Grand Challenges in Astrochemistry

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Gas-phase Chemistry in the Interstellar Medium: The Role of Laboratory Astrochemistry

"Who" and how? In this simple question the complexity of the interstellar chemistry is encapsulated. "Who" refers to what molecules are present in the interstellar medium (ISM) and "how" to the mechanisms that led to their formation. While the large number of molecules discovered in the ISM (similar to 250) demonstrates the rich chemistry occurring there, a significant number of unknown species are waiting for an identification and the processes that led to the synthesis of the identified species are still hotly debated or even unknown. Gas-phase laboratory studies in the fields of rotational spectroscopy and quantum chemistry provide an important contribution to answering the question above. An overview on the role played by rotational spectroscopy and quantum chemistry in the unraveling of the gas-phase chemistry of the interstellar medium is presented

Hyperfine structure in rotational spectra of deuterated molecules: the Hds and ND3 case studies

The determination of hyperfine parameters (quadrupole-coupling, spin-spin coupling, and spin-rotation constants) is one of the aims of high-resolution rotational spectroscopy. These parameters are relevant not only from a spectroscopic point of view, but also from a physical and/or chemical viewpoint, as they might provide detailed information on the chemical bond, structure, etc. In addition, the hyperfine structure of rotational spectra is so characteristic that its analysis may help in assigning the spectra of unknown species. In astronomical observations, hyperfine structures of rotational spectra would allow us to gain information on column densities and kinematics, and the omission of taking them into account can lead to a misinterpretation of the line width of the molecular emission lines. Nevertheless, the experimental determination of hyperfine constants can be a challenge not only for actual problems in resolving hyperfine structures themselves, but also due to the lack of reliable estimates or the complexity of the hyperfine structure itself. It is thus important to be able to rely on good predictions for such parameters, which can nowadays be provided by quantum-chemical calculations. In fact, the fruitful interplay of experiment and theory will be demonstrated by means of two study cases: the hypefine structure of the rotational spectra of HDS and ND$_3$. From an experimental point of view, the Lamb-dip technique has been employed to improve the resolving power in themillimeter- and submillimeterwave frequency range by at least one order of magnitude, thus making it possible to perform sub-Doppler measurements as well as to resolve narrow hyperfine structures. Concerning theory, it will be demonstrated that high-level calculations can provide quantitative estimates for hyperfine parameters (quadrupole coupling constants, spin-rotation tensors, spin-spin couplings, etc.) and shown how theoretical predictions are often essential for a detailed analysis of the hyperfine structure of the recorded spectra

Measurements @ mm-/sub-mm-wave spectroscopy laboratory of bologna: rotational spectroscopy applied to atmospheric studies

The physico-chemistry of the Earth's atmosphere has been one of the main subjects of studies over last years. In particular, the composition of the atmosphere is indeed very important to understand chemical processes linked to depletion of stratospheric ozone and greenhouse effect. The vertical concentration profiles of atmospheric gases can be provided by remote sensing measurements, but they require the accurate knowledge of the parameters involved: line positions, transition intensities, pressure-broadened half-widths, pressure-induced frequency shifts and their temperature dependence. In particular, the collisional broadening parameters have a crucial influence on the accuracy of spectra calculations and on reduction of remote sensing data. Rotational spectroscopy, thanks to its intrinsic high resolution, is a powerful tool for providing most of the information mentioned above: accurate or even very accurate rotational transition frequencies, accurate spectroscopic as well as hyperfine parameters, accurate pressure-broadening coefficients and their temperature dependence. With respect to collisional phenomena and line shape analysis studies, by applying the source frequency modulation technique it has been found that rotational spectroscopy may provide very good results: not only this technique does not produce uncontrollable instrumental distortions or broadenings, but also, having an high sensitivity, it is particularly suitable for this kind of investigations. A number of examples will be presented to illustrate the work carried out at the Laboratory of Millimeter/submillimeter-wave Spectroscopy of Bologna in the field of atmospheric studies

THE RENAISSANCE OF ROTATIONAL SPECTROSCOPY: THEORY MEETS EXPERIMENT FOR NEW CHALLENGES

Among different spectroscopic techniques, rotational spectroscopy, given its intrinsic high resolution and high sensitivity, is one of the most powerful tools for investigating the structure and dynamics of molecules and supramolecular systems in the gas phase. Rotational spectra contain a wealth of accurate information on structural, molecular, and spectroscopic parameters that are difficult or impossible to obtain by other experimental techniques. However, the task of extracting information from the analysis of the spectral features is challenging, time-consuming, and prone to errors. In the last decade, rotational spectroscopy has experienced huge technological improvements that have led to a revitalization of the field, also due to parallel advancements in theoretical methods and computational resources. Given these advances, the interplay of theory and experiment in rotational spectroscopy is discussed by means of representative examples that vividly illustrate what can be accomplished with theory and experiment brought together in this field. In particular, it will be shown how such an interplay can be exploited to address new challenges, exemplified by studying the nature of weak interactions in molecular adducts where the bonding pairs are made up of non-hydrogen atoms. Another fascinating challenge is offered by the open issues posed by astrochemistry. Astronomical observations of rotational spectroscopy signatures provide the unequivocal proof of the presence of chemical species in the astronomical source under consideration, and the talk will also explore the degree to which rotational spectroscopy can assist in going beyond the ``simple'' identification of interstellar species

Unveiling Bifunctional Hydrogen Bonding with the Help of Quantum Chemistry: The Imidazole-Water Adduct as Test Case

