Enthalpy and entropy factors in gas phase halogen bonding: compensation and competition

Halogen bonding in the gas phase is often thermodynamically unstable, because of the loss of entropy incurred when the translational and rotational motions of the free molecules are constrained by the formation of the complex. This produces a large negative T Delta S term, which can in many instances dominate Delta H and result in Delta G = Delta H - T Delta S > 0. This reduction in entropy is tempered, however, by the fact that weakly-bonded noncovalent complexes are generally not completely rigid, and some of the original freedom of motion and disorder remains. The stronger the interaction (more negative Delta H), the more rigid is the complex and the more negative is T Delta S. However T Delta S generally approaches a limiting value; for sufficiently strong interactions, therefore, Delta H dominates and Delta G < 0, i.e. thermodynamic stability. In solution and solid phases, the situation is complicated by additional factors, the overall effect of which can produce Delta G < 0 even in cases that are thermodynamically unstable in the gas phase. It should also be noted that Delta G > 0 does not totally preclude an interaction; it simply means that the equilibrium constant is less than one.