Article pubs.acs.org/JPCB
First Evidence of Vibrationally Driven Bimolecular Reactions in Solution: Reactions of Br Atoms with Dimethylsulfoxide and Methanol Jae Yoon Shin,† Michael A. Shaloski, F. Fleming Crim,* and Amanda S. Case*,‡ Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States ABSTRACT: We present evidence for vibrational enhancement of the rate of bimolecular reactions of Br atoms with dimethylsulfoxide (DMSO) and methanol (CH3OH) in the condensed phase. The abstraction of a hydrogen atom from either of these solvents by a Br atom is highly endoergic: 3269 cm−1 for DMSO and 1416 or 4414 cm−1 for CH3OH, 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 CH (or OH) 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.
I. INTRODUCTION The reaction of two molecules requires that they have sufficient energy to break one bond while forming another. Thus, the key to bimolecular reactions in both gases and liquids lies in the encounters of those molecules with enough energy, in an appropriate coordinate, to undergo a reaction. Molecular vibrations are often essential in thermally reactive encounters because the vibrational motion moves the atoms of molecules with respect to one another and promotes the cleavage of particular bonds. Consequently, placing energy into vibrations that excite nuclear motions along the reaction coordinate connecting reactants and products should serve as a means of controlling a reaction.1 Indeed, gas-phase experiments have proven this molecular-level control.2−9 Although there are various examples of vibrational control in the gas phase, one decisive example that comes from early work in our group is the reaction of H + HOD. The reaction of a H atom with HOD does not occur under thermal conditions. However, with the use of initial vibrational excitation the reaction proceeds at a rate similar to the gas kinetic collision rate.2 Furthermore, excitation in the OH oscillator results in hydrogen abstraction that cleaves the initially excited bond and produces an OD with a roughly 200:1 yield over that of OH.10 © 2017 American Chemical Society
Likewise, excitation in the OD oscillator favors cleavage of the OD bond by a factor of roughly 220.10 These results encourage us to explore the possibility of performing similar vibrationally driven reactions in the more complex environment of a liquid. The almost continuous interaction between the reactants and solvent complicates bimolecular reactions in solution. These interactions can hinder passage over the transition state, remove energy from the products as they form, limit the separation of products, and, perhaps most important to the reaction, change the barrier height.11 Yet, recent studies on exoergic hydrogen-abstraction reactions in solution revealed that the condensed-phase bimolecular reaction dynamics is quite similar to that in the gas phase, especially when the reaction occurs fast enough to compete with the solvent response.12−17 The solvent does not completely dampen the vibrational excitation of the product, and the excess energy of the reaction appears as product vibration. Thus, it appears that in favorable cases we can use the knowledge gained from gasphase studies to predict the reaction dynamics in the condensed Received: January 2, 2017 Revised: February 15, 2017 Published: February 16, 2017 2486
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Figure 1. Schematic of the three-beam experiment designed to promote a vibrationally driven hydrogen-abstraction reaction in a solution of Br2 in CH3OH. The 400 nm photolysis pulse generates Br atoms, and the NIR pump pulse excites either the CH or OH stretch (v = 2) of CH3OH, which drives this endoergic reaction. A 400 nm probe pulse monitors the reaction dynamics as we scan Δt2 with Δt1 fixed. The thermal reaction is negligible due to the large endoergicity.
should make the rate of the thermal reactions negligible. Within a picosecond of the photolysis pulse (Δt1 = 1 ps in Figure 1), we introduce a near-infrared (NIR) pump pulse that excites two quanta of the CH or OH stretch in CH3OH. The energy of the vibrational excitation (at 1700 nm, or 5882 cm−1, for the CH stretch and at 1500 nm, or 6667 cm−1, for the OH stretch) is far above the endothermicity for the abstraction of a methyl hydrogen (1416 cm−1) and a hydroxyl hydrogen (4414 cm−1). Therefore, if a source of reactive Br encounters the vibrationally excited CH3OH, the hydrogen-abstraction reaction can occur. To monitor the dynamics of Br atoms in solution, we follow the temporal evolution of the Br• CH3OH complex at 400 nm. (A similar scheme applies to the Br + DMSO reaction.) We examine the signal that results only when both the photolysis and NIR pump pulses are present in the sample as a means of establishing the occurrence of vibrationally driven reactions in solution.
