First Evidence of Vibrationally Driven Bimolecular Reactions in Solution: Reactions of Br Atoms with Dimethylsulfoxide and Methanol

Abstract

We present evidence for vibrational enhancement of the rate of bimolecular reactions of Br atoms with dimethylsulfoxide (DMSO) and methanol (CHOH) in the condensed phase. The abstraction of a hydrogen atom from either of these solvents by a Br atom is highly endoergic: 3269 cm for DMSO and 1416 or 4414 cm for CHOH, depending on the hydrogen atom abstracted. Thus, there is no thermal abstraction reaction at room temperature. Broadband electronic transient absorption shows that following photolysis of bromine precursors Br atoms form van der Waals complexes with the solvent molecules in about 5 ps and this Br-solvent complex undergoes recombination. To explore the influence of vibrational energy on the abstraction reactions, we introduce a near-infrared (NIR) pump pulse following the photolysis pulse to excite the first overtone of the C-H (or O-H) stretch of the solvent molecules. Using single-wavelength detection, we observe a loss of the Br-solvent complex that requires the presence of both photolysis and NIR pump pulses. Moreover, the magnitude of this loss depends on the NIR wavelength. Although this loss of reactive Br supports the notion of vibrationally driven chemistry, it is not concrete evidence of the hydrogen-abstraction reaction. To verify that the loss of reactive Br results from the vibrationally driven bimolecular reaction, we examine the pH dependence of the solution (as a measure of the formation of the HBr product) following long-time irradiation of the sample with both photolysis and NIR pump beams. We observe that when the NIR beam is on-resonance, the hydronium ion concentration increases fourfold as compared to that when it is off-resonance, suggesting the formation of HBr via a vibrationally driven hydrogen-abstraction reaction in solution.

Probing the Partial Activation of Water by Open-Shell Interactions, Cl(H2O)1–4

“…Collisions of chlorine atoms with neutral water clusters could potentially yield structures living long enough for observation, but these energy-dependent lifetimes are beyond the scope of the present analysis. A more promising prospect includes vibrational excitation, where past studies have driven this type of chemistry, − including for chlorine–water complexes. ,, Overtone excitation of an O–H stretch, for example, would already yield enough energy to reach the entire manifold of HCl and charge-transferred structures. Nonetheless, the present computations have served to identify a series of new structures even at low energies, as well as determine the “lineage” of charge-separated species in much larger clusters.…”

Bond-Breaking/Bond-Forming Reactions by Vibrational Excitation: Infrared-Induced Bidirectional Tautomerization of Matrix-Isolated Thiotropolone

“…The dream of tuning light at a vibrational frequency of a particular bond and inducing a chemical reaction only at that site is extremely challenging, even using femtosecond pulses under low-pressure gas-phase conditions . This is partially because intramolecular vibrational energy redistribution (IVR) dissipates the deposited energy fast to other modes before the reaction arises. , Alternatively, if a dominant (nonstatistical) IVR pathway exists due to the coupling between an excited vibrational mode and a reaction coordinate, this might be exploited to induce a desired chemical transformation in an elegant and selective way. , One remarkable example illustrating this possibility is the vibrationally induced unimolecular reaction of the jet-cooled Criegee intermediate syn -CH 3 CHOO, reported by Lester and co-workers − The authors demonstrated that excitation of CH stretching overtones or combination modes induces a 1,4-H shift from the methyl group to the terminal oxygen and activates decomposition of the molecule (Scheme ). Nevertheless, the application of this strategy to induce and control organic reactions remains scarcely explored. − …”

Infrared Laser Excitation Controlled Reaction Acceleration in the Electronic Ground State

“…There has been evidence that atomic Br obtained via photolysis of Br 2 reacted with the solvent (dimethyl sulfoxide (DMSO) or methanol) in a vibrationally driven hydrogen abstraction to form HBr. Further, efforts were performed to use vibrational excitation to manipulate and initiate chemical reactions successfully. , In contrast to other attempts, we demonstrated the acceleration of a thermally driven reaction, namely the alcoholysis of urethane . The bimolecular synthesis reaction between CH-ol and PHI produces cyclohexyl-carbanilate in tetrahydrofuran (THF) solution (see Figure a), with an activation energy of ∼2340 cm –1 .…”

“…In contrast to reactions in the gas-phase , solute–solvent interactions potentially limit the prospects of laser-driven bond selective chemistry and reaction rate control by means of vibrational excitation. Hence only recently, the concept of reaction rate acceleration successfully used in gas phase bimolecular reactions has been implemented in solution . There has been evidence that atomic Br obtained via photolysis of Br 2 reacted with the solvent (dimethyl sulfoxide (DMSO) or methanol) in a vibrationally driven hydrogen abstraction to form HBr.…”