Spectroscopy and Theory of Acetylene Coupling Reactions in Ti⁺(C₂H₂)_<i>n</i> Complexes

Ti⁺(C₂H₂)_<i>n</i> complexes produced by laser vaporization in a supersonic expansion are investigated with mass spectrometry, infrared laser photodissociation spectroscopy, and UV laser photodissociation. The mass distributions of the cluster ions produced are found to vary significantly with the sample rod mounting configuration in the source. For infrared spectroscopy experiments, the so-called "offset" rod mounting produces colder conditions than the "cutaway" configuration, which allows tagging the ions with one or more argon atoms for the <i>n</i> = 1 and 2 complexes. Infrared photodissociation spectra for these ions allow the identification of cation-π complexes (<i>n</i> = 1, 2) and reaction products from acetylene coupling (<i>n</i> = 2). A TiC₄ metallacycle ion is identified by experiment and theory as the dominant reaction product. Larger complexes could not be tagged with argon under our conditions and therefore infrared spectra could not be measured. Under warmer expansion conditions with the cutaway rod configuration, prominent Ti⁺(C₂H₂)₃ and Ti⁺(C₂H₂)₆ ions are formed. UV photodissociation patterns for these ions are found to be almost identical to those for the corresponding Ti⁺(C₆H₆) and Ti⁺(C₆H₆)₂ ions, suggesting that acetylene cyclization reactions have produced benzene and di-benzene complexes. Reaction path computations for both the <i>n</i> = 2 and 3 complexes investigate the energetics of the cyclization reactions in these systems.