Four conformers observed and characterized in 1-hexyne

The microwave spectrum of 1-hexyne (n-butyl acetylene) reveals the presence of four conformers that are characterized. 1-Hexyne is propane with terminal ethynyl and methyl substituents. These substituents can be arranged in anti–anti (AA), gauche–anti (GA, ethynyl gauche), anti–gauche (AG, methyl gauche), gauche–gauche (GG trans), and gauche–gauche’ (GG cis) conformations. The first four conformers are observed experimentally and their rotational constants determined. The experiments and ab initio calculations agree that the forms with anti or gauche C(ethynyl)–C–C–C conformations are nearly equally stable. The gauche C(methyl)–C–C–C configuration is higher in energy by several kJ/mol. The GG cis form is not observed and is predicted to be higher in energy. Inclusion of correlation effects in the quantum chemical calculations (MP2 vs. HF or B3LYP density functional methods) is essential for reasonable agreement between experimental results and computational models.