Molecular reorientations of triethylenediamine (TEDA) in its plastic solid phase

Abstract

Using the incoherent quasielastic neutron scattering technique, the molecular reorientations of 1·4-diazabicyclo(222)octane N(CH₂CH₂)₃N were investigated in its low-temperature and plastic phases. A sample temperature ranging from 300 K to 413 K was used with an incoming neutron wavelength λ = 10 Å. In the plastic phase quasielastic spectra are described by a model in which the molecules perform reorientational jumps between six equilibrium positions equally spaced by 60° around the molecular symmetry axis, together with a tumbling of the molecule from a [111] lattice direction to an equivalent position. Multiple scattering contribution was taken into account in the data analysis, in a semi-analytical way up to third order. The method is fully described. The two correlation times τ_M6 and τ_C4 related to rotations about molecule and crystal axes, respectively, were determined as a function of the temperature. These were found to follow the two Arrhenius laws:

Finally a comparison is made with two similar molecules: bicyclo[222] octane, HC(CH₂CH₂)₃CH, and quinuclidine, N(CH₂CH₂)₃CH.