On hydrogen bonding in 2-amino-3-hydroxymethyl-1,3-propane diol (TRIS) – variable temperature neutron single crystal and DFT study

Abstract

Geometry of hydrogen bonds in 2-amino-3-hydroxymethyl-1,3-propane diol (TRIS) was accurately determined by single crystal TOF neutron diffraction at 30 K, 100 K and 170 K. The structure of TRIS can be formally described as a structure formed by layers of molecules stacked along the b-axis. Within layers the molecules are coupled by O—H … O'/N hydrogen bonds, while the adjacent layers are linked by weaker N—H … O bonds. Weak intramolecular C—H … O contacts stabilize the molecule. Refined anisotropic displacement parameters are analyzed using the TLS approach at each temperature. Frequencies of translational and librational modes are compared to those obtained by solid state calculations. Molecular DFT calculations with simulated crystal environment were used to reveal the influence of hydrogen bonds on molecule's geometry. The shape the molecule has in crystal is not optimal under the absence of external forces and in the structure is maintained at the expense of ∼33 kcal/mol. Comparison of molecular and solid state DFT frequency calculations predicts red shifts of O—H stretching frequencies due to formation of hydrogen bonds.