Accurate Spectroscopic Parameters Research Articles

Theoretical Spectroscopic Study of Isopropyl Alcohol (CH3-CHOH-CH3)

Accurate spectroscopic parameters of isopropyl alcohol, a volatile organic compound present on Earth and in extraterrestrial atmospheres, are provided. The work pursues the study of the far-infrared region, describing the distribution of the low-lying vibrational energy levels that can be populated at very low temperatures, having effects on the detectability and identification. The potential energy surface shows 27 almost equivalent minima producing 27 subcomponents of the levels due to tunneling effects. Computations have been performed using highly correlated ab initio calculations, accurate enough to distinguish between the rotational parameters of the two quasi-identical conformers gauche and trans. A variational procedure in three dimensions depending on three interacting internal rotations, the internal rotation of the two methyl groups and the internal rotation of the hydroxyl group, is employed to compute the energies. The 27 subcomponents of the ground vibrational state can be grouped into three series of nine energy levels located around 0.000, 1.693, and 81.927 cm−1 whose energy difference is due to the OH torsion effect. The nine subcomponents integrated in each series derive from the torsion of the two methyl groups. The computations reproduce accurately the available experimental data. New predicted properties can help the spectroscopic analysis of the rotational-vibrational spectra in the gas phase and further detections of vibrationally excited isopropyl alcohol.

From the laboratory to the interstellar medium: a strategy to search for exotic molecules in space

The chemistry of the interstellar medium occurs under extreme conditions and can lead to the formation of exotic molecules. These are species that on Earth are unstable and/or highly reactive. Their discovery in space is usually based on the astronomical observation of their rotational fingerprints, which requires an accurate laboratory investigation. This is based on a strategy that starts from the interplay of experiment and theory. State-of-the-art quantum-chemical calculations are used to predict the relevant spectroscopic information required to guide the spectral recording, analysis and assignment. Rotational spectra measurements are then performed in the centimeter-/millimeter-/submillimeter-wave region, thereby exploiting efficient on-the-fly production protocols for exotic molecules. Subsequently, the spectral analysis leads to accurate spectroscopic parameters, which are then used for setting up accurate line catalogs for astronomical searches and detections. This review is based on the strategy developed and the results obtained at the ROT&Comp Lab of the University of Bologna.

Accurate Structures and Spectroscopic Parameters of Guanine Tautomers in the Gas Phase by the Pisa Conventional and Explicitly Correlated Composite Schemes (PCS and PCS-F12).

A general strategy for the accurate computation of structural and spectroscopic properties of biomolecule building blocks in the gas phase is proposed and validated for tautomeric equilibria. The main features of the new model are the inclusion of core-valence correlation in geometry optimizations by a double hybrid functional and the systematic use of wave-function composite methods in conjunction with cc-pVnZ-F12 basis sets with separate extrapolation of MP2 and post-MP2 contributions. The resulting Pisa composite scheme employing conventional (PCS) or explicitly correlated (PCS-F12) approaches is applied to the challenging problem of guanine tautomers in the gas phase. The results are in remarkable agreement with the experimental structures, relative stabilities, and spectroscopic signatures of different tautomers. The accuracy of the results obtained at reasonable cost by means of black-box parameter-free approaches paves the way toward systematic investigations of other molecular bricks of life also by non-specialists.

Ab Initio Rovibrational Spectroscopy of the Acetylide Anion.

In this work the rovibrational spectrum of the acetylide anion HCC- is investigated using high-level electronic structure methods and variational rovibrational calculations. Using a composite approach the potential energy surface and dipole surface is constructed from explicitly correlated coupled-cluster accounting for corrections due to core-valence correlation, scalar relativistic effects and higher-order excitation effects. Previous approaches for approximating the latter are critically evaluated. Employing the composite potential, accurate spectroscopic parameters determined from variational calculations are presented. In comparison to the few available reference data the present results show excellent agreement with ground state rotational constants within 0.005% of the experimental value. Intensities determined from the variational calculations suggest the bending fundamental transition ν2 around 510 cm-1 to be the best target for detection. The rather weak CD stretching fundamental ν1 in deuterated isotopologues show a second-order resonance with the (0,20,1) state and the consequences are discussed in some detail. The spectroscopic parameters and band intensities provided for a number of vibrational bands in isotopologues of the acetylide anion should facilitate future spectroscopic investigations.

