Towards a spectroscopic and theoretical identification of the isolated building blocks of the benzene-acetylene cocrystal

Isomer- and mass-selective UV and IR-UV double resonance spectra of the BA₃, B₂A, and B₂A₂ clusters of benzene (B) and acetylene (A) are presented. Cluster structures are assigned by comparison with the UV and IR spectra of benzene, the benzene dimer, as well as the BA, BA₂, and B₂A clusters. The intermolecular vibrations of BA are identified by dispersed fluorescence spectroscopy. Assignment of the cluster structures is supported by quantum chemical calculations of IR spectra with spin-component scaled second-order Møller-Plesset (SCS-MP2) theory. Initial propositions for various structures of the BA₃ and B₂A₂ aggregates are generated with model potentials based on density functional theory combined with the symmetry-adapted perturbation theory (DFT-SAPT) approach. Shape and relative cluster stabilities are then confirmed with SCS-MP2. T-shaped geometries are the dominant structural motifs. Higher-energy isomers are also observed. The detected cluster structures are correlated with possible cluster formation pathways and their role as crystallization seeds is discussed.