Dynamic and non-statistical behavior is the subject of Chapter 7 in my book. Hase and co-workers have uncovered another interesting case of dynamic behavior.¹ The reaction of interest here is F^– + CH₃OOH. A number of different critical points and reactions exist on this surface. The complex CH₃OOH^…F^– (1) lies 36.5 kcal mol^-1 below separated reactants. 1 can rearrange through TS1 (with a barrier of 24.1 kcal mol^-1) to give F^–…CH₃OOH (2). 2 can then cross a second transition state (TS2) with a barrier of 4.7 kcal mol^-1) to give CH₂(OH)₂^…F^– (3), which lies in a very deep well. The B3LYP/6-311+G(d,p) geometries of these critical points are shown in Figure 1.

1 -36.5	TS1 -12.4	2 -16.2
TS2 -11.5	3 -104.8

Figure 1. B3LYP/6-311+G(d,p) optimized geometries of the critical points on the PES for the reaction of F^– with CH₃OOH.¹ Energies in kcal mol^-1 relative to separated reactants

What drew Hase to this problem were the interesting experimental results of Blanksby, Ellison, Bierbaum and Kato.² The gas phase reaction produced HF + CH₂O + OH^–, not 3 or HF + CH₂(OH)O^–. Hase and coworkers ran a number of trajectories simulating reaction at 300 K, the experimental condition. Reactions were started at three points: (1) F^– separated by 15 Å from CH₃OOH, (2) at TS2 or (3) at a point along the intrinsic reaction coordinate (IRC) of the form HOCH₂O^–…HF.

76 of the 80 trajectories that start from TS2 result in the formation of HF + CH₂O + OH^–. The majority of the trajectories that start with separated reactants produce the complex 1 (97 out of 200), reflecting its low energy and high exit barriers. 55 of these200 trajectories remain as isolated reactants. However, 45 trajectories give HF + CH₂O + OH^–, as do all 5 trajectories that start with HOCH₂O^–…HF. No trajectories give 3, the product expected from following the IRC. The computations are in complete agreement with the experimental results; the unusual decomposition products result from following a non-IRC pathway!

Since motion along the imaginary frequency of TS2 initially is to cleave the O-O bond and the C-H bond, momentum in that direction carries the reaction over to the decomposition product rather than making a tight turn on the PES necessary to make 3. These computations show once again that reactions can follow pathways that lie far from steepest descent or IRC pathways.

References

(1) Lopez, J. G.; Vayner, G.; Lourderaj, U.; Addepalli, S. V.; Kato, S.; deJong, W. A.; Windus, T. L.; Hase, W. L., "A Direct Dynamics Trajectory Study of F^– + CH3OOH Reactive Collisions Reveals a Major Non-IRC Reaction Path," J. Am. Chem. Soc. 2007, 129, 9976-9985, DOI: 10.1021/ja0717360.

(2) Blanksby, S. J.; Ellison, G. B.; Bierbaum, V. M.; Kato, S., "Direct Evidence for Base-Mediated Decomposition of Alkyl Hydroperoxides (ROOH) in the Gas Phase," J. Am. Chem. Soc. 2002, 124, 3196-3197, DOI: 10.1021/ja017658c.