Rotational Mode Specificity in Cl + CH4(v3=1,|jNl ): Role of Reactant's

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Rotational Mode Specificity in Cl + CH4(v3=1,|jNl⟩): Role of Reactant’s Vibrational Angular Momentum Huilin Pan, Yuan Cheng,‡ and Kopin Liu* Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 10617 ABSTRACT: The effect of initial rotational states in the reaction of antisymmetric-excited CH4(v3=1,|jNl⟩) with Cl atom was investigated in a crossed-beam, product-imaging experiment over the collisional energy (Ec) range of 2−5 kcal mol−1. We found that while the initial rotational excitations exert a noticeable effect on total reactivity, they leave little imprint on the more detailed product-state and angular distributions. This finding echoes the previous conclusion in the analogous Cl + CHD3(v1=1,| NK⟩) reaction. However, the rotational enhancement factor is substantial at low Ec and then becomes insignificant at higher Ec, in contrast to the Cl + CHD3 case. A more intriguing finding is the role of the vibrational angular momentum (l) in promoting the reactivity. A heuristic picture is proposed to rationalize the observations.

differential cross sections (PDDCSs),21−23,28 a conceptual framework is yet to be developed to link the measured PDDCSs to a chemically more appealing picture of the stereorequirement of a reaction, such as the bending potentials at the reaction barrier or the reaction probability as a function of the attack angles near the transition state. Here, we present an alternative, more conventional experiment7−9,27 that might provide a complementary probe of the short-range anisotropic interactions at the transition state by examining the rotational-state selective reactivity of the title reaction. There are many atom + diatom examples pointing to the close connection between the effect of reactant rotation and the stereorequirements of the reaction. In contrast to a typical diatom, the rotational motions of the CH4(v3=1) reactant are richer with the eigenstates characterized by three quantum numbers: the rotational (N), vibrational angular momentum (l), and the total angular momentum (j) with j = N + l, and denoted as |jNl⟩.29

I. INTRODUCTION Stereorequirement of a chemical reaction is a well accepted concept in chemistry.1−5 Yet, a clear understanding of the origin of this stereospecificity is often hampered by several interwoven factors that need to be considered. Not only the landscapes of the potential energy surface (PES) both in the entrance valley4,6 and near the transition-state region1,2,5 have profound influence on reactivity, but also the kinematic factorsthe interaction time (governed by the masses and collision energy, Ec) and rotational velocities of the reactants can change the reactive outcomes in a dramatic way. To unravel the physical origin from the entangled potential and kinematic factors, detailed theoretical analyses will clearly be needed.7−19 Different experimental approaches could also prove fruitful by shedding more light onto this multifaceted problem. The stereorequirement of the reaction of Cl atom with antisymmetrically excited CH4(v3=1) was explored in a recent report by investigating the reactivity dependence on the alignment of asymptotically excited CH4(v3=1) reactants.20 Enormous effects were found, which is intriguing by itself for a nominally spherical top molecule like CH4, and paved the road for further studies. The very fact that a significant polarization dependency was observed implies little reorientation or steering effects exerted by the long-range anisotropic forces in the entrance valley;6,21−23 otherwise, the asymptotically aligned reactant will not retain its directional properties en route to the reaction barrier. It further suggests that the previously observed vibrational enhancement in reaction rate (at a fixed Ec) for unpolarized reactants24,25 must then arise from a wider range of attack angles at the transition state;14,26,27 i.e., the bending potentials near the saddle point become softer upon CH stretching excitation of CH4(v3=1). Although such a polarization experiment is enlightening and can lead to very (if not the most) detailed state-resolved, polarization-dependent © XXXX American Chemical Society

II. EXPERIMENT The experiment is essentially the same as the previous reports.26,27,30 Briefly, two doubly skimmed, pulsed molecular beams intersected in the interaction region of a time-sliced, velocity-mapped ion-imaging apparatus.31 The Cl-atom beam was generated by pulse-discharging of supersonically expanded mixture of 5% Cl2 seeded in Ne, and the rovibrationally excited CH4(v3=1,|jNl⟩) reagents were prepared in front of the first Special Issue: Piergiorgio Casavecchia and Antonio Lagana Festschrift Received: December 12, 2015 Revised: January 7, 2016

