Pronounced conformational preferences in 1,4-disubstituted but-2-ynes

Internal rotation in 1,4-disubstituted but-2-ynes XCH₂CCCH₂Y (X, Y = H, CH₃, Li, F) has been studied with the aid of ab initio molecular orbital theory. When either or both of X and Y are approximately electroneutral (H or CH₃), the barrier to internal rotation is very small. On the other hand, when X and Y are both electropositive (LiCH₂CCCH₂Li) or both electronegative (FCH₂CCCH₂F), there are pronounced preferences for conformations in which the dihedral angle XC·CY is slightly greater than 90°. When X is electropositive and Y electronegative (LiCH₂CCCH₂F), the stable rotational isomers are cis and trans with the former being preferred. For all molecules examined here the threefold component of the torsional potential function is very small.