Rotational Transition Frequencies Research Articles

Geometry of the argon…imidazole complex revealed by the microwave spectra of four isotopologues

The rotational spectra of four isotopologues of an isolated complex formed between an argon atom and imidazole, Ar…imidazole, have been recorded in the 6–19 GHz region by Fourier transform microwave spectroscopy. Rotational transition frequencies have been fitted to Watson’s S-reduced Hamiltonian to yield rotational, centrifugal distortion and nuclear quadrupole coupling constants for the complex. Rotational constants determined for the parent and three 15N-containing isotopologues allow the three-dimensional structure of the complex to be described. The two angles, θ and ϕ, which define the orientation of the Ar atom relative to the imidazole ring have been determined for the first time in addition to the distance between Ar and the center of mass of the imidazole sub-unit, R. Fitting of structural parameters to the experimentally-determined moments of inertia yields a structure where Ar is positioned above the ring plane at a distance of 3.519 Å from the center of mass of the imidazole sub-unit. In the experimentally determined, average geometry, the intermolecular axis (drawn through Ar and the center of mass of the imidazole sub-unit) is oriented at 6° from the normal to the ring plane. The experimental results allow for four alternative possibilities for ϕ with 62.0(39)° being that which is most consistent with expectations for this parameter based on previous work. The experimentally-determined nuclear quadrupole coupling constants imply that the electric field gradient at each of the nitrogen nuclei of imidazole does not significantly change on formation of the complex with Ar.

Application of Laser-Induced Acoustic Desorption for Molecular Rotational Resonance Spectroscopy.

Molecular Rotational Resonance (MRR) spectroscopy is a uniquely precise tool for the determination of molecular structures of volatile compounds in mixtures, as the characteristic rotational transition frequencies of a molecule are extremely sensitive to its 3D structure through the moments of inertia in a three-dimensional coordinate system. This enables identification of the compounds based on just a few parameters that can be calculated, as opposed to, for example, mass spectrometric data, which often require expert analysis of 10-20 different signals and the use of many standards/model compounds. This paper introduces a new sampling technique for MRR, laser-induced acoustic desorption (LIAD), to allow the vaporization of nonvolatile and thermally labile analytes without the need for excessive heating or derivatization. In this proof-of-concept study, LIAD was successfully coupled to an MRR instrument to conduct measurements on seven compounds with differing polarities, molecular weights, and melting and boiling points. Identification of three isomers in a mixture was also successfully performed using LIAD/MRR. Based on these results, LIAD/MRR is demonstrated to provide a powerful approach for the identification of nonvolatile and/or thermally labile analytes with molecular weights up to 600 Da in simple mixtures, which does not require the use of reference compounds. In the future, applications to more complex mixtures, such as those relevant to pharmaceutical research, and quantitative aspects of LIAD/MRR will be reported.

H2O-HCN complex: A new potential energy surface and intermolecular rovibrational states from rigorous quantum calculations.

In this work the H2O-HCN complex is quantitatively characterized in two ways. First, we report a new rigid-monomer 5D intermolecular potential energy surface (PES) for this complex, calculated using the symmetry-adapted perturbation theory based on density functional theory method. The PES is based on 2833 abinitio points computed employing the aug-cc-pVQZ basis set, utilizing the autoPES code, which provides a site-site analytical fit with the long-range region given by perturbation theory. Next, we present the results of the quantum 5D calculations of the fully coupled intermolecular rovibrational states of the H2O-HCN complex for the total angular momentum J values of 0, 1, and 2, performed on the new PES. These calculations rely on the quantum bound-state methodology developed by us recently and applied to a variety of noncovalently bound binary molecular complexes. The vibrationally averaged ground-state geometry of H2O-HCN determined from the quantum 5D calculations agrees very well with that from the microwave spectroscopic measurements. In addition, the computed ground-state rotational transition frequencies, as well as the B and C rotational constants calculated for the ground state of the complex, are in excellent agreement with the experimental values. The assignment of the calculated intermolecular vibrational states of the H2O-HCN complex is surprisingly challenging. It turns out that only the excitations of the intermolecular stretch mode can be assigned with confidence. The coupling among the angular degrees of freedom (DOFs) of the complex is unusually strong, and as a result most of the excited intermolecular states are unassigned. On the other hand, the coupling of the radial, intermolecular stretch mode and the angular DOFs is weak, allowing straightforward assignment of the excitation of the former.

