CH Stretching Excitation in the Early Barrier F + CHD3 Reaction Inhibits CH Bond Cleavage

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Science  17 Jul 2009:
Vol. 325, Issue 5938, pp. 303-306
DOI: 10.1126/science.1175018


Most studies of the impact of vibrational excitation on molecular reactivity have focused on reactions with a late barrier (that is, a transition state resembling the products). For an early barrier reaction, conventional wisdom predicts that a reactant’s vibration should not couple efficiently to the reaction coordinate and thus should have little impact on the outcome. We report here an in-depth experimental study of the reactivity effects exerted by reactant C-H stretching excitation in a prototypical early-barrier reaction, F + CHD3. Rather counterintuitively, we find that the vibration hinders the overall reaction rate, inhibits scission of the excited bond itself (favoring the DF + CHD2 product channel), and influences the coproduct vibrational distribution despite being conserved in the CHD2 product. The results highlight substantial gaps in our predictive framework for state-selective polyatomic reactivity.

Not all forms of energy are equally effective in driving chemical reactions. Several decades of theory and experiment in reaction dynamics culminated in the Polanyi rules (1, 2), which predict that, in reactions of an atom with a diatomic molecule, reactant vibration and translation have different impacts on the rate. When the barrier is located late along the reaction coordinate (i.e., the transition state resembles products more than reactants), the vibration is considered the more effective driver; the reverse is true for reactions with early barriers (3). A similar line of thought underlies efforts directed toward mode-selective chemistry in reactions involving polyatomics. It is now well documented, both in the gas phase (414) and at surfaces (1517), that excitation of different vibrational motions of a polyatomic reactant can exert a profound influence on chemical reactivity. An intuitively appealing picture emerging from these studies is that exciting a vibrational mode with large displacements along a particular reaction coordinate can preferentially promote the system over the barrier of that pathway, leading to mode-dependent reactivity. The atomic-level mechanism governing this selectivity remains elusive, however.

Previous mode-specific studies have focused on reactions with a late barrier, which seems sensible from the perspective of an extended Polanyi rule framework (10). In an early-barrier reaction, reactant vibrations are commonly believed not to couple efficiently to the reaction coordinate. Contrary to this current perception, we report on an experiment that poses serious challenges to our fundamental understanding, even in a qualitative sense, of the vibrational effect on reactivity.

The reaction of F + CHD3 is highly exothermic, enthalpy of reaction ~ –31 kcal/mol, and, based on Hammond’s postulate (1), can be regarded as a prototypical early-barrier reaction. Consistent with this characterization, a recent, accurate ab initio calculation of the global potential energy surface indicates a reactant-like transition state structure, Fig. 1, left (18). Experimentally, both the thermal rate constant data (19) and the crossed-beam scattering results (20) suggest a small (<0.4 kcal/mol) barrier to reaction. Previously, comprehensive crossed-beam investigations of the ground state reaction have revealed roughly equal branching to two isotopic product channels, HF + CD3 and DF + CHD2 (20); numerous rovibrational product states were populated in each channel (2123). We explored the impact on this reaction of one quantum excitation of the C-H stretching vibration (v1 = 1) of CHD3.

Fig. 1

(Left) Reaction path energetics for reactant CHD3 initially in the v = 0 (black arrow) and v1 = 1 (red) vibrational states. The curve represents schematically a cut through the multidimensional potential energy surfaces governing reactivity. The numbers in the parentheses indicate the vibrational quanta of the product pairs relevant to this study. (Right) Two normalized REMPI spectra of the probed CHD2 products, with IR-on (red) and IR-off (black), at Ec = 3.6 kcal/mol. Two product images, both with IR-on, are shown for probing of the Embedded ImageQ and Embedded ImageQ bands, respectively. Superimposed on the images are the scattering directions; the 0° angle refers to the initial CHD3 beam direction in the center-of-mass frame.

On the basis of the extended Polanyi rule framework described above, we considered several qualitative predictions of the outcome. Given the early barrier, would the vibrational excitation increase the rate relative to the ground state reaction at fixed translational energy? The previously observed equal branching to HF and DF products in the ground state reaction was a clear sign of nonstatistical behavior (20). Would initial deposition of vibration energy directly into the C–H bond facilitate its breakage to form HF + CD3 preferentially? Lastly, the localized nature of the C-H stretch mode suggests that this bond should act as a spectator if the F atom were to attack the D atoms. In keeping with this spectator model (5), would the local CH stretching motion retain its vibrational character during the reaction, favoring the CHD2 product with one-quantum excitation of the CH stretching mode (v1 = 1) over ground state CHD2 (v = 0)?

