Free energy calculations add a fascinating twist to the above picture (Figure ). As it happens, the free energy of the transition state A-BC ‡ is lower than that of the cis intermediate BC (see Figure for an explanation of our notation for transition states). In other words, the lowest vibrational state of the cis intermediate is energetically higher than the transition states separating it from the trans tautomers. A similar scenario also applies to the cis intermediate AD whose free energy is almost flush with that of transition state AC-D ‡ . Our calculations thus suggest that cis tautomers are unlikely to be observable as true intermediates. Instead, they are a part of the energy barrier through which one trans form tunnels to the other. In this respect, freebase corrole anion differs from free-base porphyrin, for which the cis tautomer is a true intermediate on the potential energy surface In the absence of a true free energy barrier for trans-to-cis tautomerization, the hydrogen bonds in H2Cor -appear to satisfy the criteria for (or very closely approach) a true low-barrier hydrogen bond. Such hydrogen bonds have been invoked (albeit controversially ) as responsible for reaction rate enhancements in enzymatic catalysis. Recently, low-barrier hydrogen bonds have also been implicated in enzyme cooperativity. Against this backdrop, free-base corrole anion may serve as a well-characterized and readily accessible model system for low-barrier N-H…N hydrogen bonds (several examples are known involving oxygen ).