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Mar 28, 2014 - been previously described.14,16 To produce HOD, helium was bubbled through a 50:50 mixture of H2O and D2O to generate a mixture of 96% ...
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Effects of Translational and Vibrational Excitation on the Reaction of HOD+ with C2H2 and C2D2: Mode- and Bond-Specific Effects in Exoergic Proton Transfer David M. Bell, Collin R. Howder, and Scott L. Anderson* Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112, United States S Supporting Information *

ABSTRACT: Reactions of mode-selectively excited HOD+ with C2H2 and C2D2 were studied over the center-of-mass collision energy (Ecol) range from 0.15 to 2.9 eV. HOD+ was prepared in each of its fundamental vibrational states: ground state (000), bend (010), OD stretch (100), and the OH stretch (001). Charge transfer is the dominant reaction at all energies, although it is inhibited by increasing Ecol, and is accompanied by hydrogen exchange. The total charge transfer cross section is similar for C2H2 and C2D2, however, the tendency toward charge transfer with hydrogen exchange (CTHE) is significantly greater for C2D2 compared to C2H2. Charge transfer shows no significant effects of HOD+ vibrational excitation, however, CTHE is significantly enhanced by vibration at Ecol < 0.62 eV. Both H+ and D+ transfer reactions (HT, and DT, respectively) are observed for both C2H2 and C2D2, with little dependence on collision energy, but with mode- and bondspecific enhancements from excitation of the OH and OD stretches. Recoil velocity measurements show that all channels are direct, except perhaps at the lowest collision energies. Mode-specific effects on the recoil velocity distributions are also observed, revealing how vibrational excitation affects reaction at different collision impact parameters.

I. INTRODUCTION Vibrationally1−16 and rotationally,8,17−21 state-selected reaction experiments have provided insight into the factors that control reactivity in different systems. In particular, mode-specific excitation can reveal dynamical effects controlling reactions that otherwise appear to be dominated by a statistical mechanisms.14,15,17,18,22−26 We have recently reported mode-selective studies in reactions of HOD+ with CO, NO2, N2O, CO2, and N2.27−31 The HOD+ system is interesting in that the modes that would be the symmetric and asymmetric stretches in H2O+ become predominantly OD stretch (100) and OH stretch (001) modes,32 allowing bond-specific effects to be observed. Most of the HOD+ reactions examined to date have shown strong mode- and bond-specific enhancements of endoergic H+ and D+ transfer (HT, and DT) reactions. In such systems, we find that excitation of the OH (or OD) stretch mode strongly enhances HT (DT) while having little effect on the competing DT (HT) reaction. A key feature in these reactions is that there is no barrier to the approach of the reactants, such that the reaction endoergicity is manifest only as the products begin to separate. Consequently, the reaction coordinates can be considered to have “late barriers”, and the strong enhancements from exciting the stretch of the bond broken in reaction are reminiscent of the preferential vibrational enhancements predicted for a model A+BC reaction with a late barrier, as demonstrated by Polanyi and coworkers.33 They found that translational energy was more efficient than vibrational energy at overcoming a barrier early on the reaction coordinate but © 2014 American Chemical Society

that vibrational energy was more efficient in overcoming late barriers. For our HOD+ reactant, energy in stretching of one of the bonds is efficiently coupled to the reaction coordinate for breaking that bond and essentially uncoupled to the reaction coordinate for breaking the spectrator (nonexcited) bond. Polyatomic systems, of course, have additional complexities, and surprising behavior is seen in some cases. For example, in some reactions (e.g., with CO2), bend excitation of HOD+ is found to enhance both HT and DT such that the overall enhancement from bend excitation is larger than that from either OH or OD stretch excitation, despite the bend being the lowest energy mode, and not intuitively coupled to either reaction coordinate. In endoergic reactions, it is easy to see why putting energy in the broken bond stretch mode enhances reaction: this motion is directly along the reaction coordinate at the point where the system has to climb the barrier to product separation. We also examined HT and DT in HOD+ + CO, where reaction is thermoneutral. In that case, OH or OD stretch excitation still enhanced HT or DT, respectively, but actually inhibited breaking the spectrator bond (i.e., DT or HT, respectively).29 The present system was chosen to probe the effects of modeSpecial Issue: A. W. Castleman, Jr. Festschrift Received: February 5, 2014 Revised: March 26, 2014 Published: March 28, 2014 8360

