ORIGIN OF APPARENT SWAIN-SCHAAD DEVIATIONS IN CRITERIA FOR TUNNELING

Vibrational analysis calculations have been conducted in an effort to understand the origins of apparently anomalous relationships reported between k(H)/k(T) and k(D)/k(T) kinetic isotope effects. The actual isotopic substitutions used in previous work are more complex than a simple comparison between H/T and D/T isotope effects. The relationships between the actual isotope effects determined in these studies amount to tests of two standards of conventional isotope effect theory, the rule of the geometric mean (or the lack of isotope effects on isotope effects) and the Swain-Schaad rule. Model calculations illustrate the importance of violations of the rule of the geometric mean (over Swain-Schaad deviations) arising in models that incorporate both explicit reaction-coordinate coupling of two isotopic sites and reaction-coordinate tunneling. Implications for experimental studies of tunneling using apparent Swain-Schaad exponents are discussed.