Dielectric phase transition and symmetry change of constituent molecules in proton–deuteron mixed crystals of squaric acid

Proton Order-Disorder Phenomena in a Hydrogen-Bonded Rhodium-η(5)-Semiquinone Complex: A Possible Dielectric Response Mechanism

A newly synthesized one-dimensional (1D) hydrogen-bonded (H-bonded) rhodium(II)-η(5)-semiquinone complex, [Cp*Rh(η(5)-p-HSQ-Me4)]PF6 ([1]PF6; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; HSQ = semiquinone) exhibits a paraelectric-antiferroelectric second-order phase transition at 237.1 K. Neutron and X-ray crystal structure analyses reveal that the H-bonded proton is disordered over two sites in the room-temperature (RT) phase. The phase transition would arise from this proton disorder together with rotation or libration of the Cp* ring and PF6(-) ion. The relative permittivity εb' along the H-bonded chains reaches relatively high values (ca., 130) in the RT phase. The temperature dependence of (13)C CP/MAS NMR spectra demonstrates that the proton is dynamically disordered in the RT phase and that the proton exchange has already occurred in the low-temperature (LT) phase. Rate constants for the proton exchange are estimated to be 10(-4)-10(-6) s in the temperature range of 240-270 K. DFT calculations predict that the protonation/deprotonation of [1](+) leads to interesting hapticity changes of the semiquinone ligand accompanied by reduction/oxidation by the π-bonded rhodium fragment, producing the stable η(6)-hydroquinone complex, [Cp*Rh(3+)(η(6)-p-H2Q-Me4)](2+) ([2](2+)), and η(4)-benzoquinone complex, [Cp*Rh(+)(η(4)-p-BQ-Me4)] ([3]), respectively. Possible mechanisms leading to the dielectric response are discussed on the basis of the migration of the protonic solitons comprising of [2](2+) and [3], which would be generated in the H-bonded chain.

Squaric acid as an internal standard for temperature measurements in 13C MAS NMR

Squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) is suggested as an internal standard for temperature measurements in 13C MAS NMR studies in the temperature range from 373 K to 520 K. Compared to previously utilized standards, squaric acid has several important advantages, such as fast kinetics of the phase transition, higher temperature range for 13C studies, and an almost constant sample volume through the transition temperature range. Thus, we found squaric acid to be suitable for the probe calibration with the reference temperature at 373.2 K. Moreover, upon partial deuteration the reference temperature can be gradually increased to 520 K. Additionally, the described procedure makes it possible to quantitatively estimate the temperature gradient across the sample. We also discuss the effect of spinning related stress on the temperature measurement technique and how this effect could be minimized.