Rotational Distortion Constants Research Articles

Pure rotational and rovibrational spectroscopy of cyclopropylamine in the far-infrared region: -NH2 torsion.

The pure rotational and rovibrational spectra of the ν27 -NH2 torsion of cyclopropylamine (CPA) in the far-infrared region were measured with a high-resolution Fourier transform infrared coupled to a synchrotron. The complex spectra reflect the presence of both trans and gauche conformers. Analysis of the pure rotational spectra (34-64cm-1) yielded accurate rotational and centrifugal distortion constants of the ground and first two torsional excited states of trans-CPA. The fundamental, hot bands and weak overtones were identified and assigned in the 200-550cm-1 range. Global analysis of over 19 000 transitions provides accurate energy levels of the torsional polyads up to vT = 3. The torsional levels and their rotational constants were in agreement with the theoretical results from quasiadiabatic channel reaction path Hamiltonian (RPH) calculations, emphasizing the need for molecular-specific theoretical treatments for large amplitude motions. Tunneling components of the torsional fundamental of gauche-CPA were assigned based on the RPH results and symmetry considerations, differing from previous experimental and theoretical work. This comprehensive spectroscopic characterization of CPA is crucial for its potential detection in the interstellar medium as a precursor to complex prebiotic molecules, providing essential data for future astronomical searches and advancing our understanding of nitrogen-containing organic molecules in space.

Millimeter to THz Spectroscopy of HC18O+ and HC17O+: Accurate Rest Frequencies for Astrophysical Studies

Heavy oxygen isotopic species of HCO+ are important optically thin astrophysical tracers. The ground-state rotational spectrum of HC18O+, DC18O+, HC17O+, and DC17O+ has been recorded in the laboratory in the frequency range from 85 GHz to 1.3 THz. The ions have been produced in the negative column of a glow-discharge plasma, and their spectrum has been recorded in absorption using a frequency-modulation submillimeter-wave spectrometer. Various sources of systematic error have been carefully accounted for in order to obtain highly accurate line-position measurements. Theoretical estimates of the molecular parameters and of the collision effects on the line shape have been obtained by high-level ab initio calculations. The analysis yielded much improved rotational and centrifugal distortion constants, thus bringing the spectroscopic characterization of these rare isotopic variants to the same level of the parent species. Also, the first experimental rotational data for DC17O+ have been provided. These results allow for the calculation of an updated set of rest frequencies to support current and future astrophysical studies. The derived data set for the widely used HC18O+ tracer reaches an accuracy of a few parts in 109 up to 1.5 THz. Such accuracy is important for the analysis of astrophysical objects targeted by Atacama Large Millimeter/submillimeter Array observations at the submillimeter regime.

Experimental observation of the ν1+3ν3 combination bands of 16O14N18O and 18O14N18O in the near infrared spectral region

The first observation of the ν1+3ν3 combination band of the nitrogen dioxide isotopologue 16O14N18O is presented. The band was measured using Fourier-Transform Incoherent Broad-Band Cavity Enhanced Absorption Spectroscopy (FT-IBBCEAS) in the region between 5870 cm−1 and 5940 cm−1. To confirm the assignment, the band was simulated using a standard asymmetric top Watson Hamiltonian using extrapolated rotational and centrifugal distortion constants. Furthermore, the first experimental observation of the ν1+3ν3 band of the 18O14N18O isotopologue is also reported. The positions of ro-vibrational lines of the ν1+3ν3 band of the naturally most abundant isotopologue 16O14N16O were used for wavenumber calibration of line positions.

Theoretical and Microwave Spectroscopic Characterization of Cyclobutenone: Planar or Puckered?

Cyclobutenone was characterized by high-resolution Fourier transform microwave spectroscopy for the first time. High-level, first-principles quantum chemical calculations at the B3LYP, CISD, MP2, and CCSD levels of theory were implemented to better understand the molecular structure and obtain model rotational and centrifugal distortion constants to aid in spectral assignment, and the results at the different levels of theory are compared. The assignment of the experimental spectrum provided fits of 2.7 kHz using Watson A-reduced and Watson S-reduced Hamiltonians. No tunneling splittings were observed, suggesting that cyclobutenone is not undergoing ring-puckering tunneling.

Inductive Effects on Intramolecular Hydrogen Bond Strength: An Investigation of the Effect of an Electron-Withdrawing CF3 Group Adjacent to an Alcohol Hydrogen Bond Donor.