To seek answers to these questions, we performed a crossed-beam scattering experiment under single-collision conditions. The experimental apparatus consisted of two rotatable pulsed molecular beams and a fixed detector assembly housed in a vacuum chamber (23, 24). The F atom beam was generated by a pulsed high-voltage discharge of 5% F2 seeded in a pulsed supersonic expansion of Ne at 6 atm. The CHD3 beam was also produced by pulsed supersonic expansion of 35% CHD3 seeded in He at 6 atm. Both beams were collimated by double skimmers and crossed in a differentially pumped scattering chamber; collision energy (Ec) was tuned by varying the intersection angle of the two molecular beams. A pulsed laser operating near 333 nm probed the nascent distribution of CHD2 and CD3 radicals at the intersection region by (2 + 1) resonance-enhanced multiphoton ionization, REMPI (23), and a time-sliced velocity imaging technique mapped the recoil vector of the CHD2+ or CD3+ ions (25). For studies with CH stretch-excited CHD3, an infrared (IR) laser was coupled to a multipass ring-reflector (26) situated just in front of the first skimmer. The narrow band of the IR laser (bandwidth ~ 0.05 cm−1) ensured preparation of CHD3 in a single rovibrational (v1 = 1, j = 2) state.

REMPI spectra of the CHD2 products from the F + CHD3 reaction at Ec = 3.6 kcal/mol are shown in Fig. 1. The ground state reaction (IR-off) produces predominantly ground state CHD2 (shown as the 000 Q head band) and smaller amounts of vibrationally excited CHD2 (the overlapped 211/311/511 bands) (27, 28). With the IR laser on, the intensities of those bands diminished and a new band 111 appeared, signifying the formation of CHD2 (v1 = 1) product. Vibrational excitation induced substantial (45%) depletion of the 000 Q head but less depletion (i.e., smaller than 000 by about 10%) of the 211/311/511 bands. To interrogate the influence of CH stretching excitation on other product states, including CHD2 (411), CD3 (000, 111, 211, 220, 311, and 411), and several combination bands (2123), we fixed the probe laser wavelengths at the peak of respective bands and recorded the signals alternatively for IR-on and IR-off. Surprisingly, they all displayed around 25% IR-associated depletion (when the concurrent 000 bands were depleted by 33%, as shown in figs. S1 and S2), suggesting that the yields of all product states except CHD2 (v1 = 1) in the F + CHD3 (v1 = 1) reaction are notably smaller than the corresponding yields in the ground state reaction. This result contrasts sharply with the behavior of the late-barrier Cl + CHD3 (v1 = 1) reaction, where IR irradiation enhanced the formation of vibrationally excited product states that are nearly absent in the ground state reaction (10).

Also overlaid in Fig. 1 are the two IR-on images of the probed CHD2 (v = 0) and (v1 = 1) products. Thanks to the time-sliced velocity imaging approach, even the raw data can be easily interpreted by inspection. The energetics of this reaction are well defined, and the vibrational CHD2 product states were selectively detected. By conservation of energy and momentum, the maximum velocities of the DF coproducts recoiling from the selected state of CHD2 were calculated for different final vibrational states and are depicted as dashed lines in Fig. 1. The successive rings in each image can thus be unambiguously assigned to the vibrational states of the dark DF coproduct. The intensity around each ring then gives an immediate impression about the preferred scattering direction of the coincidently formed DF products. Energetically, the initial rovibrational excitation of CHD3 (v1 = 1, j = 2) adds 8.63 kcal/mol to the total energy for the IR-on image, and the formation of CHD2 (v1 = 1) products requires at least 8.90 kcal/mol. This near degeneracy (only 0.27 kcal/mol of energy difference) results in energetically similar ring radii of the DF coproduct states in the F + CHD3 (v = 0) → CHD2 (v = 0) + DF (v') and F + CHD3 (v1 = 1) → CHD2 (v1 = 1) + DF (v') reactions. Remarkably, the angular distributions in the two images are also nearly identical, implicating similar pathways in the two reactions, again at marked variance with the Cl + CHD3 (v1 = 1) reaction, for which the angular distributions differ vastly from that of ground state reaction (9, 10). The 000 Q head image with IR-off proved identical to that with IR-on (fig. S1). We therefore concluded that, as predicted, F + CHD3 (v1 = 1) does not produce CHD2 (v = 0) and that the change in the 000 Q signals upon IR irradiation is due solely to depletion of ground state reactants.