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The Journal of Physical Chemistry A skimmer by an infrared (IR) optically parametric oscillator/ amplifier (OPO/A) operated at P(1), Q(1), R(0), R(1), and R(2) branches for the |jNl⟩ = |011̅⟩, |110⟩, |101⟩, |211⟩, and | 321⟩ rotational states, respectively. Owing to the long flight time (∼125 μs) from the IR-excitation region to the scattering center and relatively short depolarization lifetime of 15−20 μs,20 the rovibrationally excited CH 4 becomes largely depolarized. The notion of unpolarized reactants under such a setup with long time delay between IR-excitation and UV probe is in line with our very early attempts that failed to observe any reactant polarization effects. A multipass ring-reflector was employed to enhance the IRpumping efficiency.32 The characterizations of the IR-pumping efficiencies, n‡/n0, were performed in each day experiment, using the depletion method as detailed previously.33 The typical n‡/n0 (with uncertainty of ±0.015) are 0.13, 0.27, 0.23, 0.36, and 0.12 for P(1), Q(1), R(0), R(1), and R(2), respectively. These measured IR pumping efficiencies agree very well with the estimates from a supersonically cooled CH4 beam at Trot ∼ 8 K.33 The dominant vibrational ground state of CH3 products were detected by a (2 + 1) resonance-enhanced multiphoton ionization (REMPI) process,34−36 as exemplified in Figure 1.

Figure 2. Time-sliced raw images of the CH3(v=0) products, excited via the five branches of the CH4(v3=1←0) transition, at (a) Ec = 5.0 kcal mol−1, (b) 3.2 kcal mol−1, and (c) 2.1 kcal mol−1. The presented raw images correspond to the stretch-excited reaction only, for which the contributions of the residual (i.e., the unexcited) ground-state reaction have been properly subtracted from the IR-on images. The forward direction, 00, is defined as the initial CH4-beam direction in the center-of-mass frame. The notation of (1, 00)s refers to the (vHCl, vCH3)s product pair from the stretch-excited reaction (the subscript “s”). Note the very faint signals labeled as (0, 41)s for detecting the deformation-mode excited CH3(v4=1). Its sighting while probing the origin band CH3(000) has previously been reported in a different reaction of Cl + CH4(v3=1).25

the v3 = 1 ← 0 transition at (a) Ec = 5.0 kcal mol−1, (b) 3.2 kcal mol−1, and (c) 2.1 kcal mol−1. The raw images shown correspond to the stretch-excited reaction only, for which the contributions of the (IR-unexcited) ground-state reaction have been subtracted from the IR-on image according to the concurrent determination of n‡/n0 in each experiment. As is seen, at a given Ec the general appearances of all five images are alike. After the image analysis that accounts for the density-toflux correction,31,37 the correlated HCl(v) angular distributions (Figure 3) and resultant product speed distributions (Figure 4) are indeed invariant to the selected rotational states of CH4(v3=1). These findings are in accord with those in the Cl + CHD3(v1=1,|NK⟩) reactions,27 and suggest that the memory of initial rotational selections is largely lost once the barrier is surmounted and the ensuing dynamics is mainly governed by the transition-state properties. The vibrationally correlated HCl(v=1) and HCl(v=0) angular distributions are distinct (Figure 3), most likely reflecting two different reaction pathways. As previously surmised,25,40−43 the two pathways arise from the bifurcation of trajectories near the transition state. One path undergoes vibrationally nonadiabatic transition to the vibrational groundstate PES leading to the HCl(v=0) + CH3(v=0) products pair. The other path retains its vibrational adiabaticity and proceeds over the adiabatically (v = 1)-excited PES. On the basis of ab initio calculations,44−46 this vibrationally adiabatic excited PES exhibits a dynamical well that conceivably could trap metastable states (or reactive resonances) and mediate the adiabatically formed HCl(v=1) product yielding a “glory-like”, forwardpeaking angular distributions. This experimental conjecture of a resonance-mediated pathway has received supports from recent reduced dimensionality quantum dynamics studies.47,48 B. Correlated Vibrational Branching. The branching of this trajectory bifurcation apparently depends on Ec, as seen from the relative intensities of the two peaks in Figure 4. When

