Hans-Ullrich Siehl

Isotope Effects on nmr Spectra of Equilibrating Systems

Publisher Summary This chapter describes a method and its applications to chemical problems, which are based on a combination of equilibrium isotope effects and nmr spectroscopy. The vast majority of applications deal with deuterium isotope effects, only a few 13C-equilibrium isotope effects are discussed to demonstrate the scope of the method. The chapter introduces the nmr technique for measuring isotope effects on degenerate equilibria that is most often used,the technique based on chemical shift differences. The applications are not organized by classes of compounds throughout but for the line of argument and according to general dynamic and structural features. The selection was made primarily to illustrate the different aspects of the subject. An attempt has been made to give consistent interpretations within the framework of existing isotope effects without harming the Bigeleisen formalism and the Born–Oppenheimer approximation.

Stable vinyl cations 4: Carbon-13 nmr spectroscopic observation of an α-ferrocenyl-vinyl cation

Abstract 13 C-chemical shift data of α ferrocenyl-B-t-butyl-vinyl cation are compared with model compounds to give proof of hybridization and structure.

NMR spectroscopic and computational investigations of σ-interactions in carbocations: The β-silyl effect in vinyl cations

@The para carbon chemical shift in 1-mesityl substituted vinyl cations and the torsional barrier for the methoxy group rotation in l-p- (anisy1)vinyl cations are used to evaluate the magnitude of the silyl effect. The hyperconjugative stabilization by psilicon groups and the accompanying geometrical distortions are studied experimentally and computationally in dienyl-cations and 1-cyclopropylvinyl cations. A geminal methyl substituent enhances the interaction of a p-silyl group with the vacant p-orbital. Vinyl cations 1 are disubstituted unsaturated analogs of trisubstituted carbocations 2. The carbon Ca with the formal positive charge is sp hybridized and the Vacant p-orbital is orthogonal to a C=Cpdouble bond.

RECENT ADVANCES IN THE EXPERIMENTAL AND COMPUTATIONAL CHARACTERIZATION OF CARBOCATIONS : SILYL EFFECTS IN BICYCLOBUTONIUM IONS

Two trialkylsilyl substituted bicyclobutonium ion were investigated by NMR spectroscopy in superacid solution and by quantum chemical ab initio calculations. The 1- (trimethylsily1)bicyclobutonium ion undergoes a threefold degenerate methylene rearrangement. The 3-endo-(tert-butyldimethylsilyl)bicyclobutonium ion is the first static bicyclobutonium ion. The NMR-spectra of this carbocation are a direct proof for the hypercoordinated and puckered structure of bicyclobutonium ions.

Conformational dependence of the β-deuterium equilibrium isotope effect in acyclic carbocations. Suppression of hyperconjugation by restricted rotation in the 2,4,4,6-tetramethylheptyl cation

Steric inhibition of hyperconjunction leads to suppression of the equilibrium isotope effect which in turn is used to assign conformations of acyclic carbocations in solution.

Deuterium isotope effects in a triply degenerate cyclopropylethyl cation

The equilibrium isotope effect on the degenerate rearrangement of the 1-(2,3-dimethylclopropyl)ethyl cation is compared with theoretical and other experimental data and the implications for the interpretation of kinetic isotope effects and for the investigation of hyperco-ordinated carbocations are discussed.