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