Ab Initio Study of the Large Amplitude Motions of Various Monosubstituted Isotopologues of Methylamine (CH3-NH2)

CCSD(T)-F12 theory is applied to determine electronic ground state spectroscopic parameters of various isotopologues of methylamine (CH3-NH2) containing cosmological abundant elements, such as D, 13C and 15N. Special attention is given to the far infrared region. The studied isotopologues can be classified in the G12, G6 and G4 molecular symmetry groups. The rotational and centrifugal distortion constants and the anharmonic fundamentals are determined using second order perturbation theory. Fermi displacements of the vibrational bands are predicted. The low vibrational energy levels corresponding to the large amplitude motions are determine variationally using a flexible three-dimensional model depending on the NH2 bending and wagging and the CH3 torsional coordinates. The model has been defined assuming that, in the amine group, the bending and the wagging modes interact strongly. The vibrational levels split into six components corresponding to the six minima of the potential energy surface. The accuracy of the kinetic energy parameters has an important effect on the energies. Strong interactions among the large amplitude motions are observed. Isotopic effects are relevant for the deuterated species.

Non-relativistic eigensolutions of molecular and heavy quarkonia interacting potentials via the Nikiforov Uvarov method

In this paper, the analytical eigensolutions of the radial Schrödinger equation are obtained using the Nikiforov Uvarov method. Special cases of the modeled potential were discussed. The energy eigenvalues expressions for both quarkonia and diatomic molecular interacting systems are obtained. Furthermore, the masses of the charmonium and bottomonium mesons were computed. The vibrational energy levels, inertia rotational constant, and first two centrifugal distortion constants of the Kratzer–Fues oscillator were obtained in closed form. The results obtained are in excellent agreement with the results in the literature.

Millimeter- and submillimeter-wave spectroscopy of thioformamide and interstellar search toward Sgr B2(N)

Context. Thioformamide NH2CHS is a sulfur-bearing analog of formamide NH2CHO. The latter was detected in the interstellar medium back in the 1970s. Most of the sulfur-containing molecules detected in the interstellar medium are analogs of corresponding oxygen-containing compounds. Therefore, thioformamide is an interesting candidate for a search in the interstellar medium. Aims. A previous study of the rotational spectrum of thioformamide was limited to frequencies below 70 GHz and to transitions with J ≤ 3. The aim of this study is to provide accurate spectroscopic parameters and rotational transition frequencies for thioformamide to enable astronomical searches for this molecule using radio telescope arrays at millimeter wavelengths. Methods. The rotational spectrum of thioformamide was measured and analyzed in the frequency range 150−660 GHz using the Lille spectrometer. We searched for thioformamide toward the high-mass star-forming region Sagittarius (Sgr) B2(N) using the ReMoCA spectral line survey carried out with the Atacama Large Millimeter/submillimeter Array. Results. Accurate rigid rotor and centrifugal distortion constants were obtained from the analysis of the ground state of parent, 34S, 13C, and 15N singly substituted isotopic species of thioformamide. In addition, for the parent isotopolog, the lowest two excited vibrational states, v12 = 1 and v9 = 1, were analyzed using a model that takes Coriolis coupling into account. Thioformamide was not detected toward the hot cores Sgr B2(N1S) and Sgr B2(N2). The sensitive upper limits indicate that thioformamide is nearly three orders of magnitude at least less abundant than formamide. This is markedly different from methanethiol, which is only about two orders of magnitude less abundant than methanol in both sources. Conclusions. The different behavior shown by methanethiol versus thioformamide may be caused by the preferential formation of the latter (on grains) at late times and low temperatures, when CS abundances are depressed. This reduces the thioformamide-to-formamide ratio, because the HCS radical is not as readily available under these conditions.

Propargylimine in the laboratory and in space: millimetre-wave spectroscopy and its first detection in the ISM

