Reference isotopomer pairs

Fig. 13.2 Vapor pressures (PV,T) and liquid molar volumes (Vm,T) along the orthobaric LV equilibrium line for reference isotopomer pairs, showing their distinctly different behaviors (schematic). The thicker lines label the heavier isotopomers. The curves begin at the triple points and end at the critical points (except for He which has no triple point under orthobaric conditions). Not to scale, isotopic differences are exaggerated (Reprinted from Van Hook, W. A., Rebelo, L. P. N. and Wolfsberg, M., Fluid Phase Equilib. 257, 35 (2007), copyright 2007, with permission from Elsevier)...

The vibrational frequencies of isotopic isotopomers obey certain combining rules (such as the Teller-Redlich product rule which states that the ratio of the products of the frequencies of two isotopic isotopomers depends only on molecular geometry and atomic masses). As a consequence not all of the 2(3N — 6) normal mode frequencies in a given isotopomer pair provide independent information. Even for a simple case like water with only three frequencies and four force constants, it is better to know the frequencies for three or more isotopic isotopomers in order to deduce values of the harmonic force constants. One of the difficulties is that the exact normal mode (harmonic) frequencies need to be determined from spectroscopic information and this process involves some uncertainty. Thus, in the end, there is no isotope independent force field that leads to perfect agreement with experimental results. One looks for a best fit of all the data. At the end of this chapter reference will be made to the extensive literature on the use of vibrational isotope effects to deduce isotope independent harmonic force constants from spectroscopic measurements. 

Reference Systems Critical Property Data for Some Isotopomer Pairs... 

Extensive PVT data as well as precise values for critical property IE s are available for 3He/4He, CH4/CD4, H2/D2 and H20/D20 and these isotopomer pairs comprise an excellent reference in formulating EOS analysis of PVT IE s. Critical properties and critical property IE s for these and a few other selected isotopomer... 

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