Isotope effects on vapor phase 2nd virial coefficients

Vapor phase 2nd virial coefficient isotope effects (VCIE's) are interpreted. A useful correlation is developed between -Delta(Beta-bo)/(Beta-bo) = (-VCIE) and the reference condensed phase reduced isotopic partition function ratio [ln(fc/fg)]*. B is the second virial coefficient, bo = 2pi sigma3/3, sigma is the Lennard-Jones size parameter, and Delta is an isotopic difference, light-heavy. [ln(fc/fg)]* can be obtained from vapor pressure isotope effects for T/TCRITICAL < 0.7. Also (-VCIE) = ln(fp/fg 2), where ln(fp/fg 2) is the reduced isotopic partition function ratio describing the equilibrium between monomers and interacting pairs. At temperatures well removed from crossovers in ln(fp/fg 2) or [ln(fc/fg)]*, ln(fp/fg 2) = (0.4š0.2) [ln(fc/fg)]*.