Characteristics of the interaction in azulene···(H₂X)_n=1,2(X=O,S) clusters

A computational study of clusters containing azulene and up to two molecules of water or hydrogen sulfide was carried out to elucidate the main characteristics of these X-H···π interacting systems. For clusters with one H₂X molecule only one structure was found interacting with the aromatic cloud of azulene, with an interaction energy of -3.1 kcal/mol both for H₂O and H₂S as calculated at the CCSD(T)/AVDZ level. On the other hand, MP2 overestimates the interaction in hydrogen sulfide clusters, whereas the MPWB1K functional produces values in very good agreement with CCSD(T). A variety of structures were located for clusters with two H₂X molecules. The most stable ones are those which simultaneously present hydrogen bond between H₂X molecules and X-H···π contacts. Also, only this kind of structure presents relevant three body stabilizing contributions. On the other hand, the interaction of azulene with (H₂X)₂ dimer is stronger precisely for structures which do not present X-H···X hydrogen bond. This suggest that for larger systems, structures with the molecules distributed over the aromatic surface but without interacting among them, can be competitive with other, hydrogen bonded clusters, especially in H₂S containing systems.