Frankfurt 2006 – scientific programme

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MO: Molekülphysik

MO 15: Molecular Clusters II

MO 15.6: Talk

Monday, March 13, 2006, 17:45–18:00, H10

IR spectrum of the ethyl cation: nonclassical versus classical structure — •Horia-Sorin Andrei, Hannes Kuchelmeister, Nicola Solca, and Otto Dopfer — Institut für Physikalische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg

The competition between classical and nonclassical geometries is a fundamental issue for hydrocarbon molecules. The ethyl cation (C2H5+, protonated ethen) represents the most simple example for protonation of a C=C double bond. Calculations predict the nonclassical structure to be slightly more stable than the classical structure. However, spectroscopic evidence for this conclusion is still lacking. The present work reports IR photodissociation spectra [1] of size selected clusters of the ethyl cation with Ar and N2. The IR spectra recorded in the CH stretching range provide detailed information on the degree of delocalization of the excess proton between the two C atoms. Significantly, the structure of the additional proton depends sensitively on the microsolvation environment. Whereas Ar is a negligible perturber, leaving C2H5+ in the most stable nonclassical geometry, the stronger interaction with N2 induces a switch from a nonclassical to a classical C2H5+ ion core. The latter ligand appears to form a weak covalent bond to one C atom of the ethyl cation. The interpretation of the IR spectra is supported by rotational line profile simulations and ab initio calculations.

[1] O. Dopfer, Int. Rev. Phys. Chem. 22, 437 (2003).