Rotational Isomerism Involving an Acetylenic Carbon II : Effect of 1-Halogen Substituents on Rotamer Population and Rotational Barrier around C(sp)-C(sp<sup>3</sup>) Bonds in Bis(9-triptycyl)ethynes
l)ethynes were synthesized to examine the effect of halogen substituents on the rotational isomerism about C(sp)-C(sp3) bonds. The kinetic and thermodynamic data of the isomerization between the ap and sc rotamers were determined by the total lineshape analysis and the intensity of the 1H NMR signals, respectively. The rotationalbarriers are enhanced with the steric size of halogen atoms from 11.6