Although butane exists in staggered anti and gauche conformations, when the ethyl groups are separated by a C[triple bond]C triple bond (3-hexyne), the stable conformation changes to eclipsed, (1)C(2v). Using rotational microwave spectroscopy, we have studied another example, 3-heptyne, the C[triple bond]C elongated analogue of pentane. The most stable conformer of pentane has anti-anti (AA) conformations about the central C-C bonds (C(2v)) and the next most stable has a gauche dihedral angle (GA, C1). This microwave study determines that the extended analogue of the AA form is not staggered about the C[triple bond]C axis but eclipsed (Cs). Also, the elongated analogue of the GA conformer is also not staggered but nearly eclipsed. The conformations of low-polarity substituted acetylenes is determined by dispersion attractions between the end groups. A microwave study of the AA and GA conformers of pentane is also reported.
ABSTRACT:The microwave spectrum of 3-hexyne has recently been observed and can be assigned unambiguously to a syn-eclipsed C 2v 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 Ϯ, 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 2v conformer, ab initio calculations were carried out at the Hartree-Fock (HF) and second-order Møller-Plesset (MP2) levels of theory with several basis sets, ranging from 6-311ϩG(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 2v 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 2v structure has the lowest calculated energy using the aug-cc-pVQZ basis, in agreement with experiment.
The microwave spectrum of isobutylbenzene (2-methyl-1-phenylpropane) reveals the presence of two conformers that are characterized by their microwave spectra and by quantum chemical calculations. The more stable conformer has a gauche configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a approximately 80 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane with C(1) symmetry. The less stable conformer has a plane of symmetry, C(s), with an anti configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a 90 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane. The rotational constant values are 3070.9273(4) MHz, 736.01980(6) MHz, and 680.92889(6) MHz for the C(1) species and 2500.780(8) MHz, 885.72743(10) MHz, and 770.42036(10) MHz for the C(s) species. Quantum chemical calculations are in agreement with these structures and predict a relative energy between those two conformers of 0.4 kcal/mol.
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