Neutron diffraction, INS and DFT study of hydrogen-bonding in TRIS

Mariana Sládkovičová¹, Pavel Mach¹, Ľubomir Smrčok¹, Paula Marie Briggs-Piccoli², Alexander Kolesnikov²

¹Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-845 36 Bratislava, Slovak Republic

²Argonne National Laboratory, Argonne, Illinois, USA

The proposed project is a part of the study focusing on detailed description of H-bonds in the selected compounds by making a joint use of diffraction, spectroscopy and quantum chemistry methods. The compound tris(hydroxymethyl)-aminomethane, H₂NC(CH₂OH)₃, commonly known as TRIS (see the Figure below) can be considered as a good model system for studying the electronic structure of hydrogen bonded-solids. The structure of TRIS can be considered as a layer structure with both intra- and interlayer hydrogen bonds. The individual layers are, except for long range electrostatic forces (dipole moment of the molecule is 2.15D), also connected by weak interlayer hydrogen bonds linking N-H…O. Such a structure provides a challenge for theoretical approaches to hydrogen bonded systems since hydrogen bonding can to great extent influence the geometry of a hydrogen bonded molecule and its dynamics.

Accurate structure of TRIS was obtained from low temperature single crystal neutron diffraction. In the molecule of TRIS there are three -OH groups and one - NH₂ group that could act both as donors and acceptors of hydrogen bonds. The INS spectrum of TRIS was measured at 7 K on the HRMECS spectrometer with several incident neutron energies and was interpreted on the basis of a harmonic potential using a variety of models. The models were a single molecule, a molecule in a static field of fragments, and single unit cell with periodic boundary conditions as implemented in VASP program. INS spectrum of TRIS is basically a spectrum of hydrogen motions. Formation of total of six inter-molecular hydrogen bonds caused significant downshift of OH stretching frequencies and upshift of their bending frequencies. The same trend was observed for NH₂ frequencies but the changes were smaller. This finding goes in agreement with refined structure, which showed that NH₂ participates in the longest hydrogen bond. Hydrogen bonding has almost no influence on CH frequencies. When comparing calculated INS spectra for all models with experimental spectrum the best agreement was reached when considering not only closest hydrogen bonded neighbors but also the periodic character of the crystal structure.