Structure and relative energies of the conformers of n ‐butyl cyanide and 5‐hexynenitrile

The microwave spectrum of n ‐butyl cyanide and 5‐hexynenitrile (5‐cyano‐1‐pentyne) reveal the presence of three conformers that have been characterized. Each of these molecules can exist in anti–anti (AA), anti–gauche (AG), gauche–anti (GA), gauche–gauche (GG trans ), and gauche–gauche′ (GG′ cis ) conformations. The first three conformers have been observed experimentally and characterized in both compounds. In previous studies, we examined the microwave spectrum of the related n ‐butyl acetylene molecule and observed and characterized the mono‐ 13 C‐isotopomers of the AA and GA forms of n ‐butyl acetylene as well as the AG form, albeit at lower intensity. The GG conformers, in both cis and trans arrangements have not been identified to date for these molecules. The structures and energies of the five possible conformers of each molecule have been determined through quantum mechanical calculations at the HF and MP2 levels of theory, using both double‐zeta [6‐31G( d , p )] and triple‐zeta [6‐311+G( d , p )] basis sets. The calculated energies of the five possible conformers of each molecule are close, and a change in the relative stability order of these conformers is found at different levels of theory. For n ‐butyl cyanide and n ‐butyl acetylene, the GA form lies lowest in energy, both experimentally and at the MP2 level of theory. For 5‐hexynenitrile, the GG trans form is predicted to lie lowest in energy. The predicted conformer stability order is in agreement with a model of a net attractive electrostatic interaction between the π‐electron densities and the closest hydrogen on the next‐nearest‐neighbor carbon. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002