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LiO-: A Ground-State Triplet Biradical with a Record-Breaking Basicity

Posted on 13/10/2008

Bun Chan and Leo Radom, members of the University of Sydney node of the Centre, working in collaboration with a group led by Professor Steven Kass at the University of Minnesota, have discovered the most basic gas-phase substance yet known.

Through state-of-the-art quantum chemistry calculations, the Sydney team predicted that the lithium oxide anion (LiO-) has a gas-phase basicity (proton affinity) of 1782± 8 kJ mol-1. The previous record basicity of 1744 kJ mol-1 belonged to the methyl anion, on this corresponds to a significant raising of the bar. The Lio- result is equivalent to saying the LiOH is the weakest known gas-phase acid, having taken over this mantle from methane.

LiO- is derived formally by removal of a proton from lithium hydroxide (LiOH), and might intuitively have been expected to be a ground-state singlet. However, the calculations of the Centre researchers indicate that LiO- is a ground-state triplet biradical. In order to experimentally test the dramatic basicity prediction, the Minnesota group used electrospray ionisation mass spectrometry to generate and isolate the hard-to-synthesise LiO- from lithium oxalate. They were able to prove by exact mass measurements that LiO- was indeed the ion that they produced. In addition, by observing that LiO- can transfer an electron to molecular oxygen, they were able to determine an electron affinity for the LiO• radical. A thermodynamic cycle then enabled them to obtain a basicity for LiO- of 1781± 25 kJ mol-1, nicely consistent with the theoretical prediction.

"Given that the methyl anion stood at the top of the basicity scale for more than 30 years, this is a very exciting result," remarked Professor Kass. "And we think that it will be a major challenge for anyone to come up with a species of greater basicity than LiO-". "This is a great example of a synergistic interaction between experiment and theory," commented Professor Radom, Centre Chief Investigator from the University of Sydney.