Molecular motions in n-nonadecane C19H4: An incoherent neutron scattering study

Abstract

Incoherent neutron scattering (INS) experiments were performed on semioriented samples of n‐nonadecane C₁₉H₄₀ and of the selectivity deuterated derivatives CD₃–C₁₇H₃₄–CD₃ and C₂D₅–C₁₅H₃₀–C₂D₅ over a wide temperature range, in particular, over the domain of existence (295–305 K) of the disordered ‘‘rotator phase’’ R_I. From analyses of the inelastic and quasielastic parts of the spectra, we were able to discriminate between motions parallel or perpendicular to the main long‐chain axis. In the low frequency region (0–300 cm⁻¹), numerous lattice and internal vibrational modes, showing pronounced polarization effects, were assigned. In the quasielastic regions, large broadenings were observed in the R_I phase, providing clear evidences of both fast restricted translational motions along the chain axis and uniaxial rotational diffusions. For the rotational process, we have elaborated and made use of a model of diffusion for a particle in a twofold potential. Finally, the INS spectra of C₁₉H₄₀ in the R_I phase and of C₂₃H₄₈ and C₂₄H₅₀ in their R_II phases were compared and nicely interpreted within an unique model of ‘‘effective potential’’ by fitting the barrier heights against rotational diffusion. All these results are discussed and compared to literature data.