Rotational Spectra and Astronomical Search of Five Ethynyl Derivatives of Propene

Abstract The interstellar discovery of the five cyano derivatives of propene brings interest in analogous molecules that could be detected in similar astronomical environments. In this context, ethynyl-propene derivatives are good candidates to be observed in the Taurus Molecular Cloud (TMC-1), where other cyano and ethynyl derivatives of hydrocarbons such as ethane, allene, propene, and cyclopentadiene, among others, have been detected. We have investigated the pure rotational spectra of the five ethynyl-propene derivatives to enable their interstellar detections. These molecular species have been produced by an electric discharge and their rotational spectra were characterized using a Balle-Flygare narrow band-type Fourier-transform microwave spectrometer operating in the frequency region of 4-40GHz. The spectral analysis allowed us to derive accurate spectroscopic parameters to obtain reliable frequency predictions for astronomical searches. We do not detect any of these species in TMC-1 but we derive upper limits for the abundances for the five isomers.

Rotation-tunnelling spectrum and astrochemical modelling of dimethylamine, CH3NHCH3, and searches for it in space

ABSTRACT Methylamine has been the only simple alkylamine detected in the interstellar medium for a long time. With the recent secure and tentative detections of vinylamine and ethylamine, respectively, dimethylamine has become a promising target for searches in space. Its rotational spectrum, however, has been known only up to 45 GHz until now. Here we investigate the rotation-tunnelling spectrum of dimethylamine in selected regions between 76 and 1091 GHz using three different spectrometers in order to facilitate its detection in space. The quantum number range is extended to J = 61 and Ka = 21, yielding an extensive set of accurate spectroscopic parameters. To search for dimethylamine, we refer to the spectral line survey ReMoCA carried out with the Atacama Large Millimeter/submillimeter Array towards the high-mass star-forming region Sagittarius B2(N) and a spectral line survey of the molecular cloud G+0.693–0.027 employing the IRAM 30 m and Yebes 40 m radio telescopes. We report non-detections of dimethylamine towards the hot molecular cores Sgr B2(N1S) and Sgr B2(N2b) as well as G+0.693−0.027 which imply that dimethylamine is at least 14, 4.5, and 39 times less abundant than methylamine towards these sources, respectively. The observational results are compared to computational results from a gas-grain astrochemical model. The modelled methylamine to dimethylamine ratios are compatible with the observational lower limits. However, the model produces too much ethylamine compared with methylamine which could mean that the already fairly low levels of dimethylamine in the models may also be too high.

Spectroscopic characterization of [H, Cl, S, O] molecular system: Potential candidate for detection in Venus atmosphere.

Accurate spectroscopic parameters have been obtained for the identification of the [H, Cl, S, O] molecular system in the Venus atmosphere using computational methods. These calculations employed both standard and explicitly correlated coupled cluster techniques. All isomers possess C1 symmetry, with HOSCl being the most stable isomer. Only HOSCl and trigonal-HSOCl isomers are thermodynamically stable relative to the first dissociation limit HCl + SO. Fundamental modes of the lowest three isomers exhibit many anharmonic resonances, resulting in complex spectra. All isomers are found to be stable in the visible region as the calculation of vertical energy transition indicates. No electronic states were found to strongly absorb in the near UV-vis region.

Accurate Structures and Spectroscopic Parameters of Phenylalanine and Tyrosine in the Gas Phase: A Joint Venture of DFT and Composite Wave-Function Methods.

A general strategy for the accurate computation of conformational and spectroscopic properties of flexible molecules in the gas phase is applied to two representative proteinogenic amino acids with aromatic side chains, namely, phenylalanine and tyrosine. The main features of all the most stable conformers predicted by this computational strategy closely match those of the species detected in microwave and infrared experiments. Together with their intrinsic interest, the accuracy of the results obtained with reasonable computer times paves the route for accurate investigations of other flexible bricks of life.