Figure 1. (a) IR-on and IR-off REMPI spectra of the CH3(00) products at Ec = 5 kcal mol−1. The excited CH4(v3=1) is prepared by the R(1) branch. The vertical line marks the red shift of the IR-on spectrum, indicating a warmer rotational product from the stretchexcited reaction than the ground-state reaction. (b) Comparison of the differential REMPI spectra of the stretch-excited reaction at Ec = 3.2 and 5 kcal mol−1, for which the signals of the residual ground-state reaction (IR-off) were subtracted from the IR-on spectra according to the measured IR-pumping efficiencies. The two spectra are nearly identical within the experimental uncertainty. The two vertical arrows mark the scanned range when the product images are acquired.

The probe laser frequencies (near 333 nm) were scanned back and forth over the entire CH3(000) band, and the ion signals were acquired by a time-sliced velocity image technique.31 Thus, the recorded product images sample the full rotationalstate distribution of CH3(v=0). After the density-to-flux correction,31,37 the desired relative cross sections, correlated HCl-state and angular distributions38 were obtained. Varying the initial collisional energy Ec is readily achieved in this rotating-sources apparatus by changing the intersection angle of the two molecular beams.31,35,39

III. RESULTS AND DISCUSSION A. Correlated Angular Distributions. Figure 2 presents the time-sliced raw images of the CH3(v=0) products from the reactions with CH4 excited via the five rotational branches of B

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Figure 5. (a) Ec dependence of the vibrational branching fraction for vHCl = 1in the Cl + CH4(v3=1) reaction. A slight decline with increasing Ec is noted, yet independent of the initial rotational states. (b) Dependency of relative reactivity on the rotational state of CH4(v3=1). At each Ec, the reactivity of the rotational ground state, | 011̅⟩ excited via the P(1) branch, is set to unity. The error bar represents the statistical uncertainty from repeated experiments, encompassing the propagated errors from the IR-excitation efficiency measurements. The IR-excited rotational states are labeled by |jNl⟩.

Figure 3. Comparison of the pair-correlated differential cross sections of the CH3(v=0) product channels from reactions with five different reactant rotational states. As seen, little reactant rotational-state dependencies are discernible at all three Ec’s.

present values are significantly smaller: At the three Ec’s of this study, the present (previous) vibrational fractions are (starting from the higher Ec) 0.36 (0.42), 0.48 (0.52), and 0.56 (0.69), respectively. A similar trend was previously noted in the analogous Cl + CHD3(v1=1,|NK⟩=|10⟩) reaction,30 which was referred to the rotational probe effect: probing the low (for fixed laser wavelength) vs all rotational states (for scanning the laser wavelengths) of methyl products. We believe that the origin of this anticorrelated behavior between the probed CH3 rotor states and the coincidently formed HCl vibrational excitation is predominantly kinematic in nature. As discussed previously in the F + CD4 reaction,49−51 such anticorrelated behavior between the internal energies of the two coproducts should hold in general for a collinear H + LH′ mass combination reaction, where H(H′) and L denote heavy and light mass, respectively. In addition, by conservations of energy and angular momentum, such behavior further implies the CH3 rotational angular momentum lying preferentially antiparallel to the orbital angular momentum of the receding products.49,50 C. Vibrational Enhancement Factor and Rotational Mode Specificity. The IR-on and IR-off images for each rotational branch excitation at a fixed Ec were alternatively recorded and repeated 20−70 times for normalization. From the concurrent determination of n‡/n0, the relative reactivity of the rotationally selected Cl + CH4(v3=1,|jNl⟩) reactions, as well to the ground-state reaction, can then be obtained.26,27,52 In terms of vibrational enhancement factor, σs/σg, for R(1)excitation, this study (wavelength-scan) gives 6.8 at 5 kcal mol−1 and 40 at 3.2 kcal mol−1, which compare to the previous results (wavelength fixed at the peak of the Q-head) of 3.5 and 20, respectively.25 In other words, when all rotational states are sampled, σs/σg is approximately a factor of 2 larger than that when only low rotational states are probed. Again, this “rotational probe effect” was noted in the previous Cl + CHD3(v1=1,|NK⟩) reactiona factor of 3−4 from the

