An interpretation of the vapor phase second virial coefficient isotope effect: Correlation of virial coefficient and vapor pressure isotope effects

Experimental data on vapor phase second virial coefficient isotope effects (VCIEs) are reviewed and then interpreted using the general theory of isoto pe effects. Useful correlations are developed between -Delta (B - b(o))(B - b(o)) = (-VCIE) and [ln(f(c)/f(g))]*, where [ln(f(c)/f(g))]* is the refere nce condensed phase reduced isotopic partition function ratio, and A is the second virial coefficient, b(o) = 2 pi sigma (3)/3. sigma is the Lennard-J ones size parameter, and Delta denotes an isotopic difference, light-heavy. [ln(f(c)/f(g))]* can be straightforwardly obtained from measurements of va por pressure isotope effects for T-R = T/T-CRITICAL < 0.7. We show (-VCIE) = ln(f(p)/f(g)(2)) where ln(f(p)/f(g)(2)) is the reduced isotopic partition function ratio associated with the equilibrium between isolated gas-phase monomer species and interacting pairs. At temperatures well removed from cr ossovers in ln(f(p)/f(g)(2)) or [ln(f(c)/f(g))]*, ln(f(p)/f(g)(2)) = (0.4 /- 0.2) [ln(f(c)/f(g))]*.