Chia-Hua Wu

Methylhydroxycarbene: tunneling control of a chemical reaction.

Quantum tunneling induces the opposite outcome expected from traditional kinetic factors in a chemical rearrangement. 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 (H3C–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.

Do π-Conjugative Effects Facilitate SN2 Reactions?

Rigorous quantum chemical investigations of the SN2 identity exchange reactions of methyl, ethyl, propyl, allyl, benzyl, propargyl, and acetonitrile halides (X = F(-), Cl(-)) refute the traditional view that the acceleration of SN2 reactions for substrates with a multiple bond at Cβ (carbon adjacent to the reacting Cα center) is primarily due to π-conjugation in the SN2 transition state (TS). Instead, substrate-nucleophile electrostatic interactions dictate SN2 reaction rate trends. Regardless of the presence or absence of a Cβ multiple bond in the SN2 reactant in a series of analogues, attractive Cβ(δ(+))···X(δ(-)) interactions in the SN2 TS lower net activation barriers (E(b)) and enhance reaction rates, whereas repulsive Cβ(δ(-))···X(δ(-)) interactions increase E(b) barriers and retard SN2 rates. Block-localized wave function (BLW) computations confirm that π-conjugation lowers the net activation barriers of SN2 allyl (1t, coplanar), benzyl, propargyl, and acetonitrile halide identity exchange reactions, but does so to nearly the same extent. Therefore, such orbital interactions cannot account for the large range of E(b) values in these systems.

The Li···HF van der Waals minimum and the barrier to the deep HF–Li potential well

Molecular beam experiments (lithium atom plus hydrogen fluoride) by both Becker and co-workers (C.H. Becker, P. Casavecchia, P.W. Tiedemann, J.J.Valentini, and Y.T. Lee, J. Chem. Phys. 73, 2833 (1980)) and Loesch and Stienkemeier (H.J. Loesch and F. Stienkemeier, J. Chem. Phys. 98, 9570 (1993)) deduced a van der Waals complex of type Li···HF. In this research, molecular electronic structure theory [aug-cc-pCVQZ CCSD(T)] has been used to predict a well depth of 0.86 kcal mol−1 relative to separated Li + HF. However, the barrier from this vdW well to the more strongly bound (∼6.2 kcal mol−1) HFLi complex lies 0.43 kcal mol−1 below separated Li + HF.

Inhibiting Reductive Elimination as an Intramolecular Disulfide Dramatically Enhances the Thermal Stability of SAMs on Gold Derived from Bidentate Adsorbents

The bidentate aromatic adsorbate, 5-(octadecyloxy)-1,3-benzenedimethanethiol (R1ArmDT), with a specific design of extended S-S distance and a geometric constraint to resist cyclic disulfide formation was synthesized. The film formation and thermal stability of self-assembled monolayers (SAMs) derived from R1ArmDT were investigated and compared to those of SAMs derived from an analogous bidentate dithiol 2-(4-(octadecyloxy)-phenyl)propane-1,3-dithiol (R1ArDT), in which the two sulfur atoms can readily form a cyclic disulfide upon reductive elimination from the surface. Although the SAMs derived from R1ArmDT were less densely packed than those derived from R1ArDT, as judged by the data obtained by X-ray photoelectron spectroscopy and polarization modulation infrared reflection absorption spectroscopy, the SAMs derived from R1ArmDT were markedly more thermally stable than those derived from R1ArDT. The greater thermal stability of the R1ArmDT SAMs can be rationalized on the basis of the structure of the bidentate R1ArmDT headgroup, in which the two pendant sulfur atoms cannot access each other intramolecularly to form a cyclic disulfide upon reductive elimination from the surface.