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collision energies and masses of interest and switched the target gas flow between the scattering cell and chamber background. Integral cross sections were calculated from the ratio of reactant and product ion intensities (integrated from the TOF measurements) and corrected for ions formed outside the scattering cell using the calibrated effective length of the scattering cell and the measured pressure. TOF was used to measure both the energy of the reactant ion beam and the axial projection of the recoil velocity distributions for the products ions (vaxial). Several complete sets of cross sections for all product channels were measured for all reactant states of HOD+ as a function of collision energy. The data sets were combined to determine the average and standard deviation (indicated with error bars) of the cross sections for each product and combination of HOD+ vibrational state and collision energy. The cross section uncertainties were estimated in the following ways. The standard deviation of the cross sections over the set of runs was ∼2000 m/s.

C2H2 CT:HT + CTHE:DT C2D2 CT:CTHE:HT:DT 90:7:3 89:7:4 87: 8:5 81:10:8

77:14:5:3 78:14:5:3 76:13:6:4 69:13:10:7

Lifetimes and decay branching ratios for both complexes are shown in Table 2 for each Ecol, where complex mediation might be important. Lifetimes were calculated from the rates for each decay channel, averaged over the range of angular momenta appropriate for the collision cross section at each experimental Ecol. The Table also shows τfly by, which is simply the time it takes undeflected reactants to fly a distance of 5 Å with respect to each other. This time is intended to give an idea of the time scale relevant to direct collisions, where no complex forms. It can be seen that the set of interconverting RC complexes has dynamically insignificant lifetimes, except perhaps at the lowest Ecol, where the lifetime is still only ∼50% of τfly by. In addition, the RRKM branching out of the RC complex is almost entirely to CT, with only ∼1% branching to VA, which leads on to HT (or DT) products. The Table also shows the experimental product branching ratios for HOD+ + C2D2, where there are no mass interferences. To allow comparison with the RRKM results, we should sum the experimental HT and DT branching, producing a total HT+DT branching that ranges from 8% at the lowest Ecol to 17% at 0.62 eV. The VA complex, in contrast, has a significant lifetime in this low Ecol energy range, and because it decays both to HT products and back to RC, which should then mostly decay to CT products (with or without hydrogen exchange), the VA complex could mediate all four channels in this reaction. From the perspective of whether forward−backward symmetric vaxial distributions would be expected, we need to know τrotation, which can be calculated using the moment of inertia for VA from ab initio calculations. The angular momentum is calculated as L = μ·vrel·(σcollision /π)1/2, where μ is the reduced mass of the reactants, vrel is the relative velocity of the reactants, and σcollision is given in Figure 1. For the VA complex, τrotation varies from 0.39 ps at Ecol = 0.15 eV to 0.27 ps at Ecol = 0.62 eV. It can be seen that τcomplex ≫ τrotation, and therefore trapping into VA would lead to HT and DT products with forward− backward symmetric vaxial distributions and also contribute to a symmetric component to both CT and CTHE channels. Therefore, the question is whether this complex-mediated pathway is important, that is, does VA form in a large fraction of collisions, or are most products formed by direct mechanisms and thus avoiding this region of the potential energy surface entirely. Clearly, VA does not form in a large fraction of collisions because its decay is primarily to HT and DT products, whereas the experimental branching favors CT. Formation and decay of the VA complex might provide a mechanism for H/D scrambling needed for CTHE; however, if enough VA formed to account for the ∼13 to 14% CTHE branching, the RRKM results indicate that there should be far more HT and DT production than is observed because >90% of the VA decay is to those channels. The conclusion from the RRKM modeling is, therefore, that statistical complexes cannot be responsible for more than a small fraction of reaction, even at the lowest Ecol, and that complexes rapidly become irrelevant

