</td><td>0.171296881(27)</td><td>0.1712968222(130)</td><td>0.1674698826</td><td>0.16618853(19)</td><td>0.1661886339(210)</td></tr><tr><td>C</td><td>0.124069849</td><td>0.122761674(29)</td><td>0.1227615713(110)</td><td>0.1213779349</td><td>0.12016931(11)</td><td>0.1201687350(160)</td></tr><tr><td>Δ<sub>J</sub> × 10<sup>7</sup></td><td>0.91332</td><td>0.86030(10)</td><td>0.85976177(2800)</td><td>0.861024465</td><td>0.82088(86)</td><td>0.82187027(3200)</td></tr><tr><td>Δ<sub>JK</sub> × 10<sup>7</sup></td><td>1.9687</td><td>2.0486(58)</td><td>2.0497273(2000)</td><td>1.90984562</td><td>1.9931(50)</td><td>1.9587160(2200)</td></tr><tr><td>Δ<sub>K</sub> × 10<sup>7</sup></td><td>0.7527</td><td>0.7186(81)</td><td>0.7208732(2600)</td><td>0.4076512</td><td>0.3769(58)</td><td>0.4080925(2800)</td></tr><tr><td>δ<sub>J</sub> × 10<sup>7</sup></td><td>0.26493</td><td>0.29644(39)</td><td>0.29680413(1200)</td><td>0.246692599</td><td>0.26834(50)</td><td>0.27013902(2100)</td></tr><tr><td>δ<sub>K</sub> × 10<sup>7</sup></td><td>2.8079</td><td>3.017(18)</td><td>3.0162983(3800)</td><td>2.64538198</td><td>2.831(10)</td><td>2.8502337(5000)</td></tr><tr><td>No. IR/MW data</td><td>–</td><td>2871/8</td><td>3416/7</td><td>–</td><td>968</td><td>2915</td></tr><tr><td>J range</td><td>–</td><td>1–65</td><td>0–83</td><td>–</td><td>2–52</td><td>0–83</td></tr><tr><td>K<sub>a</sub> range</td><td>–</td><td>0–36</td><td>0–44</td><td>–</td><td>0–26</td><td>0–44</td></tr><tr><td>RMS IR/MW</td><td>–</td><td>3.3×10<sup>-4</sup>/5.2×10<sup>-6</sup></td><td>2.0×10<sup>-4</sup>/3.5×10<sup>-6</sup></td><td>–</td><td>3.3×10<sup>-4</sup></td><td>2.0×10<sup>-4</sup></td></tr></tbody></table>

Note. All constants and the RMS are given in units of cm⁻¹ and the numbers in parentheses give 1σ in units of the last digits.
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>