Theoretical study of intramolecular effects and rotational spectra for H2–AgCl complex: A corrected intermolecular potential energy surface

An efficient model was developed by optimizing the terms of intermolecular potential and monomer rotations in the Hamiltonian, and then was applied to determine the global structures and rotational transitions of H2–AgCl complex. Based on this model, a detailed study of intramolecular effects were examined for intermolecuar vibrational modes, rotational transition frequencies, and spectroscopic parameters relative to those of rigid-rotor-approximation model. Firstly, the full-dimensional equilibrium geometry is determined to be (Re, r1e, r2e, θ1e, θ2e, φe) = (2.370 Å, 0.7785 Å, 2.2579 Å, 90.0°, 180.0°, 0.0°), and the averaged structure is determined to be (Rν, r1ν, r2ν, θ1ν, θ2ν, φν) = (2.436 Å, 0.7905 Å, 2.2604 Å, 76.4°, 172.8°, 0.0°), which are in excellent agree with the available data of experimental observations. In addition, the available transitions were accurately predicted with the maximal percentage error of 0.05%. Based on the determined spectroscopic parameters, two perpendicular bands were predicted and will be helpful to guide the further investigations in the far infrared spectral region experimentally.

Laboratory rotational spectroscopy and astronomical search for 2-ethylacrolein

The rotational spectrum of 2-ethylacrolein was investigated by pulsed jet Fourier transform microwave spectroscopy complementary with quantum chemical calculations. The rotational spectra of both conformers cis and gauche and their all mono-substituted 13C and 18O isotopologues were measured and assigned in natural abundance, allowing the accurate structural determinations on their heavy-atom skeletons. The relative population ratio of cis to gauche in pulsed jets was estimated to be ≈ 3:1. With the accurate rotational transition frequencies, radio astronomical searches toward the massive star-forming region Sgr B2(N) were carried out by using the Shanghai Tianma 65 m radio telescope. No transition lines belonging to 2-ethylacrolein were detected, the upper limits of column densities for cis and gauche were derived to be 7.52 × 1013 cm−2 and 1.29 × 1015 cm−2, respectively.

Tunable itinerant spin dynamics with polar molecules

Strongly interacting spins underlie many intriguing phenomena and applications1-4 ranging from magnetism to quantum information processing. Interacting spins combined with motion show exotic spin transport phenomena, such as superfluidity arising from pairing of spins induced by spin attraction5,6. To understand these complex phenomena, an interacting spin system with high controllability is desired. Quantum spin dynamics have been studied on different platforms with varying capabilities7-13. Here we demonstrate tunable itinerant spin dynamics enabled by dipolar interactions using a gas of potassium-rubidium molecules confined to two-dimensional planes, where a spin-1/2 system is encoded into the molecular rotational levels. The dipolar interaction gives rise to a shift of the rotational transition frequency and a collision-limited Ramsey contrast decay that emerges from the coupled spin and motion. Both the Ising and spin-exchange interactions are precisely tuned by varying the strength and orientation of an electric field, as well as the internal molecular state. This full tunability enables both static and dynamical control of the spin Hamiltonian, allowing reversal of the coherent spin dynamics. Our work establishes an interacting spin platform that allows for exploration of many-body spin dynamics and spin-motion physics using the strong, tunable dipolar interaction.

High-resolution rovibrational and rotational spectroscopy of the singly deuterated cyclopropenyl cation, c-C3H2D.

Applying a novel action spectroscopic technique in a 4 K cryogenic ion-trap instrument, the molecule c-C3H2D+ has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 126 rovibrational transitions within the fundamental band of the ν1 symmetric C-H stretch were measured with a band origin centred at 3168.565 cm-1, which were used to predict pure rotational transition frequencies in the ground vibrational state. Based on these predictions, 16 rotational transitions were observed between 90 and 230 GHz by using a double-resonance scheme. These new measurements will enable the first radio-astronomical search for c-C3H2D+.

Calculations and measurements for the rotational spectrum and structure of the cyclopentadienyl thallium – benzene complex

Calculations on the structure of the cyclopentadienyl thallium – benzene complex were made using Gaussian-G16 and Spartan programs. Calculations converged to symmetric top and asymmetric top structures. Rotational transition frequencies for the cyclopentadienyl thallium – benzene complex were measured in the 4.5–8.3 GHz range using a Flygare-Balle pulsed beam microwave spectrometer. The basic spectrum from the preliminary experiments exhibited splittings of the observed transitions which would not be expected for a rigid symmetric top molecule. 11 of the measured transitions were assigned to an asymmetric top structure and fit, yielding rotational constants A = 1136.8(11) B = 378.44(11) MHz and C = 375.173(79) MHz.

