The computed stationary points have been characterized as minima or transition states by diagonalizing the Hessian matrix and analyzing the vibrational normal modes.
In this way, the stationary points can be classified as minima if no imaginary frequencies are shown or as transition states if only one imaginary frequency is obtained.
The particular nature of the transition states has been determined by analyzing the motion described by the eigenvector associated with the imaginary frequency.
The initial structure for the IRC calculation was the optimized transition state.
The geometrical structures were optimized at each point along the reaction path of IRC.
Different numbers of MaxPoints (5, 10, 15, 20, and 40) along the reaction path were adopted to examine the calculated results (MaxPoints = 40 was used unless otherwise mentioned).
For the PES calculation, the step sizes were 0.02 Å ranging from 0.95 to 1.55 Å for a C-H bond and from 2.35 to 3.85 Å for a C-O bond.