Stereodynamical control of a four-atom reaction: HD + OH → D + H(2)O.

In the HD (v = 1, j = 2) + OH → D + H2O reaction, preparing the HD axis parallel versus perpendicular to the relative velocity yields pronounced stereodynamic changes, including backward intensity and angular distributions.

• Why it matters: It remains unclear how stereodynamics in reactive collisions changes when both reactants are molecules, beyond the atom–molecule cases previously studied.
• What they did: The authors combine high-resolution crossed-beam scattering with full-dimensional quantum dynamics, using stimulated Raman pumping to set the HD axis orientation and comparing parallel and perpendicular configurations.
• The result: Scattering wavefunctions show m = ±2 accesses a noncoplanar transition state distinct from m = 0, with a nonzero dihedral angle reducing reactivity and enhancing sideward scattering for the perpendicular setup while keeping product rovibrational distributions essentially unchanged.

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