IV. DISCUSSION A. Ground-State Mechanism. For all product channels, the vaxial distributions are clearly asymmetric with respect to VCM at high energies, indicating that the reaction is dominated by direct scattering, producing product ions that are mostly backward-scattered relative to the direction of the reactant ion beam. For reactions where the charge is transferred (i.e., all channels in this system), such ions are produced in collisions where little momentum transfers along with the charge. In the limit of zero momentum transfer, the product ion would only have the thermal velocity of the neutral target gas (lab frame velocity near zero). The high intensity for backward-scattered product ions suggests, therefore, that the reaction mechanism is dominated by collisions at large impact parameters, where the charge (or H+ or D+) can transfer at long-range, and the reactants never come close enough to undergo strong scattering. Note, however, that the probability of having collisions with a particular impact parameter, b, increases with b. Therefore, if reaction is efficient at large b, then such collisions will dominate the mechanism, even if reaction is also efficient at small b. For the lowest Ecol range, the distributions can be fit with a forward−backward symmetric scattering model. However, because we cannot measure velocities reliably below ∼300 m/s, it is also entirely possible that the distributions are already asymmetric at the lowest Ecol. Nonetheless, it is worth considering the possibility that the reactions might be mediated by collision complexes with dynamically significant lifetimes. The only complexes on the reaction coordinate in Figure 3 that might have significant lifetimes are the RC complexes and VA. To test the possibility that these complexes might be involved, we calculated statistical unimolecular decay lifetimes and branching ratios for the complexes of interest, using the RRKM program developed by Zhu and Hase.47 Because Eavail is large when compared with the energy difference between RC1 and RC2/3, these complexes are better thought of as a single “RC” complex in which the system explores a range of reactantlike geometries.30 To estimate the lifetime of this interconverting RC complex, we summed the densities of states for RC1, RC2, and RC3. The HBC could also have been included as part of the RC complex, but because its energy is substantially higher, its contribution to the total density of states is negligible. Because there are no barriers separating the RC complexes from products or reactants, orbiting transition states were assumed to govern these decay pathways. TS1 was assumed to govern decay of RC to produce the strongly bound VA complex. Decomposition of the VA complex was also studied. In this case, the two channels examined were decay to HT products, assumed to be governed by the transition state leading to PC, and decay back to RC via TS1. 8367

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at higher energies, consistent with the strongly asymmetric vaxial distributions. B. Charge Transfer Dynamics. In some systems, exoergic CT takes place at truly long ranges, and the reactants never approach within contact distance. For example, in the reaction of H2CO+ with ND3,48 σTotal is 2.5 times greater than σcollision at high collision energies, mostly due to the very large and nearly Ecol-independent CT cross section. Here CT is also the dominant reaction; however, σTotal is approximately equal to σcollision over the entire energy range. This could indicate that CT occurs only in “contact” collisions or that CT also occurs at long-range but with low probability. CT at long-range is essentially an electronic transition, involving initial and final molecular orbitals that happen to be on different molecules. The cross section depends on the range at which the intermolecular interaction becomes strong enough to couple the initial and final states. In addition, for a longrange electronic transition to be likely, there must be final vibrational states that have good Franck−Condon overlap with the reactant state, and those final states must be near-resonant with the reactant state. Near-resonance means that the CT exoergicity is taken up in vibrational energy of the products rather than needing to be converted to translational or rotational energy of the products. Large rotational excitation is forbidden by angular momentum conservation, and large conversion to product recoil requires strong repulsion between the products, which clearly is not possible at long-range. In the H2CO+ + ND3 example, the large geometry change between ND3 and ND3+ results in good Franck−Condon factors for populating high levels of the umbrella bend mode, thereby accommodating the release of 0.72 eV. That does not seem to be the case for the HOD+ + C2H2/ C2D2 system, where CT is 1.2 eV exoergic. Neither HOD nor C2H2 undergoes large geometry changes in ionization, suggesting that the Franck−Condon factors should favor diagonal transitions, that is, where the initial and final vibrational quantum numbers are identical. In that case, the probability would be small for populating final vibrational states that could accommodate 1.2 eV. The probability for offdiagonal transitions can be estimated from vibrational structure in the photoelectron spectra of the two reactants. Photoionization of C2H2 produces cations, ∼66% of which are in the ground state, with 25, 7, and 1% having 1, 2, and 3 quanta, respectively, of C−C stretch excitation.49 Similarly, photoionization of water results in ∼75% ground-state cations, with the balance having one or two quanta of bend or symmetric stretch excitation. In both cases, most of the ions are vibrationless, and only a few percent have more than ∼0.5 eV of vibrational excitation. Therefore, we conclude that the envelope of Franck−Condon accessible final vibrational states does not extend to high enough energy to accommodate the 1.2 eV exoergicity, so that CT cannot occur at large interreactant separations. This situation is in contrast with analogous reaction of water cations with ethylene. Farrar and coworkers50 studied the D2O+ + C2H4 system using crossed beam methods and observed CT, CT with H/D exchange (i.e., CTHE), and DT products. They concluded that CT to ethylene occurred at long inter-reactant separations. The crossed beam data allowed them to extract product translational energy distributions and therefore also product internal energy distributions; however, the method does not lend itself to generating integral cross sections, so it is not possible to say if σTotal is greater than σcollision. To allow

