High resolution synchrotron far-infrared study of CH2D79Br: The v6 fundamental and 2v6  v6, v5 + v6  v6 and v6 + v9  v6 hot bands

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Abstract

The high resolution (0.0025 cm−1) far-infrared absorption spectrum for the v6 (v0 = 595.0733 cm−1) fundamental band of CH2D79Br has been recorded by Fourier transform spectroscopy employing a synchrotron radiation source. The spectral analysis allowed the assignment of more than 1700 rovibrational transitions attributed to the predominant a-type component. In the same spectral range, the 2v6  v6 hot band has been analyzed by selecting about 800 transitions. In addition, hot band transitions originating from v6 have been assigned in region of the v5 and v9 fundamentals. The data set of each band was treated using the Watson’s A-reduced Hamiltonian in the Ir representation.

Graphical abstract

The high resolution far-infrared absorption spectrum of CH2D79Br, recorded by Fourier transform spectroscopy employing a synchrotron radiation source, allowed the rovibrational assignment of the v6 fundamental and hot bands originating from v6.

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Research highlights

► High resolution synchrotron far-infrared study of CH2D79Br. ► Rovibrational analysis of the ν6 fundamental band. ► Upper-state parameters obtained up to sextic order centrifugal distortion terms. ► Analysis of the hot bands originating from v6. ► Effective upper-state constants determined using v6 = 1 as lower-state.

Introduction

Methyl bromide, CH3Br, is the major organic bromine species in the lower atmosphere [1], [2]. It is classified as one of the most ozone-depleting substances since it is readily photolyzed to form elemental bromine in the stratosphere which is destructive to the Earth’s ozone layer [3]. Highly accurate infrared spectroscopic data ([4] and references therein) are therefore crucial to model the opacity of this compound in the atmosphere. Analogous studies on its isotopic varieties are also of importance for refining the molecular potential energy surface of this molecule.

Since the early infrared work of Riter and Eggers [5] on the partially deuterated methyl halides, an infrared study of monodeutero methyl bromide at medium spectral resolution has been reported [6]. This work provided extensive vibrational assignments over a wide spectral range from 400 to 10 000 cm−1 supported by high-level quantum chemical calculations. Integrated infrared intensities of all the fundamental bands have been determined experimentally [7]. Since natural abundant CH2DBr is a mixture of almost equivalent percentages of 79/81Br isotopologues (50.7% and 49.3%, respectively), the rovibrational spectra of this compound are very congested. In order to access the spectroscopic parameters of the molecule, a high-resolution infrared spectrum of an isotopically enriched sample of CH2D79Br has been analyzed in the region of the v5 (A′) and v9 (A′′) fundamentals [8]. These bands of medium intensity give rise to a/b-type hybrid (v5) and c-type (v9) absorption features which, merging the ground-state combination differences (GSCDs), were deemed convenient for determining the ground-state constants.

In this work we resume the study of this molecule by analyzing the lowest frequency fundamental v6, an a/b-hybrid band where the transitions due to the change in dipole moment in the a-direction are predominant. A detailed knowledge of the v6 = 1 state constants is also important to identify and analyze the hot bands in the v5, v6 and v9 spectral regions.

Section snippets

Experimental

The preparation of CH2D79Br was accomplished according to [8]. Preliminary tests on the sample purity established that the most abundant impurity is given by the 79Br of CH3Br (∼4%), while the relative abundance of the 81Br isotopologue of CH2DBr should be lower than 1.5%, as explicitly stated in the specifications of the material used in the synthesis. The spectral interpretation was performed after exclusion of the stronger measured lines due to CH379Br [9], [10].

The high resolution

The v6 fundamental band

The v6 fundamental band is associated to the C-Br stretching mode of the molecule [6]. From symmetry considerations the vibration is expected to produce an a/b-hybrid band. However, due to the much larger change of the dipole moment along the a axis compared with that of the b axis, the interpretation of the fine rotational structure only relies on transitions belonging to the a-type component. The survey spectrum, given in Figure 1, shows the presence of a narrow central Q-branch degrading to

Results and discussions

Each data set of the four analyzed bands was treated by means of a non linear least-squares fit program where the rotational energy levels were computed with Watson A-reduced Hamiltonian in the Ir representation [16]. The ground-state constants [8] were fixed for determining that of v6 = 1 which subsequently were used as lower-state for refining the upper-state parameters of the analyzed hot bands.

Conclusion

This study reports on the first high-resolution infrared analysis of the v6 band of CH2D79Br. Accurate upper-state constants have been obtained up to sextic order centrifugal distortion terms. The many weak lines belonging to hot bands with lower-state v6 = 1 have been assigned in the same spectral range of v6 and in the v5 and v9 fundamental regions. Effective upper-state spectroscopic parameters have been determined by fixing the lower-state constants. It should be mentioned that the relative

Acknowledgements

This work was supported by the Ministero della Ricerca e dell’Università PRIN 2007 (project: ‘Trasferimenti di energia, carica e molecole in sistemi complessi’), the University of Venice (FRA funds) and the European Community – Research Infrastructure Action under the FP6 ‘Structuring the European Research Area’ Programme (through the Integrated Infrastructure Initiative ‘Integrating Activity on Synchrotron and Free Electron Laser Science’). The authors thank A. Gambi for supplying the

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