Theoretical and Computational Astrochemistry
- Andrea Lombardi (University of Perugia, Italy)
- Andrea Ferrara (Scuola Normale Superiore, Pisa, Italy)
- Savino Longo (University of Bari, Italy)
The molecular nature of the universe is nowadays at the focus of modern science in view of the recently disclosed interesting chemistry in the planets of the solar system, in the stars and in the interstellar medium, regarding the abundance of the elements and their isotopic distribution, the chemical composition, the reactions of atoms and molecules and their interactions with ultraviolet radiation and cosmic rays. In the last decades the attention to molecules in planetary atmospheres and in interstellar matter  has became a fundamental part of the research work of chemists, biologists and physicists, and it is common language in science to refer to this field of investigation as Astrochemistry. This workshop aims at reviewing the state of the art and at indicating future trends with respect to the very specific and particular role that theoretical and computational chemistry and molecular dynamics are called to play.
Tremendous advancements in astrochemistry have been fostered particularly by radio-astronomical observations and also, more recently, by extension of spectroscopic measurements to other wavebands (infrared, visible, UV).
Direct space exploration and sampling have further enlarged the perspectives of our understanding of the chemical evolution of the universe. Comparison of astronomical observations with laboratory experiments and theoretical modeling has allowed the identification of hundreds of molecule in space including also highly complex molecules containing from two to more than six atoms.
It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment such as the interstellar medium. In recent years it has become clear that not only ion/radical-molecule gas-phase reactions, but also solid state reactions on icy dust grains play an important role in the formation of new species. In order to investigate the underlying processes, laboratory based experiments have shown the need to simulate surface reactions induced by photon (UV) processing or particle (atom, cosmic ray, electron) bombardment of interstellar ice analogs.
Quantum Chemistry and structure of molecules
Astrochemical investigations have to be assisted by quantum-mechanical calculations of molecular properties, such as structures, transition frequencies etc.., to drive and interpret observations, line assignments and data analysis in the most complicated and new cases.
A paradigmatic example is that of spectroscopic techniques, which are widely used to infer information on molecular structure and dynamics and are therefore playing a crucial role in the investigation of atmospheric chemistry and interstellar medium , since the last decades. The primary target is the identification of chemical compounds in space as well as planetary atmospheres [3,4]. The relevance of this combined approach is well documented and there is an ever-growing number of papers dealing with this topic (see, for example  and ).
The formation of complex molecules, involves reactions occurring in gas-phase and at the surface of either icy particles or minerals, these latter acting as effective catalysts involving proton/electron transfer, nucleophilic/electrophilic attacks. These processes require approaches based on more refined representations of the wave function such as Møller-Plesset (MP2) and coupled cluster [e.g., CCSD(T)] methods, which are adopted when high accuracy is needed (see  and references therein). Applicability of these methods depends on the size of the systems to model. Approaches based on electron density (DFT) rather than on the wave function have become computationally cheaper alternatives to MP2 and CCSD(T), while achieving acceptable accuracy by means of well-designed electron density functionals .
To understand how the molecules in the universe can be generated, one has to look at the chemical processes taking place in the space, then chemical reaction systems have to be considered to complement the usual physical models for objects in space.
Such reactions occur in a number of environments, as astrophysical plasmas, atmosphere of stars, interstellar medium and so on, and the knowledge of their cross sections and rate constants is determinant in the modelization of these mediums. The list of interesting reactions is long. Consider for example radiative association reactions , fundamental for the early universe chemistry for the formation of simple diatomic molecules as H2, HD or LiH assumed to be crucial in the description of the spherical collapse of primordial gas clouds . Such process has also been proposed for the formation of H2O from atomic oxygen and H2 in the high atmosphere of stars in the 1,500–3,000 K temperature region . Another important class of processes is that of charge transfer between multiply charged ions and neutral targets. This is a fundamental process for the description of the interstellar medium which drives the ionization balance of charged species
The many kinds of molecular processes involve gas-phase chemistry, surface chemistry (dust fraction, adsorption, desorption), low and ultralow-temperature chemistry as well as high temperatures, suprathermal atoms and molecules .
The theoretical treatment need to be developed by using ab initio molecular calculations, and classical, semi-classical and quantum and dynamics approaches.
Two main classes of treatment can be distinguished: methods of chemical kinetics (see e.g. stochastic techniques ) and dynamical methods aimed at the detailed study of the collision and reaction dynamics at a state-to-state level. The most complete knowledge of the complex chemical environment can only be reached by combining these two approaches to obtain state-to-state kinetic models and corresponding databases .
The recent detection of molecules containing carbon having a prebiotic nature have further stimulated the debate on the origin of the building blocks of life in the universe opened by the discovery of amino acids and other protobiological molecules in meteorites , and possibly in other space environments. Then many efforts are focused on the physical, chemical and astrophysical processes by which prebiotic molecules, typically chiral, can be produced in interstellar dust and spread out all over, to the earth or other planets.
