The chemical shift of a fluorine-19 nucleus moves to higher field when deuterium is substituted for hydrogen either geminally (H-C-F -* D-C-F) or vicinally (H-C-C-F -* D-C-C-F). We have found that the vicinal chemical-shift isotope effect has a strong stereochemical dependence. When the H(D)-C-C-F dihedral angle is less than 90°, the isotope shift is about 0.15 ppm. When the nuclei are furthest apart, at 120 and 180°, the isotope shift increases to about 0.35 ppm. These results cannot be explained exclusively in terms of an electric-field model, which predicts an inverse dependence of the magnitude of the effect on the distance between the resonating and perturbing nuclei. Because the stereochemical dependence is similar to that observed for substituent inductive effects on the absolute magnitude of vicinal coupling constants and chemical shifts, we conclude that a major portion of the isotope shift results from vibrationally based inductive effects. Vicinal ( ) delocalization provides one mechanism for such a phenomenon.

Eine z.B. p‐Substitution durch Halogen bzw. eine Methyl‐, Methoxy‐ oder Methylthiogruppe führt zu einer Abhängigkeit von 1H‐ und 13C‐NMR‐Daten sowie der pKa‐Werte in.

Substitution in the aryl ring of 2-and 3-arylindenes has been used to examine the acidity of these hydrocarbons and the charge density and ion pair structure of their anions. Acidities were measured for the 2-arylindenes both in pure Me^O and in a Me2S0-H20 mixture. After allowance for the different acidities of the indicators used in the two media, the acidities of the indenes are higher in pure Me2SO (the pK.'s are lower by 1-2 units). The dispersion forces of Me2SO are more effective in stabilizing the delocalized charge of the anion than are the more localized interactions of the hydroxyl groups in the mixed solvent. The pK,'s in both solvents exhibit a linear response to the Hammett constant. Substituents that are electron withdrawing through resonance are better correlated by the " constant, suggestive of a strong resonance interaction between the aryl group and the indene ring. Such an interaction is substantiated by the chemical shift changes on deprotonation. The resonance of H4< (para position) in the unsubstituted 2-arylindenyl anion and the 13C resonance of C4. in all the 2-arylindenyl anions are shifted upheld in comparison to the neutral hydrocarbons, as a result of negative charge delocalization into the aryl ring. The sigmoidal shape of the plot of 13C chemical shifts of certain atoms in the 2-arylindenyl anions vs. gives evidence for a structural change within the anion. Only the carbons with very high charge density exhibit this behavior (Ci,3). Observations in the indene series generally parallel those in the l-aryl-4-phenylcyclopentadienes. The annelated phenyl ring in the indenes therefore causes effects similar to those of the simply bound phenyl ring in the cyclopentadienes.

that described58 for 3e. The crude solid was chromatographed over 280 g of 80-200 mesh alumina (Matheson Coleman and Bell) using petroleum ether to 5% benzene in petroleum ether as eluent.

No abstract