direct comparison with our acetylene results, we also measured integral cross sections for reaction of HOD+ (v) with C2D4, and the results are presented in the Supporting Information (Figures S4 and S5). We observe CT, CTHE, HT and DT, and σTotal that is greater than σcollision at high Ecol, consistent with the crossed beam finding of efficient long-range CT. We also observed another product channel, C2D2+ (CT with D2 elimination), which was not observed in the crossed beam study because it is endoergic (0.6 eV), with a small cross section in the Ecol range probed in that work (Ecol < 0.79 eV). The difference between acetylene and ethylene is that C2H4+ has an electronically excited state (12B2g), lying ∼2 eV above its ground state, with a relatively broad envelope of Franck− Condon accessible vibrational levels.49 As a result, the 2.11 eV CT exoergicity is readily accommodated by the production of 12B2g C2H4+ product ions with low levels of vibrational excitation. For C2H2+, the lowest energy electronically excited state lies ∼5 eV above ground state, and the neutral water product also has no electronically excited states below ∼6 eV.51 Therefore, in the HOD+ + C2H2 system, the 1.2 eV exoergicity can only be partitioned to translation, rotation, and vibration energy with products in their ground electronic state. The picture, therefore, is that CT at high Ecol occurs efficiently when the collisional interaction is strong enough to allow the exoergicity to be converted to some combination of product vibrational excitation and recoil energy. The fact that the measured recoil energies are only ∼40 to ∼60% of Eavail at high Ecol (Table 1) suggests that most of the excess energy is partitioned to product vibration and rotation. The fact that the total cross section is at the collision limit indicates that this energy conversion process is quite efficient, allowing CT to occur even in grazing collisions at impact parameters near the hard-sphere limit. At lower Ecol, CT remains efficient and σTotal increases in step with the increase in σcollision because the ioninduced dipole interaction allows collisions with larger impact parameters to be captured into “contact” range, where the energy transfer required for CT is efficient. C. Product Channel Competition. It is not surprising that the exoergic H+, D+, and charge-transfer reactions are efficient at large impact parameters. As noted, CT requires only that the interaction between the reactants is strong enough to partition enough energy into product internal and translational energy, thereby accommodating the exoergicity. Exoergic HT/DT reactions are also often found to occur with efficiency near the collision limit,46 provided that steric hindrance or spin conservation does not inhibit the reaction. In this system, the main factor limiting the HT and DT reactions is competition with the energetically favorable and facile CT channel. This competition is probably controlled by factors such as whether collisions are in favorable geometry for C−H bond formation as well as the phase of the vibrational motion of the relevant OH bond. The influence of collision geometry is also suggested by the observation that HT and DT continue to produce forwardscattered product ions at high Ecol (Figures 8 and 9), while the CT product ions (Figure 7) are exclusively backscattered. In the impulsive (i.e., high energy) regime, forward-scattered ions are expected to result primarily from small impact parameter collisions, and these intimate collisions favor proton transfer. One surprise is that CTHE is reasonably efficient, despite the evidence suggesting that intermediate complexes, which one might expect to mediate H/D exchange, are insignificant even at our lowest Ecol. A likely mechanism for CTHE is initial transfer of H+ or D+, followed by back transfer of D or H, 8368

