THE SPECTROSCOPIC CONSEQUENCES OF $C-H{\ldots}\pi$ HYDROGEN BONDING: THE $C_{6}H_{6}-C_{4}H_{2}$ COMPLEX

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2003

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Ohio State University

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Resonant two-photon ionization spectroscopy and resonant ion-dip infrared spectroscopy are used to structurally characterize the $C_{6}H_{6}-C_{4}H_{2}$ 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 $S_{0}-S_{1}$ 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 $C_{4}H_{2}$ monomer. The $C-H$ stretch fundamental of the hydrogen bonded C-H group is shifted down in frequency by about $45 cm^{-1}$ 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 $C_{4}H_{2}$ molecule in the complex.

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Author Institution: Department of Chemistry, Purdue University

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