Chemical reactivity is conventionally understood in broad terms of kinetic versus thermodynamic control, wherein the decisive factor is the lowest activation barrier among the various reaction paths or the lowest free energy of the final products, respectively. We demonstrate that quantum-mechanical tunneling can supersede traditional kinetic control and direct a reaction exclusively to a product whose reaction path has a higher barrier. Specifically, we prepared methylhydroxycarbene (H(3)C-C-OH) via vacuum pyrolysis of pyruvic acid at about 1200 kelvin (K), followed by argon matrix trapping at 11 K. The previously elusive carbene, characterized by ultraviolet and infrared spectroscopy as well as exacting quantum-mechanical computations, undergoes a facile [1,2]hydrogen shift to acetaldehyde via tunneling under a barrier of 28.0 kilocalories per mole (kcal mol(-1)), with a half-life of around 1 hour. The analogous isomerization to vinyl alcohol has a substantially lower barrier of 22.6 kcal mol(-1) but is precluded at low temperature by the greater width of the potential energy profile for tunneling.

We present evidence that the privileged use of the 3,5‐bis(trifluoromethyl)phenyl group in thiourea organocatalysis is due to the involvement of the ortho‐CH bond in the binding event with Lewis‐basic sites. We utilized a combination of low‐temperature IR spectroscopy, 2D NMR spectroscopy, nano‐MS (ESI) investigations, as well as density functional theory computations [M06/6‐31+G(d,p), including solvent corrections as well as natural bond orbital and atoms‐in‐molecules analyses] to support our conclusions that bear implications for catalyst design.

Even though quantum mechanical tunnelling has been appearing recurrently mostly in theoretical studies that emphasize its decisive role for many chemical reactions, it still appears suspicious to most organic chemists. Recent experiments in combination with powerful computational approaches, however, have demonstrated that tunnelling must be included to fully understand chemical reactivity. Here we provide an overview of the importance of tunnelling in organic chemical reactions.

A novel supramolecular system composed of diketopyrrolopyrrole electron donors and perylene derived bisimide (PDI) electron acceptors forms superstructures that undergo fast photoinduced charge separation following assembly. This bioinspired route toward functional hierarchical structures, whereby assembly and electronic properties are closely coupled, could lead to new materials for artificial photosynthesis and organic electronics.

Phenylhydroxycarbene (Ph-C-OH, 1), the parent of all arylhydroxycarbenes, was generated by high-vacuum flash pyrolysis of phenylglyoxylic acid at 600 degrees C and spectroscopically characterized (IR, UV-vis) via immediate matrix isolation in solid Ar at 11 K. The identity of 1 was unequivocally confirmed by the precise agreement between the observed IR bands and (unscaled) anharmonic vibrational frequencies computed from a CCSD(T)/cc-pVDZ quartic force field. The UV-vis spectrum of 1 displays a broad band with maximum absorption at 500 +/- 25 nm (2.5 +/- 0.1 eV) that extends to approximately 640 nm (1.9 eV), in full accord with combined CCSD(T)/cc-pVQZ and EOM-CCSD/cc-pVTZ computations that yield a gas-phase vertical (adiabatic) excitation energy of 2.7 (1.9) eV. Unlike singlet phenylchlorocarbene, 1 does not undergo photochemical ring expansion. Instead, 1 exhibits quantum-mechanical hydrogen tunneling to benzaldehyde underneath a formidable barrier of 28.8 kcal mol(-1), even at cryogenic temperatures. The remarkable hydrogen tunneling mechanism is supported by the temperature insensitivity of the observed half-life (2.5 h) and substantiated by a comparable theoretical half-life (3.3 h) determined from high-level barrier penetration integrals computed along the intrinsic reaction path. As expected, deuteration turns off the tunneling mechanism, so d-1 is stable under otherwise identical conditions.