Figure 4. CH3(v=0) product speed distributions at three Ec’s. At each Ec, the results of five different rotational branches are normalized to the same total intensities. On energetic grounds, the slow and fast peaks correspond to the HCl(v′=1) and HCl(v′=0) coproducts, respectively. Again, all results are practically invariant to the initial rotational-mode excitation.

the respective two peaks are integrated, Figure 5a summarizes the vibrational fraction of σ(vHCl=1)/Σσ(v). [Note that the very faint signals of the (0, 41)s pair indicated in Figure 2 make little contributions and do not alter the vibrational fraction.] A clear decline is seen with the increase in Ec. Compared to the previously reported vibrational fractions when the probe laser frequency was fixed at the peak of the CH3(000) band,25 the C

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The other two states, F−(j) and F+(j) with l being parallel or antiparallel to j, are linear combinations (or the superposition) of the original two vibrations and thus execute in-phase excitations constituting an ensemble of elliptical oscillations, one clockwise and the other counterclockwise.20,29 Thus, the CH4(v3=1) excited by R-branch can be viewed as an ellipsoid with again all four H atoms uniformly distributed over the surface. A crude analogy: the four H atoms are, as seen by the approaching Cl-atom, shaped like the “electron-cloud” of a diatom, in contrast to the Q-branch excited CH4(v3=1) that would be visualized as a closed-shell atom (except with an offset nuclei: the carbon atom). The ellipsoid-shaped methane with elongated C−H bonds tends to shift the saddle point even further into the product valley (i.e., more product-like). Consequently, the transition-state structure becomes more fluxional (i.e., a loose-bend transition structure) with a broader range of attack angles due to less steric hindrance between the receding HCl and CH3 fragments. In addition, as shown in a model calculation by Perdih et al.58 for collinear reactions with such a shape (i.e., a prolate type), the rotation of reactant will in general enhance the reaction rate. We surmise that it is this distinct “shape” induced by the vibrational angular momentum leading to the differential reactivity between the excited CH4(v3=1) with l = 1 (for R(0), R(1), and R(2)) and l = 0 (Q(1)). Also worth noting are the nearly identical reactivities for |110⟩ and |011̅⟩ states, which could be attributed to the “atomic-like” spherical shape of the two states. [Recalling again that for the |110⟩ state the carbon atom is offset from the center of the sphere, underscoring the polarization dependency in reactivity.20] The scenario proposed here echoes the concept of “chemical shape” introduced by Levine long ago for understanding the stereodynamics of atom + diatom reactions.4 In the present reaction the key factor in dictating the distinct shapes of the ro-vibrationally excited CH4 states appears to be the vibrational angular momentum, albeit l is merely an approximately good quantum number. It is interesting to contrast the l-propensity of the rotational specificity in this reaction to the K-propensity reported previously in Cl + CHD3(v1=1,|NK⟩).26,27 In Cl + CH4(v3=1,| jNl⟩) the rate enhancement is greater for rotational state with l = 1 (l being parallel or antiparallel to j) than that with l = 0 (l being perpendicular to j). Because j is always perpendicular to the figure axis of a rotating CH4, the l-propensity means that reaction rate will be greater for state with l being perpendicular to the figure axis. In the case of Cl + CHD3(v1=1,|NK⟩), the state with K = 0 exhibits higher reactivity than the (K > 0) states for a given N-manifold.26 Again, the reactivity prefers the rotational angular momentum N being perpendicular to the figure axis. Hence, in terms of the rotational specificity on reactivity, the body-fixed direction of the vibrational angular momentum l of CH4 seems to play a similar role as that of the K quantum number (the projection of the rotational angular momentum onto the body-fixed axis) for CHD3. A remark is needed to clarify various reported rotational specificities for the title reaction. The reactivity propensity reported in this work, e.g., R(2) > R(1) ≈ R(0) > Q(1) ≈ P(1) at Ec = 2.1 kcal mol−1 (Figure 5b), refers to the rate promotion for an unpolarized (in the space-fixed frame) reagent CH4. The underlying physics is posited here from the presence of the vibrational angular momentum whose directional property (in the molecule-fixed frame) results in distinct “shapes” of the prepared rotational states as seen by the approaching Cl-atom. A different propensity was reported previously in terms of

