Duncan and Schleyer¹ have investigated protonated acetylene and the protonated acetylene dimer. These ions are created in a pulsed supersonic nozzle/pulsed electrical discharge with a weakly bound argon atom as a tag. IR laser photodissociation spectroscopy allows for the detection of peaks down to 2000 cm^-1, a region not previously explored for this cation. The experimental IR spectrum for H⁺(C₂H₂)^.Ar has two main features: at 3146 and 2217 cm^-1. The 3146 cm^-1 corresponds to the previously observed peak² at 3142 cm^-1 and is similar to the absorption in acetylene (3136 cm^-1). MP2/6-311+G(2d,2p) computations were performed on the classical and non-classical structures of H⁺(C₂H₂), with and without a complexed argon atom. These geometries are displayed in Figure 1 and the predicted vibrational frequencies are listed in Table 1.

Figure 1. MP2/6-311+

Table 1. Relative energies (kcal mol^-1) and frequencies of protonated acetylene
and the protonated acetylene-argon cluster.¹

The argon tag only slightly perturbs the spectrum, as expected for a weakly bond atom remote from most of the hydrogen atoms. The predicted spectra of the two non-classical ions are in nice agreement with the experiment – particularly the interesting peak at 2123 cm^-1 that is due to the bridged proton. This spectra, and the confirmation of the bridging, non-classical structure, makes a nice pair with the recently reported bridging, non-classical structure of the ethyl cation,³ which I blogged on previously.

The spectrum of the H⁺(C₄H₄) ion show a doublet at 3129 and 3158 cm^-1 and two small peaks at 1261 and 1365 cm^-1. The computed structure that comes closest to matching this spectrum is for the asymmetrically bridged dimer (See Figure 2), though is much more energetic than its isomers. The authors speculate that the bridged dimer is trapped in an energy-well during the thermal expansion, which prevents the formation of the lower energy isomers.

Figure 2. Schematic drawing and relative energies of the H⁺(C₄H₄) ion.
(Note – unfortunately the authors have supplied insufficient information in the Supporting Materials to completely define the geometries of these molecules!)

References

(1) Douberly, G. E.; Ricks, A. M.; Ticknor, B. W.; McKee, W.
C.; Schleyer, P. v. R.; Duncan, M. A., "Infrared Photodissociation
Spectroscopy of Protonated Acetylene and Its Clusters," J. Phys. Chem. A, 2008, 112, 1897-1906, DOI: 10.1021/jp710808e.

(2) Gabrys, C. M.; Uy, D.; Jagod, M. F.; Oka, T.; Amano, T., "Infrared Spectroscopy of Carboions. 8. Hollow Cathode Spectroscopy of Protonated Acetylene, C2H3+," J. Phys. Chem., 1995, 99, 15611-15623, DOI: 10.1021/j100042a042.

(3) 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

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.

1	2
1.Ar(C2v)	1.Ar(Cs)

Figure 1. MP2/6-311G(2d,f) structures of 1, 2, 1^.Ar(C_2v) and 1^.Ar(C_s).

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(C_2v). This provides strong experimental evidence that ethyl cation is in fact a non-classical ion.

References

(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