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resulting in H/D scrambling. In that case, the energetics of this system probably help drive CTHE. The initial H+ or D+ transfer event is ∼0.6 eV exoergic; however, the nascent products (C2H3+ + OH, in the all-H system) are unstable by ∼0.6 eV with respect to back transfer of an H or D atom. Therefore, CTHE is in competition with the HT and DT channels, with HT and DT presumably resulting from collisions where the time scale and the reactant orientation prevent the back-transfer event, and CTHE (and CT) results from collisions where the back transfer occurs before the products can separate. D. Isotope Effects. One surprise in the results is that there appear to be large isotope effects on some channels. This analysis is somewhat complicated by the fact that for the reaction with C2H2 both CTHE and HT produce mass 27 product ions. Nonetheless, comparison of the results for C2H2 (Figure 1) and C2D2 (Figure 2) clearly shows that the product branching is strongly affected by deuteration. (See also the low Ecol branching ratios in Table 2.) Note that the DT channel is 25% larger in reaction with C 2 H 2 than with C 2 D 2 , approximately independent of Ecol. The CT cross section is also larger in reaction with C2H2 than C2D2, by 15% at low Ecol and 21% at high Ecol. The largest effect is on the CTHE channel, which is clearly much more efficient in the reaction with C2D2 than with C2H2 at low Ecol. The effect can be seen in the raw cross sections, where σCTHE in reaction with C2D2 is 75% larger than the combined cross section for HT and CTHE in reaction with C2H2. To show the effect more clearly, it is useful to, at least approximately, separate out the contributions from CTHE and HT. Note that the combined cross section for CTHE plus HT (Figure 1) is roughly Ecol-independent, averaging ∼6 Å2. Note also that the HT cross section in the reaction with C2H2 and the HT and DT cross sections in the reaction with C2D2 have similar weak dependence on Ecol and similar magnitudes, averaging between 2.7 and 3.5 Å2. It is therefore not unreasonable to approximate the HT contribution to the combined HT + CTHE signal in reaction with C2H2 as simply the average of the other three HT or DT cross sections. With this assumption, the HT contribution can be subtracted from the combined HT + CTHE cross section. The top frame of Figure 11 shows the results (cf. Figure 1). Note that the estimated HT cross section varies from 2.6 to 3.5 Å2, and with that subtracted out, the CTHE cross section is estimated to be quite small, averaging ∼2.2 Å2. The middle frame of Figure 11 compares the CT and CTHE cross sections for reaction of ground-state HOD+ with C2H2 and C2D2. It can be seen that the CTHE cross section for reaction with C2D2 is similar to the estimated CTHE cross section for C2H2 at high collision energies but that below ∼1.5 eV the CTHE cross section for C2D2 grows to over 10 Å2, while the CTHE cross section for reaction with C2H2 is estimated to remain small, so that the ratio grows to ∼3.5 at the lowest Ecol. Clearly, the corrected CTHE cross section is just an estimate; however, even with no correction at all, the CTHE cross section for C2D2 is still nearly twice that of C2H2. The bottom frame of Figure 11 shows the total cross section for CT, with or without H/D exchange (σCT + σCTHE) for both C2H2 and C2D2. It can be seen that at low Ecol, where the largest difference is seen for CTHE, the total charge-transfer cross section is essentially independent of reactant deuteration. This observation is not surprising in light of the previous discussion. CT appears to be efficient in nearly all collisions, and thus the cross section of CT should track the collision cross

Figure 11. (Top) Cross sections for the corrected production of CT and DT for the reaction with C2H2 and the average HT/DT in the reaction with C2H2 and C2D2 as a function of Ecol. The corrected CTHE values are shown as prescribed in the text. (Middle) Cross sections for the product of CTHE and CT as a function of Ecol for the reaction of HOD+ + C2H2 shown with the solid lines and open symbols and HOD+ + C2D2 shown with the dashed line and solid symbols. (Bottom) Cross section for CTHE + CT as a function of Ecol for the reaction C2H2 and C2D2 as indicated.

section, as is observed. It is clear that in the competition between CT and CTHE, deuteration of the acetylene results in a substantial increase in CTHE. It is not clear why CTHE is much more likely in reaction with C2D2 compared with reaction with C2H2; however, the fact that the CTHE enhancement for C2D2 occurs primarily at low Ecol, suggests that energetic and statistical factors may play a role. Despite complex mediation not appearing important, if there were large differences in the density of states (DOS) associated with the CT versus CTHE product channels, there might be an effect on product branching. To test this hypothesis, we calculated the DOS of the products for CT and CTHE over the low Ecol range, where the largest difference is observed between C2H2 and C2D2. The calculations were done using the direct state count method in the RRKM program of Zhu and Hase,52 averaging over angular momentum states consistent with the capture cross section at each Ecol. The ratio of the DOS for the CTHE and CT product pairs is DOS(C2HD+ + D2 O)/DOS(C2D2+ + HOD) = 1.2, for reaction with C2D2

