The structure of the simple, fundamental ethyl cation has finally been ascertained. Computational studies had long suggested the non-classical structure 1 for this cation. The classical structure 2 is a transition state for scrambling the protons. The MP2/6-311G(2d,p) geometries of both structures are shown in Figure 1.





Figure 1. MP2/6-311G(2d,f) structures of 1, 2, 1.Ar(C2v) and 1.Ar(Cs).

Dopfer1 has now obtained IR spectrum of ethyl cation by single-photon IR photodissociation spectroscopy through the reaction

C2H5+ . Ar + hν → C2H5+ + Ar

Two structures of the ethyl cation associated with Ar were optimized at MP2/6-311G(2df,2pd). (The MP2/6-311G(2d,p) structures are shown in Figure 1.) Both of their computed IR spectra have stretches at nearly identical wavenumbers as for ethyl cation 1 itself. The experimental IR spectra has absorptions at 3317 and 3037 cm-1, very close to the computed frequencies for 1.Ar(C2v). This provides strong experimental evidence that ethyl cation is in fact a non-classical ion.


(1) Andrei, H.-S.; Solcà, N.; Dopfer, O., "IR Spectrum of the Ethyl Cation: Evidence for the Nonclassical Structure," Angew. Chem. Int. Ed. 2008, 47, 395-397, DOI: 10.1002/anie.200704163