title: Temperature- and pressure-induced proton transfer in the 1:1 adduct formed between squaric acid and 4,4'-bipyridine.
abstract: 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.
authors.affiliation: School of Chemistry and EaSTCHEM Research School, The University of Edinburgh, UK.
authors.forename: David M S
authors.initials: DM
authors.lastname: Martins
authors.forename: Derek S
authors.initials: DS
authors.lastname: Middlemiss
authors.forename: Colin R
authors.initials: CR
authors.lastname: Pulham
authors.forename: Chick C
authors.initials: CC
authors.lastname: Wilson
authors.forename: Mark T
authors.initials: MT
authors.lastname: Weller
authors.forename: Paul F
authors.initials: PF
authors.lastname: Henry
authors.forename: Norman
authors.initials: N
authors.lastname: Shankland
authors.forename: Kenneth
authors.initials: K
authors.lastname: Shankland
authors.forename: William G
authors.initials: WG
authors.lastname: Marshall
authors.forename: Richard M
authors.initials: RM
authors.lastname: Ibberson
authors.forename: Kevin
authors.initials: K
authors.lastname: Knight
authors.forename: Stephen
authors.initials: S
authors.lastname: Moggach
authors.forename: Michela
authors.initials: M
authors.lastname: Brunelli
authors.forename: Carole A
authors.initials: CA
authors.lastname: Morrison
mesh_terms: D003503:Cyclobutanes / Q000737:chemistry*; D011312:Pressure; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D013696:Temperature
chemical_list: D003503:Cyclobutanes; D011522:Protons; D011725:Pyridines; C030042:squaric acid; C034306:4,4'-bipyridyl