phase and, perhaps, even carry out vibrationally driven chemistry in solution. Still, there is an additional complication to performing vibrationally driven bimolecular reactions in solution. Because of fast vibrational relaxation in solution, proper timing is critical. The reactive species must be near a vibrationally excited molecule while its vibrational energy is in the desired coordinate. Studies on bimolecular reactions in solution focus on the photolysis-induced dynamics of reactions with little or no barrier, such as hydrogen abstraction of CN, Cl, and F atoms.15−24 When examining the influence of vibrational excitation on a reaction, these thermal reactions become a part of the background, which in the condensed phase will obscure any vibrational enhancement. However, endothermic reactions that have a slow thermal rate potentially permit studies of vibrational enhancement. To this end, we study the influence of vibrational excitation on the endothermic hydrogen-abstraction reactions of Br atoms with dimethylsulfoxide (DMSO) and methanol (CH3OH). Our quantum calculations using the CBS-QB3 method show that the gas-phase enthalpies for the reactions of interest are
II. EXPERIMENT The apparatus we use to monitor vibrationally driven bimolecular reactions in solution is similar to that used in our previous work on condensed-phase photolysis reaction dynamics.25 We begin with an ultrafast Ti:sapphire laser operating at 1 kHz, producing roughly 100 fs pulses that are centered at 800 nm. We use the third harmonic of the laser at 267 nm to photolyze CHBr3 (Aldrich, 96%) or the second harmonic of the laser at 400 nm to photolyze Br2 (SigmaAldrich, ACS reagent grade ≥ 99.5%). A double-pass β barium borate optical parametric amplifier generates NIR light for vibrational excitation of the DMSO (Sigma, ≥99.5%) or CH3OH (Sigma-Aldrich, ≥99.9%) solvents. We detect the transient absorption of the Br−solvent complex at 400 nm with a pair of Si photodiode detectors. This single-wavelength detection provides the lowest noise in the difference signals. A computer-controlled mechanical translation stage controls the time delay between the photolysis and NIR pump pulses (Δt1 in Figure 1), and a second stage controls the time delay
CHBr3 in DMSO Br + CH3S(O)CH3 → HBr + •CH 2S(O)CH3
ΔH = 3269 cm−1
(R1)
Br2 in CH3OH Br + CH3OH → HBr + •CH 2OH → HBr + CH3O•
ΔH = 1416 cm−1 ΔH = 4414 cm−1
(R2) (R3)
Figure 1 outlines the Br + CH3OH reaction starting with Br2 in a CH3OH solution. First, a photolysis pulse creates Br atoms from Br2, after which the Br atoms begin to complex with the solvent to form Br•CH3OH. The substantial endothermicity 2487
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bromine precursor (CHBr3 at 267 nm and Br2 at 400 nm) quickly leads to the formation of a complex between the nascent radical and solvent. Broadband continuum probe light monitors the evolution of both the Br•−DMSO and Br•− CH3OH complexes by means of transient absorption. Figure 2 shows the transient electronic absorption for a solution of CHBr3 in DMSO following 267 nm excitation.
