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Spectroscopic and Theoretical Methods to Investigate Interstellar Medium

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Applied Chemistry".

Deadline for manuscript submissions: closed (30 November 2023) | Viewed by 10859

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Special Issue Editors

Prof. Dr. Cristina Puzzarini

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Guest Editor

Department of Chemistry "Giacomo Ciamician", University of Bologna, Via Zamboni 33, 40126 Bologna, Italy
Interests: astrochemistry; rotational spectroscopy; quantum chemistry; computational spectroscopy; detection of molecules in space; gas-phase reactivity in the ISM
Special Issues, Collections and Topics in MDPI journals

Dr. Silvia Alessandrini

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Guest Editor

Faculty of Sciences, Scuola Normale Superiore, Pisa, Italy
Interests: astrochemistry; rotational spectroscopy; quantum chemistry; computational spectroscopy; detection of molecules in space; gas-phase reactivity in the ISM

Special Issue Information

Dear Colleagues,

The interstellar medium (ISM) is characterized by harsh conditions: the temperatures and densities are very low and ionizing radiation is always present. Despite these extreme conditions, the ISM is characterized by a rich chemistry, which in turn means the presence of a large variety of molecular species. Spectroscopic techniques, ranging from MW/far-IR (rotational spectroscopy) to mid-/near-IR (vibrational spectroscopy), to UV/Vis (electronic spectroscopy), can be used to explore the molecular composition of the ISM because spectroscopic signatures provide the unequivocal proof of the presence of chemical species. However, any astronomical spectroscopy needs to be supported by laboratory studies, and this is the one of the focuses of this Special Issue. Laboratory spectroscopy is either experimental or theoretical, but currently, powerful approaches are based on the integration of experiment and theory.

A further step is to explain the formation of interstellar molecules on the basis of the reactivity of smaller species known to be present in the astronomical object under consideration. Different scenarios can be envisaged that can be all traced back to two categories of reactions: those taking place in the gas phase or those occurring on interstellar dust grains. In an analogy to the spectroscopic investigations addressed above, experimental, theoretical or integrated strategies can be exploited. These studies define another focus of this Special Issue.

In summary, this Special Issue aims at collecting both experimental and theoretical works that concern one (or more) of the key steps in the investigation and characterization of the ISM, as well as use new multidisciplinary approaches to discover the complexity of such an environment.

Prof. Dr. Cristina Puzzarini
Dr. Silvia Alessandrini
Guest Editors

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Keywords

interstellar medium (ISM)
molecular spectroscopy
computational chemistry
quantum chemistry
interstellar complex organic molecules
spectroscopic simulations
spectroscopic catalogs
reactivity in the ISM

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Published Papers (5 papers)

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Research

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17 pages, 955 KiB

Open AccessArticle

Gas-Phase Infrared Action Spectroscopy of CH₂Cl⁺ and CH₃ClH⁺: Likely Protagonists in Chlorine Astrochemistry

by Sven Thorwirth, Kim Steenbakkers, Timon Danowski, Philipp C. Schmid, Luis Bonah, Oskar Asvany, Sandra Brünken and Stephan Schlemmer

Molecules 2024, 29(3), 665; https://doi.org/10.3390/molecules29030665 - 31 Jan 2024

Cited by 1 | Viewed by 1233

Abstract

Two fundamental halocarbon ions, CH₂Cl⁺ and CH₃ClH⁺, were studied in the gas phase using the FELion 22-pole ion trap apparatus and the Free Electron Laser for Infrared eXperiments (FELIX) at Radboud University, Nijmegen (the Netherlands). The vibrational bands of a total of four isotopologs, CH₂^35,37Cl⁺ and CH₃^35,37ClH⁺, were observed in selected wavenumber regions between 500 and 2900 cm⁻¹ and then spectroscopically assigned based on the results of anharmonic force field calculations performed at the CCSD(T) level of theory. As the infrared photodissociation spectroscopy scheme employed probes singly Ne-tagged weakly bound complexes, complementary quantum-chemical calculations of selected species were also performed. The impact of tagging on the vibrational spectra of CH₂Cl⁺ and CH₃ClH⁺ is found to be virtually negligible for most bands; for CH₃ClH⁺–Ne, the observations suggest a proton-bound structural arrangement. The experimental band positions as well as the best estimate rotational molecular parameters given in this work provide a solid basis for future spectroscopic studies at high spectral resolutions. Full article