rotational bias.30 As shown in Figure 1, the acquired IR-on and IR-off REMPI spectra demonstrated that the rotational probe effect arises from the broader IR-on spectrum or a slightly warmer rotational excitation of the stretch-excited reaction than the ground-state reaction.27,30 Figure 5b presents the relative integral cross sections of rotational-state selected reactions of this study. To show the rotational-mode dependency, all data are normalized to the lowest, rotationless state |jNl⟩ = |011̅⟩ at the same Ec. Rotational excitation clearly has a beneficial impact to rate promotion at Ec < 2.0 kcal mol−1, but its effect diminishes rapidly with increasing Ec. It is interesting to contrast this behavior to a recent report on the Cl + CHD3(v1=1,|NK⟩) reaction,26,27 for which the relative rotational enhancement factor at low Ec is about the same as this reaction (∼2.5) but remains significant (∼1.5−2.0) up to Ec = 6 kcal mol−1. The different Ec dependencies of the two reactions may reflect the subtle isotopic differences of the bending potentials at saddle point. In any event, the observations that only the reactivity, not the product distributions, is influenced by the initial rotational selection lead to the following simple view of the dynamics of Cl atom with stretch-excited methane. Reactant’s rotation plays an important role for getting to the transition state, but its memory is lost once the barrier has been surmounted and the product distributions are predominantly governed by the ensuing transition-state dynamics. A full-dimensional state-to-state quantum dynamical calculation has recently been developed using the transition-state wave packet method by Manthe and co-workers.53−55 When this approach was applied to reactions with vibrationally excited reactants at total angular momentum J = 0, the loss of vibrational memory in product-state distributions have been predicted in H + CH4 → CH3 + H256 and Cl + H2O → HCl + OH.57 The works reported here and previously for the analogous Cl + CHD3(v1=1,|NK⟩) reaction27 provide the first experimental evidence of the “loss of memory” phenomenon for rotational excitations of reactants. D. Role of Vibrational Angular Momentum. It is perhaps more instructive to scrutinize the differential reactivity at Ec = 2.1 kcal mol−1 (Figure 5b), which show a rate promotion of |321⟩ > |211⟩ ≈ |101⟩ > |110⟩ ≈ |011̅⟩. On one hand, a clear propensity toward higher j seems apparent; yet, the reactant states |211⟩ and |101⟩ show comparable reactivities and the rate for |110⟩ appears about the same as that for the rotational ground-state |011̅⟩. On the other hand, the results for R(0), R(1), R(2), and Q(1) suggest that the vibrational angular momentum l also plays a significant rolethe former three cases have l = 1 and Q(1) with l = 0. One quantum excitation of the triply degenerate mode v3 = 1 exhibits a vibrational angular momentum l = 1. Because j = N + l, N can take on three values of j − 1, j and j + 1, corresponding to l being parallel, perpendicular, and antiparallel to j in the classical vector model. 20,29 As CH4 (v3=1) rotates, the degeneracy of the three energy levels is lifted by the Coriolis interactions, leading to the F−(j), F0(j), and F+(j) states for the R, Q, and P branches, respectively. For the F0(j) state or the CH4(v3=1) excited by Q-branch, l is perpendicular to j (or parallel to the figure axis). In a simple classical picture,29 all four H atoms are stretched with identical amplitudes while the C atom moves in the opposite direction, and thus the shape of the excited CH4(v3=1) remains as a sphere but with the carbonatom off-centered, and all four H atoms are uniformly distributed over the spherical surface. D

