Looking for homochirality in the inter-stellar medium.

In this report we address the question of whether some chiral molecules have a probability of being detected in the interstellar medium (ISM). To this end we rely on the Minimum Energy Principle which states that the most abundant isomer of a given generic formula should be that of lowest energy. The relative stability of the chiral molecules with respect to the other possible species of the same chemical formula are calculated by means of quantum simulations based on density functional theory (DFT). The result is that no chiral isomer in the C(3)H(6)O (acetone), C(2)H(5)ON, C(3)H(7)ON (amide), C(2)H(5)O(2)N, C(3)H(7)O(2)N (amino acid) families is the most stable species. This is also true of the C(2)(H(2)O)(2) and C(3)(H(2)O)(3) species when restricted to the sugar families, but another chiral molecule of the same chemical formula, i.e. lactic acid HOCH(CH(3))COOH is the most stable of all structures. Two other molecules with an NH(2) group, namely, NH(2)CH(CH(3))CN, the precursor of alpha-alanine and NH(2)CH(CH(3))OH, the simplest chiral molecule, are also the most stable species in their respective families. These three molecules satisfy the conditions for being detected according to the Minimum Energy Principle. With dipoles moments of 2.3, 2.7 and 1.6 Debye respectively, they make appealing targets. The present study should encourage laboratory experiments to determine rotational constants of higher precision prior to submission of observation proposals.

Looking for heteroaromatic rings and related isomers as interstellar candidates.

Finding complex organic molecules in the interstellar medium (ISM) is a major concern for understanding the possible role of interstellar organic chemistry in the synthesis of prebiotic species. The present interdisciplinary report is a prospective study aimed at helping detection of heteroaromatic compounds or at least of some of their isomers in the ISM. The thermodynamic stabilities of the C(4)H(5)N, C(4)H(4)O, C(4)H(4)S families were calculated using density functional theory (DFT). It was found that pyrrole, furan and thiophene are unambiguously the most stable isomers at the 10-50 K temperatures of the ISM. Several of the less stable isomers were synthesized and flash vacuum thermolysis experiments were performed on these species. Although the detection of pyrrole in the pyrolysis of many compounds has been reported in the literature, we observed that none of its isomers led to pyrrole in these conditions, which suggests that other formation routes are to be considered. On the other hand, these three aromatic compounds present a very high stability, few % been decomposed at 1500 K by flash vacuum thermolysis; these experiments also show a great stability of crotonitrile that is the most stable compound that can be formed in these conditions. The rotational constants, dipole moments and IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these prototype molecules.

Electronic structure of simple phosphorus containing molecules [C,xH,O,P] candidate for astrobiology (x=1, 3, 5).

The present report is a prospective study aimed at finding phosphorus containing compounds for astrobiology. Since PN, PC and HCP are the only species detected so far, it was deemed reasonable to enlarge the quest for phosphorus compounds to mixed carbon oxygen containing compounds [C,xH,O,P] analogue to the CHON family. Ab initio Møller-Plesset (MP2), Coupled Cluster (CCSD(T)) and Density Functional Theory (DFT) were used. State of the art level of theory, CCSD(T)/cc-pVQZ, was necessary to show that CH3-PH2=O is the most stable isomer, with CH3-PH-OH close by in the [C,5H,O,P] sub-family. This structure has the same C-P-O connectivity as the most stable compound of the [C,3H,O,P] sub-family, CH3-P=O but differs from the simplest [C,H,O,P] system HP=C=O. Rotational constants B=7.1377 and C=6.0636 GHz associated with a dipole moment of 4.2 Debye together with an IR spectrum with very strong bands at 1214, 2282, 2264 and 1039 cm(-1) have been calculated for CH3-PH2=O. For CH3-P=O, one has B=7.9881 and C=6.4659 GHz, a dipole moment of 2.9 Debye and four IR bands at 1198, 623, 835, 1256 cm(-1) of medium intensity. The simplest HPCO system with B=5.5206 and 5.3952 GHz and a dipole moment of 0.8 Debye has only one very strong IR frequency at 2037 cm(-1). The above values should be precise enough to encourage laboratory experiments on these prototype molecules.