Die rotationsisomerie des bicyclopropyls.: II. Die gauche/trans-isomerisierungsenthalpie und-entropie von bicyclopropyl aus ir-intensitäts-messungen. Ein experimenteller test

The determination of the enthalpy ΔH and entropy ΔS of the isomerization bicyclopropy l(trans)? bicyclopropyl(gauche) in the liquid phase by the IR intensity method is described. It is assumed that the ratio of the integral absorption coefficients of the two reference bands at 1351 cm^?1 (gauche) and 1291 cm^?1 (trans), which both belong to the same type of vibration, is temperature independent. The two values ΔH = ?160 ± 40 cal Mol^?1 and ΔS = ?0.4 ± 0.5 cal (Mol · Grad)^?1 respectively. ΔS_U = ?1.8 ± 0.5 cal (Mol · Grad)^?1 thus determined agree well with the corresponding results obtained from NMR and electron diffraction measurements. However, from the pair of reference bands at 695 cm^?1 (gauche) and 1291 cm^?1 (trans), which do not belong to the same type of vibration, strongly differing values for ΔH and ΔSresult under the same assumption as above, which apparently is not applicable in this case.It is shown through these data that the “Fateley-Test” does not provide a suitable tool to decide whether the absorption coefficients of the reference bands are temperature independent or not. The reason for this insignificance is the relatively poor accuracy and reproducibility of measured IR band intensities obtainable up to now.The relative density of bicyclopropyl between ?60°C and + 50°C was determined.