Quartic Centrifugal Distortion Constants Research Articles

Theoretical Investigation of the Reaction of O(1D) with Formamide.

Carbamic acid (H2NCOOH) is a small organic molecule that is terrestrially unstable in condensed phases under ambient conditions but could survive in the low densities and temperatures of the interstellar medium. In this work, the reaction of formamide (H2NCOH) and electronically excited oxygen atoms in the 1D state, namely, O(1D), has been investigated computationally to determine the feasibility of carbamic acid production. Geometries for carbamic acid and other potential reaction products have been calculated, as well as all pertinent transition states. In addition, harmonic and anharmonic frequency calculations were performed to determine quartic and sextic centrifugal distortion constants for all products. This work enables spectroscopic predictions that can guide the experimental search for carbamic acid. Presented here are the calculations, geometries, molecular constants, and spectral predictions for possible products of the reaction between formamide and O(1D), as well as a discussion of which products are favored.

Hunting for Sulfur-containing Molecules in Space: A Spectroscopic Characterization of Cyclopropenethione Based on High-level Ab Initio Calculations

Cyclopropenethione falls into the class of complex organic molecules but has not yet been observed in the interstellar medium or any circumstellar disks. However, its existence is very likely, and thus this study provides high-level ab initio predictions, which may serve as reference data for future observations or experimental work. Rovibrational configuration interaction theory based on multidimensional potential energy surfaces being obtained from explicitly correlated coupled-cluster calculations has been used to predict the fundamental vibrational modes, the microwave spectrum, and low-lying rovibrational transitions. Rotational constants as well as quartic and sextic centrifugal distortion constants were obtained from vibrational perturbation theory.

Rotational spectroscopy of 2,4,6-cycloheptatriene-1-carbonitrile: facilitating the search for complex cyclic molecules in the ISM.

The recent astronomical observations of the simplest aromatic nitrile benzonitrile, c-C6H5CN, followed by a five-membered and a bicyclic CN-functionalized ring in TMC-1 have provided a significant impetus to the field for searches of cyclic complex organic molecules in space. One such example is 2,4,6-cycloheptatriene-1-carbonitrile, a seven-membered ring with a -CN group attached to the sp3-hybridized carbon atom. With a permanent electric dipole moment of 4.3 D, this molecule is an excellent candidate for laboratory rotational spectroscopy. In this study, experiments were performed in the 2-8 GHz, 18-26 GHz, and 75-110 GHz frequency ranges in a supersonic expansion setup and a room temperature flow cell setup. The measurements across the broad frequency range of 2-110 GHz have enabled the identification and assignment of the vibronic ground state, singly substituted rare-atom isotopologues, and vibrationally excited states. Here, we report the precise determination of the rotational constants, quartic centrifugal distortion constants, nitrogen nuclear quadrupole coupling constants, as well as molecular structure in its vibronic ground state. The comprehensive rotational spectroscopy study of this molecule, covering a large frequency range, forms the basis for its future astronomical detection and thus for extending the pool of detected complex cyclic molecules.

The chirped pulse, Fourier transform microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane

The microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane has been recorded for the first time using the chirped pulse, Fourier transform microwave technique. Quantum chemical calculations show the two lowest energy conformers as being a twist-trans and a gauche form with the gauche form previously being shown as having two separate conformations, a gauche+ (lower in energy) and gauche- (higher in energy) form. Analysis of the spectrum provided the observation of the twist-trans conformer only, with 253 and 85 transitions being assigned to the 35Cl and 37Cl isotopologues, respectively. R-branch, a- and b-type transitions were observed. The spectrum was fit to a Watson S-reduced Hamiltonian and consisted of rotational constants, quartic centrifugal distortion constants, and nuclear quadrupole coupling constants, including the determination of the off-diagonal nuclear quadrupole coupling constant, χab. Interpretation of the structure was provided using second moments and is found to have a similar ring structure to other known silacyclopentanes. Analysis of the χzz has been carried out and compared to other similar molecules. An investigation of the known quantum chemical energies of the gauche conformer reveals that the reported B3LYP energies do not align with the observed microwave results.

