Single-Crystal Magic-Angle Spinning ¹⁷O NMR and Theoretical Studies of the Antiferroelectric Phase Transition in Squaric Acid

Abstract

We report resolution-enhanced magic-angle sample spinning (MAS) 17 O NMR studies of the paraelectricantiferroelectric phase transition (at T C ) 373 K) of the model hydrogen-bonded compound squaric acid (H 2 C 4 O 4 ). Utilization of single crystals in the MAS measurements yielded a factor of 4 decrease in the 17 O NMR line widths, as compared to powder samples. All four oxygens were clearly shown to be chemically different at T < T C . This was akin to 13 C spectra, but the 17 O peaks are much more dispersed. The peak assignment was supported by quantum theoretical calculations of the 17 O isotropic chemical shifts using a pentamer model of the crystal structure below T C . On raising the temperature, the four peaks merged and became a narrow doublet above T C , in contrast to a singlet for 13 C. Also, the mean position of the doublet was not the average of the four low-temperature peaks. This observation suggests that the phase transition involves a distortion of the whole H 2 C 4 O 4 framework, and not just the order-disorder rearrangement of the H's i.e., future models of the transition should include a displacive component, in addition to an orderdisorder part. The observation of the doublet at T > T C implies that the two O-H...O chains retain their difference in the paraelectric phase as well. This is consistent with the one-dimensional Ising chain model, according to which the SQA lattice should be visualized as a mesh of two distinct and orthogonal onedimensional chains, in contrast to the more prevalent two-dimensional C 4 O 4 square-lattice model.

Vibronic model for antiferroelectric phase transition in squaric acid, H2C4O4

“…At a first glance, this conclusion correlates well with the two-component structure of experimental high-resolution 17 O NMR spectra of the high-temperature SQA [3]. However, the fast proton exchange in the system containing both dipolar [C 4 O 4 ] 22 and dipole-less [H 4 C 4 O 4 ] 2þ particles will involve structurally different oxygen and carbon atoms into the exchange process, leading to the averaging of their chemical shifts.…”

“…However, the fast proton exchange in the system containing both dipolar [C 4 O 4 ] 22 and dipole-less [H 4 C 4 O 4 ] 2þ particles will involve structurally different oxygen and carbon atoms into the exchange process, leading to the averaging of their chemical shifts. In reality, the 17 O NMR spectra of high-temperature SQA show two-component structure [3] indicating conclusively that [H 4 C 4 O 4 ] 2þ ions must be excluded from the list of virtual states of high-temperature SQA.…”

“…For further refinement of the high-temperature SQA structure we consider it to be useful to compare quantitatively the experimental high-resolution 13 C and 17 O NMR data [2,3] with ADF-calculated nuclear magnetic shielding parameters. We suppose that in the high temperature proton disordered state the point group symmetry of H 2 C 4 O 4 molecules and of different [H 2þn C 4 O 4 ] nþ and [H 22n C 4 O 4 ] n2 ðn ¼ 1; 2Þ molecular ions will be averaged to C 2v : In such a supposition there will exist only two chemically different carbon (and oxygen) atoms related to the C-C and CyC bonds in SQA.…”

Nuclear magnetic resonance spectroscopic studies of the trihexyl (tetradecyl) phosphonium chloride ionic liquid mixtures with water

“…The success of GIAO even extends to molecular systems that include hydrogen bonding interactions [24][25][26] and ionic liquids [27].…”

Single-crystal NMR approach for determining chemical shift tensors from powder samples via magnetically oriented microcrystal arrays

“…Numerous studies focusing on S/N improvement by DNP method [5] and reduction of bulk magnetism effect [6][7][8], and studies on quadrupole nuclei [9,10], molecular conformations [11] and chemical shift tensors have been reported. Generally, a large single crystal is required to obtain sufficient S/N from the samples; this requirement limits the use of single crystals in NMR because it is difficult to grow large crystals under many circumstances.…”