Isotope-Induced Desymmetrization Can Mimic Isotopic Perturbation of Equilibria. On the Symmetry of Bromonium Ions and Hydrogen Bonds

Abstract

The desymmetrizing effect of isotopic substitution on the geometry of otherwise symmetrical structures can be surprisingly large when there is anharmonic coupling between a desymmetrizing mode and modes whose zero-point energies are strongly isotope-dependent. This phenomenon is found to account for experimental observations that had been previously interpreted as implying asymmetry in bromonium ions and in hydrogen phthalate ions in aprotic solvents. The unlabeled structures are concluded to have the higher symmetry found in computed structures. Great care must be taken in applying isotopic perturbation as a test of asymmetry.

Solvent‐Dependent Structure of Iridium Dihydride Complexes: Different Geometries at Low and High Dielectricity of the Medium

“…An alternative explanation is that for 1,t he substitution of Hb yD may lead to slight changes in geometry (see below)a nd thus an intrinsic isotopee ffect on the chemical shift may be significant. [27] The remarkable effect of solvento nJ HD and T 1 (min) for complex 1 significantly exceeds those typically observedf or hydride complexes and deserves some comments. Thet rend is that the H-H distance increases on transferring from [D 14 ]methylcyclohexane to [D 2 ]dichloromethane.…”

“…Unfortunately, a lack of reliable information about δ x − δ y does not allow a more precise evaluation of the exchange coupling. An alternative explanation is that for 1 , the substitution of H by D may lead to slight changes in geometry (see below) and thus an intrinsic isotope effect on the chemical shift may be significant …”

Spectroscopic Characterization of a [C−F−C]+ Fluoronium Ion in Solution

“…For an isotopically labeled symmetric structure, the difference in the chemical shifts of the bridging carbon atoms should be on the order of 1 ppm or less. On the other hand, a labeled equilibrating structure ( 7 ⋅SbF 5 , Figure ) should display a much larger chemical shift difference (>10 ppm) …”

Catalytic, Stereoselective Dihalogenation of Alkenes: Challenges and Opportunities

“…[25c] An additional complication in this type of dihalogenation is the extent of halogen atom bridging in the haliranium ion intermediate,a nd whether or not such intermediates are better formulated as b-halo carbocations. [68,69] As ag eneralization, cation-stabilizing substituents (e.g., gem-dialkyl substitution, aryl groups) at one end of the halonium ion, high dielectric solvents,o rc hlorine as the halogen are all factors which conspire to reduce the degree of halogen bridging. [70] This has obvious implications for both reaction stereoselectivity and -specificity,w ith weakly bridged haliranium ions typically resulting in non-stereospecific dihalogenations to give anti/syn dihalide mixtures.…”