Rovibrational analysis of the ethylene isotopologue 13C2D4 by high-resolution Fourier transform infrared spectroscopy

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Highlights

  • Covered a wide wavenumber range and high J and Kc values for the ν12 band of 13C2D4.

  • New upper state (ν12 = 1) rovibrational constants from 2068 IR transitions.

  • New experimental ground state rovibrational constants from 985 combination differences.

  • New theoretically-derived equilibrium ground state rovibrational constants.

Abstract

The Fourier transform infrared (FTIR) spectrum of the unperturbed a-type ν12 band of 13C2D4 was recorded at an unapodized resolution of 0.0063 cm−1 between 1000 and 1140 cm−1 for a rovibrational analysis. By assigning and fitting a total of 2068 infrared transitions using a Watson’s A-reduced and S-reduced Hamiltonians in the Ir representation, rovibrational constants for the upper state (ν12 = 1) up to five quartic centrifugal distortion terms were derived for the first time. The root-mean-square (rms) deviation of the fits was 0.00034 cm−1 both in the A-reduction and S-reduction Hamiltonian. The ground state rovibrational constants of 13C2D4 in the A-reduced and S-reduced Hamiltonians were also determined for the first time by a fit of 985 combination-differences from the present infrared measurements, with rms deviation of 0.00036 cm−1. The ν12 band centre of 13C2D4 was at 1069.970824(17) cm−1 and at 1069.970799(17) cm−1 for the A-reduced and S-reduced Hamiltonians respectively. The ground state constants of 13C2D4 from this experimental work are in close agreement to those derived from theoretical calculations using the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CSSD(T)/cc-pVTZ levels of theory.

Introduction

Numerous studies of ethylene and its isotopologues have been initiated since the detection of ethylene C2H4 in the atmospheres of Jupiter [1], Neptune [2], [3], [4] and Titan [5], [6] and the recognition of the need for accurate spectroscopic parameters to better understand this molecule. The vibrational structures of the C2H4 molecule and those of its isotopologues have been studied extensively by low-resolution infrared spectroscopy in the last two decades [7], [8], [9], [10]. Furthermore, in order to understand their rovibrational structures by accurate determinations of their rotational and higher order centrifugal distortion constants, many recent high-resolution infrared investigations (resolutions better than 0.01 cm−1) were made on 12C2H4 [11], [12], [13], 12C2H3D [14], trans-12C2H2D2 [15], cis-12C2H2D2 [16], [17], 12C2HD3 [18], and 12C2D4 [19]. However, high-resolution infrared spectroscopic measurement and analysis of 13C2D4 have not been reported so far. In 1973, Duncan et al. [7] identified the fundamental vibrational bands of 13C2D4 and assigned their band centres with an accuracy of 1 cm−1. So far, no studies have been made on the rovibrational structure of 13C2D4 despite the requirement for the data at infrared spectral resolution better than 0.01 cm−1.

In the last fifteen years, we have used high-resolution (0.0063 cm−1 or better) FTIR spectroscopy to measure and analyse numerous fundamental and combination bands of 12C2H4 and its various isotopologues [11], [12], [14], [15], [16], [17], [18], [19]. The main purpose was to improve or derive new ground state and upper state rovibrational constants for various isotopologues of ethylene to add to the pool of accurate data on the rovibrational structure of the ethylene molecule. The present investigation is aimed at the analysis of the a-type ν12 band of 13C2D4 which was measured using FTIR spectroscopy with a resolution of 0.0063 cm−1. By assigning and fitting infrared transitions for the ν12 band, the rovibrational constants up to five quartic terms for the ν12 = 1 state were obtained accurately for the first time. The ground state rovibrational constants for 13C2D4 up to quartic terms were also obtained for the first time by fitting ground-state combination differences (GSCDs) derived from the present infrared transitions. Furthermore, the ground state constants were calculated using the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CSSD(T)/cc-pVTZ levels of theory, and their values were compared to those derived using GSCDs from the present infrared transitions.

Section snippets

Experimental details

A Bruker IFS 125HR Michelson Fourier transform spectrophotometer in the National Institute of Education, Nanyang Technological University, Singapore, was used to record the spectrum of 13C2D4 in the 1000–1140 cm−1 region with an unapodized resolution of 0.0063 cm−1 for the rovibrational analysis of the ν12 band of 13C2D4. A globar infrared source, and a high-sensitivity liquid nitrogen cooled Hg–Cd–Te detector, and KBr beamsplitter were used. The final spectrum was produced by coadding five runs

Results and discussion

The 13C2D4 molecule is a simple asymmetric top planar molecule with D2h symmetry. Its asymmetry parameter κ is about −0.83. The ν12 vibrational mode of 13C2D4 is ascribed to the anti-symmetrical DCD in-plane bending [22]. The ν12 band is typically A-type with a prominent Q branch at about 1070 cm−1. A high-resolution (0.0063 cm−1) compressed plot of the ν12 spectrum of 13C2D4 in the 1000–1140 cm−1 region, showing the P, Q and R branches and its band centre, is shown in Fig. 1. The rotational

Conclusion

Numerous infrared transitions of the ν12 band of 13C2D4 were measured, assigned and fitted to obtain accurate upper state (ν12 = 1) constants up to five quartic terms for the first time. In addition, the accurate ground state constants of 13C2D4 were also derived for the first time from a fit of combination differences from the present infrared measurements. These ground state constants were in good agreement with those derived from theoretical calculations. These new infrared spectral data would

Acknowledgments

The authors are grateful to National Institute of Education, Singapore for the financial support of this project through research Grant number RS 7/11 TTL.

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