Phase transitions of choline dihydrogen phosphate: A vibrational spectroscopy and periodic DFT study

Author(s)

Paschoal, Vitor H. ; Ribeiro, Mauro C. C.

• Document type: Journal article
• Source: Journal of Chemical Physics; v. 160, n. 9, p. 12-pg., 2024-03-07.

Abstract

Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known structure of the low-temperature crystal, which contains the anti-conformer of [Chol](+) and hydrogen-bonded dimers of anions, was used to do periodic density functional theory calculations of the vibrational frequencies. Raman spectra indicate that the solid-solid transition at 20 degree celsius is linked to a conformational change to the gauche [Chol] conformer with a concurrent local rearrangement of the anions. The distinct bands of lattice modes in the low-frequency range of the Raman spectra vanish at the 20 degree celsius transition. Given the ease with which metastable crystals can be produced, Raman mappings demonstrate that a sample of [Chol][H2PO4] at ambient temperature can contain a combination of anti- and gauche conformers. Heating to 120 degree celsius causes continuous changes in the local environment of anions rather than melting as suggested by a recent calorimetric investigation of [Chol][H2PO4]. The monotonic change in vibrational spectra is consistent with earlier observations of a very small entropy of fusion and no abrupt jump in the temperature dependence of ionic conductivity along the phase transitions of [Chol][H2PO4]. (AU)

• FAPESP's process: 22/11983-4 - Spectroscopy signal enhancement: nanomaterials, theory, and computer simulation
• Grantee: Mauro Carlos Costa Ribeiro
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 19/00207-0 - Density functional theory applyed to vibrational dynamics of crystals of ionic systems.
• Grantee: Vitor Hugo Paschoal
• Support Opportunities: Scholarships in Brazil - Post-Doctoral