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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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Phenazine C12H8N2 and chloranilic acid C6Cl2H2O4 with the atom labels as employed in the present work.
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fig1: Phenazine C12H8N2 and chloranilic acid C6Cl2H2O4 with the atom labels as employed in the present work.

Mentions: In a completely different approach, charge flipping was applied for the direct solution of the incommensurately modulated structure in superspace (Palatinus & Chapuis, 2007 ▶; Palatinus, 2013 ▶). For the solution, the software SUPERFLIP suggested the centrosymmetric symmetry . Since we knew that the modulation is non-centrosymmetric, we have chosen the superspace group . JANA2006 was subsequently used to extract the basic structure positions and values of the first-order harmonics of the displacive modulation functions for all non-H atoms. Atoms were then named to match Fig. 1 ▶. For this model the basic structure coordinates were refined against all reflections, resulting in , and . H atoms were added at calculated positions near C atoms as in model A. Refinement of the atomic coordinates within the riding model resulted in , and . Subsequent refinement of anisotropic ADPs of the non-H atoms resulted in , and . H atoms of the hydroxyl groups were located in the difference-Fourier map and treated as in model A. Refinement of the restrained model gave , and . Refinement of the free model gave , and . Small values were applied to the displacive modulation functions of the H atoms. Refinement of the modulated structure resulted in , and . Finally, modulation parameters were introduced for the ADPs of the non-H atoms, resulting in the final fit of model C to the diffraction data as given in Table 2 ▶.


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)

Phenazine C12H8N2 and chloranilic acid C6Cl2H2O4 with the atom labels as employed in the present work.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig1: Phenazine C12H8N2 and chloranilic acid C6Cl2H2O4 with the atom labels as employed in the present work.
Mentions: In a completely different approach, charge flipping was applied for the direct solution of the incommensurately modulated structure in superspace (Palatinus & Chapuis, 2007 ▶; Palatinus, 2013 ▶). For the solution, the software SUPERFLIP suggested the centrosymmetric symmetry . Since we knew that the modulation is non-centrosymmetric, we have chosen the superspace group . JANA2006 was subsequently used to extract the basic structure positions and values of the first-order harmonics of the displacive modulation functions for all non-H atoms. Atoms were then named to match Fig. 1 ▶. For this model the basic structure coordinates were refined against all reflections, resulting in , and . H atoms were added at calculated positions near C atoms as in model A. Refinement of the atomic coordinates within the riding model resulted in , and . Subsequent refinement of anisotropic ADPs of the non-H atoms resulted in , and . H atoms of the hydroxyl groups were located in the difference-Fourier map and treated as in model A. Refinement of the restrained model gave , and . Refinement of the free model gave , and . Small values were applied to the displacive modulation functions of the H atoms. Refinement of the modulated structure resulted in , and . Finally, modulation parameters were introduced for the ADPs of the non-H atoms, resulting in the final fit of model C to the diffraction data as given in Table 2 ▶.

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