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Timestamp: 2019-04-23 20:31:08+00:00

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The rotational spectrum of monodeutero-oxirane was analysed as measured using the Zurich Gigahertz (GHz) spectrometer and our highest resolution Fourier Transform Infrared (FTIR) spectrometer system coupled to synchrotron radiation at the Swiss Light Source (SLS). 112 distinct line frequencies have been newly assigned in the GHz range (extended to 120 GHz, compared to previous work extending to only 59 GHz) including rotational states up to J = 23. We have furthermore assigned 398 lines in the far infrared or Terahertz range (0.75–2.10 THz or 25–70 cm−1) including transitions with rotational quantum numbers up to J = 59. The results are discussed in relation to the possible first astrophysical observation of an isotopically chiral molecule and in relation to molecular parity violation.
Deuterated molecules have been detected in several interstellar objects.1,2 Interestingly, the D/H ratio for some of the interstellar molecules is much higher than the galactic value. An interesting candidate for astrophysical observation to help further understanding of this effect is monodeutero-oxirane (c-C2H3DO) because this molecule is isotopically chiral3–5 (Fig. 1 shows the absolute configurations of the enantiomers of c-C2H3DO). The undeuterated parent species, oxirane or ethylene oxide (c-C2H4O), is a small cyclic molecule. Oxirane is an isomer of acetaldehyde (CH3CHO), which is more stable by about 1 eV or 100 kJ mol−1. Oxirane is kinetically stable at ordinary temperatures in the laboratory. Oxirane was first detected in Sgr B2N6 and then in several other hot-core sources,7 several other environments8 and the central molecular zone in our galactic center.9 It might be also a candidate as a carrier of some unidentified infrared bands.10 The relatively high abundance of oxirane in interstellar space indicates that monodeutero-oxirane may become the first isotopically chiral molecule to be detected by astrophysical observations.
Fig. 1 Structures of the (S)- and (R) monodeutero-oxirane (top, carbon atoms are labelled in grey, hydrogen light grey, oxygen red and deuterium violet). Axes definition in the principal inertial axis system is shown at the bottom. The c(z)-axis is perpendicular to the a(x)b(y) plane pointing toward the observer.
The current study aims to significantly extend the range of laboratory spectroscopy to support the astrophysical observation of monodeutero-oxirane by a combined GHz and THz analysis of its pure rotational spectrum. We report the measurement and analysis of the observed spectra of C2H3DO in the vibrational ground state in the ranges of 62 to 120 GHz and 25 to 70 cm−1 (0.75–2.10 THz). Some preliminary results of the present project were reported in ref. 25–27.
The synthesis has been reported in ref. 25 in more detail. In brief, in a protective atmosphere, vinyl magnesium chloride solution in THF was quenched with deuterium oxide and monodeuteroethene was introduced into a freshly prepared solution of acetylhypobromide (from silver acetate and bromine) in anhydrous tetrachloromethane. After completion of the reaction, the solution was washed with 10% sodium hydrogen sulphate, and dried over magnesium sulphate. The solvent was removed. The crude product (1-acetoxy-2-bromo[2H]ethane) was distilled under vacuum (60 °C at 30 Torr). 1-Acetoxy-2-bromo[2H]ethane and 25% sodium hydroxide in solution were combined in a flask equipped with a condenser filled with a molecular sieve (3 Å) and a nitrogen gas inlet. The flask was heated to 100 °C and monodeutero-oxirane was condensed in a cryogenic trap. The product was redistilled at 10.7 °C. The product was identified by NMR and mass spectrometry and the identity and purity were also obvious from the GHz and THz spectra, which also showed some methylene chloride CH2Cl2 present as an impurity, which could be clearly identified and thus does not negatively affect the analysis.
