Isotope effects on isotope effects. Failure of the rule of geometric mean as evidence for tunneling

The authors showed in earlier work that secondary β-tritium isotope effects in eliminations from 2-arylethyl derivatives are often larger than expected on the basis of rehybridization of the C-H(T) bond from sp3 to sp2 as the system progresses along the reaction coordinate. They suggested that a contribution of tunneling to the secondary isotope effect could be responsible for this discrepancy and presented calculations showing that a significant tunnel correction results when the bending motions of the non-transferred hydrogen are coupled with the stretching motion of the transferred hydrogen. The calculations also predict that the tunnel correction to the secondary tritium isotope effect should be diminished when the transferred atom is deuterium rather than protium. They report here experimental evidence that this is indeed the case