The potential-energy hypersurface of the addition reaction

{OH+C}_{2} H_{4}

was partially explored following two different approaches. First, the stationary points were located at the MP2(FULL)/6-31G(d,p) level and then the minimum energy path (MEP) was built starting from the MP2 saddle-point geometry. In order to improve the energetics along the MEP, single-point calculations were carried out at several higher levels, in particular, PMP2, MP4sdtq, PMP4sdtq, and QCIsd(t). In a different approach, the C–O bond length was assumed to provide an accurate parametrization of the reaction path in the vicinity of the transition state. The minimum energy structures at the MP4sdq/

6-311+ G(d,p)

level for 16 points along the

R_{C–O}

coordinate have been calculated, followed by a generalized normal-mode analysis at the MP2(FULL)/

6-311+ G(d,p)

level for each point. The initial potential information from both approaches was used to calculate canonical variational transition state (CVT) association rate constants for the temperature range 200–1000 K. Our calculations at the PMP4sdtq/6-311+G(d,p)//MP4sdq/6-311+G(d,p)[MP2(FULL)/6-311 +G(d,p)] level reproduce the inverse dependence of the rate constant with temperature at

T<565 K,

in agreement with the experimental evidence that this reaction has a negative activation energy at room temperature. The analysis of the enthalpic and entropic contributions to the Gibbs free-energy profile has allowed us to understand those negative values of the activation energy.

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