Calorimetric studies of the phase transition in iodoform

Abstract

. 66, 645 (1988).The heat capacity of crystalline iodoform prepared by vacuum sublimination has been measured from 50 to 340 K and that of iodoform prepared by recrystallization from acetone measured from 120 to 300 K. Both samples exhibit a phase transition near 260 K. The heat capacity anomaly in the sublimed material shows several small maxima which appear with variable positions and heights depending upon the thermal pretreatment of the sample. The transition, which was undetected by previously-reported spectroscopic measurements, is ascribed to an incomplete ordering of the molecular dipole.JULIANA BOERIO-GOATES et BRIAN F. WOODFIELD. Can. J. Chem. 66, 645 (1988). On ti mesurt la capacitt calorifique de l'iodoforme cristallin prtpart par sublimation sous vide 21 des temperatures allant de 50 a 340 K et celle de l'iodoforme prtpart par recristallisation de I'acCtone de 120 ? i 300 K. Les deux Cchantillons presentent une transition de phase prts de 260 K. Dans l'tchantillon prCpart par sublimination, on note une anomalie dans IacapacitC calorifique qui se traduit par plusieurs petits maxima qui apparaissent 2 des positions variables et dont les hauteurs dtpendent du prt-traitement thermique qu'a subi I'tchantillon. C'est B arrangement incomplet du dip8le moltculaire qu'on attribue la transition qui n'avait pas ttC dCtectCe au cours des mesures spectroscopiques rkalisees anttrieurement.[Traduit par la revue] Introduction Halogenated methane derivatives are of interest to theorists and experimentalists probing molecular dynamics in the solid state. These compounds also exhibit an interesting range of crystallographic structures and polymorphic phase transitions at different temperatures. The haloforms, CHX3 with X = F, C1, Br, and I, have been widely studied by techniques like X-ray (1, 2) and neutron (3, 4) diffraction, Raman and ir spectroscopy (5-7) and dielectric measurements (8) which probe structuial details of the solid-state and characterize phase transitions. However, high-precision heat capacity measurements, frequently used to study phase transitions, have been published only for fluoroform (9); differential scanning calorimetric (DSC) measurements (8) near the solid-liquid and solid-solid phase transitions in bromoform have been made, but no quantitative data were published from this study.In this paper we report heat capacity measurements of iodoform in order to resolve a discrepancy regarding the existence of a phase transition in this compound. The disagreement arises between spectroscopic (5,7) studies which show no evidence for a phase transition as the temperature is lowered, and temperature-dependent photoconductivity measurements (10) which exhibit anomalous behavior below 270 K that has been attributed to the onset of a phase transition. The manner of sample preparation differed between two of the investigations: one spectroscopic study (5) used crystals grown from acetone solution while the photo-conductivity measurements (10) were performed on samples of CHI3 prepared by vacuum sublimatio...

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