The role that chirality plays in the origin of life and then in biology and astrobiology is in connection with the origin of homochirality in biology. Many advanced hypotheses are under scrutiny and none has yet received a global consensus (see  and references therein). For example, evidence of the role of circular polarized light and in general of magnetic and electric fields, although experimentally demonstrated, appears circumstantial, because intensity of such fields has not been proved to be sufficient to induce substantial effects in the production of a specific enantiomer of a chiral species. For a review, see Avalos et al. , and also Rikken and Raupach  for asymmetric synthesis. The latest experimental observations (see, for example Stranges, ) of dichroic effects in photoionization required very intense circularly polarized synchrotron radiation, probably not available in nature. Recent investigations of the origin of asymmetry based on accurate quantum chemical calculations have also shown that parity violation due to weak forces leads to extremely small energy difference for enantiomers (see the review by Quack, , the latest papers by Faglioni et al. , and references therein).
Ray et al., , had shown that when chiral molecules are given a specific orientation in a film, asymmetry results for the scattering by polarized electrons. More recently Kim et al.  studied photoemission by absorbed chiral molecules. Vattuone et al.  and Gerbi et al.  collaborated with our group to demonstrate stereodynamic effects in scattering from surfaces of molecules aligned according to our technique described above.
Among the possibility of chiral physical fields, circularly polarized photons, and the magnetochiral effect induced by the magnetic field and unpolarized light, have been shown to be enantioselective in photochemical reactions. But in general translation-rotation motions are true chiral force fields: recently, liquid vortex motions have been shown to induce chiral discrimination in the formation of mesophase aggregates of achiral porphyrins (Ribó et al., ), although some controversy was also generated .
Busalla et al.  have given a theoretical proof that in collisions between unpolarized projectiles and chiral molecules, the differential cross sections for a molecule and its enantiomer differ if the molecules are oriented. They also showed (Musigmann et al., [26,27]) that left- and right-handed molecules can scatter unpolarized electrons differently if a chiral framework is provided by at least three non-coplanar polar vectors defined in the collision processes (see also Thompson and Blum, ; Thompson, ). Note that no experimental verifications of these predictions appear to be so far available. Very recently, molecular collisions have been proposed as a possible mechanism for chiral discrimination [30-32], while T.M. Su et al. have given a direct experimental proof of discrimination between optical rotamers of simple organic molecules induced by macroscopic translation-rotational motions 
From a theoretical and computational point of view, the many chemical processes occurring on dust grains need to be treated by methods from gas-phase and surface chemistry, using rate equations or stochastic techniques. Classical and quantum dynamics approaches are employed to calculate cross sections, needed for rate constant calculations and in the analysis of molecular spectra.
This workshop will provide an opportunity to discuss the role that theoretical and computationalchemistry has recently been called to play on the themes above mentioned. Objectives are the handling of both the vast amount of data sent by the probes exploring the solar system on the processes taking place in the atmospheres of planets and satellites, and the exploit of the information coming
from the largely unexpected radio-astronomic and spectroscopic discovery of numerous molecules of increasing complexity in interstellar space.
The main idea of the workshop is to bring together the experts in the fields of quantum chemistry and molecular dynamics with specific interest and competence on these issues, enabling them to share the last developments, establish current state-of-the-art and draw future trends and research directions. In this regard, crucial for the success of the initiative will be the active participation of young researchers and of doctoral students.
The three days of the meeting will develop a logical thread that includes
(i) – astrochemistry of the early universe (hydrogen and deuterium plasmas, helium, lithium) [34-36] and of the present universe; photodissociation ; interstellar molecules; role of molecules and dust grains in the formation of early cosmic structures; dust nucleation/growth.
(ii) – chemistry of planets and of their atmospheres; PAHs formation [38-40]; dynamics of Carbon-chain-anion.
(iii) – chemistry under extreme conditions: low and ultralow temperatures, high temperatures, suprathermal particles, with reference to proto- and exo-biology; chemical mechanisms related to the origin of homochirality in the biosphere.
The logic reflects the multiscale nature of astrochemical modeling, following a path involving
(i) – advanced techniques for molecular structure, thermodynamics and spectroscopic calculations;
(ii) – recent developments in chemical reaction dynamics, and in energy and radiation transfer theories;
(iii) – state-of-the-art methods of molecular dynamics for modeling of astrochemical environments, including Master equation and stochastic approaches;
within the scope of modern quantum chemistry and molecular dynamics approaches.
The objectives of the workshop are the following:
- to review the state-of-the art regarding the specific role of the theoretical atomic and molecular physics and of quantum chemistry with respect to the range of problems arising in analyzing data from spectroscopy and probes, and modeling astrophysical, planetary and exo-biological environments, to establish situation of already well investigated areas and possibly to individuate new ones.
- to present advances in the quantum chemical codes for the structure calculations of the numerous species progressively, individuated as of astrochemical interest, and in the exact and approximate (quantum, classical, semi-classical) approaches to the dynamics of processes of interest for modeling.
- to discuss the status of models for the chemical evolution of the universe, of the chemistry of planetary atmospheres, the possible fingerprints, of extraterrestrial life and molecular biology.
- to identify the urgent challenges of astrochemical modeling related to current scientific breakthrough and on-going or scheduled space missions.
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