The method of Stern, Van Hook, and Wolfsberg for the calculation of vapor pressure isotope effects was applied to model calculations of medium effects on /sup 13/C kinetic isotope fractionation in the decompositions (via several reaction coordinates) of a hypothetical nonlinear triatomic; molecular parameters were given values characteristic of organic compounds, while the necessary external force constants were adjusted to yield associated frequencies typicaI of moderate H bonding (30-- 90 cm/sup -1/). Influences of interactions between external coordinates were studied, as were those between external coordinates and an irternal coordinate which was not a component of the reaction coordinate. TAM was selected as the model because of its small size, all such interaction effects decreasing rapidly with increasing molecular mass and complexity. In no case studied would the medium effects calculated be measurable given present techniques; indeed, for molecules of ordinary mass and complexity, such effects would not even be a detectble increment or decrement to isotope rate effects determined with reasonable precision. Because the effects are additive, subject to certain restrictions, it is possible that medium effects of experimental signicance could be produced more » if: the isotopic molecules were relatively light; reaction temperatres were low (say, below 25 deg C); solvent-- solute interactions were drastically different in magnitude in reaction and transition states; one or more of the component interactions includes coupling to an internal coordinate (preferably isotopic) which is part of the reaction coordinate; and the force constants associated with such interactions are substantially larger than those employed here. If the last two conditions are met, the assumptions of the cell model will have been strained and some more natural technique for handling such strong interactions should be attempted. (auth) « less