Quantum indistinguishability in chemical reactions.

Quantum indistinguishability plays a crucial role in many low-energy physical phenomena, from quantum fluids to molecular spectroscopy. It is, however, typically ignored in most high-temperature processes, particularly for ionic coordinates, implicitly assumed to be distinguishable, incoherent, and thus well approximated classically. We explore enzymatic chemical reactions involving small symmetric molecules and argue that in many situations a full quantum treatment of collective nuclear degrees of freedom is essential. Supported by several physical arguments, we conjecture a "quantum dynamical selection" (QDS) rule for small symmetric molecules that precludes chemical processes that involve direct transitions from orbitally nonsymmetric molecular states. As we propose and discuss, the implications of the QDS rule include () a differential chemical reactivity of para- and orthohydrogen, () a mechanism for inducing intermolecular quantum entanglement of nuclear spins, () a mass-independent isotope fractionation mechanism, () an explanation of the enhanced chemical activity of "reactive oxygen species", () illuminating the importance of ortho-water molecules in modulating the quantum dynamics of liquid water, and () providing the critical quantum-to-biochemical linkage in the nuclear spin model of the (putative) quantum brain, among others.