Remarkable Features in the Interactions of Quadrupolar Molecules

Quadrupolar charge fields of molecules and of molecular fragments give rise to unique features in weakly interacting clusters and aggregations. Relative to dipole−dipole interactions, the interactions among quadrupolar molecules tend to allow for greater orientational distortions away from equilibrium. Potential surface regions have been found for several clusters that are attractive and yet very flat for certain directions. There is a notable slipperiness for the interactions in some of these cases. This implies significant vibrational excursions even in the ground state. Furthermore, the coupling of rotations among nearby molecules in pure clusters of quadrupolar molecules is different than for dipolar species, and it can lead to unexpectedly small internal rotation barriers. How these and other features develop and what they might imply for materials and biomolecular simulations are discussed here.