The Spectroscopic Consequences of C−H···π H-Bonding: C₆H₆−(C₄H₂)<i>_n</i> Clusters with <i>n</i> = 1 and 2

Abstract

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···π 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 π 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 S1−S0 origin transition weakly allowed and possesses an infrared spectrum that has acetylenic CH stretch absorptions due to free CH, aromatic π-bound CH, and a more weakly π-bound CH. Its structure is tentatively assigned as a cyclic, “pinwheel” structure.