Triangle Triumph

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Science  30 Nov 2007:
Vol. 318, Issue 5855, pp. 1351-1353
DOI: 10.1126/science.318.5855.1351d

How does an extra proton latch onto an ethylene? If one thinks about the H2C=CH2 molecule grasping H+, the logical structure would seem to be a triangle with the proton shared between the two carbons, because that's where the electrons doing the grasping are already mostly confined. On the other hand, the same cation should result from stripping H off ethane (H3C-CH3), and in that context it seems as though the extra H should stay on one carbon or the other, in a more classical bonding arrangement. Chemists have long puzzled over this question, and the highest-level quantum mechanical calculations favor the triangular H-sharing structure. However, direct experimental evidence has been elusive. Andrei et al. have now measured the vibrational spectrum of the C2H5+ cation in gas phase and confirmed the nonclassical geometry. The experiment relied on a highly sensitive technique in which an argon atom loosely bound to the cation is ejected (and the naked cation then mass-detected) by the energy dissipated when C-H bonds in the compound resonantly absorb infrared light. Spectral shifts of the primary bands relative to ethylene were relatively small (<50 cm−1). — JSY

Angew. Chem. Int. Ed. 46, 10.1002/anie.200704163 (2007).

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