Thérèse R. Huet

Progress toward the first observation of parity violation in chiral molecules by high-resolution laser spectroscopy†

Parity violation (PV) effects in chiral molecules have so far never been experimentally observed. To take up this challenge, a consortium of physicists, chemists, theoreticians, and spectroscopists has been established and aims at measuring PV energy differences between two enantiomers by using high-resolution laser spectroscopy. In this article, we present our common strategy to reach this goal, the progress accomplished in the diverse areas, and point out directions for future PV observations. The work of André Collet on bromochlorofluoromethane (1) enantiomers, their synthesis, and their chiral recognition by cryptophanes made feasible the first generation of experiments presented in this article.

Rotational spectrum of 13C2-methyl formate (HCOO13CH3) and detection of the two 13C-methyl formate in Orion

Context. Laboratory measurements and analysis of the microwave and millimeter-wave spectra of potential interstellar molecules are a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis provides spectroscopic parameters that are used in the assignment procedure of the laboratory spectra, and that also predict the frequencies of transitions not measured in the laboratory with a high degree of precision. Aims. An experimental laboratory study and its theoretical analysis is presented for 13 C2-methyl formate (HCOO 13 CH3) allowing a search for this isotopologue in the Orion molecular cloud. The 13 C1-methyl formate (H 13 COOCH3) molecule was also searched for in this interstellar cloud, using previously published spectroscopic data. Methods. The experimental spectra of 13 C2-methyl formate were recorded in the microwave and sub-mm energy ranges (4–20 GHz, 8–80 GHz, 150–700 GHz). The spectra were analyzed using the Rho-Axis Method (RAM), which takes the CH3 internal rotation and the coupling between internal rotation and global rotation into account. Results. Twenty-seven spectroscopic constants of 13 C2-methyl formate have been obtained from a fit of 936 transitions of the ground torsional state with a standard (unitless) deviation of 1.08. A prediction of line positions and intensities is also produced. This prediction allowed us to identify 230 13 C2-methyl formate lines in the Orion interstellar molecular cloud. We refitted all previously published ground state transitions of the 13 C1-methyl formate molecule in order to provide a prediction of its ground state spectrum. 234 lines of 13 C1-methyl formate were detected in the Orion interstellar cloud using that prediction.

Rotational Spectrum and Tentative Detection of DCOOCH3-Methyl Formate in Orion

New centimeter-wave (7-80 GHz) and submillimeter-wave (580-661 GHz) spectra of a deuterated species of methyl formate (DCOOCH3) have been measured. Transitions with a maximum value of J = 64 and K = 36 have been assigned and fitted together with previous measurements. The internal rotation of this compound was treated using the so-called rho axis method. A total of 1703 transitions were fitted using this method. Only 24 parameters were employed in the final fit, which has an rms deviation of 94.2 kHz. The dipole moment and the nuclear quadrupole coupling constants of the deuterated specie have also been obtained. This new study has permitted a tentative detection of DCOOCH3 in Orion with the IRAM 30 m telescope based on the observation of more than 100 spectral features with low blending effects among the 400 lines expected in the observed frequency domain (for which over 300 are heavily blended with other species). These 100 transitions are above noise and confusion limited without heavy blending and cannot be assigned to any other species. Moreover, none of the strongest unblended transitions is missing. The derived source-averaged total column density for DCOOCH3 is 7.8 × 1014 cm–2 and the DCOOCH3/HCOOCH3 column density ratio varies between 0.02 and 0.06 in the different cloud components of Orion. This value is consistent with the deuteration enhancement found for other species in this cloud.

Microwave and submillimeter spectroscopy and first ISM detection of 18O-methyl formate

Context. Astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict transition frequencies that were not measured in the laboratory very precisely. Aims. We present an experimental study and a theoretical analysis of two 18 O-methyl formate isotopologues, which were subsequently detected for the first time in Orion KL. Methods. The experimental spectra of both methyl formate isotopologues recorded in the microwave and sub-mm range from 1 to 660 GHz. Both spectra were analysed by using the rho-axis method (RAM) which takes into account the CH3 internal rotation. Results. We obtained spectroscopic constants of both 18 O- methyl formate with high accuracy. Thousands of transitions were assigned and others predicted, which allowed us to detect both species in the IRAM 30 m line survey of Orion KL.

Chlorofluoroiodomethane as a potential candidate for parity violation measurements

CHFClI is among the more favorable molecules for parity violation (PV) measurements in molecules. Despite the fact that calculated PV effects are two orders of magnitude smaller than in some organometallic compounds, CHFClI displays interesting features which could make possible a new experimental PV test on this molecule. Indeed, ultrahigh resolution spectroscopy using an ultrastable CO(2) laser is favored by several intrinsic properties of this molecule. For example, the high vapor pressure of CHFClI allows investigation by supersonic beam spectroscopy. Indeed, the spectroscopic constants have been accurately determined by microwave and millimetre wave spectroscopy. This is important for the subsequent selection of an appropriate absorption band of CHFClI that could be brought to coïncide with the absorption of CO(2). Partially resolved (+)- and (-)-CHFClI enantiomers with respectively 63.3 and 20.5% ees have been recently prepared and analyzed by molecular recognition using chiral hosts called cryptophanes. Finally, the S-(+)/R-(-) absolute configuration was ascertained by vibrational circular dichroïsm (VCD) in the gas phase.

