Three conformers observed and characterized in 1-hexyne

The microwave spectrum of 1-hexyne (n-butyl acetylene) reveals the presence of three conformers that have been carefully characterized. 1-Hexyne can be viewed as propane with terminal ethynyl and methyl substituents. These substituents can be arranged in anti-anti (AA), anti-gauche (AG), gauche-anti (GA), gauche-gauche (GG cis), and gauche-gauche' (GG trans) conformations. The first three conformers are those observed experimentally and their rotational constants have been determined. The GG conformers, in both cis and trans arrangements have not been positively identified to date. We have determined the structural parameters and energies of all five possible conformations through ab initio quantum mechanical calculations at the HF, MP2, and B3LYP levels of theory. The calculated energies of these five conformers are very dose, and a change in the relative stability of these conformers is found at different levels of theory. We find that inclusion of electron correlation effects is critical to our understanding of the energy ordering of these conformers. (C) 2001 John Wiley & Sons, Inc.