This combined experimental and theoretical study seeks to determine the role that inductive effects have on hydrogen bonds by an investigation into the change in intramolecular hydrogen bond strength in 2-amino-1-trifluoromethylethanol (2ATFME) relative to that in 2-aminoethanol (2AE). Toward this end, the rotational spectra of the normal, 13C, and 15N isotopologues have been measured using Fourier transform microwave spectroscopy and fit to the rotational, quadrupole coupling, and centrifugal distortion constants of the Watson A-reduction Hamiltonian. Structural parameters used to characterize the strength of the intramolecular hydrogen bond have been determined from the experimental structures of both 2ATFME and 2AE as well as from MP2/6-311++G(d,p) calculations. A comparison of these parameters in 2ATFME with those of 2AE indicates that the electron-withdrawing trifluoromethyl CF3 group strengthens the hydrogen bond. These include a 4% decrease in the distance between the donor and acceptor heavy atoms of the hydrogen bond, a 6% increase toward linearity of the OH···N angle, and a 23% decrease of the COH···N torsional angle toward planarity in 2ATFME relative to 2AE. This trend toward increased intramolecular hydrogen bond strength in 2ATFME is also observed within the ab initio structures.

Thermodynamics and spectroscopic properties of C2H5CONH2 found in Sagittarius B2(N1)

Thermodynamics and spectroscopic properties of low-lying isomers of propionamide (C2H5CONH2) have been investigated employing density functional theory and high-level coupled-cluster (CC) method. Isomer 1 (Propionamide) is found to be the thermodynamically most stable, while 2 (N, N-dimethylformamide) and 3 (1-aminopropan-2-one) lie 0.63 and 0.79 eV, respectively above 1 at CCSD/cc-pVTZ level of theory. Spectroscopic parameters such as rotational and centrifugal distortion constants, inertial axis dipole moment components, net dipole moments have been calculated using ωB97XD/cc-pVQZ and ae-CCSD(T)/cc-pVTZ methods. Molecular properties such as ionization potential, electron affinity, chemical potential, electronegativity, global hardness and softness, electrophilicity index, etc. have also been evaluated at ae-CCSD(T)/cc-pVTZ level.

Time-Domain Rotational Raman Spectroscopy of the Ethylene Dimer and Trimer by Coulomb Explosion Imaging of Rotational Wave Packets.

We report rotational Raman spectroscopy of the ethylene dimer and trimer, based on time-resolved Coulomb explosion imaging of rotational wave packets. Rotational wave packets were created in the gas-phase ethylene clusters upon nonresonant ultrashort pulse irradiation. The subsequent rotational dynamics were traced as spatial distribution of monomer ions ejected from the clusters via the Coulomb explosion process induced by a strong probe pulse. The observed images of monomer ions show multiple kinetic energy components. The time-dependence of the angular distribution for each component was analyzed, and the Fourier transformation spectra, which correspond to rotational spectra, were obtained. A lower kinetic energy component was mainly attributed to a signal from the dimer and a higher energy component mainly from the trimer. We have successfully observed rotational wave packets up to a delay time of ∼20 ns and achieved a spectral resolution of 70 MHz after Fourier transformation. Owing to this higher resolution than the previous studies, improved rotational and centrifugal distortion constants were obtained from the spectra. In addition to improving the spectroscopic constants, this study opens the way for rotational spectroscopy of larger molecular clusters than dimers through Coulomb explosion imaging of rotational wave packets. Details of spectral acquisition and analyses of each kinetic energy component are also reported.

New rovibrational subbands of Ar-D<sub>2</sub>O complex in the D<sub>2</sub>O bending mode region

