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), 2C 2 H 6 NO 2 + ·SO 4 2- ·C 2 H 5 NO 2 (or C 6 H 17 N 3 O 10 S), 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 mol-ecule that is associated with the ferroelectric behaviour of HTGS, O-H = 1.070 (3), H⋯O = 1.408 (3) [δ = 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) [δ = 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.