Quasiclassical Trajectory Study of the Rotational Mode Specificity in the O(3P) + CHD3(v1 = 0, 1, JK) → OH + CD3 Reactions

First-principles mode-specific reaction dynamics

Controlling the outcome of chemical reactions by exciting specific vibrational and/or rotational modes of the reactants is one of the major goals of modern reaction dynamics studies. In the present Perspective, we focus on first-principles vibrational and rotational mode-specific dynamics computations on reactions of neutral and anionic systems beyond six atoms such as X + C2H6 [X = F, Cl, OH], HX + C2H5 [X = Br, I], OH- + CH3I, and F- + CH3CH2Cl. The dynamics simulations utilize high-level ab initio analytical potential energy surfaces and the quasi-classical trajectory method. Besides initial state specificity and the validity of the Polanyi rules, mode-specific vibrational-state assignment for polyatomic product species using normal-mode analysis and Gaussian binning is also discussed and compared with experiment.

Stretching-mode specificity in the Cl + CH3D(v1-I, v1-II, and v4 = 1; |jK〉) reactions: dependency on the initial |jK〉 selectivity

The title reactions were studied at a collisional energy of 5.4 kcal mol^-1 in a crossed-beam product-imaging experiment. The dynamics attributes of the dominant ground-state CH₂D(0₀) and the accompanied C-D bend-excited CH₂D(6₁) products were imaged in reactions with totally 16 ro-vibrationally selected states of the CH₃D(v_i, |jK〉) reagents. We found that all three vibrational excitations yielded marked |jK〉-dependent rate-enhancements in forming the (0₀, 0/1)_s product pairs. Furthermore, for a given rotational |jK〉-mode, a vibrational-mode propensity of v₄ > v₁-I > v₁-II in rate promotion and a clear manifestation of the Fermi-phase-induced interference effect of the latter two were observed. Compared to the reactivity of the rotationless state |jK〉 = |00〉, a minute rotational-excitation of all three stretch-excited CH₃D(v_i = 1) reagents could yield significantly higher reaction rates for the product pair (0₀, 0)_s, but not so for (0₀, 1)_s. The signals in forming the (6₁, 0)_s pair were clearly notable but smaller than that of the ground-state reaction product pair, (0₀, 0)_g. An opposite propensity of v₁-II ≈ v₁-I > v₄ with a milder dependency on the initial |jK〉-states was observed. The angular distributions of the (0₀, 0)_s pairs were nearly identical for all ro-vibrationally excited reagents, displaying the typical trait for a direct abstraction of the rebound mechanism. Similar distributions were found for the (6₁, 0)_s pairs; yet, both pairs deviated substantially from the peripheral feature of the ground-state reaction pair of (0₀, 0)_g. Those of the (0₀, 1)_s pairs in reactions with v₄-excitation featured a prominent forward-peaking distribution-suggestive of a time-delayed, resonance-mediated pathway, again with little dependency on the initial |jK〉-states. As for the reactions with the two Fermi-dyads, v₁-I and v₁-II, albeit showing globally similar distributions to that for v₄, a substantial variation with the initial rotational-mode excitation could be discerned in the forward-peaking features. To unravel the impact of the Fermi-phase on the |jK〉-dependent attributes, we adopted a comparative approach by contrasting the observations in reactions with the Fermi-dyad reagents (the superposition states) to those with the pure-state reagents. Remarkable distinctions are unveiled and elucidated with the unexplained results explicitly pointed out, which call for future theoretical investigations for deeper understanding.

Rotational Mode-Specificity in the Cl + C2H6 → HCl + C2H5 Reaction

We perform rotational mode-specific quasi-classical trajectory simulations using a high-quality ab initio analytical potential energy surface for the Cl(²P_3/2) + C₂H₆ → HCl + C₂H₅ reaction. As ethane, being a prolate-type symmetric top, can be characterized by the J and K rotational quantum numbers, the excitation of two rotational modes, the tumbling (J, K = 0) and spinning (J, K = J) rotations of the reactant is carried out with J = 10, 20, 30, and 40 at a wide range of collision energies. The impacts of rotational excitation on the reactivity, the mechanism, and the post-reaction distribution of energy are investigated: (1) exciting both rotational modes enhances the reactivity with the spinning rotation being more effective due to its coupling to the C-H stretching vibrational normal modes (C-H bond elongating effect) and larger rotational energies, (2) rotational excitation increases the dominance of direct rebound over the stripping mechanism, while collision energy favors the latter, (3) investing energy in tumbling rotation excites the translational motion of the products, while the excess spinning rotational energy readily flows into the internal degrees of freedom of the ethyl radical or, less significantly, into the HCl vibration, probably due to the pronounced rovibrational coupling in this case. We also study the relative efficiency of vibrational and rotational excitation on the reactivity of the barrierless and thus translationally hindered title reaction.

Rotational Mode Specificity in the F- + CH3I(v = 0, JK) SN2 and Proton-Transfer Reactions

Quasiclassical trajectory computations are performed for the F^– + CH₃I(v = 0, JK) → I^– + CH₃F (S_N2) and HF + CH₂I^– (proton-transfer) reactions considering initial rotational states characterized by J = {0, 2, 4, 6, 8, 12, and 16} and K = {0 and J} in the 1–30 kcal/mol collision energy (E_coll) range. Tumbling rotation (K = 0) counteracts orientation effects, thereby hindering the S_N2 reactivity by about 15% for J = 16 in the 1–15 kcal/mol E_coll range and has a negligible effect on proton transfer. Spinning about the C–I bond (K = J), which is 21 times faster than tumbling, makes the reactions more direct, inhibiting the S_N2 reactivity by 25% in some cases, whereas significantly enhancing the proton-transfer channel by a factor of 2 at E_coll = 15 kcal/mol due to the fact that the spinning-induced centrifugal force hinders complex formation by breaking H-bonds and activates C–H bond cleavage, thereby promoting proton abstraction on the expense of substitution. At higher E_coll, as the reactions become more direct, the rotational effects are diminishing.

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces

We review composite ab initio and dynamical methods and their applications to characterize stationary points of atom/ion + molecule reactions.

Rotational effects on the dissociation dynamics of CHD3 on Pt(111)

MCTDH calculations suggest that experiments on the effect of alignment on dissociation of CH₄ on metal surfaces are affected by steering.

Energy efficiency in surmounting the central energy barrier: a quantum dynamics study of the OH + CH3 → O + CH4 reaction

A quantum dynamics study of the OH + CH₃ with a slightly early barrier shows that vibrational energy is more effective in promoting the reactivity than translational energy, which is just opposite to the Polanyi rules.