Investigation of Bistetramethylammonium Hydrogencyclotriphosphate—A Molecular Rotor?

• Published in: Chemistry : a European journal Vol. 24; no. 35; pp. 8756 - 8759
• Main Authors: Mangstl, Martin, Weber, Johannes, Jardón‐Álvarez, Daniel, Burghaus, Olaf, Roling, Bernhard, Schmedt auf der Günne, Jörn
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 21.06.2018
• Subjects: Activation energy; Calorimetry; Chemistry; Computer simulation; ion conductor; Line shape; Line spectra; Magnetic resonance spectroscopy; Molecular chains; NMR spectroscopy; paddle wheel; phase transition; Phase transitions; Rotation; Rotational spectra; second moment analysis; Spectrum analysis; X-ray diffraction; second moment analysis; phase transition; NMR spectroscopy; ion conductor; paddle wheel
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201800980

The crystalline phase β‐[N(CH3)4]2HP3O9 undergoes a reversible phase transition to γ‐[N(CH3)4]2HP3O9, which was studied by dynamic scanning calorimetry and X‐ray diffraction. The rotational dynamics of the anion [P3O9]3− were evident from variable temperature 31P magic angle spinning (MAS) NMR spectroscopy. The rotational dynamics could be simulated with a 3‐site jump model, which yields spectra in good agreement with experiment. An activation energy of 0.6 eV could be estimated from line shape analysis. Impedance spectra reflect a bulk proton conductivity of γ‐[N(CH3)4]2HP3O9 of 6.9×10−5 S cm−1 at 240 °C and an activation energy of approximately 1.0 eV. Thus this salt features bulk protonic motion, while local rotational anionic motion happens with activation energies of the same order, as suggested by the paddle‐wheel mechanism. The reversible phase transition of β‐[N(CH3)4]2HP3O9 was studied by dynamic scanning calorimetry and variable temperature X‐ray diffraction. NMR measurements and simulations gave evidence for rotational motion of the [P3O9]3− anion at elevated temperatures. As suggested by the paddle‐wheel mechanism, the compound features protonic motion while local rotational anionic motion happens with activation energies of the same order.