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Abstract
Incoherent inelastic scattering (INS) and quasielastic neutron scattering (QNS) experiments on polycrystalline derivatives of the propylenediammonium manganese tetrachloride compound, namely |NH3(CH2)3NH3| MnCl4, |NH3(CD2)3NH3|MnCl4 and |ND3(CH2)3ND3|MnCl4, have been performed over the 100–500 K temperature range, i.e. under three different structural phases.
A detailed assignment of the INS spectra is proposed and we observe in particular a pronounced frequency softening of the torsional τ(NH3) vibration in conjunction with an intensity enhancement when the temperature increases; this suggests quite different dynamical regimes for the NH3 groups in the disordered phases. In fact, QNS spectra of the three derivatives under study have been satisfactorily fitted using the following models: ((i) 90° jumps of only the N-H protons over two equivalent positions in the room temperature phase (
= 2·7 × 10-12 s). (ii) Two kinds of jumps for the N-H (∼ 100°) and C-H protons (∼ 110°) in the intermediate phase II, the flipping motion of the (CH2)3 backbone occurring among two positions with distinct occupation probabilities (
= 4·2 × 10-12 s and 
= 1·8 × 10-12 s). (iii) Finally, jumps of the C-H protons in conjunction with 2π/3 uniaxial reorientations of the NH3 groups in the high temperature phase I (
= 1·2 × 10-12 s and 
= 2·0 × 10-12 s). The correlation times associated with C-H jumps follow two distinct Arrhenius type laws in phases II and I, 
= 1·5 × 10-13 exp (ΔH
II/RT) and 
= 7·7 × 10-11 exp (ΔH
I/RT), where ΔH
II = 6·9 kJ mol-1 and ΔH
I = 10·1 kJ mol-1. In addition, 2π/3 reorientations of the NH3 groups in phase I satisfy the equation 
= 2·8 × 10-13 exp (ΔH′
I
RT) with ΔH′
I = 8·2 kJ mol-1; the latter activation energy is significantly lower than the potential barrier for the torsional motion as deduced from the INS torsional frequency, V
3 ⩽˜ 11·0 kJ mol-1 (493 K).
We present a comparison with other data, from optical, deuteron-N.M.R., and N.Q.R. results, and the dynamical mechanisms in the different structural forms are discussed.