15min:

GEOFFREY CHURCHILL, ALBEIRO RESTREPO, KARISSA UTZAT AND ROBERT K. BOHN, Dept. of Chemistry, University of Connecticut, Storrs, CT 06269-3060.

Ethane has a staggered equilibrium geometry with an internal rotation barrier of about 3 kcal/mol. Most chemists accept that that geometry is the result of steric repulsion among the H atoms but Lionel Goodman and others have made a strong case that the dominant effect is hyperconjugative stabilization of the anti orientation of vicinal C-H bonds, i.e., a symmetry controlled effect. When the two methyl groups are separated by a CC triple bond (CH₃-CC-CH₃), the torsional barrier drops to only 0.016 kcal/mol and the stable configuration is unknown. A corollary of ethane's stable geometry is that butane (CH₃CH₂CH₂CH₃) has stable gauche and anti conformations consistent with ethane's staggered geometry. We have observed and assigned the b-type microwave rotational spectrum of 3-hexyne, butane with a CC triple bond inserted between the two ethyl groups. The rotational constants are A = 9410.842(7) MHz, B = 1407.795(2) MHz, and C = 1269.727(2) MHz and several centrifugal distortion constants have been determined. The structure has C_2v symmetry for several reasons. The c second moment is 7.31 uA² consistent with a planar heavy atom structure, no tunneling splittings are observed indicating a single torsional minimum (therefore, not C₂), and it has a microwave spectrum and therefore a permanent dipole moment (not C_2h). The result suggests that the symmetry argument relevant to the stable conformation of ethane does not carry over to this elongated analogue of ethane.