Effect of deuterium substitution on the vapour pressure of acetonitrile

The vapour pressure difference between CH3CN and CD3CN was measured by differential capacitance manometry between - 40 and + 800C and the results can be expressed by the equation: ln (psub(H)/psub(D)) = 871.8 K2/T2 - 13.58 K/T + 0.00687. The vapour pressure of acetonitrile between the melting point and 300C has also been determined. The experimental data were interpreted within the framework of the statistical theory of isotope effects in condensed systems. The largest contribution to the vapour pressure isotope effect arises from the shifts in the CH stretching vibrations on condensation and these were found to undergo a blue shift with increasing temperature. The results also indicate that the rotation of the molecules about the 3-fold symmetry axis can be considered quasi-free whereas the rotations about the axes perpendicular to the top axis are restricted. (orig.)