The intermolecular interactions involving the water molecule play important roles in many fields of physics, chemistry, and biology. High-resolution spectroscopy of Van der Waals complexes formed by a rare gas atom and a water molecule can provide a wealth of information about these intermolecular interactions. The precise experimental data can be used to test the accuracies and efficiencies of various theoretical methods of constructing the intermolecular potential energy surfaces and calculating the bound states. In this work, the high-resolution infrared absorption spectrum of the Ar-D<sub>2</sub>O complex in the <i>v</i><sub>2</sub> bending region of D<sub>2</sub>O is measured by using an external cavity quantum cascade laser. A segmented rapid-scan data acquisition method is employed. The Ar-D<sub>2</sub>O complex is generated in a slit supersonic jet expansion by passing Ar gas through a vessel containing liquid D<sub>2</sub>O. Four new rovibrational subbands are assigned in the spectral range of 1150–1190 cm<sup>–1</sup>, namely <inline-formula><tex-math id="M1">\begin{document}$\Sigma \left( {{0_{00}}, {v_2} = 1} \right) \leftarrow \Sigma \left( {{1_{11}}} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M1.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M2">\begin{document}$\Sigma \left( {{0_{00}}, {v_2} = 1} \right) \leftarrow \Pi \left( {{1_{11}}} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M2.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M3">\begin{document}$\Sigma \left( {{1_{01}}, {v_2} = 1} \right) \leftarrow \Pi \left( {{1_{10}}} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M3.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M4">\begin{document}$\Sigma \left( {{1_{01}}, {v_2} = 1} \right) $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M4.png"/></alternatives></inline-formula><inline-formula><tex-math id="Z-20221230153751">\begin{document}$\leftarrow \Pi \left( {{1_{01}}} \right) $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_Z-20221230153751.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_Z-20221230153751.png"/></alternatives></inline-formula>. The first two subbands belong to the <i>otho</i>- species of Ar-D<sub>2</sub>O, while the latter two belong to the <i>para</i>- species. The observed rovibrational transitions together with the previously reported pure rotational spectra having the common lower vibrational sub-states are analyzed by a weighted least-squares fitting using a pseudo-diatomic effective Hamiltonian. An experimental error of 10 kHz for the far-infrared transitions and 0.001 cm<sup>–1</sup> for the infrared transitions are set in the global fitting when using Pickett’s program SPFIT, respectively. The molecular constants including vibrational substate energy, rotational and centrifugal distortion constants, and Coriolis coupling constant, are determined accurately. The previous results for the <inline-formula><tex-math id="M5">\begin{document}$\Pi \left( {{1_{11}}, {v_2} = 0} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M5.png"/></alternatives></inline-formula> substate are found to be likely incorrect. The energy of the <inline-formula><tex-math id="M6">\begin{document}$\Sigma \left( {{0_{00}}, {v_2} = 1} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M6.png"/></alternatives></inline-formula>and <inline-formula><tex-math id="M7">\begin{document}$\Sigma \left( {{1_{01}}, {v_2} = 1} \right)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221728_M7.png"/></alternatives></inline-formula>substates are determined experimentally for the first time. The band origin of Ar-D<sub>2</sub>O in the D<sub>2</sub>O <i>v</i><sub>2</sub> bending mode region is determined to be 1177.92144(13) cm<sup>–1</sup>, which is a red shift about 0.458 cm<sup>–1</sup> compared with the head of D<sub>2</sub>O monomer. The experimental vibrational substate energy is compared with its theoretical value based on a four-dimensional intermolecular potential energy surface which includes the normal coordinate of the D<sub>2</sub>O <i>v</i><sub>2</sub> bending mode. The experimental and theoretical results are in good agreement with each other. But the calculated energy levels are generally higher than the experimental values, so, there is still much room for improving the theoretical calculations.

Laboratory spectroscopy of allylimine and tentative detection towards the G+0.693-0.027 molecular cloud

Context. Substituted methanimines and ethylenes have been identified in the interstellar medium. Therefore, allylimine (CH2 =CH–CH=NH) 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 allylimine in its ground vibrational state in order to obtain a highly precise set of rest frequencies to assist its search for astronomical sources. Methods. The rotational spectra of the two most stable trans–anti and trans–syn geometrical isomers of allylimine were recorded in the laboratory in the 84–300 GHz frequency interval. Measurements were 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 parameters. Guided by new laboratory data, allylimine was searched for in space using a sensitive spectral survey of the G+0.693-0.027 molecular cloud, located at the Galactic centre. Results. Almost 1000 rotational transitions have been recorded for trans–anti and trans–syn allylimine. These new data have enabled the determination of a very accurate set of spectroscopic parameters including rotational, quartic and sextic centrifugal distortion con-stants, as well as nuclear quadrupole coupling constants. The improved spectral data allowed us to report a tentative detection for both allylimine isomers in the G+0.693-0.027 molecular cloud, located at the Galactic centre.

