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

Prealigning nonpolar reacting molecules leads to large stereodynamical effects because of their weak steering interaction en route to the reaction barrier. However, experimental limitations in preparing aligned molecules efficiently have hindered the investigation of steric effects in bimolecular reactions involving hydrogen. Here, we report a high-resolution crossed-beam study of the reaction H + HD( v = 1, j = 2) → H ₂ ( v ′, j ′) + D at collision energies of 0.50, 1.20, and 2.07 electron volts in which the vibrationally excited hydrogen deuteride (HD) molecules were prepared in two collision configurations, with their bond preferentially aligned parallel and perpendicular to the relative velocity of collision partners. Notable stereodynamical effects in differential cross sections were observed. Quantum dynamics calculations revealed that strong constructive interference in the perpendicular configuration plays an important role in the stereodynamical effects observed.

The transition state is a key concept in the field of chemistry and is important in the study of chemical kinetics and reaction dynamics.