Vibrationally excited states of 1H- and 2H-1,2,3-triazole isotopologues analyzed by millimeter-wave and high-resolution infrared spectroscopy with approximate state-specific quartic distortion constants.

In this work, we present the spectral analysis of 1H- and 2H-1,2,3-triazole vibrationally excited states alongside provisional and practical computational predictions of the excited-state quartic centrifugal distortion constants. The low-energy fundamental vibrational states of 1H-1,2,3-triazole and five of its deuteriated isotopologues ([1-2H]-, [4-2H]-, [5-2H]-, [4,5-2H]-, and [1,4,5-2H]-1H-1,2,3-triazole), as well as those of 2H-1,2,3-triazole and five of its deuteriated isotopologues ([2-2H]-, [4-2H]-, [2,4-2H]-, [4,5-2H]-, and [2,4,5-2H]-2H-1,2,3-triazole), are studied using millimeter-wave spectroscopy in the 130-375GHz frequency region. The normal and [2-2H]-isotopologues of 2H-1,2,3-triazole are also analyzed using high-resolution infrared spectroscopy, determining the precise energies of three of their low-energy fundamental states. The resulting spectroscopic constants for each of the vibrationally excited states are reported for the first time. Coupled-cluster vibration-rotation interaction constants are compared with each of their experimentally determined values, often showing agreement within 500kHz. Newly available coupled-cluster predictions of the excited-state quartic centrifugal distortion constants based on fourth-order vibrational perturbation theory are benchmarked using a large number of the 1,2,3-triazole tautomer isotopologues and vibrationally excited states studied.

Rotational spectroscopy of hydrogen-bonded binary trifluoro-propanol conformers: conformational diversity, preference and abundances in a jet expansion.

The rich conformational landscape including the associated conformational conversion paths of the hydrogen-bonded binary 3,3,3-trifluoropropanol (TFP) aggregate was explored using chirped pulse Fourier transform microwave spectroscopy and computational chemistry. To appropriately identify the binary TFP conformers responsible for the five sets of candidate rotational transitions assigned, we established a set of important conformational assignment criteria. These include an extensive conformational search, good agreement between the experimental and theoretical rotational constants, relative magnitude of the three dipole moment components, and quartic centrifugal distortion constants, and observation and non-observation of the predicted conformers. Extensive conformational searches were carried out using CREST, a conformational search tool, producing hundreds of structural candidates. The CREST candidates were screened using a multitier approach and subsequently the low energy conformers (<25 kJ mol-1) were optimized at the B3LYP-D3BJ/def2-TZVP level, leading to 62 minima within an energy window of 10 kJ mol-1. Good agreement with the predicted spectroscopic properties mentioned above allowed us to clearly identify five binary TFP conformers as the molecular carriers. Particularly, a combined kinetic and thermodynamic model was developed, which provides a satisfactory explanation for the observation and non-observation of the low energy conformers predicted. The role of the intra- and intermolecular hydrogen bonding interactions in the stability ordering of the binary conformers is discussed.

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.

Micro- and millimeter-wave spectra of five conformers of cysteamine and their interstellar search