Millimeter wave spectrum and search for vinyl isocyanate toward Sgr B2(N) with ALMA

Context. The interstellar detections of isocyanic acid (HNCO), methyl isocyanate (CH3NCO), and very recently also ethyl isocyanate (C2H5NCO) invite the question of whether or not vinyl isocyanate (C2H3NCO) can be detected in the interstellar medium. There are only low-frequency spectroscopic data (<40 GHz) available for this species in the literature, which makes predictions at higher frequencies rather uncertain, which in turn hampers searches for this molecule in space using millimeter (mm) wave astronomy. Aims. The aim of the present study is on one hand to extend the laboratory rotational spectrum of vinyl isocyanate to the mm wave region and on the other to search, for the first time, for its presence in the high-mass star-forming region Sgr B2, where other isocyanates and a plethora of complex organic molecules are observed. Methods. We recorded the pure rotational spectrum of vinyl isocyanate in the frequency regions 127.5–218 and 285–330 GHz using the Prague mm wave spectrometer. The spectral analysis was supported by high-level quantum-chemical calculations. On the astronomy side, we assumed local thermodynamic equilibrium to compute synthetic spectra of vinyl isocyanate and to search for it in the ReMoCA survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star-forming protocluster Sgr B2(N). Additionally, we searched for the related molecule ethyl isocyanate in the same source. Results. Accurate values for the rotational and centrifugal distortion constants are reported for the ground vibrational states of trans and cis vinyl isocyanate from the analysis of more than 1000 transitions. We report nondetections of vinyl and ethyl isocyanate toward the main hot core of Sgr B2(N). We find that vinyl and ethyl isocyanate are at least 11 and 3 times less abundant than methyl isocyanate in this source, respectively. Conclusions. Although the precise formation mechanism of interstellar methyl isocyanate itself remains uncertain, we infer from existing astrochemical models that our observational upper limit for the CH3NCO:C2H5NCO ratio in Sgr B2(N) is consistent with ethyl isocyanate being formed on dust grains via the abstraction or photodissociation of an H atom from methyl isocyanate, followed by the addition of a methyl radical. The dominance of such a process for ethyl isocyanate production, combined with the absence of an analogous mechanism for vinyl isocyanate, would indicate that the ratio C2H3NCO:C2H5NCO should be less than unity. Even though vinyl isocyanate was not detected toward Sgr B2(N), the results of this work represent a significant improvement on previous low-frequency studies and will help the astronomical community to continue searching for this species in the Universe.

Potential spectral lines of ethanimine (CH[formula omitted]CHNH) - an important possible precursor of amino acids

Accurate computational characterization of ethanimine (CH3NH), considered as an important possible precursor of amino acids, is now available. Ethanimine has two E and Z isomers and each isomer has two A and E substates. Some lines of Green Bank Telescope PRIMOS radio astronomy survey spectra in Sagittarius B2 North (Sgr B2(N)) have been assigned to the ethanimine. For known values of accurate energies of rotational levels in the ground vibrational state, and the spectroscopic data (rotational and centrifugal distortion constants, and electric dipole moment), we have calculated Einstein A and B coefficients (radiative transition probabilities) for rotational transitions for each substate. These radiative transition probabilities along with collisional rate coefficients are calculated using a scaling law have been used for solving a set of statistical equilibrium equations coupled with the equations of radiative transfer for each substate. We have found a number of weak MASER lines and anomalous absorption lines for each substate. These lines may play important role for identification of ethanimine in a cosmic object. Corresponding lines in two substates in a isomer have been found to show similar behaviour, except different values of their frequencies.

Disilicon carbide (Si2C) in the interstellar medium

The Si2C and SiC2 both are considered to play key role in the formation of the SiC dust grains in the atmosphere of the carbon-rich stars. The molecule of our interest Si2C has been detected in the envelope of the red supergiant star IRC+10216 first time. It is an asymmetric top molecule having electric dipole moment of 1 Debye along the b-axis of inertia. Because of zero nuclear spin of both the Carbon and Silicon atoms, it has only paratransitions. Using the given spectroscopic data (rotational and centrifugal distortion constants and electric dipole moment), for the para-Si2C, we have calculated energies of 200 lower rotational levels (having energy up to 217.8 cm-1) and the Einstein A and B coefficients for 867 radiative transitions between the levels. We have solved a set of 200 statistical equilibrium equations coupled with 867 equations of radiative transfer (Sobolev analysis), where the collisional rate coefficients are taken from a scaling law. Out of 867 radiative transitions, 13 transitions have been found showing weak MASER action, and 19 transitions showing anomalous absorption. One transition 808-717 is found to show both the MASER action as well as the anomalous absorption. These transitions in addition to the observed transitions may play important role in the identification of Si2C in the cosmic objects.

