Astronomical complex organic molecules: Quantum chemistry meets rotational spectroscopy

Abstract

Astrochemistry is an interdisciplinary field involving chemistry, physics, and astronomy, which encompasses astronomical observations, modeling, as well as theoretical and experimental laboratory investigations. In the frame of the latter, this contribution provides an overview on the computational approaches supporting and complementing rotational spectroscopy experiments applied to astrochemical studies. The focus is on the computational strategies that permit accurate computations of structural and rotational parameters as well as of energetics and on their application to case studies, with particular emphasis on the so-called "astronomical complex" organic molecules.computational astrochemistry, complex organic molecules, computational composite schemes, rotational spectroscopy, quantum-chemical calculations | I N T R O D U C T I O NFor many years, the interstellar medium (ISM) was considered too hostile for organic species to be formed. This paradigm of thought began to deteriorate roughly forty years ago with the discovery of molecules containing carbon chains and rings. As time has gone on, the pace of molecular discovery has accelerated, and the detection in the last decade of molecules showing some significant complexity, like for example, glycolaldehyde (CH 2 OHCHO), [1] acetamide (CH 3 C(O)NH 2 ), [2] and methyl acetate (CH 3 OC(O)CH 3 ), [3] has changed this view dramatically. [4] Indeed, the detection of almost 200 molecules in interstellar or circumstellar shells [5] suggests that the ISM is characterized by a rich chemistry. [4] Among the "complex" organic molecules detected, where complexity has to be considered in relative terms, that is, from an astronomical point of view, the potentially prebiotic molecules have been attracted particular interest in relation to the issue of how life has been originated. [6] The debate on the origin of the biomolecule building blocks has been further stimulated by the discovery of nucleobases and amino acids in meteorites and in other space environments (see, for example, Refs. [7,8]).Understanding the chemical evolution of the universe is one of the main aim of astrochemistry, [4,6] which is an interdisciplinary field involving chemistry, physics, and astronomy, encompassing astronomical observations, modeling, as well as theoretical and experimental laboratory investigations. The starting point for the development of astrochemical models is the knowledge whether a molecule is present in the astronomical environment considered and, if so, its abundance. In this scenario, molecular spectroscopy plays a crucial role since the astronomical observation of spectroscopic signatures provides the unequivocal proof of the presence of chemical species. [9] It has also to be mentioned that, in addition to composition (by knowing which atom, ion or molecule produces the observed transition) and the corresponding abundance, several information about astronomical objects can be inferred from the analysis of the astronomical spectrum; in particular, physical parame...

Gas-Phase Stereoinversion in Aspartic Acid: Reaction Pathways, Computational Spectroscopic Analysis, and Its Astrophysical Relevance

“…For the latter, the agreement with experimental data for rotational constants, B and C , is found to be within 1 MHz, and for constant A , it is within 10 MHz, which is even better than those predicted by the MP2/6-311++G(d,p) method, the best known in the existing literature. 59 Therefore, this level of theory can be taken to be quite reliable, though in no way, it is comparable with the state-of-the-art composite scheme forwarded by the groups of Barone and Puzzarini (see ref (64) and references therein).…”

Computational challenges in Astrochemistry

“…The second step is understanding how these molecular species are formed and how they evolve chemically toward more complex species. In this scenario, molecular spectroscopy and computational chemistry play a crucial role . On one side, astronomical observations of spectroscopic signatures provide the unequivocal proof of the presence of chemical species; on the other side, quantum chemistry allows shedding light on their formation/transformation mechanisms, which are nowadays still object of debate and often not experimentally investigatable.…”

Quantum-mechanical Rotational and Vibrational Signatures of Astrophyscially relevant Gas-Phase Stereo-isomeric Species of Leucine

“…For this, a scaling factor is determined using the experimentally known rotational parameters of a similar reference species studied in the literature. 57 This strategy is in fact parallel to the 'Bottom Up' approach but employing scaled and more accurate rotational parameters. In the present work, the gas-phase experimental rotational data is available for global minimum conformer EQ0 # of Leucine, thereby acting as the reference molecular species.…”