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Media type:
E-Article
Title:
Vapor Pressures of the Deuterated Ethanes
Contributor:
Van Hook, W. Alexander
Published:
AIP Publishing, 1966
Published in:
The Journal of Chemical Physics, 44 (1966) 1, Seite 234-251
Language:
English
DOI:
10.1063/1.1726453
ISSN:
0021-9606;
1089-7690
Origination:
Footnote:
Description:
<jats:p>The vapor pressures of all 10 deutero—protio-ethane isomers C2HiD6—i (0≤i≤6) have been measured over the temperature range 115° to 200°K [∼1 mm C2H6 to 1600 mm C2H6] by direct manometry. The isotope effects are inverse and go through maxima between 125° and 140°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 1,1- and 1,2-C2H4D2; 1,1,1- and 1,1,2-C2H3D3; and 1,1,1,2- and 1,1,2,2-C2H2D4 significant differences in vapor pressure are observed. The more unsymmetrically substituted compound has the higher vapor pressure in all cases.</jats:p>
<jats:p>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 harmonic-cell approximation using reasonable force fields. It is necessary to invoke temperature-dependent force constants for the low-energy (u=hv/kT&lt;2π) modes in order to rationalize the temperature dependency below 140°K. The agreement between the model calculation and experiment is excellent.</jats:p>
<jats:p>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.</jats:p>
<jats:p>The results constitute the second independent confirmation of the statistical theory of isotope effects in condensed systems as applied to molecules exhibiting higher-order quantum effects.</jats:p>