Rotational wave-packet imaging spectroscopy of the ethylene dimer

We report time-domain rotational spectroscopy of the ethylene dimer, (C2H4)2, by implementing time-resolved Coulomb explosion imaging. A molecular movie of rotational wave packets of the dimer has been captured with a delay time of up to 11 ns. Pure rotational spectrum of the cluster is obtained with frequency resolution of ∼ 120 MHz by Fourier transformation of the time trace of the alignment parameter. Rotational and centrifugal distortion constants are determined with much improved precision than the previous infrared studies. The present study demonstrates the capability of the newly developed time-domain approach in rotational spectroscopy of weakly bound nonpolar clusters.

A simple model for high rotational excitations of molecules in a superfluid

Recently it became possible to study highly excited rotational states of molecules in superfluid helium through nonadiabatic alignment experiments (Cherepanov et al 2021 Phys. Rev. A 104 L061303). This calls for theoretical approaches that go beyond explaining renormalized values of molecular spectroscopic constants, which suffices when only the lowest few rotational states are involved. As the first step in this direction, here we present a basic quantum mechanical model describing highly excited rotational states of molecules in superfluid helium nanodroplets. We show that a linear molecule immersed in a superfluid can be seen as an effective symmetric top, similar to the rotational structure of radicals, such as OH or NO, but with the angular momentum of the superfluid playing the role of the electronic angular momentum in free molecules. The simple theory sheds light onto what happens when the rotational angular momentum of the molecule increases beyond the lowest excited states accessible by infrared spectroscopy. In addition, the model allows to estimate the effective rotational and centrifugal distortion constants for a broad range of species and to explain the crossover between light and heavy molecules in superfluid 4He in terms of the many-body wavefunction structure. Some of the above mentioned insights can be acquired by analyzing a simple 2 × 2 matrix.

Energetics and Spectroscopic Properties of Low‐lying CaC6H2 Isomers: An Astrochemical Perspective

AbstractThe spectroscopic constants and molecular properties of low‐lying isomers of CaC6H2 have been studied using dispersion‐corrected density functional theory, MP2 and high‐level coupled‐cluster (CC) methods. 60 isomers have been identified, among which three low‐lying isomers having energy differences within 1 eV have been re‐optimized at MP2/cc‐pVTZ level. Isomer 1 (1‐calciacyclohepta‐4‐en‐2,6‐diyne) is found to be the most stable one, followed by isomer 3 (3‐calciahepta‐1,4,6‐triyne) and isomer 2 (1‐calciacyclohepta‐2,3,4‐trien‐6‐yne) that lie 20.9 and 19.7 kcal/mol above 1, respectively at MP2/cc‐pVTZ level. All three isomers are polar with a permanent electric dipole moment. Molecular properties like electron affinity, ionization potential as well as GCRD parameters, i. e., chemical potential, global hardness, etc. have been calculated at ae‐CCSD(T)/cc‐pVTZ level. NBO and AIM analyses have been performed to predict the nature of bonds in the isomers. The spectroscopic constants such as rotational and centrifugal distortion constants, inertial axis dipole moment components, etc. have been calculated using the frozen core (fc)‐CCSD(T) and all electron (ae)‐CCSD(T) methods with cc‐pVTZ basis. The ae‐CCSD(T) method with cc‐pwCVTZ‐X2C basis set has also been used to calculate spectroscopic constants. The predicted spectroscopic constants and molecular properties will facilitate in detecting these isomers in the laboratory as well as in interstellar medium (ISM).

Characterization of the Coriolis Coupled Far-Infrared Bands of syn-Vinyl Alcohol.

Rotational emission from vibrationally excited molecules are responsible for a large fraction of lines in the spectra of interstellar molecular clouds. Vinyl alcohol (VA) has two rotamers that differ in energy by 6.4 kJ/mol, both of which have been observed toward the molecular cloud, Sagittarius B2(N) [Turner and Apponi, Astrophys. J. 2001, 561, 207]. Previously, we reported an analysis of the far-infrared spectrum of the higher energy rotamer, anti-VA [Bunn et al. Astrophys. J. 2017, 847, 67], yielding rotational and higher order distortion constants in the first excited vibrational state, and here, we report an analysis of the far-infrared spectrum of the lower energy rotamer, syn-VA, whose spectrum is significantly more complicated on account of Coriolis interactions that result in perturbations to the rovibrational spectrum. We account for those perturbations with the inclusion of Coriolis coupling constants in the fit, which couples the first excited OH torsional (ν15) and CCO bending (ν11) states. Inclusion of them resulted in more physically meaningful rotational and centrifugal distortion constants, and allows for accurate pure rotational line predictions to be made up to high energies. These will be particularly useful in searches for vibrationally excited syn-VA toward warm regions of interstellar molecular clouds, where we predict that it may be significantly abundant.

