A curiously short carbon–carbon double bond?

Abstract

The discrepancy between the ‘curiously short’ carbon–carbon double bond distance in 3-ethynylcyclopropene, as measured experimentally by X-ray analysis [1.255(2) Å], compared with the value derived from high level ab initio calculations (ca. 1.28 Å), is largely due to neglect of corrections for molecular motion in the crystal.

References

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7. This discussion applies to rigid-body molecular motions. Complications arising from internal molecular motions are discussed by V. Schomaker and K. N. Trueblood, Acta Crystallogr., 1998, B54, 505 but do not affect us here.
8. H.-B. Bürgi, personal communication.
9. We thank Professor Roland Boese for kindly making these available to us.
10. A FORTRAN program by the late Professor K. N. Trueblood for analysis of ADPs; available from Dr W. B. Schweizer, Organic Chemistry Laboratory, ETH-Zentrum, 8092 Zurich, Switzerland (schweizer@org.chem.ethz.ch) or Professor E. M. Maverick, Chemistry Department, UCLA, Los Angeles CA 90024, USA (maverick@chem.ucla.edu).
11. The TLS analysis leads to positive corrections to all the carbon–carbon bond distances in the molecule. Indeed, on the basis of Fig. 1 of ref. 2, the uncorrected experimental bond lengths all tend to be shorter than the best theoretical estimates.