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Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine-chloranilic acid.

Noohinejad L, Mondal S, Ali SI, Dey S, van Smaalen S, Schönleber A - Acta Crystallogr B Struct Sci Cryst Eng Mater (2015)

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.

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.

No MeSH data available.


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Interatomic distances (Å) as a function of phase t of the modulation. The t plot for model B (in blue) is superimposed onto the t plot for model A (in black), after application of a phase shift of −0.5139 in t to model B.
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fig2: Interatomic distances (Å) as a function of phase t of the modulation. The t plot for model B (in blue) is superimposed onto the t plot for model A (in black), after application of a phase shift of −0.5139 in t to model B.

Mentions: Model A and model B give the same fit to the diffraction data (Table 2 ▶). Although modulation parameters are different, these two models are completely equivalent. They differ from each other by a phase shift (Fig. 2 ▶). Further support for model A comes from difference-Fourier maps obtained after refinements of model A and of a similar model without the acidic H atoms (see the supporting information). Model C has been obtained by solving the modulated structure by charge flipping in superspace. The modulation of model C is different from the modulations in models A and B, but is clearly higher for model C than for the other two models (Table 2 ▶). Therefore, model C provides a less good description of the modulation than models A and B do. Difficulties in obtaining the correct structure model by charge flipping are probably related to the pseudo-symmetry of the structure, with deviations from inversion symmetry being mainly the result of rearrangements of H atoms.


Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine-chloranilic acid.

Noohinejad L, Mondal S, Ali SI, Dey S, van Smaalen S, Schönleber A - Acta Crystallogr B Struct Sci Cryst Eng Mater (2015)

Interatomic distances (Å) as a function of phase t of the modulation. The t plot for model B (in blue) is superimposed onto the t plot for model A (in black), after application of a phase shift of −0.5139 in t to model B.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4383393&req=5

fig2: Interatomic distances (Å) as a function of phase t of the modulation. The t plot for model B (in blue) is superimposed onto the t plot for model A (in black), after application of a phase shift of −0.5139 in t to model B.
Mentions: Model A and model B give the same fit to the diffraction data (Table 2 ▶). Although modulation parameters are different, these two models are completely equivalent. They differ from each other by a phase shift (Fig. 2 ▶). Further support for model A comes from difference-Fourier maps obtained after refinements of model A and of a similar model without the acidic H atoms (see the supporting information). Model C has been obtained by solving the modulated structure by charge flipping in superspace. The modulation of model C is different from the modulations in models A and B, but is clearly higher for model C than for the other two models (Table 2 ▶). Therefore, model C provides a less good description of the modulation than models A and B do. Difficulties in obtaining the correct structure model by charge flipping are probably related to the pseudo-symmetry of the structure, with deviations from inversion symmetry being mainly the result of rearrangements of H atoms.

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.

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.

No MeSH data available.


Related in: MedlinePlus