Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Lee, Kwang Sei; Kweon, Jin Jung; Oh, In Hwan; Lee, Cheol Eui

doi:10.1016/j.jpcs.2012.02.013

Cited by 18 publications

(7 citation statements)

References 29 publications

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“…2, is in good agreement with experiments. Starting at around 500 K this isotope effect decreases and only around 700 K (above the decomposition temperature of H 2 SQ [38]) do the simulations predict the same d OO . MD also gives the same d OO as the PIMD simulations at 700 K, consistent with vanishing influence of quantum effects at high temperatures.…”

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confidence: 82%

Communication: Ab initio simulations of hydrogen-bonded ferroelectrics: Collective tunneling and the origin of geometrical isotope effects

Wikfeldt¹,

Michaelides²

2014

The Journal of Chemical Physics

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Ab initio simulations that account for nuclear quantum effects have been used to examine the order-disorder transition in squaric acid, a prototypical H-bonded antiferroelectric crystal. Our simulations reproduce the >100 K difference in transition temperature observed upon deuteration as well as the strong geometrical isotope effect observed on intermolecular separations within the crystal. We find that collective transfer of protons along the H-bonding chains -facilitated by quantum mechanical tunneling -is critical to the order-disorder transition and the geometrical isotope effect. This sheds light on the origin of isotope effects and the importance of tunneling in squaric acid which likely extends to other H-bonded ferroelectrics.Ferroelectric materials have been extensively examined because of their many diverse applications in, e.g., electro-optic, piezoelectric and random access memory devices [1]. Recently interest has intensified in hydrogen (H-) bonded ferroelectrics because of the discovery of above room-temperature ferroelectricity in an organic crystal and the realization that they could potentially be used as cheaper and more environmentally friendly organic electronics [2][3][4]. H-bonded ferroelectrics are also a valuable class of materials through which we can gain deeper understanding of the fundamental nature of H-bonding. Primarily this is because they are wellcharacterized crystalline materials with a range of Hbonding configurations and in most cases have been synthesized in both their standard and deuterated forms.Many H-bonded ferroelectrics exhibit phase transitions to paraelectric phases that lack long-range ordering of protons in the H-bonds. The Curie temperature (T c ) of these transitions can dramatically increase by ∼100 K upon deuteration but the physical origin of this effect is still not fully understood. An early model [5,6] explained this giant isotope effect on the basis of tunneling of protons in double well potentials. However, this model fails to account for experimentally observed geometrical isotope effects in H-bonding geometry between the protonated and deuterated crystals, where H-bonds have been observed to elongate upon deuteration [7,8] -a so called Ubbelohde effect [9,10]. Models involving a coupling between lattice modes and proton dynamics were therefore suggested [11,12]. It was argued on these grounds that tunneling is unnecessary to explain the large increase of T c upon deuteration [7]. On the other hand, neutron Compton scattering experiments on the commonly studied KH 2 PO 4 (KDP) system [13] showed that protons occupy both sites along the H-bonds on a short time-scale * wikfeldt@hi.is above its phase transition at 124 K, indicating coherent quantum tunneling, while no such coherence was found in the deuterated crystal [14]. Theoretical work [15,16] attempted to reconcile these differing interpretations by suggesting that a mechanism behind the Ubbelohde effect may itself be collective tunneling in clusters of atoms in the crystal. However, direct ab init...

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confidence: 82%

Communication: Ab initio simulations of hydrogen-bonded ferroelectrics: Collective tunneling and the origin of geometrical isotope effects

Wikfeldt¹,

Michaelides²

2014

The Journal of Chemical Physics

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“…[5] Despite the importance of its applications, no detailed structural information of the isolated molecule has been yetr eported. Squaric acid is as olid that decomposesa t2 45 8C; [6] it cannot be transferred intacti nto the vapor phase by using conventional heating methods. It has been investigated in the condensed phases, by IR, [7] X-ray diffraction, and complementary neutron diffraction spectroscopies, [8] showing at wo-dimensionals heet structure where each C 4 O 4 H 2 group is bonded to four different units.…”

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confidence: 99%

The Shape of the Archetypical Oxocarbon Squaric Acid and Its Water Clusters

Sanz-Novo

Alonso

León

2019

Chemistry A European J

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Herein, a full structural description is presented for the archetypical supramolecular synthone squaric acid (3,4‐dihydroxy‐3‐cyclobutene‐1,2‐dione), placed in the gas phase by laser ablation and characterized by chirped pulse Fourier transform microwave technique. Free from natural environmental disturbances, two different anti‐anti and syn‐anti planar forms and the corresponding water clusters have been revealed in a supersonic expansion. The substitution structure of the most stable anti‐anti conformer has also been extracted from the analysis of the rotational spectra of the 13C and 18O isotopic species in their natural abundance. The interplay between inter‐ and intramolecular interactions involving hydroxy and carbonyl groups has been analyzed by QTAIM (quantum theory of atoms in molecules) methods for squaric acid and its water clusters to understand their chemical behavior and further rationalize their role in the stabilization of these molecular systems.

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“…A second step between 210 1C and 350 1C coincides with decomposition of the linker. 31 The resulting phase was determined to be an amorphous Zr-carbonate by FTIR spectroscopy, which transforms to tetragonal ZrO 2 at 630 1C (Fig. S7 and S8 †).…”

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confidence: 99%