Single-crystal structure analysis of non-deuterated triglycine sulfate by neutron diffraction at 20 and 298 K: a new disorder model for the 298 K structure

Precise single-crystal structure analyses of the title compound, bis(glycinium) sulfate–glycine (1/1), 2C₂H₆NO₂⁺SO₄²C₂H₅NO₂ (or C₆H₁₇N₃O₁₀S), non-deuterated triglycine sulfate (HTGS) at 20 K and 298 K were undertaken using time-of-flight neutron diffraction data. At 20 K for the O—H. . .O hydrogen bond between the glycinium cation and the zwitterionic, unprotonated glycine molecule that is associated with the ferroelectric behaviour of HTGS, O—H = 1.070 (3), H. . .O = 1.408 (3) [ = 0.338 (4)], O O = 2.4777 (15) Å and O—H. . .O = 179.0 (4), which is in good agreement with previous studies. Two reasonable structures for the same three atoms were refined for the 298 K dataset. One is a single-minimum potential-energy model, with O—H = 1.090 (12), H. . .O = 1.361 (12) [ = 0.271 (17)], O O = 2.450 (7) Å and O—H. . .O = 179.2 (10), having the H atom with a large ellipticity along the bond path between the O atoms. The other is a double-minimum potential-energy model having two H atom sites with occupancies of 0.876 (8) and 0.124 (8): for the major occupancy component, O—H = 1.065 (12), H. . .O = 1.387 (12), O O = 2.451 (7) Å and O—H. . .O = 178.2 (11) and for the minor component, O—H = 1.06 (4), H. . .O = 1.41 (4), O O = 2.451 (7) Å and O—H. . .O = 166 (2). These models did not show any significant differences in R factors. In addition, the unit-cell parameters and other structural parameters of HTGS did not show any major differences compared to those of partially deuterated TGS and fully deuterated TGS for both 20 K and 298 K.