Searching for biosignatures by their rotational spectrum: global fit and methyl group internal rotation features of dimethylsulphoxide up to 116 GHz

AbstractThe identification and quantification of molecules in interstellar space and atmospheres of planets in the solar systems and in exoplanets rely on spectroscopic methods and laboratory work is essential to provide the community with the spectral features needed to analyse cosmological observations. Rotational spectroscopy in particular, with its intrinsic high resolution, allows the unambiguous identification of biomolecular building blocks and biosignature gases which can be correlated with the origin of life or the identification of habitable planets. We report the extension of the measured rotational transition frequencies of dimethylsulphoxide and its 34S and 13C isotopologues in the millimetre wave range (59.6–78.4 GHz) by use of an absorption spectrometer based on the supersonic expansion technique. Hyperfine patterns related to the methyl group internal rotation were analysed in the microwave range region (6–18 GHz) with a Pulsed Jet Fourier Transform spectrometer at extremely high resolution (2 kHz) and reliable predictions up to 116 GHz are provided. The focus on sulphur-bearing molecules is motivated by the fact that sulphur is largely involved in the intra- and inter-molecular hydrogen bonds in proteins and although it is the 10th most abundant element in the known Universe, understanding its chemistry is still a matter of debate. Moreover, sulphur-bearing molecules, in particular dimethylsulphoxide, have been indicated as possible biosignature gases to be monitored in the search of habitable exoplanets.

Effect of correlated hyperfine theory errors in the determination of rotational and vibrational transition frequencies in HD+

We revisit the problem of separating the purely rovibrational and hyperfine contributions to observed rovibrational transition frequencies. The separation of hyperfine frequency shifts played an essential role in recent works in which the value of the proton-electron mass ratio was determined from Doppler-free rovibrational spectroscopy and high-precision theory of the molecular ion. Here, we present an alternative approach to removing theoretically computed hyperfine shifts from a limited set of observed rovibrational transition frequencies, based on a least-squares adjustment. The adjustment facilitates a quantitative study of the effect of correlations between the uncertainties of lower- and upper-state hyperfine energies on the derived value of the rovibrational transition frequency. We find that correlations may lead to significant shifts of the extracted rovibrational transition frequencies. In addition, we propose and demonstrate a method to quantify the additional shift and uncertainty, which is of importance to other applications of the spectroscopic data, such as the determination of fundamental physical constants.

A theoretical study of the intermolecular interactions of H2–CuF complex: Intermolecular vibrations, isotope effects, and rotational structure

In this paper, a theoretical study has been made on the intermolecular interactions of the H2–CuF complex, including binding energy, intermolecular vibrations, isotope effects, and rotational structure. Based on different bond lengths of H2 and CuF monomers, three intermolecular potential energy surfaces (PESs) were constructed at the level of single and double excitation coupled-cluster method with a non-iterative perturbation treatment of triple excitations [CCSD(T)] with aug-cc-pVTZ basis set supplemented with bond functions. A global minimum on the PESs show that H2–CuF complex belongs to C2ν point group with a T-shaped structure. The obtained binding energy ranges from 8890 to 10050 cm−1, which increases as the increment of H–H bond length, but opposite case has been determined as the increment of Cu–F bond length. The accuracy of PESs was examined by the available data of 101–000 transition. The predicted rotational transition frequency obtained from bound state calculations can reproduce the experimental observation very well, and the predicted error is 0.1% based on the PES1 constructed with rH2 and rCuF fixed at 0.838 and 1.7409 Å. By analyzing the wave function of the bound state, the intermolecular vibrational modes were assigned unambiguously. Isotope effects were also studied and the largest error is also 0.1% compared with the available 101–000 transition data. A set of spectroscopic parameters were obtained for six isotopologues to determine rotational structure of H2–CuF complex. Upon the complex formation, the obtained structure parameters show that H–H bond length is elongated by 0.081 Å, while Cu–F value is shortened by 0.008 Å from the respective average bond lengths of free monomer.

Intermolecular rovibrational states of the H2O-CO2 and D2O-CO2 van der Waals complexes.

