Ab initio predictions and experimental confirmation of large tunneling contributions to rate constants and kinetic isotope effects for hydrogen atom transfer reactions

The interpretation of kinetic isotope effects (KIE's) involves potential energy barrier heights, vibrational effects of stretches and bends, and competition between overbarrier and tunneling mechanisms. In favorable cases, KIE's provide some of the most compelling evidence for or against detailed interpretations of the dynamics of reactive events. The present paper reports such a case in which the detailed question is the role of tunneling in hydrogen atom transfer reactions in the gas phase. In this communication the authors report new calculations and experiments on the bimolecular rate constant ratio k3/k4 O + HD → OH + D, O + HD → OD + H, which, together with earlier results for k1 and k2 O + H2 → OH + H, O + D2 → OD + D provide strong evidence for the dominance of tunneling in all four reactions at temperatures below 500 K. For the present report previous results for reactions 1 and 2 and new results for 3 and 4 are fit to Arrhenius forms k/sub i/ = A/sub i/ exp(-E/sub i//RT) and k/sub i//k/sub j/ = A/sub ij/exp (E/sub ij//RT). Since the parameters depend on the temperature range used for the fit, this is indicated in every case