Restricted Rotation Involving the Tetrahedral Carbon. X. Barriers to Rotation of Methyl Groups in 9-Methyltriptycene Derivatives.

Several 9-methyltriptycene derivatives have been synthesized and the PMR spectra examined at low temperatures. The compounds which have a methoxy or a methyl group at a peri position showed only broadening of methyl signals, whereas those with a chlorine at a peri position and another substituent at some other peri position showed a splitting of methyl signals. The compounds which have a halogen atom at a peri position are the intermediary cases in a sense that they show separation of the signal into two with 1 : 2 relative intensities but fail to show fine splittings. Activation parameters for the rotation were obtained by total line shape analysis. Substituent effects have been discussed.