Spectroscopy of a low global warming power refrigerant. Infrared and millimeter-wave spectra of trifluoroethene (HFO-1123) in the ground and some vibrational excited states

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Highlights

  • Analysis of the infrared band ν6.

  • Rotational spectra in the ground and vibrational excited states.

  • FT-IR detection of many fundamental bands.

Abstract

In the present work we carried out a combined rotational and ro-vibrational investigation on 1,1,2-trifluoroethene, a relevant unsaturated hydrofluoroolefin recently proposed as refrigerant in mixture with other halogenated compounds (like difluoromethane). By employing a frequency-modulation millimeter-wave spectrometer, the rotational spectra were recorded in the frequency ranges 80–96 GHz and 245–260 GHz for the ground and also the vibrationally excited states ν8=1, ν9=1, ν12=1, ν9=2, ν12=2, and ν9=ν12=1. In addition, the infrared spectra in the region of the ν6 band (centered at 929.8cm1) were measured with a high-resolution Fourier transform spectrometer. The data coming from the detailed rotational and ro-vibrational assignments were combined in a global fit, taking into account also the ground state transitions available in the literature. From this analysis, a very accurate set of rotational and centrifugal distortion constants was determined for the ground state and for all the vibrationally excited states here investigated.

Introduction

In the last decades, great attention has been devoted to the search for suitable replacements of the gases used for domestic and industrial purposes which strongly contribute to the atmospheric pollution, the ozone hole and the greenhouse effect. Unsaturated hydrofluoroolefins (HFO’s) are an interesting alternative to chlorofluorocarbons (CFC’s). Indeed, such molecules have a very short atmospheric lifetime, an almost zero Ozone Depletion Potential (ODP) and a low Global Warming Potential (GWP) [1].

The search of an ideal candidate for refrigerants is nowadays a crucial issue, considering that recent studies showed that a very limited number of fluids exhibit the required environmental properties [2]. 1,1,2,-trifluoroethene (CF2=CHF, HFO-1123, hereafter referred as TFE) is used nowadays in heat pumps and conditioning systems, often in mixtures with difluoromethane (CH2F2, HFC-32). In these mixtures self-decomposition does not occur [1]; also, they have low toxicity and are only mildly flammable [1], [3], proving therefore to be a valuable alternative to R-410A, a common refrigerant with high GWP.

The atmospheric importance of CF2=CHF has stimulated a number of spectroscopic studies in the past. Low resolution infrared (IR) spectra were first recorded by Mann et al. [4] and later by McKean [5]. In 2002 Jiang et al. [6] calculated the vibrational fundamental wavenumbers and the relative intensities using the Scaled Quantum Mechanical (SQM) force field procedure in combination with the hybrid three-parameter B3-PW91 density functional. Microwave transitions of the ground and some vibrationally excited states were reported a long time ago by Bhaumik et al. [7] and Wellington Davis & Gerry [8].

More recently, Leung & Marshall recorded rotational transitions of TFE between 6 and 22 GHz by Fourier transform (FT) spectroscopy for the most abundant isotopologue and the two singly 13C-substituted species. From the determined spectroscopic constants they also derived the structural parameters of the molecule [9]. High-resolution infrared studies are however limited to the very strong fundamentals ν3, ν4, and ν5 in the atmospheric window, centered at 1360, 1265, and 1173 cm1, respectively. These high-resolution spectra were recorded with a tunable diode laser and analysed by Visinoni et al. [10], [11], [12]. The authors pointed out the presence of several resonances and determined some parameters for the interacting states.

The infrared atmospheric window has been only partially analysed and this work aims to a more complete spectroscopic characterization of this region and of the low-lying vibrational states. The goal is to provide the necessary laboratory data useful for the atmospheric detection of this molecule. The infrared spectrum was recorded at high resolution (0.004 cm1) by FT-IR spectroscopy between 500 and 1500 cm1, where the ν3, ν4, ν5, ν6 (929 cm1), ν7 (623 cm1) and ν10 (750 cm1) fundamental bands are located with the objective to assign and analyse for the first time ν6, ν7, and ν10 and to re-investigate ν3, ν4, and ν5. Given the complexity of the ro-vibrational structure, in this paper we focused only on the ν6 fundamental vibrational band and on the detection of pure rotational transitions in the ground state and in the low energy ν8=1, ν9=1, ν12=1, ν9=2, ν12=2, and ν9=ν12=1 excited vibrational states. The rotational spectra were recorded in the frequency ranges 80–96 GHz and 245–260 GHz using a frequency-modulation millimeter-wave spectrometer.

In this work we present therefore a combined rotational and ro-vibrational investigation from which very accurate spectroscopic parameters were determined for the ground state and the investigated excited vibrational states.

Section snippets

Millimeter spectrometer

Rotational spectra were recorded in the frequency ranges 80–96 GHz and 245–260 GHz using a frequency-modulation millimeter-wave spectrometer [13], [14]. The radiation source is a Gunn diode (J.E. Carlstrom Co.) emitting in the spectral range 80–115 GHz with an output power up to 50 mW. A passive multiplier (WR3.4X2, Virginia Diodes) is used to reach the higher frequencies. The diode’s radiation is phase-locked to a harmonic of a digital frequency synthesizer (HP8672A, 2–18 GHz) and its

General features

From a spectroscopic point of view, trifluoroethene is a planar near-prolate asymmetric-top molecule belonging to the Cs symmetry point group, having an asymmetry parameter κ=0.74. The molecular geometry of TFE with respect to its principal axes is shown in Fig. 2.

Its permanent electric dipole moment (μ=1.30(6) D) lies in the ab plane, with the b component (μb=1.30(6) D) much greater than that of the a component (μa=0.075(15) D) [7]. Of the 12 fundamentals, all infrared active, 9 are

Discussion

The fitting procedure, the spectral simulation and the calculation of the ro-vibrational term values were carried out by employing the ATIRS software [21] and the SPFIT/SPCAT program suite [22]. The rotational and ro-vibrational data were analysed in a global fit together with the literature data for the ground state only [7], [8], [9]. The transition frequencies of Ref. [7] relative to vibrationally excited states were not used in our global fit, not only because they are less precise than our

Conclusions

In this paper we report the detection of the rotational spectrum of TFE in the ground and in the vibrationally excited states ν9=1, ν12=1, ν8=1, ν9=2, ν12=2, and ν9=ν12=1. Moreover, the fundamental ν6 ro-vibrational band has been observed. All the data were analysed in a global fit and sets of spectroscopic parameters for each state were determined in the A-reduction scheme. The quality of the fit is very good, as shown by the values of the statistical errors, the precision of the parameters

CRediT authorship contribution statement

Filippo Tamassia: Conceptualization, Validation, Methodology, Investigation, Resources, Writing - review & editing, Supervision, Project administration. Mattia Melosso: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization, Supervision, Project administration. Luca Dore: Software, Writing - review & editing, Resources, Funding acquisition. Michele Pettini: Investigation, Formal analysis. Elisabetta Canè: Writing - review & editing.

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.

Acknowledgments

This study was supported by Bologna University (RFO funds), MIUR (Project PRIN 2015: STARS in the CAOS, Grant number 2015F59J3R), and Ca’ Foscari University, Venice (AdiR funds).

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