P-quinone dimers: H-bonding vs stacked interaction. Matrix-isolation infrared and ab initio study

Matrix-isolation IR spectroscopy and ab initio calculations have been used to investigate the structure and the vibrational spectrum of the quinone dimer formed in low-temperature Ar matrices. A specially developed experimental technique was applied to separate the bands of the quinone dimer from the bands of the quinone monomer in the IR spectra. The composition-of the matrix samples was precisely controlled with a lowtemperature quartz micr.obalance. As a result, a set of bands assigned to the quinone dimer were identified. Ab initio calculations at the MP2/6-31+G*, MP2/6-31++G**, and DFT/B3LYP/6-31++G** levels of theory have been carried out to determine the relative energies and the vibrational spectra of the two stable configurations of the quinone dimer found in the calculations. These configurations are a planar complex with two weak C-H⋯O hydrogen bonds and a stacked complex stabilized by the dispersion forces. The MP2 calculations of the interaction energies corrected for the basis set superposition error (BSSE) predict the two dimers to be equally stable. The comparison of the observed IR frequency shifts with the theoretically predicted shifts indicate that only the planar configuration is responsible for all of the experimentally observed dimer bands. Thus, we conclude that the stacked dimer is absent from the matrix. The influence of the matrix environment on the stability shift in favor of the planar dimer is discussed.