between these two pump pulses and the probe pulse (Δt2 in Figure 1). We find the optimum signal when the NIR pump pulse arrives roughly a picosecond after the photolysis pulse (Δt1 = 1 ps), and if the delay time, Δt1, is more than about 5 ps (or less than 0 ps, that is, the NIR pump pulse arrives earlier than the photolysis pulse), the difference signal disappears entirely. All of the three laser beams cross each other in the 0.12 M sample solution. A peristaltic pump circulates the sample through a cell with MgF2 windows and a 1 mm thick poly(tetrafluoroethylene) spacer. We divide our probe light into a signal pulse that passes through the sample and a reference pulse that goes around the sample. We use two fast mechanical choppers, each of which blocks and unblocks the photolysis and NIR pump beams at a rate of 500 and 250 Hz, respectively. The combinations of the two chopper states result in four different measurements: photolysis (on)−NIR (off), photolysis (off)−NIR (on), photolysis (on)−NIR (on), and photolysis (off)−NIR (off). We calculate three transient absorption signals based on these combinations of the two chopper states: the signals induced by (1) the photolysis pulse only, (2) the NIR pulse only, and (3) both the photolysis and NIR pulses. In addition, we calculate a difference signal by taking signal (3) and subtracting the contributions from the other two signals, (1) and (2). We also obtain broadband transient absorption measurements without vibrational excitation. To obtain the probe pulse for these broadband measurements, we focus a few nanojoules of 800 nm light into a CaF2 substrate to generate broadband continuum light that covers roughly 330−1100 nm. A pair of modified Czerny−Turner spectrometers, each with a 600 grooves/mm holographic grating, disperse signal and reference pulses onto matched Si photodiode arrays (512 pixels). This arrangement delivers a resolution of 0.7 nm/pixel and about 350 nm of spectral coverage. We calibrate the spectral window collected by the arrays using holmium absorption features. We employ mechanical shutters to control the introduction of the pump and probe pulses, allowing for calculation of the transient absorption. We use only one of the translation stages described above, since there is no vibrational excitation in these measurements. We use the Gaussian 09 suite of programs with the CBS-QB3 method to calculate the overall reaction energies. The CBSQB3 method extrapolates the energy to the complete basis set limit and is frequently used for estimating reaction energies with high accuracy.21,26,27
III. RESULTS AND DISCUSSION We first describe the characterization of the Br•−solvent complexes using broadband transient absorption. Then, we present the results of our three-beam experiments using singlewavelength detection at 400 nm. IR-driven depletion of the Br•−solvent complexes indicates that vibrationally driven chemistry occurs in the title reactions. Finally, we describe our efforts to monitor the HBr reaction product to confirm that hydrogen-atom abstraction occurs. The amount of product is too small for detection above the limiting noise of our apparatus by time-resolved IR absorption. Thus, we measure changes in the pH of the solution following long-time irradiation to test for the formation of the HBr product as a result of vibrational excitation. III.I. Probing the Br•−Solvent Complex. We monitor hydrogen abstraction by a Br atom in two solutions, CHBr3 in DMSO and Br2 in CH3OH. In both cases, photolysis of the
Figure 2. Broadband electronic transient absorption measurements for the reaction of CHBr3 in a DMSO solution following 267 nm photolysis: (a) contour plots, (b) spectra at selected delay times, and (c) time evolution at 425 nm. We obtain the solid line in (c) by fitting the data using the Smoluchowski model.
Panel (a) is a two-dimensional representation of the transient absorption data, and panels (b) and (c) are cuts along the time and wavelength axes, respectively. The figure shows rapid growth of a broad absorption band centered near 450 nm. In a previous study, we observed the formation of the iso-CHBr2− Br• isomer and the CHBr3−Br• complex following photolysis of CHBr3 in CCl4. In a concentrated CHBr3 solution (90% v/ v), an absorption feature near 390 nm appears within a few picoseconds and then decays as a band near 490 nm grows in with a time constant for the rise of 19 ps.25 In dilute CHBr3 2488