Images acquired for the overlapped 211/311/511 bands as well as other product states all exhibit very similar features, regardless of whether IR irradiation is applied (figs. S1 and S2). This result confirms the conjecture from the IR-attenuated REMPI bands that the formation of those product states from the stretch-excited reactant are at most a small fraction of the corresponding product yields in the ground state reaction. On the basis of the degrees of the attenuation of all other CHD2 and CD3 product states, we estimated their collective yields in F + CHD3 (v1 = 1) to be no more than the single product state of CHD2 (v1 = 1). This highly mode-specific behavior is quite unexpected and important. Although enhanced production of CHD2 (v1 = 1) in F + CHD3 (v1 = 1) is anticipated by the spectator model, we did not expect it to be the only detectable product state. The observation of a diminishingly small cross section for the H-atom transfer channel upon C-H stretching excitation, that is, HF < DF, is counterintuitive and exemplifies a strong bond selectivity. We inferred that, upon vibrational excitation, the long-range anisotropic interaction of F atom with a stretched/compressed C-H must change in such a way that it effectively steers the trajectory away from the transition state, practically shutting down the C–H bond scission channel. The mechanism is reminiscent of the stereodynamical effect, induced by the anisotropic van der Waals forces in the reactant valley, that was proposed previously for a preferential formation of DCl over HCl in the Cl + HD ground state reaction (29).

More quantitative information about the predominant CHD2 (v1 = 1) + DF channel can be deduced through image analysis that accounts for density-to-flux transformation (25). Figure 2 summarizes the resultant dynamical attributes from images at Ec = 1.2 kcal/mol. Depicted in Fig. 2A are the product speed distributions of the two images, IR-on and IR-off, acquired at the peak of the CHD2 (111) band. From the CHD2 (000) depletion measurement, we found that ~ 30% of CHD3 reactants were stretch-excited in this case. By scaling down the IR-off distribution by 0.3 and subtracting it from the IR-on data set, we obtained the genuine distribution for the CHD2 (v1 = 1) product from F + CHD3 (v1 = 1) (the red curve in Fig. 2B). On energetic grounds, the product pair labeled as (vDF, 11) can readily be identified. [The superscript indicates the state pair that is produced in the F + CHD3 (v1 = 1) reaction.] Also displayed in Fig. 2B is the speed distribution from the IR-off image acquired at the CHD2 (000) peak (in black); the DF in this case is vibrationally hotter than in the IR-excited reaction. Such distinct distributions provide compelling evidence that, although the initial C-H stretching excitation of CHD3 transforms adiabatically into the C-H stretching motion of the CHD2 product, it also influences the vibration branching of the DF coproduct. Hence, the locally excited CH bond does not act entirely as a spectator when the F atom attacks the unexcited CD bond.

Fig. 2

(A) The normalized product speed distributions P(μ) deduced from the IR-on and IR-off images of the CHD2 (Embedded Image) band at Ec = 1.2 kcal/mol. (B) The genuine speed distribution of the F + CHD3 (v1 = 1) → DF (v) + CHD2 (v1 = 1) reaction depicted in red after analysis as described in the text. The distribution in black is derived from the IR-off image of the CHD2 (Embedded Image) band, corresponding to the ground state reaction. On energetic grounds, the peak features can be assigned to the state pairs of the two products, as labeled. The small energetic difference of the two reactions is evident from the slight shift of the peak positions of the (4, 00) and (4, 11) product pairs. (C and D) Pair-correlated angular distributions from the stretch-excited reaction and ground state reaction, respectively; the quantum number v′ refers to the DF coproduct.

Unlike the speed distributions, the product angular distributions of the two reactions (Fig. 2, C and D) are practically identical (the subtle difference is merely due to the different DF vibrational branching ratios), confirming the initial impression from inspection of the raw images. This contrast in state and angular distributions of the two reactions is instructive. In general the correlated angular distribution, for which the scattering angle should be governed mainly by the trajectories of the two heavy atoms, must also carry the imprint of the initial impact parameter and orientation, that is, a global trait. The correlated DF vibrational distribution, on the other hand, reflects mostly the concerted motions of the transferred D atom and the methyl moiety near the transition state region and is thus more susceptible to local interactions. As such, we reasoned that the differential DF vibrational branching of the two reactions could provide a clue as to how the initial C-H stretching motion affects subsequent events along the reaction trajectory. Because the effects of slight changes in local interaction potential on reactivity tend to more readily manifest at lower collision energies, we examined the Ec dependence of the correlated vibrational branching of DF products in the ground state (Fig. 3A) and stretch-excited (Fig. 3B) reactions. The disparity in the two correlated DF vibrational distributions is substantial for Ec < 2 kcal/mol but becomes negligibly small as Ec rises. (Figs. S3 and S4 show the speed and angular distributions, respectively, at higher collision energies.)

