中央研究院 原子與分子科學研究所

The transition state, which gates and modulates the reactive flux, serves as the central concept in our understanding of activated reactions. The barrier height of the transition state can be estimated from the activation energy taken from thermal kinetics data or from the energetic threshold in the measured excitation function (the dependence of reaction cross sections on initial collision energies). However, another critical and equally important property, the angle-dependent barrier to reaction, has not yet been amenable to experimental determination until now. Here, using the benchmark reaction of Cl + CHD₃(v1 = 1) as an example, we show how to map this anisotropic property of the transition state as a function of collision energy from the preferred reactant bond alignment of the backward-scattered products ‒ the imprints of small impact-parameter collisions (left figure). The deduced bend potential at the transition state agrees with ab initio calculations (right figure). We expect that the method should be applicable to many other direct reactions with a collinear barrier.

Vibrational motions of a polyatomic molecule are multifold and can be as simple as stretches or bends or as complex as concerted motions of many atoms. Different modes of excitation often possess different capacities in driving a bimolecular chemical reaction, with distinct dynamic outcomes. Reactions with vibrationally excited methane and its isotopologs serve as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. Here, some recent progress in this area is briefly reviewed. Particular emphasis is placed on the key concepts developed from those studies. The interconnections among mode and bond selectivity, Polanyi’s rules, and newly introduced vibrational-induced steric phenomena are highlighted.

The concept of geometrical constraints and steric hindrance in reactions is implanted deeply in a chemist’s ‘chemical intuition’. However, until now a true three-dimensional view of these steric effects has not been realized experimentally for any chemical reaction in full. Here we report the complete three-dimensional characterization of the sterics of a benchmark polyatomic reaction by measuring the dependence of the product state-resolved angular distributions on the spatial alignment of the reactive bond in a crossed molecular beam experiment. The results prove the existence of two distinct microscopic reaction mechanisms. Detailed analysis reveals that the origin of the stereodynamics in the HCl(v = 0) + CD₃(0₀) product channel can be captured by a textbook line-of-centres collision model. In contrast, a time-delay pathway, which includes a sharp switch from in-plane to out-of-plane scattering in the forwards direction, appears to be operative in forming the excited HCl(v = 1) + CD₃(0₀) product pair.

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.

Wereport a comprehensive study of the quantum-state correlation property of product pairs from reactions of chlorine atoms with both the ground-state and the CH stretch-excited CHD₃. In light of available ab initio theoretical results, this set of experimental data provides a conceptual framework to visualize the energy-flow pattern along the reaction path, to classify the activity of different vibrational modes in a reactive encounter, to gain deeper insight into the concept of vibrational adiabaticity, and to elucidate the intermode coupling in the transition-state region. This exploratory approach not only opens up an avenue to understand polyatomic reaction dynamics, even for motions at the molecular level in the fleeting transition-state region, but it also leads to a generalization of Polanyi’s rules to reactions involving a polyatomic molecule.