Pinpointing the role of geometric phase: During chemical reactions, electrons usually rearrange more quickly than nuclei. Thus, theorists often adopt an adiabatic framework that considers vibrational and rotational dynamics within single electronic states. Near the regime where two electronic states intersect, the dynamics get more complicated, and a geometric phase factor is introduced to maintain the simplifying power of the adiabatic treatment. Yuan et al. conducted precise experimental measurements that validate this approach. They studied the elementary H + HD reaction at energies just above the intersection of electronic states and observed angular oscillations in the product-state cross sections that are well reproduced by simulations that include the geometric phase. Science , this issue p. 1289

Access via Vibration: Molecular beam studies over the past decade have elucidated many subtle quantum mechanical factors governing the influence of vibrational excitation on the outcome of elementary chemical reactions. However, these studies have generally had to focus on reagents that can be easily made to vibrate by direct absorption in the infrared (IR). Wang et al. (p. 1499 ) show that a variation on stimulated Raman pumping can efficiently excite the IR-inactive stretch vibration in the diatomic molecule, hydrogen deuteride (HD). As a result, they can probe the influence of that vibration on the outcome of the HD + F reaction. Through a combined spectroscopic and theoretical investigation, they uncover Feshbach resonances along the reaction coordinate that are only accessible through vibrational preexcitation.

A few very brief pauses in the action: Chemical reactions proceed by the cumulative effect of trillions upon trillions of collisions between atoms and molecules. Usually, a given collision bounces the participants right back out again, either in their original form or with the atoms shuffled around into distinct products. In certain cases, the reacting partners experience a brief lull, termed a resonance, before they rearrange. Yang et al. report the discovery of particularly short-lived resonances in certain reactive collisions of chlorine atoms with vibrationally excited hydrogen deuteride (HD). Their results suggest that similar, as yet overlooked, resonances may lurk in other reactions of vibrationally excited molecules. Science , this issue p. 60

Intriguing interference mechanism: Quantum interference (QI) effects play a fundamental role in the dynamics of chemical reactions. Xie et al. detected unusual QI oscillations in the differential cross section measured in the recoil scattering direction of the prototypical elementary reaction H + HD → H ₂ + D (see the Perspective by Aoiz). Topological analysis showed that this pattern originates from the QI between a direct abstraction and previously unknown rebounding insertion pathways, which are affected by the geometric phase at energies far below the conical intersection. The QI observed between these two distinctive pathways in the three-atom system is a clear example of the quantum nature of chemical reactivity. Science , this issue p. 767 ; see also p. 706

The O + O₂ isotope exchange reactions play an important role in determining the oxygen isotopic composition of a number of trace gases in the atmosphere, and their temperature dependence and kinetic isotope effects (KIEs) provide important constraints on our understanding of the origin and mechanism of these and other unusual oxygen KIEs important in the atmosphere. This work reports a quantum dynamics study of the title reactions on the newly constructed Dawes-Lolur-Li-Jiang-Guo (DLLJG) potential energy surface (PES). The thermal reaction rate coefficients of both the ¹⁸O + ³²O₂ and ¹⁶O + ³⁶O₂ reactions obtained using the DLLJG PES exhibit a clear negative temperature dependence, in sharp contrast with the positive temperature dependence obtained using the earlier modified Siebert-Schinke-Bittererova (mSSB) PES. In addition, the calculated KIE shows an improved agreement with the experiment. These results strongly support the absence of the “reef” structure in the entrance/exit channels of the DLLJG PES, which is present in the mSSB PES. The quantum dynamics results on both PESs attribute the marked KIE to strong near-threshold reactive resonances, presumably stemming from the mass differences and/or zero point energy difference between the diatomic reactant and product. The accurate characterization of the reactivity for these near-thermoneutral reactions immediately above the reaction threshold is important for correct characterization of the thermal reaction rate coefficients.