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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.

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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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Schematic representation of resonance within the anion Hca− of chloranilic acid.
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fig4: Schematic representation of resonance within the anion Hca− of chloranilic acid.

Mentions: The largest variation in bond lengths within the FE-IC phase is found for the hydrogen bond O1—H1o1⋯N1, with a variation of Δd(O1—H1o1) = 0.25 Å and Δd(N1—H1o1) = 0.42 Å (Table 3 ▶). All other bonds are much less affected by the modulation, with a maximum variation of 0.06 Å for C3—O1 in H2ca and of 0.019 Å for C14—C15 in Phz (see the supporting information). The next largest variations of bond lengths are found for C3—C2, C1—C2 and C1—O4 (Table 4 ▶). These bonds are precisely those involved in resonance stabilization of the Hca− ion, as it is obtained after transfer of the proton within the O1—H1o1⋯N1 hydrogen bond. Further evidence for this interpretation comes from t-plots (Fig. 3 ▶), which show that an elongation of the O1—H1o1 bond (interpreted as proton transfer) correlates with an elongation of the C3—C2 and C1—O4 bonds, for which resonance represents the admixture of single-bond character into these formally double bonds (Fig. 4 ▶). Concomitantly, C1—C2 and C3—O1 have become shorter due to the admixture of double-bond character into formally single bonds. A similar variation of bond lengths is found in the crystal structure of the FE-II phase (Table 4 ▶). The results support the model of partial proton transfer (see §3.3).


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)

Schematic representation of resonance within the anion Hca− of chloranilic acid.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig4: Schematic representation of resonance within the anion Hca− of chloranilic acid.
Mentions: The largest variation in bond lengths within the FE-IC phase is found for the hydrogen bond O1—H1o1⋯N1, with a variation of Δd(O1—H1o1) = 0.25 Å and Δd(N1—H1o1) = 0.42 Å (Table 3 ▶). All other bonds are much less affected by the modulation, with a maximum variation of 0.06 Å for C3—O1 in H2ca and of 0.019 Å for C14—C15 in Phz (see the supporting information). The next largest variations of bond lengths are found for C3—C2, C1—C2 and C1—O4 (Table 4 ▶). These bonds are precisely those involved in resonance stabilization of the Hca− ion, as it is obtained after transfer of the proton within the O1—H1o1⋯N1 hydrogen bond. Further evidence for this interpretation comes from t-plots (Fig. 3 ▶), which show that an elongation of the O1—H1o1 bond (interpreted as proton transfer) correlates with an elongation of the C3—C2 and C1—O4 bonds, for which resonance represents the admixture of single-bond character into these formally double bonds (Fig. 4 ▶). Concomitantly, C1—C2 and C3—O1 have become shorter due to the admixture of double-bond character into formally single bonds. A similar variation of bond lengths is found in the crystal structure of the FE-II phase (Table 4 ▶). The results support the model of partial proton transfer (see §3.3).

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