Stable isotope studies. Annual progress report, December 1, 1978-November 30, 1979

The BBIR-Type precision cryostat was debugged, and interfacing of the cryostat with an LSI-11-based microcomputer was completed. A program for the cryostat control and data acquisition was written. A spiral quartz gauge for the absolute pressure measurements up to 1500 torr has been calibrated against a dead-weight gauge. The vapor pressures of 12CHF3, 13CHF3, and 12CDF3 have been measured in the cryostat at temperatures between -139.850C and -80.810C. The carbon isotope effects observed are inverse effect, i.e., P(13C) > P(12C), while the hydrogen isotope effect observed are normal effect, P(H) > P(D). The observed carbon vapor pressure isotope effects point to a second cross-over temperature at 218.660K, at which P(13C) = P(12C). A plot of T 1n(P'/P) against 1/T for the observed D/H vapor pressure isotope effects indicates a positive intercept with the ordinate. These anomalies are explainable on the basis of the blue-shift in the C-H stretching frequency in fluoroform. Besides the isotopic species of fluoroform, methylfluoride was also purified to reduce all chemical impurities to a level below 10 ppM. The construction of nitrogen-15 fractionation system using gaseous nitric oxide under pressure and liquid dinitrogen trioxide, which was started during 1978, was continued. A new scheme for the determination of unit stage separation factor (α) for isotope exchange reactions has been devised,which combines the advantages of the cascade method and the single-stage method. An analytical expression has been developed for the effect of a differential change in a vibrational force constant upon isotopic reduced partition function ratio