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Bimolecular Reactions |
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| We are the condensed-phase, bimolecular reactions lab, where right now we are interested in looking at reactions of the type
Br• + H-R → HBr + R• These reactions open themselves to vibrational mediation due to their inherent endoergicity: they simply will not occur without our help. It is the ultimate goal of this work to open the above reaction pathway through the use of vibrational excitation in the H-R reactant molecule. . Past Work We begin with producing the bromine radical and understand subsequent interactions with the surrounding solvent before we can begin to undertake study of the full reaction. Our first forays into these studies were with pure bromoform (CHBr3) samples, wherein the bromoform acts both as reactant and solvent. We irradiate our sample with a 100-fs pulse of 267-nm light, generating a Br/CHBr2 radical pair and probe it with white light continuum to measure the spectrum of the transient species. By controlling the path length difference between the pump and probe light, we generate a 2D UV-Vis spectrum that spans ~350-700 nm (~1 nm resolution) and 0-1 ns (~0.5 ps resolution). In these studies (as well as others from this lab), we re-learn the lesson that a radical should not be thought of as an independent species in solution, but will often associate with the surrounding solvent molecules.  Current Work. Using the assignment of the two peaks as corresponding to bromine absorptions in the solvent, we plunge ahead to attempt to drive this endothermic reaction with vibrational excitations. The cartoon of an endothermic, hydrogen-transfer reaction pathway indicates how vibrations can help the reaction complex reach the transition point.  We are now setting up our experiment to include an infrared pump that will excite C-H fundamental stretches, and is tunable over a broad enough range to include many hydrogen atom sources. Our expectation is to see a non-recovering loss of the Br signal, and we may soon search for the new H-Br vibration that would result from a hydrogen abstraction. Future Work. The bromine atom is a dynamic species in our sample, and we will first look to see if reactivity changes when we excite the H-R vibration at different points in the bromine's evolution and thermalization. Might it react better if the Br is still "hot," or do we expect that the 2D potential energy surface will lead us towards looking for reactivity with "cold" Br? Furthermore, we need to extend these thoughts of our 2D PES (decidedly a gas-phase, isolated molecule ideology) to a solvent-based picture. To this end, we are beginning some DFT calculations of our bromine complexes, with hopes of illuminating our findings with a more complete description of the surface that controls this reaction. It is hoped that our surfaces will be used to perform some molecular dynamics simulations so that we may have yet another handle on the factors affecting our reaction. |
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