VAPOUR PRESSURE DIFFERENCES OF ISOTOPIC COMPOUNDS:INFRA-RED CONTRIBUTION OF THE DISPERSION EFFECT AS THE CAUSE FOR GREATER VOLATILITY OF HEAVY MOLECULES

摘要

Examination of the contribution made by infra-red, intramolecular vibrations to Van der Waal forces of attraction emanating from the dispersion exchange effect has shown that this infra-red exchange effect favours greater volatility of molecules containing heavy isotopes. The effect is brought about because the numerical value of the part of attractive forces due to infra-red absorption bands is greater for light molecules than for heavy ones. In consequence of this effect, the energy as a function of distance of two (or more) molecules is not the same for light and heavy molecules.nIn the case of definite examples, the relative volatility of light and heavy molecules will not only depend on the dispersion exchange energy, but also on other effects, particularly the zero point energy of the vibration of the entire molecule with respect to its position of minimum potential energy in the liquid and solid material.