Do identical polar diatomic molecules form stacked or linear dimers?

Abstract

The interaction of two identical polar neutral molecules is modeled by two equal but oppositely charged point particles at a fixed distance of separation. The total Coulomb potential energy of this system is calculated as a function of this distance, x, and the distance, y, between their centers of mass. We find that when the y/x ratio is less than about 3.1619, the linear configuration has the lower energy, whereas when this ratio exceeds 3.1619, a stacked structure is more stable. Interestingly, when two real polar molecules, such as HF interact, neither of these structures are formed. It is ‘hydrogen bonded’. The NaF dimer on the other hand, has a global minimum as stacked structure at a y/x ratio of about 1, significantly smaller than 3.1619!