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)(+)center dot SO42-center dot C2H5NO2 (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 center dot center dot center dot 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 center dot center dot center dot O = 1.408 (3) [delta = 0.338 (4)], O center dot center dot center dot O = 2.4777 (15) angstrom and O-H center dot center dot center dot O = 179.0 (4)degrees, 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 center dot center dot center dot O = 1.361 (12) [delta = 0.271 (17)], O center dot center dot center dot O = 2.450 (7) angstrom and O-H center dot center dot center dot O = 179.2 (10)degrees, 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 center dot center dot center dot O = 1.387 (12), O center dot center dot center dot O = 2.451 (7) angstrom and O-H center dot center dot center dot O = 178.2 (11)degrees and for the minor component, O-H = 1.06 (4), H center dot center dot center dot O = 1.41 (4), O center dot center dot center dot O = 2.451 (7)degrees and O-H center dot center dot center dot O = 166 (2)degrees. 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.