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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.


Related in: MedlinePlus

Selected interatomic distances (Å) as a function of phase t of the modulation. (a) The O—H and N—H distances within the two hydrogen bonds. (b) C—C and C—O distances of the resonance system stabilizing the Hca− anion, as well as the C6—O2 distance not involved in resonance. Notice the different length scale on the vertical axes for panels (a) and (b).
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fig3: Selected interatomic distances (Å) as a function of phase t of the modulation. (a) The O—H and N—H distances within the two hydrogen bonds. (b) C—C and C—O distances of the resonance system stabilizing the Hca− anion, as well as the C6—O2 distance not involved in resonance. Notice the different length scale on the vertical axes for panels (a) and (b).

Mentions: More precisely, the crystal structure of the FE-I phase contains one crystallographically independent molecule H2ca with two independent O atoms involved in O—H⋯N hydrogen bonds, denoted by O1 and O2 (Gotoh et al., 2007 ▶). The basic structure of the FE-IC phase is the same as the crystal structure of the FE-I phase, so that the FE-IC phase contains modulated atoms O1 and O2. Finally, the FE-II phase represents a twofold superstructure of the structure of the FE-I phase. Together with a reduction of the point symmetry to triclinic, this results in four crystallographically independent molecules H2ca with atoms O1A through to O1D derived from O1, and atoms O2A through to O2D derived from O2 (Noohinejad et al., 2014 ▶). In all three phases, the hydrogen bonds O2—H1o2⋯N2 are not involved in superstructure formation. For the FE-IC structure, Table 3 ▶ and Fig. 3 ▶(a) show that bond lengths involving the O2, H1o2 and N2 atoms exhibit only a weak dependence on phase t of the modulation. For the FE-I and FE-II structures this property has been previously determined by Gotoh et al. (2007 ▶) and Noohinejad et al. (2014 ▶), and it is summarized in Table 3 ▶. Therefore, the hydrogen bonds O2—H1o2⋯N2 do not play a direct role in the ferroelectricity of this compound.


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)

Selected interatomic distances (Å) as a function of phase t of the modulation. (a) The O—H and N—H distances within the two hydrogen bonds. (b) C—C and C—O distances of the resonance system stabilizing the Hca− anion, as well as the C6—O2 distance not involved in resonance. Notice the different length scale on the vertical axes for panels (a) and (b).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig3: Selected interatomic distances (Å) as a function of phase t of the modulation. (a) The O—H and N—H distances within the two hydrogen bonds. (b) C—C and C—O distances of the resonance system stabilizing the Hca− anion, as well as the C6—O2 distance not involved in resonance. Notice the different length scale on the vertical axes for panels (a) and (b).
Mentions: More precisely, the crystal structure of the FE-I phase contains one crystallographically independent molecule H2ca with two independent O atoms involved in O—H⋯N hydrogen bonds, denoted by O1 and O2 (Gotoh et al., 2007 ▶). The basic structure of the FE-IC phase is the same as the crystal structure of the FE-I phase, so that the FE-IC phase contains modulated atoms O1 and O2. Finally, the FE-II phase represents a twofold superstructure of the structure of the FE-I phase. Together with a reduction of the point symmetry to triclinic, this results in four crystallographically independent molecules H2ca with atoms O1A through to O1D derived from O1, and atoms O2A through to O2D derived from O2 (Noohinejad et al., 2014 ▶). In all three phases, the hydrogen bonds O2—H1o2⋯N2 are not involved in superstructure formation. For the FE-IC structure, Table 3 ▶ and Fig. 3 ▶(a) show that bond lengths involving the O2, H1o2 and N2 atoms exhibit only a weak dependence on phase t of the modulation. For the FE-I and FE-II structures this property has been previously determined by Gotoh et al. (2007 ▶) and Noohinejad et al. (2014 ▶), and it is summarized in Table 3 ▶. Therefore, the hydrogen bonds O2—H1o2⋯N2 do not play a direct role in the ferroelectricity of this compound.

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