The rotational spectra of monodeutero-oxirane in the GHz region were measured using our Zurich GHz spectrometer described in detail recently.28,29 It consists of an Agilent synthesizer (E8257D PSG, Agilent Technologies Option 520 UNX) working in the range 250 kHz to 20 GHz with a frequency resolution of 0.001 Hz and frequency locked to a 10 MHz-rubidium standard (FS725, Stanford Research Systems) for short-term stability and to a GPS standard (TM-4, Spectrum Instruments Inc.) for long-term stability leading to a frequency uncertainty of only Δν/ν = 10−11. The signal is frequency multiplied by a factor of 6 (S10MS-AG, OML Inc.) and coupled out to free space via a conical W-band antenna (QWH-WCRR, QuinStar Technology Inc.) leading to an operation in the frequency range of 67 to 118 GHz. The beam is focused through Teflon lenses into a 2.5 m cell equipped with two Teflon windows and mounted in a Brewster angle to the incoming and outcoming beam. Finally, the beam is detected using a two-channel indium antimonide (InSb) bolometer [QFI/2(2), QMC Instruments Ltd] after passing a second Teflon lens. All transitions were collected using the frequency modulation technique (second harmonic lock-in detection) with a modulation frequency of 50 kHz. The final combined uncertainty in the line frequencies is estimated to be about 200 kHz.
The high sensitivity of this setup has been demonstrated in our recent high resolution spectroscopic studies of 1,2-dithiine29,30 and deuterated phenols.31 The survey spectra were recorded first between 69 and 113 GHz in 3 GHz segments at p = 25 μbar, followed by subsequent close examinations of each individual line by choosing a narrow bandwidth (∼5 MHz) with longer integration time for the best signal-to-noise (S/N) ratios. This procedure allowed for measurements of weaker transitions. Eventually, with better predictions based on an improved effective Hamiltonian, the measurement of individual lines was extended to 62 GHz (lower limit) and 120 GHz (upper limit).
The spectrometer is connected via transfer optics to the switchyard of the infrared port at the SLS, consisting of one parabolic, two toroidal and one flat mirror. The toroidal mirrors are necessary due to the strong astigmatism of the synchrotron beam. It has the shape of a curved 3D filament. A parallel beam entering the source chamber of the spectrometer is focused onto the aperture using a parabolic mirror with focal length 41.8 cm, yielding a diameter as small as 0.5 mm, and passes on into the interferometer. The Maximum Optical Path Differences (MOPD) of dMOPD = 11.7 m leads to a best possible unapodized instrumental bandwidth of 0.00052 cm−1 or 16 MHz (Full-Width at Half-Maximum, FWHM). This spectrometer is a further development of our IFS 120/125 Prototype 2001.32,34,35 The rotational spectra in the range 25 to 70 cm−1 (0.75–2.10 THz) were recorded with the maximum dMOPD of 11.7 m. A 6 μm Mylar beamsplitter, an aperture of 3.15 mm and a liquid He-cooled bolometer were used for the measurements. The scanner velocity (modulation frequency) was 30 kHz. Several spectra with pressures between 0.3 and 10 mbar have been recorded. Fig. 2 shows an overview of the measured THz spectra of monodeutero-oxirane. All wavenumbers were calibrated with residual H2O vapor lines between 50 and 90 cm−1 as compared to values reported in the HITRAN database.36 Of course, neither the H2O impurity lines nor the other impurity lines (CH2Cl2) have any negative effects on the high resolution analysis of the monodeutero-oxirane spectrum, which was unambiguously assigned. As only a small and precious sample was available for this isotopomer, we decided against any attempts of further purification, which was not necessary for our analysis and would have jeopardized the sample.
Fig. 2 An overview of the THz spectrum of C2H3DO between 0.75 and 2.5 THz. Decadic absorbance lg(I0/I) is shown where the cutoff at maximum absorbance has the value of 2, pathlength l = 10 m, pressure = 0.8 mbar and temperature T = 295 K. The insert in the top-left corner shows an example of the typical line with full-width at half-maximum of about 0.0006 cm−1 in the measured range.
In general, wavenumbers measured in this spectral range are estimated to be accurate to within about 0.0001 cm−1. The data given in the line lists of the ESI† correspond to calibrated line maxima. The insert in Fig. 2 shows a typical linewidth (FWHM) of 0.0006 cm−1. The Doppler width is about 0.00009 cm−1 at 50 cm−1, indicating that the resolution is instrument limited. The measured line widths indicate the absence of significant pressure broadening in agreement with expected widths from pressure broadening (less than about 10−4 cm−1 for the conditions of Fig. 2).
