Etude par Calorimetrie, Diffraction des Rayons X et Diffusion Raman d'une Transition de Phase dans (C 4 H 4 P) Mn (CO) 3

Abstract

Heat capacity measurements (95-300K), X-ray diffraction (78-300K) and low frequency Raman spectroscopy (10-350K) have evidenced an order-disorder phase transition in phosphacymantrene, (C₄ H₄P) Mn (CO)₃. This transition has been characterized by a monoclinic ←→ triclinic structural change at about 110 K and by a pretransitional phenomenon. The measured transition enthalpy and entropy are 480 ± 10J.mil⁻¹ and 4.17 ± 0.08J.K⁻¹ mol⁻¹, at 115 K, respectively. A complete assignment of the observed Raman bands in h₄ and d₄ derivatives is proposed. From the temperature dependence of frequencies, intensities and half-widths of some Raman bands we have discussed the order, the nature and the mechanism of the phase transition: intermolecular interactions appear to be mainly involved in the mechanism and an activation energy roughly equal to 2100 ± 840 J. mol⁻¹ has been determined.