The vapor pressures of all 10 deuteromdash;protiohyphen;ethane isomers C2HiD6mdash;i(0le;ile;6) have been measured over the temperature range 115deg; to 200deg;K sim;1 mm C2H6to 1600 mm C2H6 by direct manometry. The isotope effects are inverse and go through maxima between 125deg; and 140deg;K. At the maxima they are on the order of 1.2 per D atom. Deviations from the law of the mean are small, but for the three sets of equivalent isomers 1,1hyphen; and 1,2hyphen;C2H4D2; 1,1,1hyphen; and 1,1,2hyphen;C2H3D3; and 1,1,1,2hyphen; and 1,1,2,2hyphen;C2H2D4significant differences in vapor pressure are observed. The more unsymmetrically substituted compound has the higher vapor pressure in all cases.The data are interpreted in the light of the statistical theory of isotope effects in condensed systems. A model calculation is made within the framework of this theory in its harmonichyphen;cell approximation using reasonable force fields. It is necessary to invoke temperaturehyphen;dependent force constants for the lowhyphen;energy (u=hv/kT2pgr;) modes in order to rationalize the temperature dependency below 140deg;K. The agreement between the model calculation and experiment is excellent.The calculations reveal that the torsional motion, which must be blue shifted on condensation, is the single most important contributor to the isotope effects between equivalent isomers.The results constitute the second independent confirmation of the statistical theory of isotope effects in condensed systems as applied to molecules exhibiting higherhyphen;order quantum effects.
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