Matrix-isolation FTIR spectroscopy of benzil: probing the flexibility of the CC torsional coordinate

The infrared spectrum and conformational flexibility of benzil, (C6H5CO)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 (S1 and T1), whose lowest-energy conformations correspond, for both S1 and T1 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 So 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