Knowledge Bank

Resonant two-photon ionization spectroscopy and resonant ion-dip infrared spectroscopy are used to structurally characterize the $C6H6-C4H2$ complex. The complex takes on a T-shaped geometry in which the diacetylene molecule sits on the six-fold axis of benzene, with one of its $C-H$ groups hydrogen bonded to the $\pi$ cloud of benzene. The direction and magnitude of the electronic frequency shift of the complex's transitions relative to benzene monomer, the forbidden $S0-S1$ origin transition, and the acetylenic $C-H$ stretch vibrational fundamentals are all consistent with this structure. The complexation with benzene localizes the C-H stretch vibrations of the two acetylenic $C-H$ groups. Both $C-H$ stretch fundamentals are clearly observed, one unshifted and one shifted from its frequency in $C4H2$ monomer. The $C-H$ stretch fundamental of the hydrogen bonded C-H group is shifted down in frequency by about $45cm-1$ and gains significant intensity, as one would expect from a hydrogen bonded XH group. This complex is a good candidate for half-collision reaction dynamics following excitation of the $C4H2$ molecule in the complex.