The spectroscopic consequences of C-H•••π H-bonding:: C6H6-(C4H2)n clusters with n=1 and 2

The ultraviolet and infrared spectra of the C6H6-(C4H2)(n) complexes with n = 1 and 2 have been formed and studied in a supersonic expansion using resonant two-photon ionization (R2PI), resonant ion-dip infrared spectroscopy (RIDIRS), and IR-UV hole-burning spectroscopy. A T-shaped structure is deduced for the complex, with the C4H2 centered, end on, over the benzene ring. This C-H...pi H-bond shifts the frequency of the vibronic transitions of C6H6 by 161 cm(-1) to the blue. The acetylenic CH stretch fundamental of C4H2 is localized and split by formation of the 7 H-bond. The H-bonded CH stretch fundamental is lowered in frequency by about 40 cm(-1), increased in intensity by more than a factor of 2, and split by a Fermi resonance that is turned on by the complexation. The C6H6-(C4H2)(2) complex has a structure that makes the S-1-S-0 origin transition weakly allowed and possesses an infrared spectrum that has acetylenic CH stretch absorptions due to free CH, aromatic T-bound CH, and a more weakly pi-bound CH. Its structure is tentatively assigned as a cyclic, pinwheel structure.