Benzene...carbon monoxide and benzene...formaldehyde complexes are studied using abinitio methods with the highest calculations at the MP4SDTQ/6–31+G**//MP2/6–31+G** level. The benzene...carbon monoxide dimer forms a π complex of Cs symmetry where the CO top is nearly parallel with the benzene plane. In the benzene...formaldehyde π complex the dimer is without any symmetry. In this arrangement a weak hydrogen bond is expected between the elements where benzene acts as the acceptor, while in a linear benzene...formaldehyde alignment benzene has been identified as a weak hydrogen bond donor to the carbonyl oxygen. Changes in the intramolecular geometric parameters upon dimerization are small. Interaction energy of the benzene...CO dimer seems to be underestimated compared to the experimental value. No experimental value has been found for the benzene...formaldehyde binding energy. The calculated value is more negative by 0.7 kcal/mol with reference to the benzene...CO dimer. Calculated intermolecular vibrational frequencies are in partial agreement with the experiment. The stretching frequency of the benzene...CO dimer is well reproduced, intermolecular bending and torsional frequencies are overestimated. The intramolecular vibrational frequencies for the monomers show over and underestimation in the high and low frequency ranges, respectively. Experimental results in the literature suggest an almost free internal rotation of the CO top above benzene. The calculated barrier to internal rotation is 0.01 kcal/mol in good agreement with the experimental value. Based on this theoretical value the model with the almost free internal rotation was supported. Analysis for the benzene...formaldehyde dimer suggests more hindered rotation, if at all, with a H2CO top.

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