Long-range intramolecular attractions in alkynes

Date of Completion

January 2006


Chemistry, Physical




High-resolution, pulsed jet, Fourier transform microwave spectroscopy is used in separate studies of two means of intramolecular stabilization, namely, CH/π interaction and dispersive attraction. The smallest branched terminal alkyne capable of two simultaneous CH/π interactions, 4-methyl-1-pentyne, is measured in the first investigation, and three disubstituted acetylenes are the focus of the second phase: 3-hexyne, 3-heptyne, and 2-methyl-3-hexyne. ^ The conformer of 4-methyl-1-pentyne with C1 symmetry is shown to have closer approach of the hydrogen atom nearest to the triple bond than expected from nominal bond angles, adding support to the previously proposed CH/π interaction. A second conformer with two simultaneous CH/π interactions is definitively identified for the first time. Isotopomers with 13C in natural abundance are seen with this structure, demonstrating significant population at the 2 K rotational measurement temperature. Computations at the MP2/6-311++G(2d,2p) level of theory show modest stabilization by two CH/π interactions with one triple bond vs. a single interaction, though less than the roughly additive effect shown by others when two triple bonds are involved. ^ The molecules 3-hexyne and 3-heptyne may freely orient their end-groups around the triple bonds by internal rotation. Both have stable structures with the centers of charge density aligned, consistent with an attraction through dispersive force. Extensive computations through MP2/AUG-cc-pVQZ//MP2/AUG-cc-pVTZ show an extremely flat potential energy surface. Vibrational levels calculated for this potential suggest that only the observed structure of 3-hexyne ( C2v symmetry) is significantly populated at 2 K. The potential function also implies a stabilizing role when the bonds on carbon atoms C2 and C5 are eclipsed. The observed form of 3-heptyne, with a torsional angle of -15° relative to eclipsing of the bonds, demonstrates a dominant role of dispersive attraction in determining such structures. ^ Based on this result, possible orientations are anticipated for 2-methyl-3-hexyne. A comprehensive attempt has been made to observe one or more conformers, yielding sufficient lines to assign any structure obeying a semi-rigid rotor Hamiltonian. Such an assignment is not possible, apparently from tunneling perturbations. This is consistent with the even flatter potential energy surface predicted if the effects calculated for 3-hexyne are considered transferable. ^