Temperature- and pressure-induced proton transfer in the 1:1 adduct formed between squaric acid and 4,4′-bipyridine

David M. S. Martins, Derek S. Middlemiss, Colin R. Pulham, Chick C. Wilson, Mark T. Weller, Paul F. Henry, Norman Shankland, Kenneth Shankland, William G. Marshall, Richard M. Ibberson, Kevin Knight, Stephen Moggach, Michela Brunelli, Carole A. Morrison

Research output: Contribution to journalArticlepeer-review

77 Citations (SciVal)


We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.
Original languageEnglish
Pages (from-to)3884-3893
Number of pages10
JournalJournal of the American Chemical Society
Issue number11
Early online date2 Mar 2009
Publication statusPublished - 25 Mar 2009


Dive into the research topics of 'Temperature- and pressure-induced proton transfer in the 1:1 adduct formed between squaric acid and 4,4′-bipyridine'. Together they form a unique fingerprint.

Cite this