Context. Cysteamine (NH2CH2CH2SH), a molecule of potential astrobiological interest, has not yet been detected in the interstellar medium. Furthermore, the sulfur- stituted isomer of ethanolamine (or 2-aminoethanol) has been recently detected in the molecular cloud G+0.693–0.027. Aims. In order to conduct a new interstellar search for cysteamine in the molecular cloud G+0.693–0.027, its pure rotational spectrum needs to be investigated in the laboratory. Methods. A pulsed-jet Fourier transform microwave spectrometer and a Stark-modulated free-jet millimeter-wave absorption spectrometer were used to measure the purely rotational spectrum of cysteamine in the range of 6.5-18 GHz (46.12–16.66 mm) and 59.6–120.0 GHz (5.03–2.72 mm), respectively. We used a deep spectral line survey toward the molecular cloud G+0.693-0.027 obtained with the IRAM 30 m and Yebes 40 m radiotelescopes to search for cysteamine. Results. We assigned 815 rotational transition lines of five conformers (gGt, gGg, g′Gg, g′Gg′, and g'Gt) to fit the rotational constants, quartic centrifugal distortion constants, and the 14N nuclear quadrupole coupling constants. For four conformers (gGt, gGg, g′Gg, and g′Gg′), the 34S isotopologs were observed, and for two of them (gGg and g'Gg), the 13C and 15N isotopolog spectra were also detected; all in natural abundance. The five conformers of cysteamine were not detected toward the G+0.693–0.027 molecular cloud. We derived upper limits for their molecular abundances compared to molecular hydrogen of &lt;(0.2–1.3) × 10–10. The relative abundances with respect to the oxygen counterpart ethanolamine, previously detected toward this cloud, are NH2CH2CH2OH/NH2CH2CH2SH &gt; 0.8–5.3.

Determination of the "Privileged Structure" of 8-Hydroxyquinoline.

An accurate semi-experimental equilibrium structure of 8-hydroxyquinoline (8-HQ) has been determined combining experiment and theory. The cm-wave rotational spectrum of 8-HQ was recorded in a pulsed supersonic jet using broadband dual-path reflection and narrowband Fabry-Perot-type resonator Fourier-transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants and 14 N quadrupole coupling constants are determined. Rotational constants of all 13 C, 18 O and 15 N singly substituted isotopologues in natural abundance and those of a chemically synthesized OD isotopologue were used to obtain geometric parameters for all the heavy atoms and the hydroxyl hydrogen from a number of structure determination models. Theoretical approaches allowed for the determination of a semi-experimental equilibrium structure, in which computed rovibrational and electronic corrections were utilized to convert vibrational ground state constants into equilibrium constants. Despite the molecule having only a horizontal plane of symmetry and possessing 11 individual heavy atoms, microwave spectroscopy has allowed for a reliable and accurate structure determination. A mass dependent, structure was determined and proved to be equally reliable by comparison with the B3LYP-D3(BJ)/aVTZ equilibrium structure.

ВРАЩАТЕЛЬНЫЕ СПЕКТРЫ РЯДА ПОЛИЦИКЛИЧЕСКИХ АРОМАТИЧЕСКИХ УГЛЕВОДОРОДОВ

A theoretical study of the rotational spectra of a number of polycyclic aromatic hydrocarbons (PAHs) was carried out on the basis of quantum-chemical calculations by PRIRODA program in the PBE/3ζ approximation. Calculations are given for neutral molecules, their radical anions and radical cations in the rigid top approximation. Twelve neutral PAHs under study have no dipole moment and are not of interest for rotational spectroscopy. Thirteen neutral PAHs have a dipole moment not exceeding 0.09 Debye and, under certain conditions, can be studied in the microwave region. The remaining six compounds are PAHs with methyl and phenyl substituents, their dipole moments are 0.32-0.65 Debye, which makes it possible to study their microwave spectrum. For radical ions, the situation with the dipole moment is as follows: if a neutral molecule does not have a dipole moment, then the corresponding radical ion does not have it either; if the dipole moment of a neutral molecule is nonzero, then for radical ions it increases at least several times. For example, in the benzo [b] chrysene radical anion, the μa-component component of the dipole moment increases by 250 times, the μb-component by 7 times, and the total dipole moment by about 100 times. This effect is more pronounced for radical anions than for radical cations. In this case, it becomes possible to detect the spectrum of if not a neutral molecule, then at least one of its charge states. For a number of compounds, the patterns of various spectroscopic parameters depending on the number of carbon atoms in PAHs were found. In the B3LYP/6-31G(d,p) approximation, quartic centrifugal distortion constants were calculated for five compounds, and their effect on the rotational spectrum was estimated: for many compounds under study, they can have a significant effect on the microwave spectrum. With an increase in PAHs, the centrifugal constants decrease indicating an increase in the rigidity of the molecules. Thus, substituted neutral PAHs, as well as a number of radical ions, may be of interest for experimental studies in laboratory conditions and in space.

