Abstract
The thermochemistry of the hydride transfer equilibrium (CD/sub 3/)/sub 3/C/sup +/ + (CH/sub 3/)/sub 3/CH reversible (CH/sub 3/)/sub 3/C/sup +/ + (CD/sub 3/)/sub 3/CH was measured by pulsed high-pressure mass spectrometry. Measurements over a wide temperature range, 145-600K, allowed an accurate determination of the values: ..delta..H/sup 0/ = -0.57 +- 0.02 kcalmol; ..delta..S/sup 0/ = -0.57 +- 0.08 cal(mol x K); ..delta..G/sup 0//sub 298/ = -0.40 +- 0.07 kcal x mol; and K/sub 298/ = 1.97 +- 0.20. The direction of the observed isotope effect is consistent with C-H bond weakening in the ion due to hyperconjugation. The kinetics of the reaction show a slow rate and a large negative temperature coefficient, with k/sub 300/ = 0.36 and k/sub 600/ = 0.00625 x 10/sup -10/ cm/sup 3/ s/sup -1/, i.e., reaction efficiencies of about 0.03 to 0.0005. The observed negative temperature coefficient, k = AT/sup -5.8/, is larger than those observed for more exothermic hydride transfer reactions. The approach to collision rate is abrupt.
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