We present quantum five-dimensional bound-state calculations of the fully coupled intermolecular rovibrational states of H2O-CO2 and D2O-CO2 van der Waals (vdW) complexes in the rigid-monomer approximation for the total angular momentum J values of 0, 1, and 2. A rigid-monomer version of the recent ab initio full-dimensional (12D) potential energy surface of H2O-CO2 [Q. Wang and J. M. Bowman, J. Chem. Phys. 147, 161714 (2017)] is employed. This treatment provides for the first time a rigorous and comprehensive description of the intermolecular rovibrational level structure of the two isotopologues that includes the internal-rotation tunneling splittings and their considerable sensitivity to rotational and intermolecular vibrational excitations, as well as the rotational constants of the two vdW complexes. Two approaches are used in the calculations, which differ in the definition of the dimer-fixed (DF) frame and the coordinates associated with them. We demonstrate that with the approach introduced in this work, where the DF frame is fixed to the CO2 moiety, highly accurate results are obtained using significantly smaller basis sets in comparison to those for the alternative approach. In addition, the resulting wavefunctions tend to lend themselves better to physical interpretation and assignment. The H2O-CO2 ground-state internal-rotation tunneling splittings, the rotational transition frequencies, and the rotational constants of both vdW complexes are in excellent agreement with the experimental results. The calculated intermolecular vibrational fundamentals agree well with the scant terahertz spectroscopy data for these complexes in cryogenic neon matrices.

Rotational Rest Frequencies and First Astronomical Search of Protonated Methylamine

We report first laboratory rest frequencies for rotational transitions of protonated methylamine, CH3NH3+, measured in a cryogenic 22-pole ion trap machine and employing an action spectroscopy scheme. For this prolate symmetric top molecule thirteen transitions between 80 and 240 GHz were detected in the ground vibrational state, covering JK = 2K − 1K up to JK = 6K − 5K with K = 0, 1, 2. Some transitions exhibit noticeable structure that is attributed to internal rotation splitting. As the CN radical and several of its hydrogenated and protonated forms up to methylamine, CH3NH2, are well known entities in the laboratory and in space, protonated methylamine, CH3NH3+, is a promising candidate for future radio astronomical detection.

Synchrotron-based far-infrared spectroscopy of [formula omitted]: Extended ro-vibrational analysis and new line list up to 3360 cm[formula omitted

The far-infrared spectrum of HC3N has been recorded at high resolution between 70 and 500 cm−1using synchrotron radiation. Four prominent features, i.e., ν7, ν6−ν7, ν4−ν6, and 2ν7 have been identified in the spectrum together with many associated hot bands. In addition, rotational transitions for the interacting v4=v7=1, (v6=2,v7=1), (v5=1,v7=2), and v7=5 vibrationally excited states have been recorded in the millimeter/submillimeter region. The newly assigned transitions, together with those reported previously, form a comprehensive data set including about 17 000 transitions, which involves almost all the vibrational states of HC3N lying below 1300 cm−1 plus some excited states with energies between 2075 and 3550 cm−1. These data have been fitted to an effective Hamiltonian which takes into account rotational and vibrational l-type resonance effects, together with a number of anharmonic interaction terms. On average, all the analysed data are reproduced within the experimental accuracy. About 90 000 rotational and ro-vibrational transition frequencies have been computed on the basis of the spectroscopic constants obtained from the global fit in order to support data interpretation and astronomical searches in the interstellar medium and planetary atmospheres. Part of these data is included in the 2020 release of the HITRAN database.

Toward a global model of the interactions in low-lying states of methyl cyanide: Rotational and rovibrational spectroscopy of the [formula omitted] state and tentative interstellar detection of the [formula omitted] state in Sgr B2(N)

Rotational spectra of methyl cyanide were recorded newly and were analyzed together with existing spectra to extend the global model of low-lying vibrational states and their interactions to υ4=1 at 920 cm−1. The rotational spectra cover large portions of the 36–1439 GHz region and reach quantum numbers J and K of 79 and 16, respectively. Information on the K level structure of CH3CN is obtained from IR spectra. A spectrum of 2ν8 around 717 cm−1, analyzed in our previous study, covered also the ν4 band. The assignments in this band cover 880–952 cm−1, attaining quantum numbers J and K of 61 and 13, respectively.The most important interaction of υ4=1 appears to be with υ8=3, ΔK=0,Δl=+3, a previously characterized anharmonic resonance. We report new analyses of interactions with ΔK=-2 and Δl=+1, with ΔK=-4 and Δl=-1, and with ΔK=-6 and Δl=-3; these four types of interactions connect all l substates of υ8=3 in energy to υ4=1. A known ΔK=-2,Δl=+1 interaction with υ7=1 was also analyzed, and investigations of the ΔK=+1, Δl=-2 and ΔK=+3,Δl=0 resonances with υ8=2 were improved, as were interactions between successive states with υ8⩽3, mainly through new υ8⩽2 rotational data.A preliminary single state analysis of the υ4=υ8=1 state was carried out based on rotational transition frequencies and on ν4+ν8-ν8 hot band data. A considerable fraction of the K levels was reproduced within uncertainties in its entirety or in part, despite obvious widespread perturbations in υ4=υ8=1.In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to υ4=1, we report the tentative detection of υ4=υ8=1 toward the main hot molecular core of the protocluster Sagittarius B2(N) employing the Atacama Large Millimeter/submillimeter Array.