Ab initio study of spectroscopic properties at anharmonic force fields of LiNH2.

This work presents a systematic investigation of the spectroscopic properties at anharmonic force fields of ground electronic state ([Formula: see text]) of LiNH2, which are calculated using second-order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT) with hybrid GGA and meta-hybrid GGA (M06-2X) exchange-correlation functional. Two high angular momentum basis sets of 6-311+g (2d, p) and 6-311++g (3df, 2pd) are used. The equilibrium geometries, ground-state rotational constants, harmonic frequencies, and quartic and sextic centrifugal distortion constants of LiNH2 are calculated and compared with corresponding experimental or theoretical data. The predicted accuracy of the calculated constants has been confirmed by analyzing the deviations with respect to experiment. In addition, the anharmonic constants, vibration-rotation interaction constants, force constants, and Coriolis coupling constants of LiNH2 are firstly obtained. The infrared spectrum is predicted and together with the first prediction on the higher-order anharmonic constants contributes to a better understanding of the vibrational and rotational characteristics of LiNH2, thus revealing its internal structure. Graphical abstract The IR spectra and the magnified IR spectra at 3500 cm-1 in harmonic approximations of LiNH2 using B3P86, M06-2X and MP2 methods combining with 6-311++g(3df,2pd).

Theoretical and experimental studies of the isotope effects for He–CO2 and Ne–CO2 complexes

In this work, we have studied the isotope effects for the He–CO2 and Ne–CO2 complexes by means of theoretical calculations and experimental measurements, which were carried out using a distributed quantum cascade laser to probe a pulsed supersonic jet expansion. Firstly, infrared spectra have been recorded for the He/Ne–12C18O2 complexes. Spectroscopic parameters including band origin ν0, rotational constants A, B, C, and centrifugal distortion constants ΔJK were obtained by fitting a Watson A-reduced Hamiltonian with 13 assigned rovibrational transitions for He–12C18O2. For Ne–12C18O2, the observed spectrum produces a set of spectroscopic parameters including the band origin, rotational constants and all the quartic centrifugal distortion constants with more than 100 rovibrational transitions (40 new transitions). Secondly, we have calculated the rovibrational energy levels, vibrational shifts, and rotational constants for the He/Ne–CO2 complexes based on potential energy surfaces (PESs) and bound state calculations for ground and vibrationally excited states. The obtained results show that the spectroscopic characteristics (vibrational shifts and rotational constants) for Ne–CO2 are analogous to those of Ar–CO2, while those for He–CO2 show some differences especially for the rotational constants. Finally, according to the available experimental data and our theoretical calculations, infrared spectra were predicted for six isotopologues with C2v symmetry of Ne–CO2 complex.

Rotational spectroscopy and precise molecular structure of 1,2-dichlorobenzene

Supersonic expansion rotational spectroscopy at 4–18 GHz with rectangular pulse and chirp excitation was used to measure spectra of twelve isotopic species of 1,2-dichlorobenzene at conditions of completely resolved double chlorine hyperfine structure by using a newly constructed slit expansion nozzle. Four species: 37Cl13 and the three singly substituted 13C species have been assigned for the first time. Spectroscopic constants for six isotopic species with single deuterium substitution were considerably improved by using chemically synthesised samples. Hyperfine constants in the field gradient principal axes system were determined and used as a test of self-consistency of the analysis. Additional room-temperature measurements up to 275 GHz allowed assignment of rotational transitions in first excited states of the three lowest normal modes. The determined values of vibrational changes in rotational constants, inertial defects, and quartic and sextic centrifugal distortion constants, allowed calibration of harmonic and anharmonic force field calculations, resulting in determination, among others, of precise semi-experimental equilibrium, reSE, geometry of 1,2-dichlorobenzene. It is found that r(C-C) and ∠(C-C-C) in the phenyl ring of 1,2-dichlorobenzene show very small changes relative to benzene.

