Contributed Talk

Spectroscopic Characterization of Molecules in Space by Theoretical Chemistry: From Rovibrational Spectroscopic Constants to CO₂/SO₂ IR Line Lists

¹Xinchuan Huang, ²David Schwenke, ²Timothy Lee (¹SETI Institute, ²NASA Ames Research Center)

Recently, theoretical chemistry has proven to be of vital significance in the prediction of spectroscopic properties necessary for the detection of interstellar molecular species. For example, Cernicharo and co-workers have identified C₅N^- in an astronomical spectroscopic observation on the basis of highly accurate spectroscopic constants determined from ab initio calculations because no laboratory experimental data is yet available. Combining 2nd-order perturbation theory and rovibrational variational calculations with highly accurate quartic force fields (QFF) fitted from CCSD(T) energies extrapolated to the complete basis set limit and enhanced by inclusion of corrections for core-valence electron correlation, scalar relativity, and higher order correlation, now we are able to provide reliable geometrical structures, spectroscopic constants, and vibrational fundamentals for those molecules that are difficult to study in laboratory experiments. Our recent studies include the cis- and trans-HOCO radical/anion/cation, cyclic and bent HC₂N and C₃H³⁺ and their isotopologues. Agreement between the best computed results and the available experimental data are usually about 10 to 100 MHz for rotational constants and 0-5 cm^-1 for vibrational fundamentals. In addition, we have been working on highly accurate IR line lists for some molecules considered to be "weeds" and thus their spectra need to be eliminated or assigned so that the "flowers" can be identified. Recent progress made for CO₂ and SO₂ IR line lists, where 0.01-0.03 cm^-1 accuracy for line positions and better than 85-95% for IR intensities is obtained, will be discussed. It is hoped that these accurate results will be useful in the interpretation of high-resolution astronomical observations or laboratory experiments.

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