Chlorine nuclear quadrupole coupling in chlorodifluoroacetyl chloride: Theory and experiment

doi:10.1016/j.jms.2010.07.001

Journal of Molecular Spectroscopy

Volume 263, Issue 2, October 2010, Pages 127-134

https://doi.org/10.1016/j.jms.2010.07.001 Get rights and content

Abstract

A potential energy scan of chlorodifluoroacetyl chloride, ${CF}_{2} Cl$ single bond C( double bond O)Cl, at the MP2/6-311+G(d) level of theory predicts stable gauche and trans conformers, with $E_{gauche} < E_{trans}$ . Ab initio calculations were made of approximate equilibrium structures of these and, on these structures, calculations were made of ³⁵Cl and ³⁷Cl nuclear quadrupole coupling constant tensors. Coupling constants here predicted, as well as rotational constants and molecular dipole moments, are applied to aid analyses of experimental microwave spectra. Chirped pulse Fourier transform microwave spectroscopy has been used to rapidly record the rotational spectra of four isotopologues of the title molecule, ${CClF}_{2} C (O) Cl$ , namely ³⁵Cl single bond ³⁵Cl, ³⁵Cl³⁷Cl, ³⁷Cl³⁵Cl, and ³⁷Cl³⁷Cl. Only the gauche conformer was observed under the experimental conditions. For the four isotopologues a total of 464, 219, 197, and 77 transitions have been recorded, respectively. With the exception of the ³⁷Cl³⁷Cl isotopologue sufficient data was available to determine all Cl quadrupole coupling tensors. Comparisons between the theoretical and experimental results are made.

Introduction

High resolution microwave spectroscopy continues to be a powerful tool for molecular characterization, providing insight into molecular geometric and electronic structures [1], [2]. However, heavy asymmetric top molecules containing two or more quadrupolar nuclei, while prevalent in nature, can still provide significant challenges regarding transition quantum number assignments. Although numerous examples exist in the literature where the aforementioned problem has been solved, the solution is far from being a routine procedure. Given the biological importance of asymmetric tops possessing, say, two or more chlorines, or nitrogens, or mixtures (e.g. sucralose, tryptophan, sertraline, guanine [3]), methodologies that aim to assist microwave spectral assignments are highly valued.

Chlorodifluoroacetyl chloride (CDFAC), ${CF}_{2} {Cl}^{-} C (O) Cl$ , provides a particularly stiff challenge with regard to “assignmentology”. It possesses two quadrupolar chlorine nuclei in quite different local environments unrelated through symmetry considerations. Furthermore the quadrupole moments of the Cl isotopes are relatively small in magnitude $(Q (^{35} Cl) = - 81.65 (80)$ mb and $Q (^{37} Cl) = - 64.35 (64)$ mb [4]) resulting in quite small hyperfine splittings. This effect will be exaggerated given the relatively heavy mass of the species $(m_{r} = 148 amu)$ , i.e. only higher J transitions will be observed in traditional regions.

For CDFAC the possible existence of two conformers, trans and gauche, with four chlorine isotopologues each, means that the spectra will likely be dense and overlapping. Furthermore, both conformers are predicted to have very small electric dipole moment components along the a-axis, potentially precluding a straightforward spectral assignment from the normal harmonic progressions of a-type transitions.

In the present work we have successfully recorded the pure rotational spectrum of four isotopologues of the gauche conformer of CDFAC using a chirped pulse Fourier transform microwave spectrometer, which provides spectra with reliable relative transition intensities. Analyses of the recorded spectra has been greatly facilitated through a theoretical approach which determines accurate molecular geometries and hyperfine constants. This approach is fully described.

Chlorodifluoroacetyl chloride has been the subject of prior structural and spectroscopic studies. The first was a Raman and infrared study on the gas, liquid and solid phases [5]. The study demonstrated that $Δ H$ between the trans and gauche conformers is $\approx 1 kcal {mol}^{- 1}$ . The second was a gas electron diffraction experiment [6] which produced structural parameters for the dominant gauche conformer. This work reports the first microwave spectroscopic investigation of chlorodifluoroacetyl chloride.

Section snippets

Theoretical

A molecule like chlorodifluoroacetyl chloride – with two quadrupolar chlorine nuclei, four isotopic species (35,35; 35,37; 37,35; and 37,37), and possibly two stable conformers (Section 2.1) – would be expected to show rather complex microwave spectra. It is the primary purpose of our theoretical work to predict for this molecule ³⁵Cl and ³⁷Cl nuclear quadrupole coupling constants (nqcc’s) of sufficient accuracy to assist with assignment of its experimental microwave hyperfine structure. Along

Experimental methods

Chlorodifluoroacetyl chloride was purchased from Synquest Labs (99%) and was used without further purification. CDFAC, boiling point = 26–27 °C, was dissolved in argon making an $\approx 3 %$ solution at a stagnation pressure of 7000 Torr. This solution was pulsed by means of a solenoid valve (Parker–Hannefin Series 9) into a region between two horn antenna held in a vacuum at less than $10^{- 3}$ Torr. A 4 μs pulse of a fast linear frequency sweep (chirp) of microwave radiation spanning a 2 GHz region was broadcast

Spectral analysis

As discussed above there are potentially two conformers for CDFAC. However, only the gauche conformer was observed in this work. gauche-CDFAC exhibited a ground-state rotational spectrum appropriate to a near prolate, asymmetric rotor that was clearly complicated by the presence of two quadrupolar nuclei, and four isotopologues with significant natural abundance. Transitions allowed by two components of the electric dipole moment were observed, $μ_{b}$ and $μ_{c}$ , although the latter were weak. Each

Conclusion

A theoretical study of chlorodifluoroacetyl chloride predicts stable gauche and trans conformers. Microwave rotational spectra of the lower energy gauche conformer have been observed for the first time. The trans conformer looks like a good candidate for study with a waveguide spectrometer. Assignment of the observed, complex, high resolution rotational spectrum has been greatly facilitated through an accurate quantum chemistry calculational method, which holds promise for the continued future

Acknowledgments

This work has been supported by a National Science Foundation Chemistry Research Instrumentation and Facilities: Instrument Development (CRIF:ID) award, CHE-0820833.

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Chlorine nuclear quadrupole coupling in chlorodifluoroacetyl chloride: Theory and experiment

Abstract

Introduction

Section snippets

Theoretical

Experimental methods

Spectral analysis

Conclusion

Acknowledgments

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