Charge Density Analysis of an Organic Ferroelectric. Croconic Acid: an Experimental and Theoretical Study

Abstract

The charge density in the high density (ρ = 1.912 g·cm -3 ) organic ferroelectric, croconic acid, was determined from low temperature (20 K) X-ray diffraction data, and DFT theoretical calculations. The high density was associated with strong intermolecular interaction IntroductionThe recent discovery of the ferroelectric properties of croconic acid (4,5-dihydroxy-4-cyclopentene-1,2,3-trione, C 5 H 2 O 5 ) crystals [1] has solicited new interest in this high density (ρ = 1.912 g·cm -3 ) organic compound, both in applications and research science. For example, a successful attempt has recently been made to deposit thin films of croconic acid using Matrix-assisted pulsed laser deposition (MAPLE). [2] Additional density functional theoretical calculations, [3,4] X-ray photoelectron spectroscopy combined with ab initio methods, [5] as well as studies by NQR and NMR spectroscopy [6] reflect the increasing attraction of this compound in the last two years. Although croconic acid has been known since 1825, when it was discovered by L. Gmelin [7] who named it "croconic" due to its yellow color, and despite being used as a colorant and for preparation of various dyes for a long time, the publication of its crystal structure first appeared in 2001. [8] This work was followed by further structural studies on the monovalent [Li + , K + , Na + , Rb + , Cs + , (NH 4 ) + ] cation salts and croconic acid itself [9] to understand the nature of hydrogenbond interactions and packing forces in such crystals. Nevertheless, the investigation of the charge density distribution in the croconic acid crystal has not been reported.In this paper we present an extensive study of the electron density distribution and its derivatives to gain insight into the high density and ferroelectric properties of this compound. Three different methods were applied and the results compared with each other: data obtained from a precise X-ray diffraction * Prof. Dr. A. A. Pinkerton Fax: +1-419-530-4033 E-Mail: A.Pinkerton@utoledo.edu [a]1969 energies. The spontaneous polarization is correlated with the derived molecular dipole moment (9.3 Debye). Improvements in multipole refinement protocols are described. Limitations in the multipole model with respect to heteronuclear bond properties are discussed.

Quantifying the Inter- and Intramolecular Interactions in Crystalline Phthalic Acid

“…We have previously observed values for O···H (1.4−11.3 kJ mol −1 ), O···C (2.6−8.7 kJ mol −1 ), C···C (3.4−4.5 kJ mol −1 ), and H···H (1.3−5.2 kJ mol −1 ). 17,22,31,39,42 However, together they provide sufficient energy to modify any prediction of crystal packing based on hydrogen bonded motifs alone.…”

Hirshfeld atom refinement for modelling strong hydrogen bonds

“…The other feature showing the superiority of HAR is the presence of circular regions of negative deformation density around the nuclei of the nonhydrogen atoms, a characteristic that is not observed for the multipole-based models. This deficiency of the standard multipole model has been noted before and there are efforts to improve the description of the core regions by expanding the standard multipole model (Zhurov & Pinkerton, 2013;Fischer et al, 2011;Bindzus et al, 2014). In the case of HAR_aniso, deformation density in the region of the hydrogen maleate ring is the most symmetric, which is particularly clear for the intramolecular hydrogen bond and allows evaluation of the influence of the intermolecular interactions on the density and structure of the anion.…”

Experimental determination of core electron deformation in diamond

“…The more flexible EHC model was able to reveal highly interesting polarization features of the core electrons, which reflect the chemical bonding environment in the diamond and silicon structures. Zhurov & Pinkerton (2013) have also addressed the inadequacy of the HC model in describing the ED near nuclear positions. They resolved this issue by introducing a multipolar model with a double monopole in its valence description.…”