Mono- and di-deuterated ammonias: Far-infrared spectra and spectroscopic parameters in the ground state

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Highlights

  • NH2D and ND2H transitions are identified in the high resolution far infrared spectra.

  • For NH2D, 2153 precise transitions in the ground state with J up to 24 are assigned.

  • For ND2H, 3214 precise transitions in the ground state with J up to 28 are assigned.

  • Watson-type S reduced Hamiltonian, representations IIIr and IIIl, was adopted.

  • The model successfully reproduces the transitions at experimental accuracy.

Abstract

Transitions of NH2D and ND2H have been detected in the high resolution spectrum of 14ND3 recorded at unapodized resolution of 0.00096 cm−1 in the region 60–600 cm−1 using the Bruker IFS 125 Fourier transform spectrometer located at the synchrotron far-infrared beamline, Canadian Light Source. In this study we report on the observation and analysis of the spectrum for the ground state of NH2D and ND2H. 2153 rotation and rotation-inversion transitions, with J up to 24, were assigned for NH2D, and 3214 transitions, with J up to 28, for ND2H. The newly assigned transitions, together with those present in the literature, were fitted on the basis of a Watson-type S reduced Hamiltonian using the IIIr and IIIl representations for NH2D and ND2H, respectively. The model Hamiltonian includes terms up to the 10th power of the angular momentum and vibration–rotation interaction terms between the inversion states. Precise and extended sets of spectroscopic effective parameters were derived for both isotopologues as results of least squares weighted fit procedures that reproduce the transition wavenumbers at experimental accuracy.

Introduction

The partially deuterated ammonias, NH2D and ND2H, are evergreen targets of spectroscopic investigations [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. They are, like ammonia, NH3, prototype molecules for the study of the hindered inversion motion at the pyramidal nitrogen atom, a classic problem in tunneling dynamics ([16] and references therein). The inversion Hamiltonian for NH2D and ND2H was developed by Danielis et al. [17] and applied to the spectroscopic analysis of the ground and vibrationally excited states. In the past decades spectroscopic studies on both molecules have been also stimulated by their detections in the cold, dense starless or pre-stellar clouds and in the low mass and high mass protostellar environments [18], [19], [20], [21], [22], [23], [24], [25], [26]. From their first detection, dated in 1985 for NH2D [18] and in 2000 for ND2H [21], they became of interest for astrophysicists since the measurement of the amount of deuterium fractionation in ammonia is a powerful probe to trace the evolutionary stage of dense cores from both physical and chemical points of view [27], [28], [29], [30], [31]. The emissions observed by the radio-telescopes at 85.9 and 110.1 GHz, correspond to the 11,1 → 10,1 inversion-rotation transitions in the ground state for the ortho and para NH2D, respectively. For ortho and para ND2H the pure rotation 11,0 → 10,1 emissions are observed at 110.8 GHz and 110.9 GHz.

The pure rotation or inversion-rotation transitions in the ground state (GS) of NH2D and ND2H were observed in the microwave (MW), millimeter-wave (mmw), and sub-millimeter-wave (sub-mmw) regions [1], [2], [3], [4]. The first quantitative description of the GS energy levels for both molecules was presented in Ref. [3]. Cohen and Pickett firstly analyzed the rotation inversion spectra using a Watson-type Hamiltonian, including the interaction between the inversion states [4]. Accurate GS parameters for both isotopologues were reported from global analyses of MW data [4], of Fourier transform infrared (FTIR) transitions of ν2 [5], FTIR and diode laser spectra of ν2 [6], [7], [8], and combining the data from the literature [1], [2], [3], [4] with pure rotation and rotation-inversion transitions in the MW and far infrared (FIR) regions [9]. More recently, the experimental data set of ND2H was extended up to 2.6 THz with kilohertz accuracy, improving the values of the spectroscopic parameters for the GS [10]. The new predictions with extended quantum number range and higher accuracy have been included in the Cologne database for molecular spectroscopy (CDMS) [32]. They are important for astronomical observations as the modern radio-telescopes are characterized by high sensitivity and better spectral resolution. Very recently, the rotational spectra of NH2D and ND2H have been recorded by means of Lamb-dip techniques at sub-mm wavelengths by Melosso et al. [11], [12] and many hyperfine lines have been measured. In addition, for ND2H, accurate FIR transitions have been detected in spectra recorded from 45 to 220 cm−1 with the synchrotron radiation at the SOLEIL [12] synchrotron facility, France. Improved sets of spectroscopic parameters and of nuclear quadrupole coupling and spin-rotation constants were determined for the GS of NH2D and ND2H [11], [12]. Accurate estimates of the rotational partition functions and of the dipole moments components useful for astronomical purposes were also calculated [12]. As far as the vibrational excited states are concerned, the ν2 fundamental was studied first [5], [6], [7], [8]. The analysis of the stretching ν1, ν3a, ν3b, and of the ν4a and ν4b bending fundamentals were reported by Snels et al. [13], [14], [15].

