The detection of molecules in space has largely depended upon having reliable reference data for comparison to astronomical spectra. However, unexplained spectral features still arise from every telescopic observation of the interstellar medium (ISM) while the list of experimentally comparable species grows short. In exploring possible carriers for these features, the astrochemist's creativity can be limited by the available experimental resources. Quantum chemistry has no such hinderances where any molecule that can be dreamed can be examined as simple input to the program, and computational accuracies are improving continually.
Modern, high-level, coupled cluster quartic force field methods are able to reproduce known vibrational transitions to better than 1.0 cm-1 in some cases. A prime example for the benefit of quantum chemistry in astrochemical spectral analysis comes in the mass-57 proton bound complexes comprised of carbon monoxide and/or nitrogen molecules. The isobaric composition of these compounds frustrates mass selection experiments. However, our quantum chemical work has shown that the bright proton "rattle" fundamental frequencies for four of these species are quite disparate and distinguishable. Another difficult class of molecules to examine experimentally is noble gas molecular cations. Since the detection of ArH+ in the Crab nebula in 2013, noble gas molecules are being pursued for their possible existence in the ISM. Our work has shown that six covalent-level noble gas molecules and even three proton-bound complexes, like the mass-57
36Ar-H-20Ne+ complex, are stable enough to be observed rotationally and
vibrationally in space. Finally, anions are hypothesized to form in the ISM via dipole-bound excited states, and the rovibrational spectra for many such candidate anions has also been provided via quantum chemistry in our group.
In the strange world of astrochemistry, quantum chemistry opens the doors to exploration in novel ways allowing theory and experiment to work together to solve some of the most fascinating problems in astrophysics.