Природа тонкой структуры вращательных уровней основного X-=SUP=-2-=/SUP=-Sigma-=SUP=-+-=/SUP=--состояния радикала CN

By means of ab initio high-level quantum-chemical calculations of non-diagonal matrix elements of spin-orbital and electron-rotational coupling between the ground X2Σ+ and excited (1--4)2Π states the observed regular effect of γ-doubling of the rotational levels of the X2Σ+ state was shown to be mainly determined by intramolecular interactions of the mentioned state with remote states (2--4)2Π. In terms of the nonadiabatic model of the effective radial Hamiltonian of the isolated electronic state, it was possible to create the analytical potential of the X2Σ+ state and the corresponding function γ (R) reproducing the frequencies of rotational and vibrational-rotational transitions (for the lowest vibrational levels ν≤3) of the CN molecule at the experimental (spectroscopic) level of accuracy.

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 withJ ≤ 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, and15N singly substituted isotopic species of thioformamide. In addition, for the parent isotopolog, the lowest two excited vibrational states,v12 = 1 andv9 = 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.

Automated Construction of Potential Energy Surfaces Suitable to Describe van der Waals Complexes with Highly Excited Nascent Molecules: The Rotational Spectra of Ar-CS(v) and Ar-SiS(v).

Some reactions produce extremely hot nascent products which nevertheless can form sufficiently long-lived van der Waals (vdW) complexes-with atoms or molecules from a bath gas-as to be observed via microwave spectroscopy. Theoretical calculations of such unbound resonance states can be much more challenging than ordinary bound-state calculations depending on the approach employed. One encounters not just the floppy, and perhaps multiwelled potential energy surface (PES) characteristic of vdWs complexes, but in addition, one must contend with excitation of the intramolecular modes and its corresponding influence on the PES. Straightforward computation of the (resonance) rovibrational levels of interest, involves the added complication of the unbound nature of the wave function, often treated with techniques such as introducing a complex absorbing potential. Here, we have demonstrated that a simplified approach of making a series of vibrationally effective PESs for the intermolecular coordinates-one for each reaction product vibrational quantum number of interest-can produce vdW levels for the complex with spectroscopic accuracy. This requires constructing a series of appropriately weighted lower-dimensional PESs for which we use our freely available PES-fitting code AUTOSURF. The applications of this study are the Ar-CS and Ar-SiS complexes, which are isovalent to Ar-CO and Ar-SiO, the latter of which we considered in a previously reported study. Using a series of vibrationally effective PESs, rovibrational levels and predicted microwave transition frequencies for both complexes were computed variationally. A series of shifting rotational transition frequencies were also computed as a function of the diatom vibrational quantum number. The predicted transitions were used to guide and inform an experimental effort to make complementary observations. Comparisons are given for the transitions that are within the range of the spectrometer and were successfully recorded. Calculations of the rovibrational level pattern agree to within 0.2% with experimental measurements.

Microwave spectroscopy of planar and quasi-planar organic molecules: Indole and 1,2,3,4-tetrahydroquinoline

In this study, 3179 and 3813 rotational transition frequencies for indole and 1,2,3,4-tetrahydroquinoline, respectively, were analyzed using the PGOPHER software. In both analyses, all the matrix elements of the sextic rotational Hamiltonians were fitted simultaneously to the hyperfine quadrupole terms. The global fits of all the transition frequencies provided the most precise ground state molecular parameters for both molecules, so far. The presented experimental results are in concordance with the density functional theory calculations.

Rotational spectra in seven excited vibrational states of 1,2,3,4-tetrahydroquinoline

In this study, more than 7000 rotational line frequencies in vibrationally excited states (ν57, ν56, 2ν57, ν57 + ν56, ν55, 3ν57, ν54) of 1,2,3,4-tetrahydroquinoline (THQ) were measured, assigned and analyzed. The line assignments were facilitated by the Loomis-Wood, Fortrat and other specific diagrams. The rotational transition frequencies were least-squares fitted to sextic A-reduced Hamiltonians. Based on the analyses, the rotational and centrifugal distortion constants of the above-mentioned excited vibrational states were determined with very high accuracies.