Theoretical Calculations, Microwave Spectroscopy, and Ring-Puckering Vibrations of 1,1-Dihalosilacyclopent-2-enes

High level theoretical CCSD/cc-pVTZ computations have been carried out to calculate the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2SiCPF2) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl2). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr2) was obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm-1 barrier to planarity, 2SiCPF2 has a planar ring system, 2SiCPCl2 has a calculated tiny 4 cm-1 barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm-1. Microwave spectra of seven isotopic species of 2SiCPF2 were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The a-type and b-type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent, 29Si, 30Si, and all singly-substituted 13C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar. The experimentally determined rotational constants and geometrical parameters agree very well with the theoretical values and confirm the planarity of the five-membered ring. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid.

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 &gt;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.

Analysis of several high-resolution infrared bands of perdeutero-spiropentane, C5D8

Perdeutero-spiropentane (C5D8) has been synthesized and infrared spectra were taken from 400 to 4000 cm−1 at low sample pressures in a 22 m multipass cell. The spectral resolution (0.0015 cm−1) reveals thousands of individual rovibrational transitions for several fundamental bands occurring in the spectral region of 500–1100 cm−1. C5D8 ground state constants reported in a previous study of the ν15 parallel band at 1054 cm−1[1] were improved significantly in the present work by adding combination-differences derived from ν16 parallel and ν24 perpendicular band analyses. The larger data set at higher J values also allowed determination of the ground state K″ = 2 splitting constant (Δ2″ = 0.76(2) × 10−9 cm−1). For ν16, the band origin is 847.16933(5) cm−1 and changes in upper state rotational constants are ΔA = –0.0013411(5) and ΔB = –0.0002021(1) (in units of cm−1). Overlapping, and in Fermi resonance with ν16, is a second parallel band which, from Gaussian calculations at the anharmonic level, is assigned as the b2 combination ν8 + ν12, with band origin of 840.5434(1) and ΔA = –0.0003500(7) and ΔB = –0.0003502(1) cm−1. For ν24 analogous values are 556.41453(2) and ΔA = –0.00032295(9) and ΔB = –0.00016782(4) cm−1. The analyses also yield accurate values for the changes in both quartic and sextic centrifugal distortion constants and, for ν24, values for the l-doubling constants q+ and q−. Theoretical calculations of the rovibrational parameters aided in the analysis and the numerical values are in good agreement with experimental results.

Exploring the limits of the Data-Model-Theory synergy: “Hot” MW transitions for rovibrational IR studies

In order to further improve the accuracy of rovibrational IR line lists generated from the “Best Theory + Reliable High-resolution Experiment” (BTRHE) strategy from 0.01 to 0.05 cm−1, or 300–1500 MHz, to ∼10 MHz, we explore the current limits of the Data-Model-Theory synergy by examining the accuracy and consistency of existing data, then propose that “hot” bands in microwave (MW) spectra is the solution we need for future enhancements. The Ames SO2J = 0–20 rovibrational energy levels computed on the semi-empirically refined Ames-2 potential energy surface (PES) are fit to the Effective Hamiltonian (EH) model regularly used in the experimental infrared (IR) analysis for SO2 isotopologues. In the fitted EH(Ames) model, the rotational constants A/B/C and all 5 quartic centrifugal distortion constants display clear, systematic, and consistent patterns along the vibrational state energy or quanta. Such consistent patterns may facilitate the vibrational assignments for MW hot bands and extract more information from high temperature MW spectra. Some EH(Expt) analyses were carried out with the lowest order Coriolis Coupling term, C1. Their constants should not be directly compared with other EH(Expt) and EH(Ames) results. After excluding them, our δ = EH(Ames)- EH(Expt) analyses for 5 isotopologues (626, 636, 646, 628 and 828) indicates some loss of accuracy and consistency starting from vibrational states as low as 2ν2 or 1000 cm−1. Some EH parameters, e.g. δK, may have relative deviations as large as 50–100% and totally lose any recognizable patterns. This simply means that current EH(Expt) models do not have the system-wide consistency we need to further refine the EH(Ames) and Ames rovibrational IR line lists. A large part of such defects are probably inherited from the limited precision of experimental line positions, i.e. 1E-3 ∼ 1E-4 cm−1, or 3–30 MHz. This is confirmed in a series of truncation tests using the Ames data. Although the EH(Ames) consistency may help identify unreliable rovibrational EH(Expt) parameters, and make reliable predictions for minor isotopologues and unobserved vibrational bands, we believe only the highly accurate “hot” MW transitions can provide real enhancements for EH(Expt) accuracy and consistency. “Hot” MW spectra should play a more significant role in the future synergy of Data, Model, and Theory in the field of rovibrational IR studies.

