Analysis of certain factors in the direct dynamics method: Variational rate constant of the NH3+OH→NH2+H2O reaction

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Abstract
The minimum energy path for the NH3+OH→NH2+H2O reaction was traced and the coupling between the reaction coordinate and normal modes was analyzed along it. Following the ‘‘direct dynamics’’ methodology, the rate constants were calculated for the temperature range 200–4000 K using the variational transition-state theory with two approximations to calculate the tunneling effect. With respect to the construction of the minimum energy path, the influence of the number of points calculated along this path, the scale factor in the Hessian matrix, and different ways of scaling the potential energy surface were analyzed. As this reaction presents a large reaction-path curvature, the small curvature approximation was not suitable for including the tunneling effect and a fitted Eckart potential was used. The variational effects were concluded to be negligible for this reaction, and good agreement with experimental values was found for T≥300 K when using the PSAC4/A1 curve (scaled by 0.8 and shifted to s=0), the Hessian matrix scaled by 0.9, and the fitted Eckart factor for the tunneling correction.