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.

The structures of the phenol (Ph)-acetylene (A) clusters PhA1,2,3 and Ph2A1 are assigned on the basis of isomer and mass specific IR-UV double resonance spectroscopy and compared to the structure of the PhA cocrystal. The structures of the PhA1,2,3 clusters are dominated by phenol-acetylene π-hydrogen bonds whereas Ph2A1 binds via OH···OH···C≡C interaction with dominating Ph-Ph hydrogen bond like in the phenol dimer and acetylene attached to the free OH group of the proton acceptor phenol. The macroscopic crystal is a clathrate of phenol with acetylene with hydrogen bridges only between the phenol molecules and not between phenol and acetylene. (1) A possible aggregation pathway is proposed in which larger phenol clusters like Ph6 are cyclic with no free OH available anymore to which acetylene could attach as proton acceptor.