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The Journal of Physical Chemistry A (space-fixed) alignment effect, R(0) > R(1) > Q(1) > P(1).20 It corresponds to differential reactivity of distinct collision geometries by polarizing the reactant and thus mainly reflects the degree of reactant alignment prepared by IR absorption via different rotational branches. Accordingly, the paramount concern becomes the alignment of j; j is a good quantum number, excluding the nuclear spin. The variation of the above two propensities could then be regarded as a manifestation of the subtle roles of the directional properties of j and l in governing different dynamical observables. The term of rotational specificity or propensity will then depend on the dynamical attribute of interest.

(4) Levine, R. D. The Chemical Shape of Molecules: An Introduction to Dynamical Stereochemistry. J. Phys. Chem. 1990, 94, 8872−8880 and references therein.. (5) Jiang, B.; Li, J.; Guo, H. Effects of Reactant Rotational Excitation on Reactivity: Perspectives from the Sudden Limit. J. Chem. Phys. 2014, 140, 034112. (6) Liu, K. Perspective: Vibrational-Induced Steric Effects in Bimolecular Reactions. J. Chem. Phys. 2015, 142, 080901. (7) Xu, Y.; Xiong, B.; Chang, Y. C.; Ng, C. Y. Communication: Rovibrationally Selected Absolute Total Cross Sections for the Reaction H2O+(X2B1; v1+v2+v3+ = 000; N+Ka+Kc+) + D2: Observation of the Rotational Enhancement Effect. J. Chem. Phys. 2012, 137, 241101. (8) Xu, Y.; Xiong, B.; Chang, Y.-C.; Ng, C. Y. The Translational, Rotational, and Vibrational Energy Effects on the Chemical Reactivity of Water Cation H2O+(X2B1) in the Collision with Deuterium Molecule D2. J. Chem. Phys. 2013, 139, 024203. (9) Li, A.; Li, Y.; Guo, H.; Lau, K.-C.; Xu, Y.; Xiong, B.; Chang, Y.-C.; Ng, C. Y. Communication: The Origin of Rotational Enhancement Effect for the Reaction of H2O+ + H2 (D2). J. Chem. Phys. 2014, 140, 011102. (10) Kornweitz, H.; Persky, A.; Levine, R. D. The Effect of Reagent Rotation on Steric Requirement of Elementary Exchange Reaction. Chem. Phys. Lett. 1986, 128, 443−448. (11) Mayne, H. R. Effect of Reactant Rotation on Reactivity: A Comparison of Classical and Quantum Effects in a Model System. Chem. Phys. Lett. 1986, 130, 249−253. (12) Mayne, H. R.; Minick, S. K. Effects of Reactant Rotation on Reactivity: Comparison of Exact Coplanar Results and a Model Calculation for H + H2. J. Phys. Chem. 1987, 91, 1400−1404. (13) Mayne, H. R. Effect of Reactant Rotation on Hydrogen Atom Transfer. J. Am. Chem. Soc. 1990, 112, 8165−8166. (14) Sokolovski, D.; Connor, J. N. L.; Schatz, G. C. CentrifugalSudden Hyperspherical Study of Cl + HCl → ClH + Cl Reaction Dynamics on “ Tight-bend” and “Loose-bend” Potential Energy Surfaces. Chem. Phys. 1996, 207, 461−476. (15) Zhang, Z.; Zhang, D. H. Effects of Reagent Rotational Excitation on the H + CHD3 → H2 + CD3 Reaction: A Seven Dimensional TimeDependent Wave Packet Study. J. Chem. Phys. 2014, 141, 144309. (16) Song, H.; Guo, H. Vibrational and Rotational Mode Specificity in the Cl + H2O → HCl + OH Reaction: A Quantum Dynamical Study. J. Phys. Chem. A 2015, 119, 6188−6194. (17) Song, H.; Li, J.; Jiang, B.; Yang, M.; Lu, Y.; Guo, H. Effects of Reactant Rotation on the Dynamics of the OH + CH4 → H2O + CH3 Reaction: A Six-Dimensional Study. J. Chem. Phys. 2014, 140, 084307. (18) Czako, G. Quasiclassical Trajectory Study of the Rotational Mode Specificity in the O(3P) + CHD3(v1 = 0, 1, JK) → OH + CD3 Reactions. J. Phys. Chem. A 2014, 118, 11683−11687. (19) Yan, P.; Meng, F.; Wang, Y.; Wang, D. Energy Efficiency in Surmounting the Central Energy Barrier: A Quantum Dynamics Study of the OH + CH3 → O + CH4 Reaction. Phys. Chem. Chem. Phys. 2015, 17, 5187−5193. (20) Pan, H.; Yang, J.; Wang, F.; Liu, K. Imaging the Stereodynamics of Cl + CH4(v3 = 1): Polarization Dependence on the Rotational Branch and the Hyperfine Depolarization. J. Phys. Chem. Lett. 2014, 5, 3878−3883. (21) Wang, F.; Lin, J.-S.; Liu, K. Steric Control of the Reaction of CH Stretch-Excited CHD3 with Chlorine Atom. Science 2011, 331, 900− 903. (22) Wang, F.; Liu, K.; Rakitzis, T. P. Revealing the Stereospecific Chemistry of the Reaction of Cl with Aligned CHD3(v1 = 1). Nat. Chem. 2012, 4, 636−641. (23) Wang, F.; Liu, K. Steric Effects in the Cl + CHD3(v1 = 1) Reaction. Chin. J. Chem. Phys. 2013, 26, 705−709. (24) Kawamata, H.; Tauro, S.; Liu, K. Unravelling the Reactivity of Antisymmetric Stretch-Excited CH4 with Cl by Product PairCorrelation Measurements. Phys. Chem. Chem. Phys. 2008, 10, 4378−4382.

