EXPONENTIAL RELATIONSHIPS AMONG MULTIPLE HYDROGEN ISOTOPE EFFECTS AS PROBES OF HYDROGEN TUNNELING

For hydrogen transfer reactions with moderate degrees of tunneling, the semiclassical relationship of deuterium, tritium to protium, tritium isotope effects is predicted to break down, such that ( kD kT)3.26 < kH kT. By contrast, kinetically complex reactions produce ( kD kT)3.26 > kH kT. We now show that these limiting cases may not pertain under the conditions of (i) measured secondary kinetic isotope effects which fall below their equilibrium limits and (ii) extensive hydrogen tunneling. Although condition (i) could lead to a false signature of tunneling, ambiguities of interpretation can be avoided by the simultaneous measurement of exponential relationships for primary and secondary isotope effects. With the exception of reactions where the H-transfer process is known to be fully rate determining, condition (ii) may lead to an underestimate of quantum effects as judged by exponential relationships. © 1992.