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The Journal of Physical Chemistry B solutions, the intensity of the 390 nm band is smaller and the 490 nm band appears at 450 nm, but the time constants for the appearance of these features remain the same. We assigned the 390 nm feature to the CHBr3−Br• complex and the 450 nm feature to the iso-CHBr2−Br• isomer.25 However, in this study we find that the rise of the 450 nm feature is complete within ∼5 ps, after which it continuously decays. Further, our rather dilute CHBr3 solution shows no clear absorption feature at 390 nm at early time delays, suggesting that the photolysis of CHBr3 in DMSO results in a different transient species from that formed in CCl4. Sumiyoshi et al. observed a similar transient feature in a pulsed-radiolysis study of DMSO and CBrCl3 mixtures, and they assigned this band at 450 nm to the absorption of a Br•−DMSO complex.28 Thus, it appears that the stronger interaction between the Br atom and DMSO (a polar solvent) compared to that with CCl4 (a nonpolar solvent) and the small number density of CHBr3 relative to that of DMSO favors the formation of the Br•−DMSO complex over that of the CHBr3−Br• complex and iso-CHBr2−Br• isomer. Figure 3 shows the results of electronic transient absorption measurements for a solution of Br2 in CH3OH following 400 nm excitation. Panel (a) is a two-dimensional representation of the transient absorption data, and panels (b) and (c) are cuts along the time and wavelength axes, respectively. This figure indicates the presence of an absorption band near 380 nm. The 380 nm feature appears immediately following the arrival of the photolysis pulse at 0 ps, maximizes after ∼5 ps, and decays quickly until ∼30 ps, after which the rate of decay slows. As with the CHBr3 in DMSO mixture, we infer that the Br• radical forms a complex with the solvent and that there is no other reaction pathway, such as isomerization, for this Br•−CH3OH complex. Thus, we ascribe the 380 nm feature to the absorption of the Br•−CH3OH complex. We observe the formation of a Br•−solvent complex in both the CHBr3 in DMSO and Br2 in CH3OH solutions. The rise associated with the formation of these complexes occurs in roughly 5 ps and is followed by a decay to roughly 60% of the maximum. This temporal behavior reflects the formation of the complex and subsequent recombination of these Br•−solvent complexes with their radical congeners (CHBr2• or Br•, which are either f ree in a solution or complexed with the solvent) to re-form the bromine precursor. Because the hydrogenabstraction reactions of the Br• radical with these solvents are highly endothermic (see Reactions R1−R3), we discount the possibility of thermally driven reactions between the Br atoms and the CH3OH or DMSO solvent. We fit the temporal evolution of each Br•−solvent complex band using the Smoluchowski model (Figures 2c and 3c). The Smoluchowski model describes well the diffusive recombination and diffusion-limited reactions in a solution29 and is widely used to characterize the reaction dynamics in the condensed phase.21,30−32 In the Smoluchowski model, the time-dependent survival probability P(t), or concentration, of the Br•−solvent complex is ⎧ ⎛ r − R ⎞⎫ ⎪ R rec rec ⎪ ⎜⎜ 0 ⎟⎟⎬exp( −k1t ) P(t ) = [Br]0 ⎨ 1 − erfc ⎪ ⎪ r0 ⎝ 4Drect ⎠⎭ ⎩
Figure 3. Broadband electronic transient absorption measurements for the reaction of Br2 in a CH3OH solution following 400 nm photolysis: (a) contour plots, (b) spectra at selected delay times, and (c) time evolution at 380 nm. We obtain the solid line in (c) by fitting the data using the Smoluchowski model.
of the radical recombination partners. This relative diffusion constant is the sum of the diffusion constants for each of the recombining partners, and we calculate it from the Stokes− Einstein equation
D=
kBT 6πηa
(2)
where kB is the Boltzmann constant, T is the temperature, η is the solvent viscosity, and a is the radius of the diffusing particle. We estimate the radius of the Br•−solvent complex as a sum of the radii of the Br atom and the solvent molecule, and we calculate the radii of methanol and DMSO from each molar volume. Because the density of the CHBr2 radical is not known, we calculate the radius of CH2Br2 and assume that the radius of CHBr2 is close to this value.
(1)
where [Br]0 is the initial concentration of the Br•−solvent complex; Rrec is the effective radius between the radical recombination partners, after which recombination occurs; r0 is the relative distance between the radical recombination partners following dissociation; and Drec is the relative diffusion constant 2489
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Figure 4. Electronic transient absorption measurements using a single-wavelength probe at 400 nm for the reaction of a CHBr3 in DMSO solution when (a) the 267 nm photolysis pulse is present, (b) the NIR pump pulse at 1680 nm is present, and (c) both the photolysis pulse and NIR pump pulse are present with Δt1 = 1 ps. We subtract (a) and (b) from (c) to obtain (d). Thus, (d) represents the difference signal that arises only when both the photolysis and NIR pump light interact with the sample.
