L. Margulès

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.

The equilibrium N–H bond length

The equilibrium C-H bond length has been determined up to now for about 40 polyatomic molecules. These data are used to demonstrate the existence of quantitative correlations betweenre(C-H), isolated C-H bond stretching frequency and average distancerg. It is also shown that ab initio calculations are often reliable to calculate the absolute value ofre(C-H), if an empirical correction is made. Some other correlations are also discussed. Finally, accuratere(C-H) values are predicted for simple molecules.

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.

Rotational spectrum of deuterated and

Context. Ethyl cyanide is an abundant molecule in hot molecular clouds. Its rotational spectrum is very dense and several hundreds of rotational transitions within the ground state have been identified in molecular clouds in the 40−900 GHz frequency range. Lines from 13 C isotopically substituted ethyl cyanide were identified in Orion. Aims. To enable the search and the possible detection of other isotopologues of ethyl cyanide in interstellar objects, we have studied the rotational spectrum of deuterated ethyl cyanide: CH2DCH2CN (in-plane and out-of-plane) and CH3CHDCN and the spectrum of 15 N substituted ethyl cyanide CH3CH2C 15 N. Using these experimental data, we have searched for these species in Orion. Methods. The rotational spectrum of each species in the ground state was measured in the microwave and millimeter-submillimeter wavelength range using a waveguide Fourier Transform spectrometer (8−17 GHz) and a source-modulated spectrometer employing backward-wave oscillators (BWOs) (150−260 and 580−660 GHz). More than 300 lines were identified for each species, for J values in the range 71−80 and Ka values in the range 28−31 depending on the isotopologues. The experimental spectra were analyzed using a Watson’s Hamiltonian in the A-reduction. Results. From the fitting procedure, accurate spectroscopic constants were derived for each of the species. These new sets of spectroscopic constants enable us to predict reliably the rotational spectrum (lines frequencies and intensities) in the 4−1000 GHz frequency range and for J and Ka up to 80 and 31, respectively. Combined with IRAM 30 m antenna observations of Orion, this experimental study allowed us to detect 15 N substituted ethyl cyanide CH3CH2C 15 N for the first time in Orion. The derived column density and rotational temperature are 10 13 cm −2 and 150 K for the plateau and 3 × 10 14 cm −2 and 300 K for the hot core. The deuterated species were searched for but were not detected. The upper limit to the column density of each deuterated isotopologues was 10 14 cm −2 .

\mathsf{^{15}}

Context. Ethyl cyanide is an abundant molecule in hot molecular clouds. Its rotational spectrum is very dense and several hundreds of rotational transitions within the ground state have been identified in molecular clouds in the 40−900 GHz frequency range. Lines from 13 C isotopically substituted ethyl cyanide were identified in Orion. Aims. To enable the search and the possible detection of other isotopologues of ethyl cyanide in interstellar objects, we have studied the rotational spectrum of deuterated ethyl cyanide: CH2DCH2CN (in-plane and out-of-plane) and CH3CHDCN and the spectrum of 15 N substituted ethyl cyanide CH3CH2C 15 N. Using these experimental data, we have searched for these species in Orion. Methods. The rotational spectrum of each species in the ground state was measured in the microwave and millimeter-submillimeter wavelength range using a waveguide Fourier Transform spectrometer (8−17 GHz) and a source-modulated spectrometer employing backward-wave oscillators (BWOs) (150−260 and 580−660 GHz). More than 300 lines were identified for each species, for J values in the range 71−80 and Ka values in the range 28−31 depending on the isotopologues. The experimental spectra were analyzed using a Watson’s Hamiltonian in the A-reduction. Results. From the fitting procedure, accurate spectroscopic constants were derived for each of the species. These new sets of spectroscopic constants enable us to predict reliably the rotational spectrum (lines frequencies and intensities) in the 4−1000 GHz frequency range and for J and Ka up to 80 and 31, respectively. Combined with IRAM 30 m antenna observations of Orion, this experimental study allowed us to detect 15 N substituted ethyl cyanide CH3CH2C 15 N for the first time in Orion. The derived column density and rotational temperature are 10 13 cm −2 and 150 K for the plateau and 3 × 10 14 cm −2 and 300 K for the hot core. The deuterated species were searched for but were not detected. The upper limit to the column density of each deuterated isotopologues was 10 14 cm −2 .

N ethyl cyanides: CH

The equilibrium structures of a few small molecules containing halogens: methyl halides, cyanogen halides, haloethynes, and halocyanoethynes (from chloride to iodide) are reviewed and redetermined if necessary using high-level ab initio calculations and experimental data from microwave and infrared spectroscopies. The correlation of the r(C–X) (X=Cl, Br, I) bond length with other properties (electronegativity or another bond length) is analyzed. It is shown that simple empirical correlations may be used to make a reliable prediction of the r(C–X) bond length.

\mathsf{_3}

Context. Formamide is the simplest bearer of peptide bond detected in the interstellar medium (ISM). Aims. There is still a lack of laboratory data on its rotational spectrum in the THz domain. Methods. We measured the rotational spectrum of formamide in the frequency range 400–950 GHz. The ground and first excited vibrational state of the normal species as well as the ground state of 13 C isotopic species were analysed. Results. The results obtained represent an extension by a factor of two in frequency range compared to previous studies. Of all transition frequencies in the dataset about 45% are new measurements. A reliable set of rotational constants allows accurate predictions of transition frequencies in the THz domain. Based on the spectroscopic results, the ν12 = 1 excited vibrational state of formamide was detected in the IRAM 30 m line survey of Orion KL for the first time in the ISM.

CHDCN and CH

The anharmonic force field up to quartic terms has been calculated for both CF2 and CCl2 at the CCSD(T) level of theory with large basis sets. The calculated spectroscopic parameters are in excellent agreement with the available experimental data. The equilibrium structure of CCl2 has been calculated at the CCSD(T) level of theory with the cc-pV5Z basis set (including core correlation corrections). It is in excellent agreement with the semi-experimental equilibrium structure derived from the experimental rotational constants and the ab initio rovibrational interaction parameters. The heats of formation of CF2 and CCl2 have been calculated using the W2 method and are −193.43 and 227.96 kJ mol−1, respectively.

\mathsf{_2}

Abstract The possibility of calculating accurate ab initio bond angles is examined using a sample of 29 molecules (35 independent angles) containing only first row atoms and whose equilibrium structures are known. Three different correlated methods are compared: MP2, CCSD(T), and DFT, using the hybrid functional B3LYP. The convergence of Dunnings correlation consistent polarized valence basis sets, cc-pVnZ is also studied. It is found that the CCSD(T) method is consistently the most accurate; the DFT/B3LYP being slightly less reliable than MP2. It is shown that when convergence of the basis set is achieved (which is dependent on the kind of bonding) and when the effect of diffuse functions on electronegative atoms is taken into account, a high accuracy may be obtained: 0.03° for the median of absolute deviations or 0.07° for the mean absolute deviation. It does not exclude the possibility that the ab initio method may fail in some particular case, for instance when a large amplitude motion is involved. The MP2/cc-pVQZ method gives a mean absolute deviation of 0.22° to be compared with the 0.07° of the CCSD(T) method. To obtain these results, it was necessary to reanalyze the structure of a few molecules, particularly, a new and more accurate structure is proposed for nitroxyl, HNO and hydrogen peroxide, H 2 O 2 .