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M. Kogler, B. Stöger:
"Hydrogen-bonding in mono-, di- and tetramethylammonium dihydrogenphosphites";
Zeitschrift für Kristallographie - Crystalline Materials, 236 (2021), 33 - 41.

Abstract: The crystal structures of methylammonium and dimethylammonium dihydrogenphosphite (MA⋅H₂PO₃, I2/a and DMA⋅H₂PO₃, P2_#{1}/c) are built of infinite chains of hydrogen bonded H₂PO₃^- anions. The chains are connected by the ammonium cations via hydrogen bonding to di- (DMA⋅H₂PO₃) and triperiodic (MA⋅H₂PO₃) networks. Tetramethylammonium dihydrogenphosphite monohydrate (TMA⋅H₂PO₃⋅H₂O) features temperature dependent dimorphism. The crystal structure of the high-temperature (HT, cubic P2₁3) and low-temperature (LT, orthorhombic P2₁2₁2₁) phases were determined at 150 and 100 K, respectively. The hydrogen bonding network in the HT phase is disordered, with H₂PO₃ and H₂O being located on a threefold axis and is ordered in the LT phase. On cooling, the point symmetry is reduced by an index of 3. The lost symmetry is retained as twin operations, leading to threefold twinning by pseudo-merohedry. The hydrogen-bonding networks of the HT and LT phases can be represented by undirected and directed quotient graphs, respectively.

hydrogen bonding; phase transition; symmetry relations; twinning; voltage graphs.

http://dx.doi.org/10.1515/zkri-2020-0088