The present work on the GS of the mono- and di-deuterated ammonias has been prompted by the detection of their absorption lines in the FTIR spectra of 14ND3, recorded from 60 to 600 cm−1 at the Canadian Light Source, CLS, in Saskatoon, Canada, by means of the synchrotron radiation. While assigning the ND3 transitions, many rotation and rotation-inversion transitions in the GS of the partially deuterated ammonias were identified and assigned up to J max = 24/28, Ka, max = 20/21, and Kc, max = 22/27 for NH2D/ND2H, respectively. 2153/3214 FIR transitions have been added to the data present in the literature extending the Jmax = 18 value previously observed. The new analyses allowed for the determination of rotational term values up to J = 24/28 and of precise and extended sets of spectroscopic parameters for the GS of both species.

The paper is structured as follows: the experimental details are given in Section 2; the description of the spectra and the procedure adopted for the assignments are given in Section 3; the theoretical model and the results of the fits are described in Section 4 while the conclusions are drawn in Section 5.

Section snippets

Experimental details

The spectra were recorded using the Bruker IFS 125 FT spectrometer located at the FIR beam line, CLS [33]. Three spectra were obtained at 295 K using two identical samples of 14ND3 supplied by Sigma-Aldrich with a stated isotopic purity of 99%. The recorded spectra showed a composition of about 60% of ND3, 30% of ND2H, 10% of NH2D, and traces of NH3, H2O, and HOD. The spectra were recorded in the region between 60 and 600 cm−1 at 0.00096 cm−1 unapodized instrumental resolution. Optimum

Description of the spectra and assignments

The partially deuterated ammonias are light asymmetric tops with asymmetry parameter κ=2B-A-CA-C equal to −0.31 (NH2D) and −0.13 (ND2H), values far from the prolate or oblate limits. The usual complex pattern of the energy levels in asymmetric tops is further complicated by the splitting of the rotational levels into inversion doublets and by the perturbations induced by vibration–rotation interactions. The components of the inversion doublets are designed as ortho and para states for Ka+Kc+v

Spectral analysis

An accurate quantitative reproduction of the rotational energy levels of partially deuterated ammonias is achievable only treating simultaneously the splitting of the levels due to inversion and the perturbations from the vibration–rotation interactions [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The analysis followed the same approach and used the computer code adopted in Ref. [9], appropriately extended to analyze transitions with J values up to 28. The same reference coordinate

Conclusions

The rotation and rotation-inversion transitions of NH2D and ND2H have been detected and assigned in the high resolution FIR spectrum of 14ND3 recorded at CLS using the synchrotron radiation from 60 to 600 cm−1. The existing experimental databases have been extended by the addition of 2153 and 3214 precise FIR transitions involving the v = 0,1 inversion levels of the GS, with J up to 24 and 28 for NH2D and ND2H, respectively. A Watson-type Hamiltonian in the S reduction, IIIr (NH2D) and IIIl (ND2

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

EC and FT acknowledge the financial support from the Università di Bologna (RFO). We thank Dr. B. Billinghurst for recording the ammonia spectra at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the

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