Solution-phase reaction dynamics

Gaining control

Using infrared light to control the outcome of a chemical reaction is problematic in solution because of numerous interactions and non-specific sample heating. Now, condensed-phase results showing the vibrational enhancement of an otherwise thermally driven reaction may reinvigorate discussion of the practical applications of vibrational control.

For bimolecular reactions to occur, the molecules involved need to have both sufficient energy and the appropriate nuclear arrangement to cleave one bond while creating another. Molecular vibrations are often essential in thermally driven reactive encounters because the vibrational motion changes the positions of the nuclei with respect to one another and, thus, can promote the breaking of particular bonds. Chemists have mastered many aspects of synthesis; however, using vibrations to manipulate the chemistry of reacting molecules is not one of them. While placing energy into specific vibrations (using infrared light) and determining their influence on the subsequent reaction may lead to exciting synthetic applications, it also provides fundamental insight into the chemistry involved.

Although there are various examples of vibrational control in bimolecular, gas-phase reactions1, the almost continuous interaction between reactants and solvent complicates bimolecular reactions in solution2. These interactions can hinder passage over the transition state, remove energy from the products as they form, limit the separation of products and change the barrier height to reaction. Furthermore, fast vibrational relaxation in solution adds an additional complication to vibrational control in bimolecular, condensed-phase reactions.

Evidence of a vibrationally driven bimolecular reaction in solution has recently been reported3. The researchers (in the spirit of full disclosure, I was one of them) used vibrational excitation of the solvent to drive an endothermic reaction, namely hydrogen abstraction by a bromine atom. To experimentally determine the influence of vibrational excitation on a reaction, a comparison is made between signals obtained with and without the infrared excitation. As a result, any thermal reactions that occur without the infrared excitation become background signal, which can obscure vibrational enhancement. The endothermic abstraction reactions studied generated very small backgrounds, which was crucial to identifying the influence of the vibrational excitation. In contrast, writing in Nature Chemistry, Heyne and co-workers report that they are able to observe vibrational enhancement in an otherwise thermally driven bimolecular reaction in solution4. This result increases the allure of using vibrational control to alter the course of a reaction in the condensed phase.

The use of infrared light to drive chemical reactions relies on the excitation of a vibration that maps onto the reaction coordinate, thus carrying the system from reactants to products. The degree to which the vibrational motions resemble the reaction coordinate is crucial in determining its control of the reactivity. In the gas phase, excitation of a vibration by use of a nanosecond laser pulse (having good energy resolution) creates a stationary state that persists long enough to become the reaction coordinate of a bimolecular reaction. In the condensed phase, where collisions occur roughly every 100 fs, a shorter laser pulse is required to obtain dynamic information. Excitation of a vibration by use of a femtosecond laser pulse (having poorer energy resolution) creates a vibrational population that evolves with time. For this non-stationary state to drive the chemical reaction effectively, it must retain some character that resembles the reaction coordinate when the vibrationally excited molecule finds a reactive partner.

Heyne and co-workers describe acceleration of the formation of urethane and polyurethane following vibrational excitation of their reaction mixtures. Specifically, they excite the OH and NCO stretching vibrations of their alcohol- and isocyanate-containing reactants, respectively (see Fig. 1). Because fast vibrational relaxation occurs in solution, the critical question raised by the results is whether the acceleration is due to mode-specific behaviour or simply a consequence of heating the sample. Infrared light excites particular vibrational motions, as opposed to the statistical population of vibrational motion obtained by heating. However, initially excited molecules can quickly undergo intramolecular vibrational relaxation to populate other vibrational modes within the molecule and, on a longer timescale, intermolecular energy transfer to populate vibrational modes in the surrounding molecules5,6. Heyne and co-workers use ultrafast infrared-pump, infrared-probe experiments to follow the picosecond reactant decay and product rise following O–H stretch excitation to show that the enhancement in the product formation occurs in roughly 10 ps, a timescale that is considered too fast to be the result of a generalized heating of the solvent. Although there is a clear vibrational dependence on the rate, the 350 fs experimental time resolution does not allow them to explicitly tie the O–H stretching vibration to the reaction coordinate.

Figure 1: Vibrational enhancement of bimolecular reactions.
figure1

a, Schematic plot of a generic binary reaction and the potential influence of vibrational excitation. b, Heyne and co-workers4 excited the OH and NCO stretching vibrations of their reactants to enhance the rate of formation of urethane and polyurethane. Shown here is the urethane synthesis; vibrationally excited phenylisocyanate and cyclohexanol react to produce cyclohexyl-carbanilate. Polyurethane was made by starting with toluene-2,4-diisocyanate and 2,2,2-trichloroethane-1,1-diol.

While vibrationally mediated chemistry has strong roots in fundamental physical chemistry, the ability to exploit vibrations to control the course of a chemical reaction has many potential practical applications. Heyne and co-workers recognized the promise of applying vibrational control to photolithography and, as such, used their infrared laser-driven acceleration of polyurethane formation to write squares onto their sample window. With this work, Heyne and co-workers reaffirm the significance of vibrational control in fundamental molecular reaction dynamics and, in addition, advance the idea of vibrationally controlling a chemical reaction towards more practical applications. Yet, with a measured increase in the reaction rate of 10% and an overall increase in the quantum yield of about 0.3% upon O–H excitation, much work awaits to make vibrationally driven chemistry a commercially viable process.

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Correspondence to Amanda S. Case.

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Case, A. Gaining control. Nature Chem 10, 113–114 (2018). https://doi.org/10.1038/nchem.2937

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