As a three-membered heterocyclic molecule, the heavy atom skeleton of the ground state structure of monodeutero-oxirane is co-planar. Based on an early microwave study of C2H4O by Gwinn and co-workers, which showed one electric dipole moment component in the molecular plane μb = 1.88 ± 0.01 D using Stark effect measurements,11 we expect a similar μb in C2H3DO and the components μa and μc due to the deuterium substitution to be nonzero but quite small in absolute value (see, for instance, CH3D,40 CD3H41 and CH2D242 for comparison). The principal axis system for C2H3DO is given in Fig. 1.
The assignment of the GHz spectra was assisted by the previously reported rotational constants in ref. 12, which were based on the analysis of 20 transitions with Jmax = 8 and Kc,max = 4. 112 distinct lines obeying b-type selection rules for KaKc quantum numbers (oe ↔ eo and ee ↔ oo)43,44 were assigned using the PGOPHER program45,46 in the range of 62–120 GHz with Jmax = 23 and Kc,max = 14. The newly assigned transitions were fitted separately and the resulting rotational parameters are given in Table 1. The root-mean-square deviation drms of the fit is less than 30 kHz, with the typical linewidths (FWHM) being about 200 kHz. The rotational constants agree with those obtained previously using microwave spectroscopy.12 The centrifugal distortion constants are determined here for the first time. Subsequently, this enlarged set of improved spectroscopic parameters led to a reasonably good prediction for the pure rotational spectra with higher J value transitions in the THz region. After further transitions were added to the assigned line list of the GHz spectra and fitted, the simulated spectrum was refined leading to the assignment of more transitions. This iterative process continued until all the stronger transitions had been accounted for. An additional 398 distinct lines were assigned between 0.75 and 2.10 THz (25 to 70 cm−1) and co-fitted with transitions assigned in the GHz region as well as MW lines reported by ref. 12. The resulting spectroscopic parameters of the global fit are presented in Table 2.
a Values in parentheses indicate uncertainties as 1σ in units of the last digits given. Rotational constants without parenthesis were held fixed at the values of the ground state obtained by fitting only transitions in the MW and GHz region. b Values from ref. 22. c Fixed at the ab initio estimate from ref. 22.
The assignment and analysis of the pure rotational spectra measured in the GHz region over a range of ∼58 GHz provide a much broader coverage of energy levels than the earlier microwave study.12 This allowed for a new and accurate determination of centrifugal distortion constants of the ground state of monodeutero-oxirane, not available previously, in addition to more accurate rotational constants. The small drms ≈ 30 kHz of the fit using only three rotational constants and five centrifugal distortion constants suggests that the effective Hamiltonian employed here provides an accurate description of the observed spectra in this range. This success in modelling is evident in the comparison between experimental and simulated GHz spectra shown in Fig. 3 using the spectroscopic parameters in Table 1. Fig. 4 shows a comparison of simulations based on the early microwave work12 and our new analysis. Clearly, the prediction based on a model Hamiltonian with only three rotational constants cannot be extrapolated to guide field observations beyond the measurement range. In this case, the extended spectral range was vital in providing more accurate predictions of line frequencies, especially those with high J and Kc values which are more sensitive to centrifugal distortion constants.
Fig. 4 A comparison of the GHz spectrum of C2H3DO (from top to bottom): (1) observed spectrum; (2) simulated spectrum based on early MW work in ref. 12; (3) calculated (unscaled) spectrum (CCSD(T)/cc-pVTZ) in ref. 22; (4) calculated (scaled) spectrum (CCSD(T)/cc-pVTZ) in ref. 22; (5) simulated spectrum with our constants. The quantum numbers of the transitions are arranged in the form of .