Context. Small imines containing up to three carbon atoms are present in the interstellar medium (ISM). As alkynyl compounds are abundant in this medium, propargylimine (2-propyn-1-imine, HC ≡C−CH =NH) thus represents a promising candidate for a new interstellar detection. Aims. The goal of the present work is to perform a comprehensive laboratory investigation of the rotational spectrum of propargylimine in its ground vibrational state in order to obtain a highly precise set of rest frequencies and to search for it in space. Methods. The rotational spectra of E and Z geometrical isomers of propargylimine have been recorded in the laboratory in the 83–500 GHz frequency interval. The measurements have been performed using a source-modulation millimetre-wave spectrometer equipped with a pyrolysis system for the production of unstable species. High-level ab initio calculations were performed to assist the analysis and to obtain reliable estimates for an extended set of spectroscopic quantities. We searched for propargylimine at 3 mm and 2 mm in the spectral survey of the quiescent giant molecular cloud G+0.693-0.027 located in the central molecular zone, close to the Galactic centre. Results. About 1000 rotational transitions have been recorded for the E- and Z-propargylimine, in the laboratory. These new data have enabled the determination of a very accurate set of spectroscopic parameters including rotational, quartic, and sextic centrifugal distortion constants. The improved spectral data allowed us to perform a successful search for this new imine in the G+0.693-0.027 molecular cloud. Eighteen lines of Z-propargylimine were detected at level >2.5σ, resulting in a column-density estimate of N = (0.24 ± 0.02) × 1014 cm−2. An upper limit was retrieved for the higher energy E isomer, which was not detected in the data. The fractional abundance (with respect to H2) derived for Z-propargylimine is 1.8 × 10−10. We discuss the possible formation routes by comparing the derived abundance with those measured in the source for possible chemical precursors.

Internal methyl rotation and molecular structure of trifluorotoluenes: microwave rotational spectra of 2,3,4- and 2,4,5-trifluorotoluene

The microwave rotational spectra of 2,3,4- and 2,4,5-trifluorotoluenes, along with all 13C isotopic species in natural abundance, have been recorded in the frequency range 8–27 GHz employing pulsed-jet Fourier transform microwave spectroscopy. The analysis of the spectra in the lowest torsional state has yielded the rotational constants, centrifugal distortion constants, three-fold barrier to methyl rotation, and the direction of the internal rotation axis in the moment of inertia principal axes systems of these trifluorotoluenes. For both molecules, the molecular constants of their eight isotopologues have been used to obtain the substitution rs structures of the ring and the methyl-carbon. The potential barriers hindering the internal rotation of the methyl top in 2,3,4- and 2,4,5-trifluorotluene are 2.5878(80) and 2.2809(23) kJ/mol, respectively.

Rotational spectroscopy and astronomical search for glutaronitrile

Context. Nitriles constitute almost 15% of the molecules observed in the interstellar medium (ISM), surprisingly only two dinitriles have been detected in the ISM so far. The lack of astronomical detections for dinitriles may be partly explained by the absence of laboratory rotational spectroscopic data. Aims. Our goal is to investigate the rotational spectrum of glutaronitrile, N≡C−CH2−CH2−CH2−C≡N, in order to allow its possible detection in the ISM. Methods. The rotational spectrum of glutaronitrile was measured using two different experimental setups. A Fourier transform microwave spectrometer was employed to observe the supersonic jet rotational spectrum of glutaronitrile between 6 and 20 GHz. In addition, the mmW spectrum was observed in the frequency range 72−116.5 GHz using a broadband millimetre-wave spectrometer based on radio astronomy receivers with fast Fourier transform backends. The spectral searches were supported by high-level ab initio calculations. Results. A total of 111 rotational transitions with maximum values of J and Ka quantum numbers 54 and 18, respectively, were measured for the gg conformer of glutaronitrile. The analysis allowed us to accurately determine the rotational, nuclear quadrupole coupling, quartic and sextic centrifugal distortion constants. These rotational parameters were employed to search for glutaronitrile in the cold and warm molecular clouds Orion KL, Sgr B2(N), B1-b and TMC-1, using the spectral surveys captured by IRAM 30 m at 3 mm. Glutaronitrile was not detected, and the upper limits’ column densities were derived. Those are a factor of 1.5 and 5 lower than those obtained for the total column densities of the analogous succinonitrile in Orion KL and Sgr B2, respectively.

Investigation of amino acetonitrile (NH2CH2CN) — a precursor of glycine, in the interstellar medium

Known values of rotational and centrifugal distortion constants, in conjunction with electric dipole moment, have been used to calculate energies for the ground rotational levels, ground vibrational state, and ground electronic state of amino acetonitrile (NH2CH2CN), which is of interest for studies of life in the universe. Probabilities for radiative transitions between the rotational levels are also calculated. Such transition probabilities are used in conjunction with scaled values for the collisional rate coefficients for large velocity gradient calculations. A line 110–111 at 450.31 MHz is found to show anomalous absorption. Two observed lines 909–808 at 80.947 GHz and 120,12–110,11 at 107.283 GHz, and another nine lines, 918–817, 716–615, 817–716, 919–818, 1019–918, 808–707, 100,10–909, 110,11–100,10, and 707–606, are found to show emission features. There are several other observed lines, which are found weaker than these 11 lines. These 10 additional lines may help in the identification of amino acetonitrile in the interstellar medium.