During my stay in the Jäger group I have studied several weakly bound complexes of simple hydrocarbons, such as methane, ethylene, and acetylene. The molecular level investigation of these relatively simple systems can be regarded as a starting point of a systematic study into larger clusters, and eventually the solvation processes involving organic species. By gradually increasing the size of the cluster, fundamental aspects of solvation processes, such as the size dependence of the structural and dynamical properties of clusters, may be studied as they change from the behavior of small molecular associations to that of bulk phases.

Large amplitude internal motions occur in all the seven systems studied, namely Ar-ethylene, Ne-ethylene, Ar-acetylene, Ne-acetylene, Kr-methane, Ar-methane, and CO-methane, as a consequence of the weak binding energies of these systems. Rotational spectra are complicated by the large amplitude internal motions. Transition doubling was observed in the spectra of Ar-ethylene and Ne-ethylene, which is attributed to a tunneling motion of the ethylene subunit. The presence of a strong coupling between large amplitude radial and angular internal motions was inferred from the spectra of rare gas-acetylene complexes. Rotational transitions within various internal rotor states were measured for the Kr-methane and CO-methane dimers. This is evidence for a nearly free internal rotation of methane within these weakly bound complexes. The recorded spectrum of CO-methane closely resembles that of Kr-methane, indicating that CO also undergoes nearly free internal rotation within the dimer.

In addition to the spectroscopic method, several systems were also studied with ab initio methods. The obtained theoretical results provided further information about the internal dynamics of these systems. It is evident from the ab initio minimum energy paths that in Ne-ethylene, the internal rotation of the ethylene subunit about the C=C bond is preferred mainly because the radial movement involved with this motion is not as pronounced as in the internal rotation about the c-principal axis of the ethylene monomer. The strong radial-angular coupling is also evident from the ab initio potential energy surface of Ne-acetylene. Ab initio energy calculations of Kr-methane and Ar-methane predict that the angular dynamics does not change significantly when the binding partner of methane changes from Kr to Ar. The dipole moment calculation of these two dimers provided qualitative interpretation for several spectral observations.

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