Showing papers by "K. N. Houk published in 2001"

The magnitude of [C-H...O] hydrogen bonding in molecular and supramolecular assemblies.

Abstract: Ab initio calculations at the MP2/6-311++G** level on model systems (N-methylpyridinium complexes of dimethyl ether and dimethyl phosphate anion) provide quantitative measures of the large stabilization energies that arise from [C-H...O] contacts in charged systems. These attractive interactions control (i) the self-assembly of bipyridinium-based catenanes and rotaxanes in solution, (ii) the self-organization of left-handed Z-DNA with alternating [dC-dG] sequences in the solid state, and (iii) the binding of pyridinium derivatives with single- and double-stranded DNA. Slightly attractive interactions occur between the donor ether and phosphate moieties and a neutral pyridine molecule in the gas phase. Electrostatic potential and solvation calculations demonstrate that [C-H...O] interactions which involve a cationic [C-H] donor are dominated by electrostatic terms.

Cyano, Amino, and Trifluoromethyl Substituent Effects on the Claisen Rearrangement

Abstract: Ab initio quantum mechanical methods with the B3LYP/6-31G* method were used to model the reactants, transition states, and products of Claisen rearrangements of allyl vinyl ethers substituted at all positions with CN, NH2 or CF3 groups. The calculations predict that 1-CN, 1-CF3-cis, 5-NH2, 6-CN, and 6-CF3 substituents increase the activation barriers, in agreement with experimental results on substituent effects by CN, CF3, and the related 5-OCH3 example. All other substituents lower activation energies. A Marcus theory type analysis was applied to separate the intrinsic and thermodynamic contributions to the activation energies.

Quantitative Predictions of Substituent and Solvent Effects on the Regioselectivities of Nitrile Oxide Cycloadditions to Electron‐Deficient Alkynes.

Abstract: Hybrid density functional theory calculations with the B3LYP/6-31G ∗ method were used to calculate the activation barriers of nitrile oxide cycloadditions to the unsymmetrical alkynes cyanoacetylene and methyl propiolate. Inherent electronic effects and solvent polarity both influence regioselectivity.