Effects of aryl and arylethynyl substituents at the 1-position on rotational barrier around C(sp)–C(sp3) bonds and bending deformation of acetylenic carbons in bis(9-triptycyl)ethynes

The barriers to rotation around C(sp)–C(sp3) single bonds in (1,4-dimethyl-9-triptycyl)(1-X-9-triptycyl)ethynes [X=phenyl, mesityl, phenylethynyl, or (1-naphthyl)ethynyl] were determined by the dynamic NMR method to examine the effect of nonspherically shaped aromatic substituents on the rotational barriers. The rotational barriers increase in the order of the pheny, arylethynyl and mesityl substituted compounds from 15.8 to 18.8 kcal/mol. This result suggests that not only the bulkiness but also the shape and flexibility of the substituents are important factors in determining the rotational barrier. Significant bending deformations of the acetylenic carbons (∼166°) were found in the X-ray structures and the optimized structures by the MM2 calculations.