One common mechanism by which catalysts operate is by lowering the transition state barrier to reaction. ÌýWe are investigating the ability of the hydrogen bonding of water to the transition state to catalyze simple organic reactions such as dehydration and decarboxylation. ÌýQuantum chemistry calculations demonstrate that hydrogen bonding at the transition state can dramatically lower the barrier to reaction. ÌýThis is expected to result in an acceleration of the reaction in a thermal solution phase environment. ÌýOur interest is in photochemistry, where the reaction is activated by light rather than thermal excitation. ÌýIn this case, it is not clear whether the barrier lowering will promote the reaction since hydrogen bonding will also provide an avenue for energy dissapation. ÌýWe study photocatalysis on the ground-state potential energy surface using small water clusters, e.g. CH2FOH•(H2O)nÌýand CH2(OH)2•(H2O)n. ÌýThese species undergo photo-elimination Ìýreactions, e.g. CH2FOH + hνÌý→ CH2O + HF. ÌýThe reactions are theoretically modeled using on-the-fly trajectory dynamics simulations following OH-overtone excitation. ÌýA competition occurs between the reactive channel and the dissapation channel represented by water evaporation from the cluster. ÌýIt is found that the reaction is oftenÌýanticatalyzedÌýby the presence of the water, where the reaction does not occur until the photon energy is well above the uncatalyzed barrier. ÌýOur group is studying photocatalysis as a non-statistical dynamical problem since the mere observation of barrier lowering is insufficient to establish the catalytic effect of hydrogen bonding.Ìý
- Kramer, Z. C., Takahashi, K., Vaida, V. & Skodje, R. T. Will water act as a photocatalyst for cluster phase chemical reactions? Vibrational overtone-induced dehydration reaction of methanediol. The Journal of chemical physics 136, 164302 (2012).
- Kramer, Z. C., Takahashi, K. & Skodje, R. T. Water catalysis and anticatalysis in photochemical reactions: Observation of a delayed threshold effect in the reaction quantum yield. Journal of the American Chemical Society 132, 15154–15157 (2010).
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