One common mechanism by which catalysts operate is by lowering the transition state barrier to reaction. We are investigating the ability of the hydrogen bonding of water to the transition state to catalyze simple organic reactions such as dehydration and decarboxylation. Quantum chemistry calculations demonstrate that hydrogen bonding at the transition state can dramatically lower the barrier to reaction. This is expected to result in an acceleration of the reaction in a thermal solution phase environment. Our interest is in photochemistry, where the reaction is activated by light rather than thermal excitation. In 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. We study photocatalysis on the ground-state potential energy surface using small water clusters, e.g. CH2FOH•(H2O)n and CH2(OH)2•(H2O)n. These species undergo photo-elimination reactions, e.g. CH2FOH + hν → CH2O + HF. The reactions are theoretically modeled using on-the-fly trajectory dynamics simulations following OH-overtone excitation. A competition occurs between the reactive channel and the dissapation channel represented by water evaporation from the cluster. It is found that the reaction is often anticatalyzed by the presence of the water, where the reaction does not occur until the photon energy is well above the uncatalyzed barrier. Our 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.
- 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).