We can also rationalize the large barrier between the CIP and SSIP states for the H = 6.70 Å system based on changes in ion solvation. While most systems are fully solvated at the CIPto-SSIP barrier, ions at this barrier in the the H = 6.70 Å slit (structure "b" in Fig. ) are under-coordinated relative to the free ion limit. We hypothesize that this under-coordination destabilizes the transition state from CIP to SSIP, yielding the large barrier observed in the PMF. 9