By fitting our data to this model, we determine Rrec and r0 to be 0.19 (±0.04) and 0.51 (±0.08) nm, respectively, for the Br2 in CH3OH solution and 0.14 (±0.09) and 0.48 (±0.08) nm, respectively, for the CHBr3 in DMSO solution. We calculate the recombination yield from the ratio of Rrec/r0 and find it to be 0.37 for the Br2 in CH3OH solution and 0.29 for the CHBr3 in DMSO solution. These values are comparable to the recombination yield of the Cl•−solvent complex in neat CH2Cl2, which we previously determined to be 0.32.31 The exponential term in eq 1 describes the reactive loss of the complex, with an associated rate of k1. The fit to our data gives k1 ≈ 0 because the reaction of Br atoms with the solvent is negligible, as described above. However, the Cl•−solvent complex can (and does) react with the CH2Cl2, resulting in k1 = 1.36 × 107 M−1 s−1.31 The distinct difference between k1 for the endothermic (Br) and exothermic (Cl) hydrogenabstraction reactions shows that there are indeed no thermal reactions between the Br atoms and solvent molecules. III.II. IR-Driven Br•−Solvent Complex Depletion. Broadband detection contains valuable spectroscopic information, making it the preferred method for probing dynamics. However, single-point detection has less noise by roughly 2 orders of magnitude in our apparatus. Because the IRdependent difference signal is only about 1 mOD, we use single-point detection to obtain the best signal-to-noise ratio. We initiate the reaction by first photolyzing the bromine precursor and then irradiating the sample with NIR light intended to excite the first overtone of the CH (or OH) stretch of the solvent molecules. We scan the time delay between the two pump pulses and the probe pulse (varying Δt2
with Δt1 fixed) and record the changes in the intensity of the 400 nm probe light, which reflects the formation or loss of the Br•−solvent complex. During one scan, we record the four traces described above: the time evolution of the probe induced by (1) the photolysis pulse only, (2) the NIR pump only, (3) both the photolysis and NIR pump pulses, and (4) the difference signal calculated by subtracting (1) and (2) from (3). This final trace, that is, the difference signal, reveals the time evolution of the probe pulse that occurs only when both the photolysis and NIR pump pulses interact with the sample. III.II.I. CHBr3 in DMSO. Figure 4 shows a typical three-beam scan obtained for a solution of CHBr3 in DMSO. We show the temporal evolution of the Br•−DMSO complex that results from the 267 nm photolysis pulse (Figure 4a), the 1680 nm NIR pump pulse (Figure 4b), and the photolysis pulse followed by the NIR pump pulse (Figure 4c). Figure 4a exhibits the same dynamics as that in the two-beam, continuum-probe experiment shown in Figure 2c. Fitting this trace using the Smoluchowski model, we find parameters that are the same within experimental uncertainty as those in the two-beam experiment. The equivalence of these data confirms that the 400 nm probe is reporting on the same species detected with the broadband probe. Because there is no Br•−solvent complex formed without the photolysis pulse, the signal that results from the NIR pump alone (Figure 4b) contains only a coherence signal at Δt2 = 0. At first glance, Figure 4c looks like Figure 4a, but it becomes apparent in the difference signal (Figure 4d) that together the photolysis and NIR pump pulses induce dynamics beyond that attributable to their simple sum. It is useful to recall that the difference signal disappears when the 2490
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The Journal of Physical Chemistry B time delay between the photolysis pulse and NIR pump pulse is greater than 5 ps (the time scale for Br•−DMSO formation). The high sensitivity of the difference signal to the photolysis− NIR pump time delay and the need for Δt1 ≤ 5 ps suggest that the difference signal reflects the fate of the Br• radical. The observed difference signal shows a dip at Δt2 = 0 ps, which recovers to a steady level that remains below the initial baseline. The recovery of the signal is best fit by a biexponential, but the recovery times are inconsistent. However, we consider the difference signal as an indication of complex formation and tentatively interpret its behavior as arising from the interaction of the radical and the hot solvent. At short time delays, the solvent molecules have sufficient vibrational energy to prevent the prompt formation of Br•− DMSO complexes. The absence of complex formation appears as a large depletion in the difference signal. As the time delay increases, the solvent molecules begin to relax vibrationally, allowing the formation of some Br•−DMSO complexes. This increase in complex formation appears as a recovery in the difference signal. The effect of the vibrationally excited solvent on complex formation explains the dip and recovery that we observe in our difference signal, but not the long-time offset. We