Charge Resonance Interaction in Aromatic Clusters

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We propose to probe the charge resonance (CR) interaction in cold aromatic cluster ions isolated in the gas phase by means of mass spectrometry, infrared and electronic laser spectroscopy, and quantum chemical calculations. CR is based on charge delocalization and is a strong fundamental intermolecular force that complements other attractive interactions in charged clusters such as electrostatic, induction (polarization), and dispersion forces. Because of its importance, CR has been characterized extensively at low resolution in the condensed phase. However, gas-phase studies - which do not suffer from strong perturbations of the environment - have so far been restricted to mass spectrometry and optical spectroscopy, both of which do not provide precise and direct information about the strength of the CR and the involved charge distribution. Herein, we propose a new approach in which we utilize high-resolution infrared spectroscopy of cold and mass-selected aromatic cluster cations in the gas phase to probe the central properties of CR interaction (structure, energetics, dynamics) at high precision by correlating the strength of the CR with vibrational frequency shifts. Significantly, this approach enables us for the first time to experimentally evaluate the strength of the CR and the degree of charge delocalization in isolated aromatic cluster ions at high precision. This combined experimental and computational approach has recently been pioneered and benchmarked by our group for the case of the pyrrole dimer cation and shall now be extended to a more general and systematic characterization of the CR in a variety of aromatic cluster ions to provide new and reliable insight into its properties at the molecular level. To this end, we will investigate the dependence of the strength and dynamics of the CR on a variety of parameters, including (1) microsolvation by polar and nonpolar ligands (symmetry breaking and restoring, variation of type and strength of perturbation), (2) substitution of functional groups of pyrrole and substitution of one pyrrole by another aromatic molecule (change in ionization energy), and (3) delocalization of CR in larger pyrrole clusters. We will also explore the application of various schemes of energy decomposition analysis to evaluate the various contributions to the CR.