Observed fundamentals in infrared spectra of pure liquids are red-shifted by the so-called "dielectric effect". This dielectric shift is an excited state effect, and the observed fundamentals must be corrected before one deduces the isotope effects on the zero-point vibrational energy which are needed in the theoretical evaluation of vapor pressure isotope effects. A simple formula is applied to calculate the dielectric shift, which requires only the molar concentration and the integrated absorption coefficient for the fundamental.
I. Background on Vapor Pressure Isotope EffectsA cell model of a molecular liquid is often employed in theoretical considerations on vapor pressure isotope effects [1,2]. In this cell model, the degrees of freedom of the N-atomic molecule consist of 3N-6 (3N-5 for a linear molecule) internal molecular harmonic vibrations and of six additional ones (five for a linear molecule) which are harmonically bound to the cell and which correspond to translational and rotational motion. The contribution to the vapor pressure isotope effect of the 3N-6 internal vibrational motions can usually be well expressed just in terms of the isotope effect on the zero-point energy difference between these motions in the gas phase and in the liquid phase. Moreover, this quantity is frequently deduced from the observed fundamentals in the liquid phase and in the gas phase. Often the observed fundamentals are converted into appropriate force constants for liquid phase and gas phase molecules, and these force constants are then used to deduce isotope effects on the zero-point energy difference between the 3N-6 internal vibraional degrees of freedom in gas and in liquid.In a study of 13 C/ 12 C vapor pressure isotope effects in carbon disulfide, Jancso and Van Hook [3] found that, when the usual procedure is followed of deducing the isotope effect on the zero-point energy Reprint requests to Prof. M. Wolfsberg,