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Project Progress

Understanding mass-independent fractionation in sulfur isotopes

Understanding sulfur isotope fractionation in the early Earth geochemical record is a key astrobiology goal and an important component of our proposed work. James Lyons is taking both experimental and theoretical approached to this problem. Collaborative experiments on SO₂ dissociation with J. Farquhar at the University of Maryland are still in progress. (This work will be submitted soon for publication.) Lyons is planning sulfur experiments here at UCLA in Fall 2005.

On the theoretical side, Lyons initiated collaboration with a quantum chemist (J. Francisco, Purdue University) to investigate various sulfur molecule structures and sulfur isotope exchange rate coefficients. Their first objective was to see how well ab initio models could reproduce recent measurements of S₃ structure. Thiozone, S₃, is of interest because S₃ formation occurs be a reaction isovalent to the O₃ formation reaction, O + O₂ ‡ O₃. Since O₃ is well known to exhibit mass-independent fractionation during formation, it is reasonable to expect that S₃ will do the same. The ab initio calculations show that S₃ structure (bond length and angle, vibrational frequencies) is very well reproduced by a high-level coupled-cluster theory. Isotopic shift patterns were also computed (I. Williams, University of Bath) for the two different S₃ structures. The calculations also yield an accurate potential energy surface, which will be useful for future studies of mass-independent fractionation in S₃. This work has been accepted for publication in the Journal of Chemical Physics.