Rotational spectroscopy and astronomical search for glutaronitrile★.

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. 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. 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. 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 30m at 3mm. 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.

DFT meets the segmented polarization consistent basis sets: Performances in the computation of molecular structures, rotational and vibrational spectroscopic properties

Quantum-chemical calculations assist the analysis of laboratory spectra, and often provide the only means to determine spectroscopic data that cannot be accessed experimentally. For the purpose, reliable predictions of structural and spectroscopic parameters are required. Although coupled cluster theory in conjunction with to large basis sets and composite schemes can reach impressive accuracies for structural, thermochemical and spectroscopic properties, it is still limited to small sized molecules. DFT represents the working option for medium to large molecular systems. In this context, systematic investigations are required aimed at characterizing the performances of the different DFT model chemistries. In this work, the accuracy of the popular hybrid B3LYP and the double hybrid B2PLYP functionals coupled to the segmented polarization consistent (aug-)pcs-n basis sets in the prediction of molecular structures and rotational- and vibrational spectroscopic parameters are investigated using a benchmark set of molecules of both atmospheric and astrochemical relevance. For comparison purposes, different flavors of Dunning’s triple-ζ basis sets and the SNSD basis set, are also employed. The convergence behavior of the pcs-n hierarchy with n = 1–4 is also addressed to some extent. The results indicate the B3LYP-D3 functional in conjunction with the aug-pcs-1 or SNSD basis sets as a cost-effective model chemistry for applications in the field of rotational and vibrational spectroscopies. Improved accuracy is obtained by coupling the B2PLYP-D3 functional with the aug-pcs-2 or aug-cc-pVTZ triple-ζ basis sets that show an accuracy around 0.003 Å and 0.3° for bond lengths and angles, 1% and 3% for rotational and quartic centrifugal distortion constants, respectively, 12 cm−1 for fundamental frequencies and 3 km mol−1 for IR intensities. The B2PLYP-D3/maug-cc-pVTZ-dH level keeps the same accuracy, with slightly larger deviations for intensities.

2-Propionylthiophene: planar, or not planar, that is the question.

The long-standing ambiguity of the molecular planarity when an alkyl group is attached to a system with conjugated double bonds is a great challenge for both experiments and theory. This also holds true for the case of 2-propionylthiophene (2PT) where a propionyl group is attached at the second position of the planar, aromatic thiophene ring. Results from quantum chemistry at the MP2 level of theory, showing that in the two conformers syn- and anti-2PT the ethyl group of the propionyl moiety is slightly tilted out of the thiophene ring plane, conflict with those from the other methods, stating that the ethyl group is in-plane with the thiophene ring. In the microwave spectrum, both syn- and anti-2PT were observed, and their geometry parameters such as the rotational and quartic centrifugal distortion constants were precisely determined. The experimental heavy atom skeleton obtained by isotopic substitutions revealed a tiny, but non-zero tilt angle of the ethyl group out of the thiophene plane, thereby convincingly confirming the non-planarity of 2-propionylthiophene. This conclusion was further supported by the inertial defects calculated from the experimental rotational constants. Finally, splittings arising from the internal rotation of the terminal methyl group were analysed, yielding torsional barriers of 806.94(54) cm-1 and 864.5(88) cm-1 for the two observed conformers, respectively.