and DOS(C2HD+ + H 2O)/DOS(C2H 2+ + HOD) = 0.8, for reaction with C2H 2 8369

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that excitation of the OH or OD stretch selectively enhances the transition to the corresponding HT or DT product channel. One factor that may play a role in the vibrational dynamics is that this system involves the transfer of a light ion (L = H+ or D+) between two heavy moieties (H1 = OD, H2 = C2H2 or C2D2). This type of reaction, H1-L-H2, is best described using mass-weighted coordinates,41 which account for the fact that motion along different coordinates corresponds to motion of molecular fragments with different masses. The mass-weighted coordinates are scaled and skewed such that every direction is kinematically equivalent. The resulting angle between the reactant and product valleys is small for an H-L-H system, and in this situation, getting around the tight bend to reach the product valley may be inefficient, even though the process is exothermic. Vibrational motion of the bond to be broken corresponds to motion along the product separation coordinate and therefore can enhance the probability that the system successfully makes the transition from the reactant valley to the HT/DT product valley. Another consideration is that while HT and DT are ∼0.6 eV exoergic, they are also ∼0.6 eV energetically uphill with respect to CT products. Therefore, if CT occurs early in a collision, as reactants approach, and if the ∼1.2 eV CT energy release is randomized among the active degrees of freedom, there might not be enough energy in the degree of freedom corresponding to the HT or DT reaction coordinate to drive the system into those product channels. In this scenario, the HT and DT channels could be considered to be endoergic, with a late barrier; therefore, the OH or OD stretch excitations might be expected to enhance transition to HT or DT products, provided that the system still “remembers” the reactant vibrational excitation after the initial CT process. Finally, consider that when either stretch is active and at the outer turning point of the stretch, that is, long O−H bond distances, the effect will be to increase the size of the HOD+ and thus the cross section associated with collisions bringing the excited H or D atom into contact with the neutral reactant. For example, this might allow H+ or D+ stripping-type collisions at larger impact parameters and thus increase the cross section. Such an effect should appear in the vaxial distributions at high Ecol as a selective enhancement of the strongly backwardscattered part of the distributions. Figure 10 shows the experimental vaxial distributions for DT (C2H2D+ production) in collisions with C2H2, comparing the distributions for HOD+ in its ground state or with one quantum of OD stretch excitation. The distributions are scaled so that their integrated areas match the integral cross section, allowing the effects of vibration on different parts of the distribution to be seen directly. At low Ecol, the enhancement is approximately the same in all parts of the distribution, but for high Ecol, it can be seen that OD stretch excitation primarily enhances the strongly backward-scattered part of the distribution, indicating an enhancement of stripping-type dynamics. A similar type of mode-, bond-, and impact parameter-specific enhancement was observed at high Ecol for the reaction of HOD+ + N2O.30 F. Vibrational Effects on CTHE. As shown in Figure 5, all modes of HOD+ vibrational excitation enhance CTHE for C2D2 at low Ecol, with the two stretch modes having similar effects (∼50%), and the bend is roughly half as effective. Enhancement is observed only for Ecol below ∼0.5 eV, that is, in the range where collision time and DOS effects also appear to enhance CTHE branching for reaction with C2D2. For the reaction with C2H2, we do not feel that the approach used

