Molecular properties required for astrochemical modeling can sometimes be obtained more easily through quantum chemical calculations than laboratory measurements. I will discuss two examples, both required method development. The circular polarization of methanol masers carries information on magnetic field strengths, which play a crucial role in models that try to explain how stars much heavier than our sun can be formed. Spectroscopic determination of the Zeeman parameters that relate spectral shifts of torsion-rotation transitions to magnetic field strength, first attempted in the early fifties, has not yet been achieved. Electronic structure codes are available to compute the rotational g-tensor of methanol, i.e., the coupling of the overall rotation to a magnetic field. However, the calculation of the Zeeman parameters of methanol is complicated by the internal rotation of the methyl group with respect to the hydroxyl group, and the hyperfine couplings arising the nuclear spins of the four hydrogen atoms. Ro-vibrational emission of CO is observed in protoplanetary disks and star-forming regions. Extracting information on densities and temperatures from the observed intensities requires knowledge of Einstein A coefficients and also collisional properties when the environments are not in local thermal equilibrium. The rates of ro-vibrational H+CO collisions are particularly difficult to compute due to the strong chemical interaction. As a result, we only recently succeeded in obtaining converged results for coupled channels calculation. The Nead-Infrared spectrograph and the Mid-Infrared instruments on board the planned James Webb Space Telescope mission will be able to observe molecules with no permanent dipole moment such as carbon dioxyde, acetylene and methane. Very little is know experimentally on ro-vibrational collision rates for these molecules. I will discuss a new methods which treats rotation exactly and includes the ro-vibrational coupling in the Born approximation. We are applying this method to compute collision rates for these polyatomic molecules.

Probing magnetic fields with methanol masers and ro-vibrational collision rates for modeling protoplanetary disks

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.