Conformational studies of 3-hexyne

摘要

The microwave spectrum of 3-hexyne has recently been observed and can be assigned unambiguously to a syn-eclipsed C-2 nu conformation. Owing to its nonpolar nature, a trans-conformer (C-2h) would not be observable in microwave studies. A gauche conformer (C-2), which would exhibit tunneling splittings because there are two equivalent structures corresponding to a torsional angle of +/-tau, was also not experimentally observed. In the present study, we determine whether computational studies can be carried out with sufficient accuracy to support these experimental observations. The torsional barrier to rotation in this molecule is expected to be of the order of 10-20 cal/mol, similar to that observed in 2-butyne. This very low barrier offers a significant computational challenge. In an attempt to find a rigorous explanation for the observed stability of the C-2 nu conformer, ab initio calculations were carried out at the Hartree-Fock (HF) and second-order Moller-Plesset (MP2) levels of theory with several basis sets, ranging from 6-311+G(d,p) to aug-cc-pVQZ. We find that the calculated potential energy surface for 3-hexyne is extremely flat for variation of the torsional angle. The relative stability of the conformers of 3-hexyne is quite dependent on the size of the basis set and tight convergence criteria must be used in the search for stable minima. Stable structures of nearly identical energy were found for the syneclipsed C-2 nu conformation and a gauche C-2 structure. The trans-C-2h, structure is found to be a transition state for the basis sets that were used in this study. The C,, structure has the lowest calculated energy using the aug-cc-pVQZ basis, in agreement with experiment. (c) 2006 Wiley Periodicals, Inc.