Nuclear quantum effects, tunneling and reaction dynamics

6 problems

• Low-temperature rate enhancement of the OH plus methanol reaction

  Partially resolved

  The reaction of hydroxyl radicals with methanol runs nearly a hundred times faster at 63 K than at 200 K, the opposite of the usual slowing on cooling. Quantum tunneling of a hydrogen atom has been proposed, but whether the speed-up survives at the very low gas densities of interstellar clouds is disputed.

• Rules for vibrational mode selectivity in polyatomic reactions

  Partially resolved

  For three-atom reactions, empirical rules (the Polanyi rules) say whether energy put into vibration or into collision speed better promotes reaction. For larger molecules such as methane these rules fail in some measured cases, and no general replacement is established.

• A quantum transition-state theory valid in the deep-tunneling regime

  Partially resolved

  Transition-state theory estimates a reaction rate from the flow across the top of the energy barrier without following full trajectories. Its quantum version, built on ring polymers (chains of beads that represent a quantum particle), is unique and exact in some limits but can fail when tunneling dominates.

• Accurate quantum time-correlation functions for anharmonic liquids

  Open

  Infrared spectra and diffusion constants of a liquid come from how it changes in time, and for light nuclei that motion is quantum. Exact quantum simulation of it is exponentially costly, and the approximations in use produce known artifacts such as shifted or split spectral peaks.

• Predictive theory of roaming reaction branching fractions

  Open

  In a roaming reaction a molecule starts to break apart, but the loose fragment moves around the rest of the molecule and then removes an atom from it instead of leaving. Simulations reproduce known cases but cannot yet predict when roaming will be important.

• Quantitative H2O versus D2O differences from first-principles quantum simulation

  Open

  Heavy water, in which hydrogen is replaced by deuterium, has its density maximum near 11 C instead of 4 C and is about 23 percent more viscous. These differences come only from the quantum motion of nuclei, so they test whether simulations treat that motion correctly.