3-HEXYNE, $(CH_{3}CH_{2}-CC-CH_{2}CH_{3}), (CH_{3}CH_{2}-CC-CH_{2}CH_{3})$, HAS $C_{2v}$ SYMMETRY
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Date
2004
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Publisher
Ohio State University
Abstract
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 $(CH_{3}-CC-CH_{3})$, the torsional barrier drops to only 0.016 kcal/mol and geometry is that butane $(CH_{3}CH_{2}CH_{2}CH_{3})$ 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 $7.31 uA^{2}$ consistent with a planar heavy atom structure, no tunneling splittings are observed indicating a single torsional minimum (therefore, not $C_{2}$), and it has a microwave spectrum and therefore a permanent dipole moment (not $C_{2h}$). The result suggest that the symmetry argument relevant to the stable conformation of ethane does not carry over to this elongated analogue of ethane.
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Author Institution: Dept. of Chemistry, University of Connecticut