Conformational relaxation of S-(+)-carvone and R-(+)-limonene studied by microwave Fourier transform spectroscopy and quantum chemical calculations

S-(+)-carvone (C10H14O, 5-isopropenyl-2-methylcyclohex-2-en-1-one) and R-(+)-limonene (C10H16, 4-isopropenyl-1-methylcyclohexene) have been characterized in the gas phase using a Fourier transform microwave spectrometer coupled to a supersonic molecular beam. Two conformers—with the isopropenyl group in the equatorial position—have been detected for each compound and described by a set of molecular parameters including the principal rotational constants and the quartic centrifugal distortion parameters. Quantum chemical calculations indicate that a third conformer might not be observed due to relaxation processes in the jet. The gas phase results are compared with the liquid phase IR-Raman-VCD spectra.

The far infrared spectrum of naphthalene characterized by high resolution synchrotron FTIR spectroscopy and anharmonic DFT calculations

Using synchrotron radiation, we performed the rotationally resolved Fourier transform infrared absorption spectroscopy of three bands of naphthalene C10H8, namely ν(46)-0 (centered at 782 cm(-1), 12.7 μm), ν(47)-0 (centered at 474 cm(-1), 21 μm), and ν(48)-0 (centered at 167 cm(-1), 60 μm). The intense CH bending out of plane ν(46)-0 band was recorded under supersonic jet-cooled conditions using a molecular beam (the Jet-AILES apparatus) and the low frequency ν(47)-0 and ν(48)-0 bands were measured at room temperature in a long absorption path cell. The simultaneous rotational analysis of these bands permitted us to refine the ground state (GS) and ν(46) rotational spectroscopic constants and to provide the first sets of constants for the ν(47) and ν(48) modes. The experimental rotational constants were then used as reference data to calibrate theoretical models in order to provide new insights into the accuracy of anharmonic calculations. The B97-1 functional associated with the cc-pVTZ and ANO-RCC basis sets gave a consistent set of results, for rotational constants and fundamental frequencies. The data presented here pave the way for the search of naphthalene through its far-infrared spectrum in different objects of the interstellar medium.

Magnetic hyperfine coupling of a methyl group undergoing internal rotation: a case study of methyl formate.

The hyperfine structure of methyl formate was recorded in the 2-20 GHz range. A molecular beam coupled to a Fourier transform microwave spectrometer having an instrumental resolution of 0.46 kHz and limited by a Doppler width of a few kHz was used. A-type lines were found split by the magnetic hyperfine coupling while no splittings were observed for E-type lines. Symmetry considerations were used to account for the internal rotation of the methyl top and to derive effective hyperfine coupling Hamiltonians. Neglecting the spin-rotation magnetic coupling, the vanishing splittings of the E-type lines could be understood and analyses of the hyperfine patterns of the A-type lines were performed. The results are consistent with a hyperfine structure dominated by the magnetic spin-spin coupling due to the three hydrogen atoms of the methyl group.

THz spectroscopy and first ISM detection of excited torsional states of 13C-methyl formate

Context. An astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range of organic molecules to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict the transitions frequencies very precisely that were not measured in the laboratory. Aims. We present the experimental study and its theoretical analysis for two 13 C-methyl formate isotopologues to detect these two isotopologues for the first time in their excited torsional states, which lie at 130 cm −1 (200 K) in Orion-KL. Methods. New spectra of HCOO 13 CH3 ( 13 C2) methyl formate were recorded with the mm- and submm-wave spectrometer in Lille from 50 to 940 GHz. A global fit for vt = 0 and 1 was accomplished with the BELGI program to reproduce the experimental spectra with greater accuracy. Results. We analysed 5728 and 2881 new lines for vt = 0 and 1 for HCOO 13 CH3. These new lines were globally fitted with 846 previously published lines for vt = 0. In consequence, 52 parameters of the RAM Hamiltonian were accurately determined and the value of the barrier height (V3 = 369.93168(395) cm −1 ) was improved. We report the detection of the first excited torsional states (vt = 1) in Orion-KL for the 13 C2 and 13 C1 methyl formate based on the present analysis and previously published data. We provide column densities, isotopic abundances, and vibrational temperatures for these species. Conclusions. Following this work, accurate prediction can be provided. This permits detecting 135 features of the first excited torsional states of 13 C-methyl formate isotopologues in Orion-KL in the 80−280 GHz frequency range, without missing lines.

13C-Methyl Formate: Observations of a Sample of High-mass Star-forming Regions Including orion-KL and Spectroscopic Characterization

This work was supported by the National Science Foundation under grant 1008800. We are grateful to the Ministerio de Economia y Competitividad of Spain for the financial support through grant No. FIS2011-28738-C02-02 and to the French Government through grant No. ANR-08-BLAN-0054 and the French PCMI (Programme National de Physique Chimie du Milieu Interstellaire). This paper makes use of the following ALMA data: ADS/JAO. ALMA#2011.0.00009.SV.ALMAis a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. C.F. thanks Dahbia Talbi, Eric Herbst, and Anthony Remijan for enlightening discussions. Finally, we thank the anonymous referee for helpful comments.