(This article belongs to the Special Issue Spectroscopic and Theoretical Methods to Investigate Interstellar Medium)

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14 pages, 428 KiB

Open AccessArticle

CCSD(T) Rotational Constants for Highly Challenging C₅H₂ Isomers—A Comparison between Theory and Experiment

by Venkatesan S. Thimmakondu and Amir Karton

Molecules 2023, 28(18), 6537; https://doi.org/10.3390/molecules28186537 - 9 Sep 2023

Cited by 1 | Viewed by 1888

Abstract

We evaluate the accuracy of CCSD(T) and density functional theory (DFT) methods for the calculation of equilibrium rotational constants (

A_{e}

B_{e}

, and

C_{e}

) for four experimentally detected low-lying C

_{5}

_{2}

isomers (ethynylcyclopropenylidene (2 [...] Read more.

We evaluate the accuracy of CCSD(T) and density functional theory (DFT) methods for the calculation of equilibrium rotational constants (

A_{e}

B_{e}

, and

C_{e}

) for four experimentally detected low-lying C

_{5}

_{2}

isomers (ethynylcyclopropenylidene (2), pentatetraenylidene (3), ethynylpropadienylidene (5), and 2-cyclopropen-1-ylidenethenylidene (8)). The calculated rotational constants are compared to semi-experimental rotational constants obtained by converting the vibrationally averaged experimental rotational constants (

A_{0}

B_{0}

, and

C_{0}

) to equilibrium values by subtracting the vibrational contributions (calculated at the B3LYP/jun-cc-pVTZ level of the theory). The considered isomers are closed-shell carbenes, with cumulene, acetylene, or strained cyclopropene moieties, and are therefore highly challenging from an electronic structure point of view. We consider both frozen-core and all-electron CCSD(T) calculations, as well as a range of DFT methods. We find that calculating the equilibrium rotational constants of these C

_{5}

_{2}

isomers is a difficult task, even at the CCSD(T) level. For example, at the all-electron CCSD(T)/cc-pwCVTZ level of the theory, we obtain percentage errors ≤0.4% (

C_{e}

of isomer 3,

B_{e}

and

C_{e}

of isomer 5, and

B_{e}

of isomer 8) and 0.9–1.5% (

B_{e}

and

C_{e}

of isomer 2,

A_{e}

of isomer 5, and

C_{e}

of isomer 8), whereas for the

A_{e}

rotational constant of isomers 2 and 8 and

B_{e}

rotational constant of isomer 3, high percentage errors above 3% are obtained. These results highlight the challenges associated with calculating accurate rotational constants for isomers with highly challenging electronic structures, which is further complicated by the need to convert vibrationally averaged experimental rotational constants to equilibrium values. We use our best CCSD(T) rotational constants (namely, ae-CCSD(T)/cc-pwCVTZ for isomers 2 and 5, and ae-CCSD(T)/cc-pCVQZ for isomers 3 and 8) to evaluate the performance of DFT methods across the rungs of Jacob’s Ladder. We find that the considered pure functionals (BLYP-D3BJ, PBE-D3BJ, and TPSS-D3BJ) perform significantly better than the global and range-separated hybrid functionals. The double-hybrid DSD-PBEP86-D3BJ method shows the best overall performance, with percentage errors below 0.5% in nearly all cases. Full article

(This article belongs to the Special Issue Spectroscopic and Theoretical Methods to Investigate Interstellar Medium)

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23 pages, 850 KiB

Open AccessArticle

Ab Initio Rovibrational Spectroscopy of the Acetylide Anion

by Benjamin Schröder

Molecules 2023, 28(15), 5700; https://doi.org/10.3390/molecules28155700 - 27 Jul 2023

