Structure and Energetics of Weakly Bound Complexes. Systems Incorporating NH3 and PH3

Neutral molecules such as NH3 and PH3 form complexes of widely varying strength. A great deal is known about their interactions with cations like H+ and Li+ and the nature of the very strong binding. Gaps remain in the understanding of complexes of medium strength such as hydrogen and lithium bonds. The latter two types of interactions are compared and contrasted with particular attention to the underlying causes of their differing vibrational spectra. Whereas the Coulombic force is of prime importance in both strong and medium-strength interactions, it plays a subordinate role to exchange and dispersion in weak complexes such as (NH3)2 and Ar··XH3. The very strong anisotropy of exchange appears to guide the angular features of these complexes while dispersion is responsible for their bound nature