Matrix-Isolation FTIR Spectroscopy of Benzil: Probing the Flexibility of the C−C Torsional Coordinate

The infrared spectrum and conformational flexibility of benzil, (C 6 H 5 CO) 2, are studied by matrix-isolation FTIR spectroscopy, supported by DFT calculations. It is shown that the low-frequency (ca. 25 cm -1 ), large-amplitude torsion around the C−C central bond strongly affects the structural and spectroscopic properties exhibited by the compound. The equilibrium conformational distribution of molecules with different O C−C O dihedral angles, existing at room temperature in the gas phase, and trapped in a low-temperature ( T = 9 K) inert matrix can be changed either by in situ irradiation with UV light (λ > 235 nm) or by annealing the matrix to higher temperatures ( T ≈ 34 K). In the first case, the increase of the average O C−C O angle results from conformational relaxation in the excited electronic states (S 1 and T 1 ), whose lowest-energy conformations correspond, for both S 1 and T 1 states, to a nearly planar configuration with the O C−C O dihedral angle equal to 180°. In the second case, the decrease of the average value of the O C−C O dihedral angle is a consequence of the change in the S o C−C torsional potential, resulting from interactions with the matrix media, which favors the stability of the more polar structures with smaller O C−C O dihedral angles.