Computational Protocol for the Identification of Candidates for Radioastronomical Detection and Its Application to the C3H3NO Family of Isomers

The C3H3NO family of isomers is relevant in astrochemistry, even though its members are still elusive in the interstellar medium. To identify the best candidate for astronomical detection within this family, we developed a new computational protocol based on the minimum-energy principle. This approach aims to identify the most stable isomer of the family and consists of three steps. The first step is an extensive investigation that characterizes the vast number of compounds having the C3H3NO chemical formula, employing density functional theory for this purpose. The second step is an energy refinement, which is used to select isomers and relies on coupled cluster theory. The last step is a structural improvement with a final energy refinement that provides improved energies and a large set of accurate spectroscopic parameters for all isomers lying within 30 kJ mol-1 above the most stable one. According to this protocol, vinylisocyanate is the most stable isomer, followed by oxazole, which is about 5 kJ mol-1 higher in energy. The other stable species are pyruvonitrile, cyanoacetaldehyde, and cyanovinylalcohol. For all of these species, new computed rotational and vibrational spectroscopic data are reported, which complement those already available in the literature or fill current gaps.

Rotational spectroscopy of oxirane-2,2-[formula omitted], [formula omitted]-CD[formula omitted]CH[formula omitted]O, and its tentative detection toward IRAS 16293-2422 B

We prepared a sample of oxirane doubly deuterated at one C atom and studied its rotational spectrum in the laboratory for the first time between 120 GHz and 1094 GHz. Accurate spectroscopic parameters up to eighth order were determined, and the calculated rest frequencies were used to identify c-CD2CH2O tentatively in the interstellar medium in the Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293-2422. The c-CD2CH2O to c-C2H4O ratio was estimated to be ∼0.054 with Trot=125 K. This value translates to a D-to-H ratio of ∼0.16 per H atom which is higher by a factor of 4.5 than the ∼0.036 per H atom obtained for c-C2H3DO. Such increase in the degree of deuteration referenced to one H atom in multiply deuterated isotopologs compared to their singly deuterated variants have been observed commonly in recent years.

Potential energy surfaces and bound state calculations for Ne–CO complex: rovibrational spectra and isotope effects

The intermolecular potential energy surfaces (PESs) of Ne–CO complex have been constructed by using the CCSD(T)-F12a method with the aug-cc-pVQZ basis set. These PESs were subsequently applied to investigate the microwave and infrared spectra via bound state calculations. Firstly, compared to the available microwave data, the pure rotational transitions with K = 0 and J ≤ 4 for this complex can be well reproduced. Furthermore, a set of accurate spectroscopic parameters were deduced for the fundamental (v CO = 1) and overtone (v CO = 2) bands. The vibrational shift for the normal isotopologue (Ne–12C16O) was determined to be –0.0683 cm−1, which is in excellent agreement with the experimental observation of –0.0711 cm−1. Isotope effects were also investigated, and the determined rotational constants and vibrational shifts display a fairly good linear relationship, which is classified into two types according to the 12C and 13C in the CO monomer.

Accurate structures and spectroscopic parameters of α,α-dialkylated α-amino acids in the gas-phase: a joint venture of DFT and wave-function composite methods.

Accurate computations of structural, conformational and spectroscopic properties in the gas phase have been performed for two α,α-dialkylated α-amino acids, namely aminoisobutyric acid and cyclopropylglycine. Thanks to the integration of modern double hybrid functionals and wave-function methods, several low-energy structures of the title molecules could be analyzed employing standard computer resources. The computed features of all the most stable conformers of the target amino acids closely match the corresponding spectroscopic parameters issued from microwave spectroscopic studies in the gas-phase. Together with their intrinsic interest, the accuracy of the results obtained with reasonable computer times paves the way for accurate investigations of other flexible bricks of life.

High-resolution ro-vibrational and rotational spectroscopy of HC3O.