The ubiquitous role of water and its amphiprotic nature call for a deeper insight into the physical-chemical properties of hydrogen-bonded complexes formed with building blocks of biomolecules. In this work, the semiexperimental (SE) approach combined with the template model (TM) protocol allowed the accurate determination of the equilibrium structure of two isomeric forms of the imidazole-water complex. In this procedure, the integration of experiment (thanks to a recent rotational spectroscopy investigation) and theory is exploited, also providing the means of assessing the reliability and accuracy of different quantum-chemical approaches. Overall, this study demonstrated the robustness of the combined SE-TM approach, which can provide accurate results using affordable quantum-chemical methods. Finally, the structural and energetic characteristics of these complexes have been examined in detail and compared with those of analogous heterocycle-water adducts, also exploiting energy decomposition analyses

Methanimine as a key precursor of imines in the interstellar medium: the case of propargylimine

A gas-phase formation route is proposed for the recently detected propargylimine molecule. In analogy to other imines, such as cyanomethanimine, the addition of a reactive radical (C$_2$H in the present case) to methanimine (CH$_2$NH}) leads to reaction channels open also in the harsh conditions of the interstellar medium. Three possible isomers can be formed in the C$H_2$NH + C$_2$H reaction: Z- and E-propargylimine (Z-,E-PGIM) as well as N-ethynyl-methanimine (N-EMIM). For both PGIM species, the computed global rate coefficient is nearly constant in the 20-300 K temperature range, and of the order of 2-3 $\times$ 10$^{-10}$ cm$^3$ molecule$^{-1}$ s$^{-1}$, while that for N-EMIM is about two orders of magnitude smaller. Assuming equal destruction rates for the two isomers, these results imply an abundance ratio for PGIM of [Z]/[E] $\sim$ 1.5, which is only slightly underestimated with respect to the observational datum.Comment: 10 pages, 4 figures, 2 tables. Accepted in ApJ

NON-COVALENT INTERACTIONS AND INTERNAL DYNAMICS IN PYRIDINE-AMMONIA: A COMBINED QUANTUM-CHEMICAL AND MICROWAVE SPECTROSCOPY STUDY

The 1:1 complex of ammonia with pyridine has been characterized by using state-of-the-art quantum-chemical computations combined with pulsed-jet Fourier-Transform microwave spectroscopy. The computed potential energy landscape pointed out the formation of a stable $sigma$-type complex, which has been confirmed experimentally: the analysis of the rotational spectrum showed the presence of only one 1:1 pyridine – ammonia adduct. Each rotational transition is split into several components due to the internal rotation of NH$_3$ around its $C_3$ axis and to the hyperfine structure of both $^{14}$N quadrupolar nuclei, thus providing the unequivocal proof that the two molecules form a $sigma$-type complex involving both a N-H$cdots$N and a C-H$cdots$N hydrogen bond. The dissociation energy (BSSE and ZPE corrected) has been estimated to be 11.5 kJ$cdot$mol$^{-1}$. This work represents the first application of an accurate, yet efficient computational scheme, designed for the investigation of small biomolecules, to a molecular cluster

Quantum chemistry meets rotational spectroscopy for astrochemistry: increasing molecular complexity

For many years, scientists suspected that the interstellar medium was too hostile for organic species and that only a few simple molecules could be formed under such extreme conditions. However, the detection of approximately 180 molecules in interstellar or circumstellar environments in recent decades has changed this view dramatically. A rich chemistry has emerged, and relatively complex molecules such as C$_{60}$ and C$_{70}$ are formed. Recently, researchers have also detected complex organic and potentially prebiotic molecules, such as amino acids, in meteorites and in other space environments. Those discoveries have further stimulated the debate on the origin of the building blocks of life in the universe. Rotational spectroscopy plays a crucial role in the investigation of planetary atmosphere and the interstellar medium. Increasingly these astrochemical investigations are assisted by quantum-mechanical calculations of structures as well as spectroscopic and thermodynamic properties to guide and support observations, line assignments, and data analysis in these new and chemically complicated situations.\footnote{V. Barone, M. Biczysko, C. Puzzarini 2015, Acc. Chem. Res., 48, 1413} However, it has proved challenging to extend accurate quantum-chemical computational approaches to larger systems because of the unfavorable scaling with the number of degrees of freedom (both electronic and nuclear). In this contribution, it is demonstrated that it is now possible to compute physicochemical properties of building blocks of biomolecules with an accuracy rivaling that of the most sophisticated experimental techniques. We analyze the spectroscopic properties of representative building blocks of DNA bases (uracil and thiouracil), of proteins (glycine and glycine dipeptide analogue), and also of PAH (phenalenyl radical and cation)

PREBIOTIC MOLECULES IN INTERSTELLAR SPACE: ROTATIONAL SPECTROSCOPY OF CYANOMETHANIMINE AND ETHANIMINE

Ethanimine and cyanomethanimine are possible precursors of amino acids, and thus they are considered important prebiotic molecules that may play an important roles in the formation of biological building blocks in the interstellar medium. In addition, their identification in Titan's atmosphere would be important for understanding the abiotic synthesis of organic species. For both molecules, an accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the E and Z isomers has been carried out by means of a composite scheme based on coupled-cluster techniques. By combining the computational results with new millimeter-wave measurements, up to 300 GHz for ethanimine and to 420 GHz for cyanomethanimine, the rotational spectra of both isomers can be accurately predicted up to 500 GHz for ethanimine and 700 GHz for cyanomenthanimine. For the latter, spectral features have been searched in the mm-wave range using the high-sensitivity and unbiased spectral surveys obtained with the IRAM 30-m antenna in the ASAI context, thus sampling the earliest stages of star formation from starless to evolved Class I objects