Spectroscopic and Computational Characterization of 2-Aza-1,3-butadiene, a Molecule of Astrochemical Significance.

Being N-substituted unsaturated species, azabutadienes are molecules of potential relevance in astrochemistry, ranging from the interstellar medium to Titan’s atmosphere. 2-Azabutadiene and butadiene share a similar conjugated π system, thus allowing investigation of the effects of heteroatom substitution. More interestingly, 2-azabutadiene can be used to proxy the abundance of interstellar butadiene. To enable future astronomical searches, the rotational spectrum of 2-azabutadiene has been investigated up to 330 GHz. The experimental work has been supported and guided by accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the two possible forms, trans and gauche. The trans species, more stable by about 7 kJ/mol than gauche-2-azabutadiene, has been experimentally observed, and its rotational and centrifugal distortion constants have been obtained with remarkable accuracy, while theoretical estimates of the spectroscopic parameters are reported for gauche-2-azabutadiene.

Rotational spectrum of Isochroman

The ground state rotational spectrum of isochroman (3,4-dihydro-1H-2-benzopyran, C9H10O) has been studied by using a pulsed jet Fourier transform microwave spectrometer in the range of 5-20 GHz, complemented with theoretical calculations. Accurate spectroscopic parameters including rotational and centrifugal distortion constants were determined for the parent species and all mono-substituted 13C isotopologues in natural abundance, which allow the determination of the accurate geometry of the carbon skeleton. The semi-experimental structure was evaluated by taking into account the vibrational corrections calculated from the B3LYP-D3(BJ)/6-311++G(d,p) anharmonic force field. The electrostatic potential surface above the benzene ring with different saturated cycles (n = 3-7) has been calculated at the MP2/6-311++G(d,p) level, implying that the electron withdrawing effect of electronegative O and N atoms can be transferred from the adjacent saturated ring to the benzene ring.

Laboratory observation and astronomical search of 1-cyano propargyl radical, HCCCHCN

Context. The reaction between carbon atoms and vinyl cyanide, CH2CHCN, is a formation route to interstellar 3-cyano propargyl radical, CH2C3N, a species that has recently been discovered in space. The 1-cyano propargyl radical (HC3HCN), an isomer of CH2C3N, is predicted to be produced in the same reaction at least twice more efficiently than CH2C3N. Hence, HC3HCN is a plausible candidate to be observed in space as well. Aims. We aim to generate the HC3HCN radical in the gas phase in order to investigate its rotational spectrum. The derived spectroscopic parameters for this species will be used to obtain reliable frequency predictions to support its detection in space. Methods. The HC3HCN radical was produced by an electric discharge, and its rotational spectrum was characterized using a Balle-Flygare narrowband-type Fourier-transform microwave spectrometer operating in the frequency region of 4–40 GHz. The spectral analysis was supported by high-level ab initio calculations. Results. A total of 193 hyperfine components that originated from 12 rotational transitions, a- and b-type, were measured for the HC3HCN radical. The analysis allowed us to accurately determine 22 molecular constants, including rotational and centrifugal distortion constants as well as the fine and hyperfine constants. Transition frequency predictions were used to search for the HC3HCN radical in TMC-1 using the QUIJOTE survey between 30 and 50 GHz. We do not detect HC3HCN in TMC-1 and derive a 3σ upper limit to its column density of 6.0 × 1011 cm−2.

Microwave Spectrum of Two-Top Molecule: 2-Acetyl-3-Methylthiophene.

The microwave spectrum of 2-acetyl-3-methylthiophene (2A3MT) was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer and could be fully assigned to the anti-conformer of the molecule, while the syn-conformer was not observable. Torsional splittings of all rotational transitions in quintets due to internal rotations of the acetyl methyl and the ring methyl groups were resolved and analyzed, yielding barriers to internal rotation of 306.184(46) cm-1 and 321.813(64) cm-1 , respectively. The rotational and centrifugal distortion constants were determined with high accuracy, and the experimental values are compared to those derived from quantum chemical calculations. The experimentally determined inertial defect supports the conclusion that anti-2A3MT is planar, even though a number of MP2 calculations predicted the contrary.