The ro-vibrational and pure rotational spectra of the linear ion HC3O+ have been investigated in a 4 K cryogenic ion trap instrument. For this, a novel action spectroscopic technique, called leak-out-spectroscopy (LOS, Schmid et al., J. Phys. Chem. A 2022, 126, 8111), has been utilized and characterized. In total, 45 ro-vibrational transitions within the fundamental band of the ν1 C-H stretching mode were measured with a band center at 3237.132 cm-1, as well as 34 lines from the combination band ν2 + ν4, and 41 lines tentatively identified as the combination band ν2 + ν5 + ν7, interleaved and resonant with ν1. Surprisingly, also two hot bands were detected despite the cryogenic operation temperature. Based on the novel action spectroscopy approach, a new double-resonance rotational measurement scheme was established, consisting of rotational excitation followed by vibrational excitation. Seven rotational transitions were observed between 89 and 180 GHz. Highly accurate spectroscopic parameters were extracted from a fit using all available data. In addition, a pulsed laser system has been employed to record a low resolution vibrational spectrum, in order to demonstrate the compatibility of such lasers with the LOS method.

Hunting for interstellar molecules: rotational spectra of reactive species.

Interstellar molecules are often highly reactive species, which are unstable under terrestrial conditions, such as radicals, ions and unsaturated carbon chains. Their detection in space is usually based on the astronomical observation of their rotational fingerprints. However, laboratory investigations have to face the issue of efficiently producing these molecules and preserving them during rotational spectroscopy measurements. A general approach for producing and investigating unstable/reactive species is presented by means of selected case-study molecules. The overall strategy starts from quantum-chemical calculations that aim at obtaining accurate predictions of the missing spectroscopic information required to guide spectral analysis and assignment. Rotational spectra of these species are then recorded by exploiting the approach mentioned above, and their subsequent analysis leads to accurate spectroscopic parameters. These are then used for setting up accurate line catalogs for astronomical searches.

Laboratory rotational spectroscopy and astronomical search of ethynyl substituted naphthalene

ABSTRACT The recent interstellar detection of cyanonaphthalenes brings interest in related aromatic molecular species that could be present in similar astronomical environments. In this context, ethynyl derivatives of naphthalene are promising candidates to be observed in the Taurus Molecular Cloud (TMC-1), where cyanonaphthalenes together with cyano- and ethynyl-derivatives of cyclopentadiene and benzene have been detected. To enable the interstellar searches for ethynyl derivatives of naphthalene, their pure rotational spectra need to be investigated in the laboratory. We have observed for the first time the rotational spectra of 1- and 2-ethynylnaphthalene species using a broad-band Fourier-transform microwave spectrometer operating in the 2–8 GHz frequency region. Accurate spectroscopic parameters are derived from the analysis of the experimental spectra, allowing for reliable predictions for astronomical searches. Our searches in TMC-1 for both isomers provide upper limits for the abundances of these species.

High resolution FTIR and diode laser spectroscopy of trifluoromethylacetylene and tetrafluoromethane in a supersonic jet expansion

We report high resolution infrared spectra in the CF stretching fundamental ranges of trifluoromethylacetylene (CFC≡C-H, 3,3,3-trifluoropropyne) and of tetrafluoromethane (CF, R14, PCF-14, Halocarbon14). For the experiments a diode laser setup combined with a supersonic jet expansion is described, achieving approximate rotational temperatures between 5 and 10 K for CF, using a pulsed slit nozzle providing effective optical path lengths of 3 to 4 cm in a single pass. Slightly higher rotational temperatures (near 30 K) for CFCCH greatly simplify the rotational-vibrational structure of the spectra for this molecule, for which we report also high resolution Fourier transform (FTIR) spectra at room temperature with nearly Doppler limited resolution (instrumental bandwidth 0.001 cm) using the Zürich prototype spectrometer ZP 2001 (Bruker IFS125). The analysis of the CF-symmetric stretching (A) fundamental provides accurate spectroscopic parameters of the ground and excited vibrational states with a band centre = (1253.4985 ± 0.0015) cm from a least squares adjustment to more than 1100 line data with a root mean square deviation less than 0.0003 cm. The low temperature spectra of the degenerate (F) fundamental of CF show a very simple structure of line clusters which can be analysed with an approximate term formula. The spectra are essentially instrument limited with a smallest effective line width near 0.002 cm. The assignment of the P(1) line at 1283.33693 cm in the supersonic jet spectra (about 10 K) provides a band centre in terms of the vibrational energy of the ( = 1, J = 0, F (F)) level at (1283.7193 ± 0.002) cm. Also the assignment of the isolated R(0) line at 1283.48420 cm provides the energy of the well defined ( = 1, J' = 1, A (A)) level. The results are discussed in relation to the data from the HITRAN tables, earlier FTIR data, and new frequency comb based data for CF. We also discuss possible studies of nuclear spin symmetry conservation in CF, for which lower temperatures would be required, and a concept to study parity violation in CF by a level crossing experiment using molecular beam spectroscopy.

