A fitting formula was recently proposed (Zhang & Law 2009) to approximate the falloff curves of unimolecular reaction rate constants, yielding improved performance as compared with previous formulas in all reactions tested except for the strongly tunneling-affected reaction C2H3 → C_2H_2+H. In this work, we first formulated the effects of tunneling on the shape of the falloff curves based on Troe's weak collision model of unimolecular reaction rates. An alternate fitting formula, which includes an additional tunneling-correction parameter, is proposed from the formulation and examined for several tunneling models. The present expression shows improved performance in fitting tunneling-affected reaction rates as compared to previous formulas.
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