Coordination of Methanol Clusters to Benzene: A Computational Study

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Abstract
Benzene–methanol cluster structures were investigated with theoretical chemistry methods to describe the microsolvation of benzene and the benzene-methanol azeotrope. Benzene–methanol (MeOH) clusters containing up to six methanol molecules have been calculated by ab initio [MP2/6-311++G(d,p)//MP2/6-31+G(d,p) + BSSE correction] method. The BSSE was found quite large with this basis set, hence, different extrapolation schemes in combination with the aug-cc-pVxZ basis sets have been used to estimate the complete basis set limit of the MP2 interaction energy [ΔE(MP2/CBS)]. For smaller clusters, n ≤ 3, DFT procedures (DFTB+, MPWB1K, M06-2X) have also been applied. Geometries obtained for these clusters by M06-2X and MP2 calculations are quite similar. Based on the MP2/CBS results, the most stable C6H6(MeOH)3 cluster is characterized by a hydrogen bonded MeOH trimer chain interacting with benzene via π···H–O and O···H–C(benzene) hydrogen bonds. Larger benzene–MeOH clusters with n ≥ 4 consist of cyclic (MeOH)n subclusters interacting with benzene by dispersive forces, to be denoted by C6H6 + (MeOH)n. Interaction energies and cooperativity effects are discussed in comparison with methanol clusters. Besides MP2/CBS calculations, for selected larger clusters the M06-2X/6-311++G(d,p)//M06-2X/6-31+G(d,p) procedure including the BSSE correction was also used. Interaction energies obtained thereby are usually close to the MP2/CBS limit. To model the benzene–MeOH azeotrope, several structures for (C6H6)2(MeOH)3 clusters have been calculated. The most stable structures contain a tilted T-shaped benzene dimer interacting by π···H–O and O···H–C (benzene) hydrogen bonds with a (MeOH)3 chain. A slightly less negative interaction energy results for a parallel displaced benzene sandwich dimer with a (MeOH)3 chain atop of one of the benzene molecules.

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