Computational study of the interaction in X-(CH3NH2)n clusters (X = F, Cl; n=1-4).: The balance between ion-molecule and molecule-molecule interactions

A computational study of clusters consisting of a fluoride or chloride ion and up to four methylamine molecules was conducted using ab initio and density functional theory (DFT) methods. A large number of structures corresponding to minima on the potential surfaces for the different clusters were obtained that included interior structures (where the methylamine molecules lie around the halide ion) and surface structures (where a cluster of methylamine molecules interacts as a unit with the ion). On the basis of the results, fluoride ion tends to form no surface structures; the most stable structure for each cluster is of the interior type. On the other hand, chloride ion forms surface structures where methylamine molecules interact with one another via N-(HN)-N-... bonds. Whether or not a surface structure can be formed depends on the balance between ion-molecule and molecule-molecule interactions. In interior structures, the stabilizing effect arises from strong ion-molecule attractions; in surface structures, ion-molecule interactions weaken at the expense of interactions between methylamine molecules. In the fluoride clusters, ion-molecule interactions are so strong that the potential formation of hydrogen bonds between methylamine molecules cannot overcome the tendency of the molecules to interact with the ion and form interior structures. On the other hand, ion-molecule interactions are weaker for chloride, so the hydrogen bonding between molecules is strong enough for surface structures to be formed. At some minima, the N-(HX)-X-... interaction was replaced with less stable C-(HX)-X-... bonds; in the chloride clusters, however, such a stability loss was offset by the interaction between methylamine molecules, and the resulting structures were as stable as those of the interior forms, The N-H symmetric stretching frequencies are red-shifted by the effect of the interaction; also, they differ between interior and surface structures. Thus, the interior structures give signals that span only a narrow frequency range. The structures forming N-(HN)-N-... hydrogen bonds exhibit the most markedly red-shifted signals, which, in addition, span a broader frequency range.