For the assignment of the THz spectra, which consist of transitions with higher J and Kc values, additional higher order centrifugal constants up to the sextic terms were used for the convergence of the fit. The global fit of combined MHz, GHz and THz transitions has a drms of only 0.000079 cm−1 (∼2 MHz). This small drms is yet another illustration of the excellent agreement between the observed spectra and the effective Hamiltonian employed. A comparison of the experimental THz spectra and the simulation based on spectroscopic parameters reported in Table 2 is presented in Fig. 5. Although most simulated spectral lines, if not all, are accounted for in both the GHz and THz spectra, as shown in Fig. 3 and 5, it can be seen that a few lines still remain unassigned, especially in the THz range. We first noticed these extra lines in the GHz spectra, where clusters of lines with different linewidths are present. The patterns look like hyperfine splittings from nuclear quadrupole couplings which are expected to be negligible in C2H3DO at this resolution. Fig. 3 shows an example of such line structures. Furthermore, Fig. 6 shows that the simulated spectra of methylene chloride (CH235Cl2) match many unassigned lines in the experimental THz spectra well. Thus, we concluded that these extra lines are due to contaminants including at least dichloromethane, and have no spectroscopic implications other than causing spectral overlapping and, in some cases, congestion. Interestingly, having impurities in our experimental sample has demonstrated the power of high resolution spectroscopy in providing unambiguous identification of molecules in a mixture, which is the cornerstone of remote sensing.
Fig. 5 A comparison of observed and simulated rotational spectra of C2H3DO in the THz region. Decadic absorbance lg(I0/I) is shown where the cut-off at maximum absorbance has the value of 0.9, pathlength l = 10 m, pressure = 0.8 mbar and temperature = 295 K.
Fig. 6 A comparison of (from top to bottom): (1) observed THz spectrum; (2) simulated C2H3DO spectrum; (3) simulated CH235Cl2 spectrum. Decadic absorbance lg(I0/I) is shown where the cut-off at maximum absorbance has the value of 0.55, pathlength l = 10 m, pressure = 0.8 mbar and temperature = 295 K.
The recent ab initio calculation by Puzzarini et al.22 provided ground state spectroscopic parameters at the CCSD(T)/cc-pVTZ level. In general, the calculated parameters are close to experimental values determined here (see Table 2). In fact, the rotational constants were calculated with a relative deviation of less than 0.15%. This small uncertainty is, indeed, an excellent achievement in terms of ab initio theory by experts in the field. However, it would not be sufficient in spectroscopy at high resolution when it comes to a desired “line-by-line” assignment in searches in the field, in which the signal of molecules of interest is often mixed with the signal associated with other components of the mixture being examined. Fig. 4 shows that not only are the transitions shifted by about 100 MHz at 104 GHz but in addition, the simulated spectrum also failed to reproduce the rotational structure of C2H3DO, upon which field assignments strongly rely. This would add ambiguity to the assignment effort, particularly when congested, complicated spectra are being examined. Puzzarini et al. in ref. 22 further scaled the calculated parameters using the theoretical best estimate and experimental data for the main isotope. The scaling factor improved the calculation considerably and this can be seen in Fig. 4: two strong lines 127 6–126 7 and 116 6–115 7 are within 5 MHz of the predictions, however, transitions with higher Ka values are still off by more than 100 MHz. On the other hand, the fact that the scaling factor obtained from the normal oxirane works reasonably well on the mono-deuterated species is hardly surprising, because this highly strained three-membered heterocycle is expected to be rigid (as opposed to non-rigid in the sense of Longuet–Higgins3,51) and the deuterium substitution leads to a minor change of rotational constants only.
We enjoyed help and support from as well as discussions with Irina Bolotova, Hans Hollenstein, Carine Manca Tanner, Roberto Marquardt, Frédéric Merkt, Georg Seyfang and Jürgen Stohner. Our research is funded by ETH Zürich, in particular the Laboratory of Physical Chemistry, as well as a presidential grant, the Swiss National Science Foundation, an ERC Advanced Grant and the COST project MOLIM. K. K. thanks the Marie Heim-Vögtlin Program of the SNSF for financial support. The research leading to these results has in particular also received funding from the European Union's seventh Framework Program (FP7/2007-2013) ERC grant agreement no. 290925 [SPEQUACHIRAL2].
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