title: Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine-chloranilic acid.
abstract: The co-crystal of phenazine (Phz) and chloranilic acid (H2ca) becomes ferroelectric upon cooling through the loss of inversion symmetry. Further cooling results in the development of an incommensurate ferroelectric phase, followed by a lock-in transition towards a twofold superstructure. Here we present the incommensurately modulated crystal structure of Phz-H2ca at T = 139 K with a symmetry given by the superspace group P2(1)(½ σ(2) ½)0 and σ(2) = 0.5139. The modulation mainly affects the positions of the protons within half of the intermolecular hydrogen bonds that are responsible for the spontaneous polarization in all three low-temperature phases. Evidence for proton transfer in part of the hydrogen bonds is obtained from the correlated dependence on the phase of the modulation of the lengths of bonds involved in resonance stabilization of the acidic anion, and much smaller variations of bond lengths of atoms not involved in the resonance mechanism. Incommensurability is explained as competition between proton transfer favored for single hydrogen bonds on the basis of pKa values and avoiding unfavorable Coulomb repulsion within the lattice of the resulting ionic molecules.
authors.affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Leila
authors.initials: L
authors.lastname: Noohinejad
authors.affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Swastik
authors.initials: S
authors.lastname: Mondal
authors.affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Sk Imran
authors.initials: SI
authors.lastname: Ali
authors.affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Somnath
authors.initials: S
authors.lastname: Dey
authors.affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Sander
authors.initials: S
authors.lastname: van Smaalen
authors.affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Schönleber
keywords: ferroelectric materials; incommensurate structure; protron transfer
references.citation: J Am Chem Soc. 2007 Oct 31;129(43):12920-1
references.pmid: 17924632
references.citation: Acta Crystallogr C. 2007 Jan;63(Pt 1):o17-20
references.pmid: 17206046
references.citation: J Am Chem Soc. 2005 Apr 13;127(14):5010-1
references.pmid: 15810822
references.citation: J Chem Phys. 2006 Aug 28;125(8):084715
references.pmid: 16965047
references.citation: Nat Mater. 2005 Feb;4(2):163-6
references.pmid: 15665837
references.citation: Acta Crystallogr B. 2013 Feb;69(Pt 1):1-16
references.pmid: 23364455
references.citation: Acta Crystallogr A. 2011 Jan;67(Pt 1):45-55
references.pmid: 21173472
references.citation: J Am Chem Soc. 2012 Jan 18;134(2):1036-46
references.pmid: 22142304
references.citation: Nat Mater. 2008 May;7(5):357-66
references.pmid: 18432209