BINDING-ENERGY AND STRUCTURE OF VAN-DER-WAALS COMPLEXES OF BENZENE

In this work we demonstrated that newly developed spectroscopic techniques yield precise information on the structure and energetics of small van der Waals molecules. For the prototype systems of benzene with noble gases we found precise van der Waals distances from high-resolution UV spectroscopy. Rigid structures with sharp rotational lines have been detected not only for benzene‒noble gas dimers but also for trimers with two Ar atoms attached to the benzene ring. Rotationally resolved spectroscopy of van der Waals vibronic bands is helpful for their assignment. The resulting van der Waals vibrational energy and the vibrationally averaged rotational constants contain information on the intermolecular potential essential for modeling of physical and chemical properties of these prototypes complexes. Energetic data are obtained in a direct way from observation of the dissociation of dimer ions produced by two-photon ionization in a time-of-flight mass spectrometer. Delayed pulse field ionization of the excited high Rydberg states of the dimers allows one to produce vibrational state-selected dimer ions and to monitor their decay as a function of the excited vibrational state. In this way information on the binding energy of neutral and ionic dimers is achieved. In van der Waals molecules of larger components, more than three intermolecular vibrations exist. At present the application of the high-resolution and the pulsed field ionization technique does not lead to reasonable results probably because of the lowfrequency van der Waals modes congesting the S1 ← S0 spectrum as well as the ion spectrum. Here the breakdown technique observing metastable daughter ion efficiency curves is applicable, yielding energetic information on the dissociation energy of neutral and ionic complexes. The strong increase of the binding energy in ionic dimers consisting of molecules with delocalized π electrons and similar ionization energy point to a strong contribution from charge-transfer resonance interaction to the intermolecular bonding. It is the purpose of future work to apply the experimental methods described in this work to van der Waals molecules consisting of even larger components. Precise structural and energetic information on van der Waals molecules with four or more weakly bound noble gases attached to an aromatic molecule components might be accessible if lower vibrational temperatures can be achieved in the molecular beams. This would lead to a rigidity of the larger complexes allowing e.g. the application of highresolution techniques. © 1994, American Chemical Society. All rights reserved.