IV. CONCLUSIONS In summary, the rotational mode specificity in the Cl + CH4(v3=1) reaction is studied in a crossed-beam imaging experiment. We found that the initial rotational-state selectivity affects the total reactivity but leaves the more detailed productstate and angular distributions unchanged, in keeping with the previous findings in the Cl + CHD3(v1=1) reaction. This “loss of memory” observation suggests a stepwise process for the title reactions. The initial step is governed by the capability of reactants to attain the transition state, which depends on the initial rotational state. Once the barrier is surmounted, the memory of the initial rotation is lost and the product distributions are primarily controlled by the ensuing transition-state dynamics, the second step. The experimental results presented here also suggest a hierarchy, at least for the low rotational states at low Ec, in the dependency of rotational mode specificity on the rotational quantum numbers. First, a finite vibrational quantum number (l > 0) is more efficacious to promote the reaction rate than the l = 0 state. Then for a given l-manifold the higher j state will yield a faster rate. A simple physical picture in terms of vibrational-induced reactant shapes is proposed to elucidate the observed effect by l.

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AUTHOR INFORMATION

Corresponding Author

*K. Liu. E-mail: [email protected]. Phone: +886-223668259. Present Address ‡

Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan, 430071 People’s Republic of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. M. Kirste for assistances in the initial stage of this project. This work was financially supported by Academia Sinica and the Minister of Science and Technology of Taiwan (NSC102-2119-M-001-002-MY3).

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REFERENCES

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DOI: 10.1021/acs.jpca.5b12156 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.5b12156 J. Phys. Chem. A XXXX, XXX, XXX−XXX