Is it possible to detect homodimers of organic acids in the interstellar medium or circumstellar shells?

The two smallest organic acids have been already detected in the interstellar medium or circumstellar shells, they are the formic acid (H-COOH) [1] and the acetic acid (CH3-COOH) [2]. On the other hand, two substituted propenes, such as acrylonitrile (CH2=CH-CN) [3] and acrolein (CH2=CH-CHO) [4], have been observed. It seems then reasonable to assume the presence of the unsaturated acrylic acid (CH2=CH-COOH). So far no weakly bound molecular complexes have been detected in the interstellar medium or circumstellar shells. However, pairs of carboxyl groups bind strongly (about 60 kJ/mol), given that both groups act as proton donor and acceptor forming a ring containing two hydrogen bonds, so the dimers of organic acids could be the most reasonable candidates for this kind of research. Rotational spectroscopy is a powerful tool to identify molecules in gas phase. When coupled with supersonic expansion of the sample, it allows: (i) to reach very low rotational temperatures (few kelvins) obtaining simplified spectra and (ii) to stop the collisions within molecules, making it possible the investigation of usually unstable species like radicals, ions and weakly bond molecular complexes. In order to show a rotational spectrum, a species must have a permanent electric dipole moment (), so the homodimers of formic and acetic acid are not visible, because the dipole moments of the units are antiparallel and their sum is zero. The case of acrylic acid is different because, depending on the cis or trans orientation of the double bonds, it has two almost isoenergetic possible conformations (Etrans-Ecis=58(20) cm-1 [5]): when two cis or two trans conformers form a dimer mu=0, but when a cis unit binds a trans one mu=/0 and the dimer can be studied by rotational spectroscopy. We observed the pulsed jet Fourier transform microwave spectrum of the cis-trans dimer of acrylic acid and its hydroxyl deuterated isotopologues. For each species we observed two series of lines due to the concerted double transfer of the hydroxyl protons between the monomers. Based on the experimental and quantum chemical calculated data, a flexible model [6] was applied to characterize the double minimum potential energy surface that generate the tunneling doubling.