Efficient terahertz absorption gas sensor with Gaussian process regression in time- and frequency-domain

The terahertz region presents promising opportunities for quantitative gas sensing in practical environments thanks to its access to molecular fingerprints and immunity to particulate scattering. With Terahertz time-domain spectroscopy (THz-TDS), molecular absorption and dispersion spectra can be rendered using femtosecond-laser-based interferometric system. However, procurement of accurate spectroscopic parameters in this uncharted spectral region and quantitative reduction of the broadband data remain challenging. We propose and validate two end-to-end machine learning models based on Gaussian process regression (GPR) for efficient THz-TDS sensing in both time- and frequency-domain (TD and FD). Quantitative CO sensing is accomplished for proof-of-concept demonstration of the method. Both TD- and FD-GPR models demonstrate accurate predictions in the presence of measurement noise and ambient interference, and the root-mean-square errors are below 0.6 % with both simulated and experimental data. It is concluded that while the FD model benefits from immunity to dispersion distortions, the TD model noses out for its baseline-free nature and better elimination of temporally introduced noises. This work presents the first experimental demonstration of THz-TDS for quantitative gas sensing exploiting both TD and FD signals, showing promising prospects for real-time, multi-species, multi-parameter sensing under challenging scenarios such as chemically reactive or open-path atmospheric environments.

The absorption spectrum of nitrous oxide between 7647 and 7918 cm−1

We revisit the room temperature absorption spectrum of natural nitrous oxide recorded by high sensitivity cavity ring down spectroscopy between 7647 and 7918 cm−1 (1.31–1.26 μm) reported by Liu et al. (doi: 10.1016/j.jms.2011.03.025). In this previous analysis, 2272 transitions could be assigned in the region and no intensity information was provided. In the present work, on the basis of improved predictions of the effective operator models, the assignments could be extended to more than 3200 transitions and, more importantly, line intensities are provided for all the transitions. The assigned transitions belong to 49 bands of the 14N216O, 14N15N16O, 15N14N16O, 14N218O, and 14N217O isotopologues, 16 being newly assigned. All the identified bands correspond to the ∆P= 14 series of transitions, except for four ∆P= 12 and two ∆P= 13 bands of the 14N216O isotopologue (P= 2V1+V2+4V3 is the polyad number where Vi= 1-3 are the vibrational quantum numbers). For comparison, the HITRAN2020 database includes only the ν1+3ν3 band of the main isotopologue and a number of calculated line parameters for the fourth minor isotopologue, 14N218O. The band-by-band analysis provides accurate spectroscopic parameters of 46 bands from a fit of the measured line positions. Nine resonance perturbations were identified including a few extra lines for two 14N216O bands. The interaction mechanisms and the perturbers were assigned on the basis of the effective Hamiltonian models. The comparison with current spectroscopic databases (HITRAN, HITEMP, NOSD) is discussed. All show significant deviations compared to our observations (e.g. for ∆P= 13 band intensities). The reported results will be thus valuable to more accurately account for the absorption of nitrous oxide in the region.

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.

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex.

The 1:1 benzofuran–formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint “rotational spectroscopy–quantum chemistry” strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low‐energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone‐pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi‐experimental equilibrium structure for a molecular complex of such a dimension.