Cited by 2 | Viewed by 1456

Abstract

In this work the rovibrational spectrum of the acetylide anion HCC

^{-}

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

ν_{2}

around 510 cm

^{- 1}

to be the best target for detection. The rather weak CD stretching fundamental

ν_{1}

in deuterated isotopologues show a second-order resonance with the

(0, 2^{0}, 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. Full article

(This article belongs to the Special Issue Spectroscopic and Theoretical Methods to Investigate Interstellar Medium)

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13 pages, 484 KiB

Open AccessArticle

Spectroscopic Constants and Anharmonic Vibrational Frequencies of C(O)OC, c-C₂O₂ and Their Silicon-Containing Analogues

by Olivia A. Harwick and Ryan C. Fortenberry

Molecules 2023, 28(11), 4563; https://doi.org/10.3390/molecules28114563 - 5 Jun 2023

Viewed by 2453

Abstract

Comets are likely to contain various carbon oxide molecules potentially including C(O)OC and c-C

_{2}

_{2}

on their surfaces and comae, as well as their silicon-substituted analogues possibly playing a role in the formation of interstellar dust grains. In this work, high-level [...] Read more.

Comets are likely to contain various carbon oxide molecules potentially including C(O)OC and c-C

_{2}

_{2}

on their surfaces and comae, as well as their silicon-substituted analogues possibly playing a role in the formation of interstellar dust grains. In this work, high-level quantum chemical data are provided to support such potential future astrophysical detection through the generation of predicted rovibrational data. Laboratory-based chemistry would also benefit from such aforementioned computational benchmarking considering these molecules’ historic computational and experimental elusiveness. Coupled-cluster singles, doubles, and perturbative triples, the F12b formalism, and the cc-pCVTZ-F12 basis set garner the rapid, yet highly trusted F12-TcCR level of theory leveraged presently. This current work points to all four molecules’ strong IR activity, coupled with large intensities, thus suggesting the potential for JWST detection. Although Si(O)OSi possesses a permanent dipole moment significantly larger than those of the other molecules of present interest, the significant abundance of the potential precursor carbon monoxide suggests that the dicarbon dioxide molecules may yet be observable in the microwave region of the electromagnetic spectrum. Thus, this present work details the likely existence and detectability of these four cyclic molecules, providing updated implications compared to previous work performed both experimentally and computationally. Full article

(This article belongs to the Special Issue Spectroscopic and Theoretical Methods to Investigate Interstellar Medium)

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Review

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39 pages, 11881 KiB

Open AccessReview

Infrared Spectra of Small Radicals for Exoplanetary Spectroscopy: OH, NH, CN and CH: The State of Current Knowledge

by Svatopluk Civiš, Adam Pastorek, Martin Ferus, Sergei N. Yurchenko and Noor-Ines Boudjema

Molecules 2023, 28(8), 3362; https://doi.org/10.3390/molecules28083362 - 11 Apr 2023

Cited by 4 | Viewed by 3052

Abstract

In this study, we present a current state-of-the-art review of middle-to-near IR emission spectra of four simple astrophysically relevant molecular radicals—OH, NH, CN and CH. The spectra of these radicals were measured by means of time-resolved Fourier transform infrared spectroscopy in the 700–7500 cm⁻¹ spectral range and with 0.07–0.02 cm⁻¹ spectral resolution. The radicals were generated in a glow discharge of gaseous mixtures in a specially designed discharge cell. The spectra of short-lived radicals published here are of great importance, especially for the detailed knowledge and study of the composition of exoplanetary atmospheres in selected new planets. Today, with the help of the James Webb telescope and upcoming studies with the help of Plato and Ariel satellites, when the investigated spectral area is extended into the infrared spectral range, it means that detailed knowledge of the infrared spectra of not only stable molecules but also the spectra of short-lived radicals or ions, is indispensable. This paper follows a simple structure. Each radical is described in a separate chapter, starting with historical and actual theoretical background, continued by our experimental results and concluded by spectral line lists with assigned notation. Full article

(This article belongs to the Special Issue Spectroscopic and Theoretical Methods to Investigate Interstellar Medium)

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