In essence, the DOS slightly favors CTHE for reaction with C2D2 but slightly favors CT in the reaction with C2H2, which might lead to a factor of ∼1.5 (= 1.2/0.8) increase in the cross section of CTHE in reaction with C2D2, compared with C2H2. The experimental increase in CTHE is twice as large (∼3.5, if the separation of HT and CTHE channels for C2H2 is correct). Another possible contributing factor is collision time scale; that is, the lower frequency vibrational modes for C2D2, may give additional collision time that increases the H/D exchange probability by giving more time for back transfer of an H or D atom. The vaxial distributions for CT (Figures 7 and Figure S2 in the Supporting Information) show similar degree of asymmetry for C2H2 and C2D2; however, the rotational period of the collision complex, that is, the “clock” that determines how collision time is reflected in asymmetry of the vaxial distributions, is slower for C2D2. It is not obvious that either of these effects would result in such a large increase in the CTHE magnitude for C2D2, but the combination of DOS and collision time effects, together, may account for the difference. E. Mode-Specific Effects on HT/DT. The cross sections for the HT and DT production in the reaction of vibrationally excited HOD+ + C2H2 and C2D2 are shown in Figures 4 and 5, respectively. Both reactions exhibit strong mode- and bondspecific effects on the HT and DT channels. Excitation of the OH and OD stretches selectively enhances the HT and DT channels, respectively; that is, exciting vibrations localized in one of the HOD bonds selectively enhances the reaction breaking that bond. OH and OD stretch excitation have little effect on DT and HT, respectively; that is, exciting the spectrator bond has little effect, except at low Ecol, where there is weak enhancement. Bend excitation enhances both HT and DT nonselectively, primarily at low Ecol. As discussed in the Introduction, this type of bond/modespecific enhancement is observed to be significantly stronger in endothermic HT/DT reaction of HOD+ with various neutral reactants.27−31 In the reaction of HOD+ with N2 (1.0 eV endoergic) the enhancement near threshold is ∼400−500% for the broken bond stretch. In the similar reaction of HOD+ with CO2 (0.54 eV endoergic), the enhancement near threshold is ∼150−200%. The spectrator stretch in both reactions causes no enhancement and in some instances actually causes a small inhibition. Analogous effects on H versus D atom transfer reactions in neutral systems have been reported by the Crim and Zare groups. For example, the reaction HOD + H → OH + HD or OD + H2 is endoergic (0.66 eV) with a significant 0.94 eV barrier.9,10,53 Crim and coworkers studied the reaction using overtone vibrational excitation to overcome the barrier, comparing the reaction of HOD with four quanta of the OH stretch versus five quanta of the OD stretch. Reaction essentially was exclusive to rupture of the excited bond. Zare and coworkers used translationally hot H atoms (1.5 eV) to overcome the barrier, in conjunction with HOD excited with one quantum of either the OH or OD stretches,54 and found that reaction was still quite mode/bond specific. The mode/bond-specific reaction in these endoergic reactions is consistent with the vibrational enhancement predicted for late barrier reactions by Polanyi and coworkers,33 as discussed in the Introduction. The same is true for our previous studies of endoergic HT and DT reactions of HOD+ with various neutrals. In the HOD+ + acetylene system, the HT and DT reactions are strongly exoergic, and thus there is no barrier to reaction, late or otherwise. Nonetheless, it is clear 8370

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above to approximately separate the CTHE and HT contributions to the mass 27 signal is reliable enough to address vibrational effects. Nonetheless, it is clear that there are no dramatic vibrational effects on CTHE in reaction with C2H2. Note that the combined CTHE + HT channel for C2H2 (Figure 6) is significantly enhanced by OH stretch excitation, with a weaker effect from the bend excitation, approximately independent of Ecol. For reasons previously discussed, both the OH stretch and bend enhancements almost certainly result from enhancements of the HT channel. If the mechanism for CTHE is, as previously suggested, initial HT or DT followed by back transfer of a D or H atom before the products can separate, then the same factors enhancing either HT or DT might be expected to enhance CTHE. This is exactly what is observed, at least in CTHE in reaction with C2D2. The OH and OD stretch modes strongly enhance HT and DT, respectively, and both enhance CTHE as well. The bend mode provides a weaker enhancement but to both HT and DT, and bending excitation also provides a modest enhancement of CTHE. It is likely that similar effects are present for CTHE in reaction with C2H2 if we were able to measure that channel without interference from HT.

ASSOCIATED CONTENT

S Supporting Information *

vaxial distributions for the reaction of HOD+ with C2H2 and integral cross sections for the reaction of HOD+ with C2D4. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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V. CONCLUSIONS The reaction of HOD+ with C2H2 (C2D2) is dominated by CT (including CTHE) with smaller cross sections for HT and DT channels. The total cross section is at but does not exceed the collision limit, suggesting that long-range CT is not efficient but that CT and other reactions occur with unit probability in more intimate collisions. All channels appear to be direct in the sense that the collision time is less than the rotation period of any complexes that might form, and this observation is consistent with the calculated reaction coordinate. The probability that CT is accompanied by H/D exchange is much greater in reaction with C2D2 as compared with C2H2, and this effect is tentatively attributed to a combination of DOS and collision time effects. At low Ecol, all vibration states of HOD+ enhance CTHE in reaction with C2D2 but have little effect on CTHE in reaction with C2H2. The HT and DT channels show strongly mode- and bond-specific chemistry. Excitation of the OH or OD bonds enhances the HT and DT channels, respectively, having much smaller effects on breaking of the spectrator bond.

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*E-mail: [email protected]. Tel: (801)585-7289. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants CHE-0647124 and CHE1111935 from the Chemistry division of the National Science Foundation. 8371

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