MOLECULAR MOTIONS OF ALKYLENEDIAMMONIUM CHAINS IN THE LAYER-TYPE COMPOUNDS (NH3-(CH2)3-NH3)MNCL4, (NH3-(CH2)4-NH3)MNCL4, (NH3-(CH2)5-NH3)MNCL4 - AN INCOHERENT NEUTRON-SCATTERING STUDY

Molecular motions of alkylenediammonium cations in manganese tetrachloride compounds (in short 2CnMn), namely, (NH3-(CH2)n-NH3)MnCl4, (ND3-(CH2)n-ND3)MnCl4, and (NH3-(CD2)n-NH3)MnCl4 with n = 3, 4, 5, have been studied in their dynamically disordered phases by means of incoherent neutron scattering (INS) over the 100-500 K temperature range in order to afford vibrational, kinetic and structural information. A detailed band assignment of the INS spectra is proposed, and the frequency softening of the torsional τ(NH3) vibrations is evaluated. Quasi-elastic neutron scattering (QNS) spectra of the various derivatives under study have shown that the dynamical regimes of the -NH3+ groups and of the -(CH2)n-backbones are different. QNS profiles for 2C4Mn and 2C5Mn compounds were satisfactorily fitted by the model of a conformational equilibrium between two "trans" and two "twisted" states while the -NH3+ groups were performing additional independent oscillations about the -C-N axes: the corresponding correlation times and activation energies were then evaluated. A model of molecular geometry for twisted forms has been proposed in which the distortion of the molecules occurs through cooperative torsions of the -CH2- units along the main molecular axis. The dynamical regimes in the various structural phases are then discussed in connection with the already suggested crystal structures. Finally, our results are compared to recent Raman and X-ray diffraction data on the related cadmium 2C5Cd derivative and on calorimetric measurements on 2C5Mn: this leads to a consistent and unified picture of the various phase-transition mechanisms in these two-dimensional "composite" materials. © 1990 American Chemical Society.