Benzene clustered with N2, CO2, and CO: Energy levels, vibrational structure, and nucleation

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Abstract
Two-color time-of-flight mass spectroscopy is employed to study the van der Waals (vdW) clusters of benzene(N2)n (n≤8), benzene(CO2)n (n≤7), and benzene(CO)n (n=1,2) created in a supersonic molecular jet. Potential energy calculations of cluster geometries, normal coordinate analysis of vdW vibrational modes, and calculations of the internal rotational transitions are employed for the assignment of the benzene(solvent)1 cluster spectra in the 000 and 610 regions of the benzene 1B2u←1A1g transition. The respective vibronic and rotational selection rules for these clusters are determined based on the appropriate point groups and molecular symmetry groups of the clusters. Good agreement between the calculated and experimental spectra is obtained with regard to the vdW vibrational and internal rotational modes. The solvent molecules rotate nearly freely with respect to benzene about the benzene–solvent bond axis in the benzene(solvent)1 clusters. In the excited state a small ∼20 cm−1 barrier to rotation is encountered. Studies of larger clusters (n>2) reveal a broad red shifted single origin in the 610 spectra. A linearly increasing cluster energy shift is observed as a function of cluster size. The cluster energy shifts are not saturated by one solvent molecule on each side of the aromatic ring; several solvent molecules effectively interact with the solute π electronic cloud. Both homogeneous and inhomogeneous nucleation take place for the clusters studied depending on the ratio of the solvent–solvent binding energy to the cluster binding energy.