focus on the offset in the difference signal that persists even at the longest time delay. We observe this offset consistently in every scan. The magnitude of this offset varies from scan to scan, but it is always between 1 and 2 mOD. This offset in the temporal evolution of the Br•−DMSO complex shows the fraction of Br• radicals that are no longer complexed to the DMSO molecules. In the absence of reaction or another loss mechanism, all of the potentially reactive Br would be in the Br•−DMSO complex. Therefore, the offset reflects the loss of reactive Br. Two potential loss channels for Br• are radical recombination and reaction with a vibrationally excited DMSO molecule. Thus, this offset could signify the promotion of an endothermic reaction by vibrational excitation. The NIR pump wavelength dependence of the long-time offset is a test of the need for resonant vibrational excitation of the solvent. Figure 5a shows a static absorption spectrum of DMSO in the NIR region. There are two absorption features, one at 1680 nm and another at 1725 nm. The band at 1680 nm is likely the first overtone transition of the CH stretch in DMSO, but the origin of the 1725 nm band is undetermined.33 (The NIR pulse, having a bandwidth of roughly 170 cm−1, can also excite two quanta of the CHBr3 CH stretch at 1693 nm.34 However, the concentration of CHBr3 in the sample is 2 orders of magnitude lower than that of DMSO (0.12 vs 14 M), making any potential contribution to the long-time offset negligible.) Figure 5b shows the difference signals, for which we tune the NIR excitation pulse to three different wavelengths. We observe a large long-time offset when the NIR pulse is resonant with either the CH stretch overtone transition (1680 nm) or the 1725 nm feature. Further, there is no longtime offset with the NIR pulse at 1850 nm, away from either of the transitions. Thus, Figure 5 demonstrates the direct relationship between vibrational excitation of DMSO and the persistence of the long-time offset. III.II.II. Br2 in CH3OH. Figure 6 shows a typical three-beam scan obtained for a solution of Br2 in CH3OH. We show the time evolution of the Br•CH3OH complex that results from the 400 nm photolysis pulse (Figure 6a), the 1700 nm NIR pulse (Figure 6b), and the photolysis pulse followed by the NIR pulse (Figure 6c). Figure 6a exhibits the same dynamics as that in the two-beam, continuum-probe experiment shown in
Figure 5. NIR pump wavelength dependence of the difference signal for solutions of CHBr3 in DMSO: (a) absorption spectrum of DMSO in the overtone region (arrows indicate where we tune the NIR pump) and (b) difference signal with the NIR pump at 1680 nm (red), 1725 nm (blue), and 1850 nm (black). The solid lines are aids and do not represent fits to the data.
Figure 3c, again verifying that the 400 nm probe is tracking the desired species. Also, the signal that results from the NIR pump alone (Figure 6b) contains only the coherence signal. The difference signal obtained for the Br2 in CH3OH solution in Figure 6d resembles the one obtained for CHBr3 in DMSO and once again requires Δt1 ≤ 5 ps. The consistent offset that we observe for this solution is a bit smaller than that in the CHBr3 in DMSO solution, and it fluctuates from roughly 500 μOD to 1 mOD. Once more, we attribute the temporal evolution of the difference signal to the effect of the vibrationally excited solvent on Br•−solvent complex formation, and we credit the offset to the loss of reactive Br. Just as that for the CHBr3 in DMSO solution, we examine the NIR excitation wavelength dependence of the long-time offset observed for the Br2 in CH3OH solution. Figure 7a shows a static absorption spectrum of CH3OH in the NIR region. The first overtone of the CH stretch appears at 1700 nm, whereas we presume the broad absorption band at 1573 nm to be the first overtone of the OH stretch. Figure 7b shows the difference signals obtained using four different NIR wavelengths: 1300, 1500, 1700, and 1900 nm. When we tune the NIR wavelength to 1500 nm, resonant with (presumably) the first overtone of the OH stretch, we observe the largest offset. On changing the NIR wavelength to 1700 nm, resonant with the first overtone of the CH stretch, we observe the next largest offset. Excitation of either the CH or OH stretching overtone of the CH3OH solvent results in a significant offset in the difference signal. It is intriguing to 2491
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Figure 6. Electronic transient absorption measurements using a single-wavelength probe at 400 nm for the reaction of a Br2 in CH3OH solution when (a) the 400 nm photolysis pulse is present, (b) the NIR pump pulse at 1700 nm is present, and (c) both the photolysis pulse and NIR pump pulse are present, with Δt1 = 1 ps. We subtract (a) and (b) from (c) to obtain (d). Thus, (d) represents the difference signal that arises only when both the photolysis and NIR pump light interact with the sample.
