Über die bestimmung von isomerisierungsenthalpien und -entropien mit der IR-intensitätsmethode

Among the many sources of error which may affect the results for the enthalpy ΔH and entropy ΔS of isomerization determined by the IR-intensity method, the influence of the temperature dependence of the absorptivities of the reference bands, α_g(T) and α_t(T), is studied theoretically. Based on the general case I, the following three special cases II–IV are examined, in which increasingly stronger constraints are imposed on the compound-specific functions α_g(T) and α_T(T): II. Constant ratio α_g(T)/α_T(T), III. α_g(T) and α_T(T) both temperature independent and different, IV. α_g(T) and α_T(T) both temperature independent and equal. For each case the formulae for the evaluation of ΔH and ΔS as well as the kind and the number of the quantities to be measured are given. Moreover, in the cases I and II new methods for the analysis of the measured data are suggested. In each case the plots of the normalized band intensities $\text{A}{}_{\mathrm{g}}$ and $\text{A}{}_{\mathrm{t}}$ and the temperature T versus each other in different two-dimensional diagrams have been examined to find out whether one of the simpler special cases is applicable for the respective pair of reference bands. It is shown that these plots are not a suitable tool because of the relatively poor accuracy and reproducibility of measured IR-band intensities obtainable up to now.