Structural and spectroscopic features of proton hydrates in the crystalline state. Solid-state DFT study on HCl and triflic acid hydrates

• Published in: Molecular physics 2018-01, Vol.116 (2), p.251-262
• Main Authors: Vener, M. V., Chernyshov, I. Yu, Rykounov, A. A., Filarowski, A.
• Format: Article
• Language: English
• Subjects: Banded structure; Cations; Chemical bonds; Crystal structure; Crystal structure databases analysis; Crystallinity; Density functional theory; electron-density features; Hydrates; IR-active bands; Molecular structure; nitric acid trihydrate; Protons; Solid state; strong and moderate intermolecular H-bonds; Symmetry; Triflic acid
• ISSN: 0026-8976; 1362-3028
• DOI: 10.1080/00268976.2017.1380860

Crystalline HCl and CF 3 SO 3 H hydrates serve as excellent model systems for protonated water and perfluorosulphonic acid membranes, respectively. They contain characteristic H 3 O + , H 5 О + 2 , H 7 О + 3 and H 3 O + (H 2 O) 3 (the Eigen cation) structures. The properties of these cations in the crystalline hydrates of strong monobasic acids are studied by solid-state density function theory (DFT). Simultaneous consideration of the HCl and CF 3 SO 3 H hydrates reveals the impact of the size of a counter ion and the crystalline environment on the structure and infrared active bands of the simplest proton hydrates. The H 7 O + 3 structure is very sensitive to the size of the counter ion and symmetry of the local environment. This makes it virtually impossible to identify the specific features of H 7 O + 3 in molecular crystals. The H 3 O + ion can be treated as the Eigen-like cation in the crystalline state. Structural, infrared and electron-density features of H 5 О + 2 and the Eigen cation are virtually insensitive to the size of the counter ion and the symmetry of the local crystalline environment. These cations can be considered as the simplest stable proton hydrates in the condensed phase. Finally, the influence of the Grimme correction on the structure and harmonic frequencies of the molecular crystals with short (strong) intermolecular O-H···O bonds is discussed.