Considering the large excess of the solvent (vs the solute), it is feasible that a portion of the reactive Br is lost because of a vibrationally driven hydrogen-abstraction reaction. There is also the potential for absorption of the NIR pump pulse by the Br•− solvent complexes, which may result in dissociation of the complex or an intramolecular hydrogen-atom transfer reaction. However, the sensitivity of the difference signal to the time delay between the photolysis and NIR pump pulses indicates that its temporal evolution and the loss of reactive Br result mainly from changes to the Br• radical (the dominant form of reactive Br when Δt1 < 5 ps) and not the Br•−solvent complexes (the dominant form of reactive Br when Δt1 ≥ 5 ps). The apparent nonreaction of the Br•−solvent complexes with vibrationally excited solvent molecules may result from an increased barrier height or a geometry that hinders their approach to the transition state. III.III. HBr Product Formation. Detecting the reaction product, HBr, would provide direct evidence of a hydrogenabstraction reaction. Moreover, observing the formation of the product in real time could provide information about the details of the reaction dynamics. Recent studies of bimolecular reactions in the condensed phase obtained detailed reaction dynamics, such as energy disposal into products and relaxation dynamics of vibrationally excited products, by monitoring the formation of products via time-resolved IR spectroscopy.21−24 We use the time-resolved IR spectrometer with tunable mid-IR light near 4000 nm previously described in our work on Clatom reactions21 to search for the HBr product, but we see no signal above the limiting noise in the apparatus.
consider the possibility of bond-selective chemistry, but the nature of vibrational relaxation in the condensed phase complicates our simple gas-phase picture. The significant offset following CH and OH stretch excitation may indicate that the Br• radical readily abstracts either the methyl or hydroxyl hydrogen atoms when the CH or OH stretches are excited (bond-selective chemistry). However, it may simply reflect shared excitation by our ultrafast NIR pump pulse or intramolecular vibrational relaxation to populate similar vibrational states that undergo hydrogen abstraction. Alternatively, we can adjust the NIR wavelength so that it is nonresonant with the vibrational transitions of the CH3OH solvent. The 1300 and 1900 nm, nonresonant data show a smaller offset. Ideally, the long-time offset would cease to exist when the NIR wavelength is off-resonance, as in the case of the CHBr3 in DMSO solution. However, considering the roughly 170 cm−1 bandwidth of our NIR excitation pulse, detuning the NIR wavelength from the CH3OH absorption bands found in the FTIR spectrum does not guarantee that there is no vibrational excitation from our excitation pulse. The inset in Figure 7a shows the NIR pulse bandwidth for comparison with the FTIR absorption spectrum, providing a measure of the effectiveness of these “off-resonance” NIR wavelengths. The results of the three-beam experiments on the Br2 in CH3OH and CHBr3 in DMSO solutions demonstrate chemistry initiated by vibrational excitation. The long-time offset observed in the difference signal for both of these systems reports the loss of some of the reactive Br due to the vibrational excitation. This loss occurs as a result of either radical−radical reactions or abstraction of a hydrogen atom by the Br• radical. 2492
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of the order of 106,37 indicating the presence of little to no molecular HBr. The possibility of reactive loss of HBr by protonation of CH3OH suggests that we might detect its initial formation by measuring the pH of the CH3OH solution with Br2 following long-time irradiation with photolysis and NIR light. At various instances over the course of this Br-atom reaction work, we have measured the pH of the solutions, where the NIR light being irradiated was on- and off-resonance, and have consistently found that the pH is lower following irradiation with 1700 nm light (resonant with the CH stretch overtone) than that upon irradiation with 1900 nm light (nonresonant). Although we find that resonant irradiation always results in a lower pH than that of nonresonant irradiation, the absolute values of these readings are not always the same, likely reflecting the amount of water present in the undried methanol. To quantify the influence of NIR excitation, we systematically measure the pH of a 0.03 M Br2 in CH3OH solution following 4 h of irradiation with photolysis and NIR light (1700 and 1900 nm) using a pH meter with an electronic probe. To obtain reliable results from the pH probe, we dilute our irradiated sample 10-fold with distilled water to ensure deprotonation of the methanol (in favor of formation of the hydronium ion). Consistently, we find a fourfold increase in the hydronium ion concentration when the laser is resonant with the CH stretch (1700 nm) versus that when it is off-resonant (1900 nm). The increased acidity of the solution when the NIR wavelength is on-resonance confirms the notion that vibrational excitation promotes the hydrogen-abstraction reaction of the Br• radical with CH3OH.
