Vapor pressure isotope effect in liquid methylene fluoride

The vapor pressures of the isotopic methylene fluorides /sup 12/CH/sub 2/F/sub 2/, /sup 12/CD/sub 2/F/sub 2/ and /sup 13/CH/sub 2/F/sub 2/ have been measured at temperatures between 149 and 244/sup 0/K by differential manometric techniques in a precision cryostat. Over the entire temperature range of the measurements, P(/sup 12/CH/sub 2/F/sub 2/) > P(/sup 12/CD/sub 2/F/sub 2/) and P(/sup 12/CH/sub 2/F/sub 2/) < P(/sup 13/CH/sub 2/F/sub 2/). The data are best represented by T 1n(f/sub c//f/sub g/) = (632.26 +- 97.62)/T + (19.175 +- 1.016) - (0.0532 +- 0.0025)T for the D/H effect and by T 1n(f/sub c//f/sub g/) = -(31.64 +- 1.97)/T - (0.4069 +- 0.0107) for the /sup 13/C//sup 12/C effect. A series of calculations using Wilson's FG matrix method and a modified Schachtschneider-Snyder program resulted in temperature dependent interactions between the HCF angle bending motions and the molecular rotations about the z and y axes. These interactions in the liquid give satisfactory explanation of the D/H and /sup 13/C//sup 12/C vapor pressure isotope effects, spectroscopic data, and are qualitatively consistent with the structure of CH/sub 2/F/sub 2/ clusters obtained by ab initio molecular orbital calculations.