Fig. 3

Collisional energy dependence of the correlated DF vibrational branching ratio for the ground state reaction (A) and the stretch-excited reaction (B). The vertical dashed arrows indicate the respective energetic thresholds for forming the DF (v' = 4) state.

Returning to the REMPI spectra shown in Fig. 1, we noted that the enhancement of CHD2 (111) band intensity is smaller than the concurrent depletion of the CHD2 (000) band. Because CHD2 (v1 = 1) is the only product state showing any appreciable formation with IR irradiation, the implication of this discrepancy is that one quantum of CH stretching excitation of CHD3 may actually decelerate the reaction. Repeating the image measurements for both CHD2 (v1 = 1) and CHD2 (v = 0) and normalizing them under different collision energies, we obtained the reactive excitation function σ(Ec), which is the dependence of the integral cross section on Ec, for the ground state and stretch-excited reactants (Fig. 4). To determine the relative cross sections, we exploited an IR-ultraviolet double resonance technique to quantify the relative detection sensitivity S111 of the 111Q and 000Q REMPI bands (30) and found that probing the CHD2 (111Q) band is 1.9 ± 0.7 times more sensitive than probing the CHD2 (000Q) band. Taking this factor into account, Fig. 4 shows that one quantum excitation of CH stretch mode of CHD3 slows down the reaction rate by about 10-fold and that within our experimental uncertainty the ratio of σ(11)/σ(00) in the DF + CHD2 isotope channel exhibits little if any Ec dependency. Because numerous product states are formed abundantly in the ground state reaction, the suppression of overall reactivity by a stretch-excited CHD3 reactant is more significant than that shown in Fig. 4.

Fig. 4

(Top) Normalized excitation functions of the ground state and stretch-excited reactions. (Bottom) Dependence of the ratio of two excitation functions on the collisional energy. The scale on the right ordinate is from the raw results, whereas that on the left gives the true reactivity ratio. The solid curve and the horizontal dashed line are the two possible fits to the experimental results. The vertical bar associated with each data point represents one standard deviation error limit of four to five repeated measurements.

As alluded to earlier, most studies of vibrational effects on chemical reactivity have focused on late barrier reactions. Several studies of atom plus diatom reactions with early barriers showed vibrational enhancement (3133), although extra translational energy accelerated the reactions even more, corroborating the Polanyi rules (3, 10). In contrast, we observed a pronounced negative impact of reactant vibration on total reactivity in F + CHD3 (v1 = 1). Moreover, the initial C-H stretching motion plays a subtle yet decisive role in controlling dynamical attributes. It not only effectively blocks the H atom transfer channel, yielding a counterintuitive product isotope distribution, but also remotely affects the cleavage of the unexcited CD bond, resulting in different pair-correlated distributions in DF + CHD2. In contrast, previous experiments on the Cl + CHD3 (v1 = 1) reaction (5, 9, 10) demonstrated a rate enhancement by vibration at fixed translational energy, a preferential cleavage of the excited CH bond yielding more HCl than DCl, and the formation of numerous rovibrationally excited CD3 and CHD2 products that are not populated in the ground state reaction. Such a sharp contrast of the reactive behaviors between CHD3 (v1 = 1) with F and CHD3 (v1 = 1) with Cl is particularly illuminating and underscores the pivotal role of the barrier location in directing mode-selective chemistry. Clearly, a conceptual framework of vibrational effects on chemical reactivity is far from complete.

Supporting Online Material

Figs. S1 to S4


References and Notes

  1. In the vibronic band notation used herein, 211 designates the REMPI transition involving the v2 vibrational mode with one quantum excitation each in the ground electronic state (the subscript) and the electronically excited state (the superscript).
  2. This work was supported by the National Science Council of Taiwan, Academia Sinica, and the Air Force Office of Scientific Research (grant no. AOARD-09-4030).

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