Figure 7. NIR pump wavelength dependence of the difference signal for solutions of Br2 in CH3OH: (a) absorption spectrum of CH3OH in the overtone region (arrows indicate where we tune the NIR pump) and (b) difference signal with the NIR pump at 1300 nm (black), 1500 nm (red), 1700 nm (blue), and 1900 nm (green). The solid lines are aids and do not represent fits to the data. The inset in (a) shows the spectrum of the IR excitation pulse.
IV. SUMMARY We demonstrate that vibrational excitation of the solvent alters the dynamics of a Br•−solvent complex in DMSO and CH3OH. The difference signals obtained from three-beam experiments (photolysis, NIR pump, and probe) show that together the photolysis and NIR pump pulses induce dynamics that results in a long-time loss of reactive Br. This observed offset depends on the NIR wavelength, becoming large only when the NIR pump is resonant with a vibrational transition of the solvent. The loss of reactive Br may arise from radical recombination or from the reaction of the Br• radical with a vibrationally excited solvent molecule. The sensitivity of the difference signal to the time delay between the photolysis and NIR pump pulses illustrates the need for free Br• radicals, and an increase in the acidity of the sample following long-time irradiation with resonant NIR light versus nonresonant NIR light demonstrates that the loss of reactive Br results, at least in part, from the vibrationally enhanced hydrogen-abstraction reaction. Thus, we infer that vibrational excitation opens an endothermic hydrogen-abstraction channel in solution.
Our inability to detect HBr is perhaps not surprising. On the basis of the extinction coefficient of the Br•−DMSO complex at the absorption maximum (430 nm)28 and the IR absorption cross section of HBr in the gas phase,35 along with the assumption that the 1 mOD offset in the difference signal is entirely due to the loss of the Br•−DMSO complex through a hydrogen-abstraction reaction, we estimate a signal of 17 μOD for transient absorption by HBr. This signal is comparable to our current noise level, making detection of the HBr product using time-resolved IR detection unlikely. Additionally, we bubble HBr gas into each of the solvents in an attempt to obtain the static IR absorption spectrum and ensure that we are using an appropriate IR window for our probe light. For the HBr in DMSO solution, we observe a slight color change and no sign of HBr in the IR absorption spectrum. For the HBr in CH3OH solution, we observe a very small feature in the IR absorption spectrum near 2474 cm−1 that has a sizeable interference from the CH3OH absorption. These results are consistent with our inability to measure the HBr product by either transient or static IR absorption spectroscopy, indicating instead that the chemistry of the HBr product in solution prevents the formation of a detectable concentration. In DMSO, the formation of bromosulfonium bromide may drive the loss of the HBr product36 and, also, account for the color change observed when bubbling HBr in DMSO. The protonation of CH3OH by HBr has an equilibrium constant
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (F.F.C.). *E-mail:
[email protected] (A.S.C.). ORCID
Amanda S. Case: 0000-0002-2739-7488 Present Addresses ‡
Department of Chemistry, Augsburg College, Minneapolis, Minnesota 55454, United States (A.S.C.). 2493
DOI: 10.1021/acs.jpcb.7b00035 J. Phys. Chem. B 2017, 121, 2486−2494
Article
The Journal of Physical Chemistry B †
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Department of Chemistry, Stanford University, Stanford, California 94305, United States (J.Y.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We gratefully acknowledge support from the National Science Foundation (1321931).
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DOI: 10.1021/acs.jpcb.7b00035 J. Phys. Chem. B 2017, 121, 2486−2494
