Order–disorder phase transition in an anhydrous pyrazole-based proton conductor: the enhancement of electrical transport properties

Abstract

The crystal structure of 1H-pyrazol-2-ium hydrogen oxalate has been studied at 100 K. It consists of two-dimensional layers built with one-dimensional chains that contain pyrazolium and oxalate acids bonded by N-HO and O-HO hydrogen bonds. According to the X-ray data and the Quantum Theory of Atoms in Molecules, it was shown that weak and moderate hydrogen bonds are present in the crystal at room temperature. The thermal stability was studied with the DSC, TGA, and DTG methods: three endothermic peaks are observed at 384, 420, and 469 K. Conductivity measurements have been performed in the temperature range from 300 to 433 K. At 383 K the pyrazole-oxalic acid framework loses its rigidity and the crystal undergoes an ordered-disordered phase transition. At this temperature, the value of the activation energy of proton conductivity changes from 1.14 to 2.31 eV. The proton conduction pathways and the transport mechanism have been studied with theoretical methods.

Spectroscopic investigations of the new anhydrous proton‐conducting compound of pyrazole with oxalate acid

“…The crystal structure of 1 H ‐pyrazole‐2‐ium hydrogen oxalate (PyrHOx) crystal shows that the chemical unit consists of two protonated pyrazolium cations PyrH + and two oxalate ions . In Table S1, we have collected the dimensions of the pyrazolium at 300 K, structural data for protonated (positively charged, PyrH + ), and unprotonated (neutral, Pyr) pyrazole molecules; internal coordinates of PyrH + are shown in Figure S1. The X‐ray data were compared with DFT calculations performed at the theory level B3LYP/6‐311++G(d,p) for PyrH + and Pyr molecules.…”

“…Moreover, we have performed the DFT calculations of Raman‐ and IR‐active normal modes of the pyrazolium ion. In the optimization process, the initial geometry has been taken from X‐ray data . The most intensive bands related to the internal vibrations of pyrazolium cation are marked in Figure .…”

“…To study the vibrational properties of PyrHOx, the theoretical calculations were performed with Gaussian 03 program using the 6‐311++G(d,p) Pople style basis set and the B3LYP hybrid density functional, which combines Beckeapos;s three‐parameter nonlocal exchange potential with the nonlocal correlation functional of Lee, Yang, and Parr . The initial geometry of the investigated molecules was taken from the X‐ray data performed for the single crystal of PyrHOx at 300 K and applied in the geometry optimization job. The results of optimizations correspond to energy minima because no imaginary wavenumbers were found.…”

“…To the best of our knowledge, Karthiga Devi et al have so far provided only an initial interpretation of the vibrational structure of this salt. According to the X‐ray data, the system of weak or moderate HBs and strong anisotropy of electrical conductivity can be expected . The electrical conductivity measurement shows that at 383 K, the value of the activation energy changes from 1.14 to 2.31 eV.…”

“…One of the most interesting dicarboxylic acids is the oxalic acid that exhibits intramolecular and intermolecular HBs in dimers . Recently, a new proton anhydrous conducting material, the oxalic acid salt 1 H ‐pyrazole‐2‐ium hydrogen oxalate (PyrHOx), was synthesized . The X‐ray studies of PyrHOx carried out at room temperature shows that the crystal belongs to a triclinic system with space group, a = 3.7286(7) Å, b = 9.836(2) Å, c = 10.487(2) Å, α = 117.35(3)°, β = 97.01(3)°, γ = 93.65(3)°, V = 335.92(14) Å 3 , Z = 2.…”