The proton magnetic resonance spectrum of triptycene. The effect of magnetic anisotropy on the proton shifts

Abstract

The ring proton spectrum of triptycene (2.2 mole % in CS2) was analyzed after the methine proton was decoupled. The modified ring current model accounts for most of the internal shift. Bond anisotropies and electron density contributions are also present. The latter account for the mean proton shift. The agreement between the predicted and calculated shifts is excellent for the proton meta to the methine substituent, but a discrepancy of 0.1 p.p.m. is found for the ortho proton. Correction for the ring anisotropy effects produces excellent agreement of the methine proton shift with the empirical correlation between the C—H proton shifts and the 13C–H coupling constants. The ring proton–proton coupling constants are little different from those in benzene, but a small strain in the bonds near the methine carbons is implied.