The relative efficiency of different forms of the energy invested in a chemical reaction has been widely studied both experimentally and theoretically in the past decades. For the smallest A + BC atom + diatom reactions, the first rules of thumb of energy efficiency have been established:1 translational energy promotes such a reaction if the reactants have to surmount an early barrier, that is, the reaction has a reactant-like transition state (TS), while vibrational energy enhances a late-barrier reaction having a product like TS structure with an elongated B-C bond. In the case of more complex chemical reactions involving a polyatomic molecule, the question of mode-specificity naturally rises: which vibrational mode of the polyatomic reactant should be excited to promote (or inhibit) the reaction? Or, even more specifically, how can one reach the selective cleavage of a chemical bond? These issues have also been extensively investigated,2−25 involving even 7- and 9-atomic systems, as well.26−33
However, a somewhat less studied phenomenon is when the rotational motion of a polyatomic reactant molecule is excited. In atom + diatom reactions, where the rotation of the diatom can be characterized by the J rotational quantum number, rotational excitation effects have been the focus of several experimental, classical, and quantum dynamics studies.34−41 Stepping forward, in the case of polyatomic reactants, rotational mode-specificity can also be defined and investigated, as a symmetric/asymmetric polyatomic rotor can also be characterized by K/KaKc quantum number/labels, the projections of the J total rotational angular momentum to the body-fixed axes. Thus, the K/KaKc quantum number/labels, which can adopt values in the [0, ±1, …, ±J] interval, necessarily introduce rotational mode-specificity as their different values correspond to different rotational modes (states) of the reactant molecule. Accordingly, rotational mode-specific studies for the H2O+ + H2/D2,42,43 H/F/Cl + H2O,44−46 F/Cl/OH + CH4,47−50 H/Cl/O + CHD3,51−54 and the F- + CH3F/CH3Cl/CH3I55,56 reactions involving asymmetric, spherical, and symmetric top polyatomic reactants have been carried out.
Experiments found significant rotational promotion in the case of the H2O+(J, Ka, Kc) + D2 reaction,42 which was later explained by simulations as the facilitated reorientation of the H2O+ molecule due to rotational excitation.43 In the case of the H + CHD3 reaction, a seven-dimensional quantum dynamics study found basically no effect of rotational excitation up to J = 2;51 however, for Cl + CHD3(v1 = 1) → HCl + CD3, a joint crossed-beam, quasi-classical, and quantum dynamics investigation showed that the tumbling rotation (J, K = 0) of CHD3 significantly enhances the reactivity, while the spinning rotation of the reactant around the C-H axis (K = J) has only a minor effect.52 It was shown for this,52 and also for the O(3P) + CHD3(v1 = 0,1) → OH + CD3,54 reaction that rotational excitation does not affect the scattering angle distribution of the products, but the initial attack angle distributions indicated the enlargement of the reactive cone of acceptance with increasing J. Interestingly, for the OH + CH3 → O + CH4 reaction, quantum dynamics simulations found that the rotational excitation of both of reactants hinders the reaction.57,58 Rotational mode-specific computations involving the F- + CH3F/CH3Cl bimolecular nucleophilic substitution (SN2) reactions observed substantial rotational inhibition as well.55 In the former case, both the spinning (K = J) and the tumbling (K = 0) rotation of the CH3F reactant had a similar inhibiting effect, whereas for the latter reaction, the tumbling rotation was found to be less effective in hindering reactivity.55 Very recently, the F- + CH3I reaction was also studied and showed considerable rotational hindrance in the case of the SN2 channel for both tumbling and spinning excitations, whereas the proton-transfer reaction was noticeably promoted with increasing J by exciting the spinning reactant rotational mode; however, it was left unaffected by tumbling excitation.56
In the present work, we study the effect of rotational excitations for a 8-atomic reactant molecule, namely, ethane, in the Cl(2P3/2) + C2H6 → HCl + C2H5 reaction by performing quasi-classical trajectory (QCT) simulations on our recently developed high-quality ab initio full-dimensional potential energy surface (PES).59 Ethane is a prolate-type symmetric top, characterized by the J and K rotational quantum numbers, and we focus on the two limiting cases, (1) K = 0, referring to the rotation around the axis perpendicular to the C-C bond, that is tumbling rotation, and (2) K = ±J, denoting the spinning rotation around the C-C bond. We investigate the different rotational-mode excitations on the reactivity, the mechanism, and the energy flow during the reactions. We also compare the relative efficacy of rotational, translational, and even vibrational31 form of energy investments.