(a) Synthesis and structural characterization. Free-base 5,10,15-tris(p-cyanophenyl)corrole (H3Cor) was synthesized via the water-methanol method reported by Gryko et al., with all spectroscopic data matching those reported earlier. The free-base corrole anion, H2Cor -, was obtained as the tetrabutylammonium salt, TBA[H2Cor], in ~50% yield by treating the neutral free base with tetrabutylammonium fluoride (TBAF) in dichloromethane, followed by crystallization from a mixture of dichloromethane and hexane. The salt was fully characterized by standard spectroscopic techniques, including UV-vis absorption, fluorescence, FT-IR, and 1 H NMR spectroscopies, ESI mass spectrometry, and single-crystal X-ray diffraction analysis (see Supporting Information). Deep blue crystals of TBA[H2Cor] were isolated from a solution of the free-base corrole and TBAF in CH2Cl2-hexane mixtures upon slow evaporation at room temperature. The crystal structure (Figure and Table ) reveals several notable features: (a) First, the corrole core is almost strictly planar, unlike that of a neutral corrole, which is invariably strongly puckered as a result of steric repulsion among the three central NH hydrogens. (b) Second, the four central nitrogens are arranged in the form of an isosceles trapezoid in which the longest side (~3.0 Å) is antipodal to the direct pyrrole-pyrrole C1-C19 linkage, while the other three sides are each about 2.6 Å. (c) Third, an ordered trans arrangement of the central NH hydrogens is observed, with each NH hydrogen within hydrogen-bonding distance of two neighboring unprotonated nitrogens. Note that the apparently short N-H bond distances ~0.9 Å merely reflect the fact that X-ray diffraction measures electron density, not nuclear position, which, for hydrogen, is a key distinction. It is clear nonetheless that each NH group engages in two hydrogen bonds of different lengths, a point that we will revisit below with DFT calculations. (d) Finally, the crystal structure reveals supramolecular chains of H2Cor units held together by C-H...N interactions involving the p-cyanophenyl groups. Locally, the C-H...N interactions form R22(10) and R21(5) rings, to use Etter's terminology. Such a structure results in voids, which are occupied by tetrabutylammonium cations (See Supporting Information for details). and). Consistent with the crystal structure (Figure ), the ground state corresponds to either AC or BD, i.e., the protons are attached to either pyrrole rings A and C or pyrrole rings B and D. The two tautomers are not identical, as a result of the asymmetry created by the meso-phenyl groups, but are equienergetic for all practical purposes. Both tautomers exhibit bifurcated hydrogen bonding, where each central NH interacts with two neighboring unprotonated nitrogens, much as in a free-base porphyrin. Unlike in porphyrin, however, the bifurcated hydrogen bonding here is asymmetric, with each NH group interacting via an ultra-short hydrogen bond (~1.8 Å) and a somewhat longer hydrogen bond (~2.4 Å). The shorter hydrogen bonds always involve a pair of pyrrole rings on one side of the molecular pseudo-C2 axis (such as pyrroles A and B, or C and D), whereas the longer hydrogen bonds span a pair of pyrrole rings on opposite sides of the pseudo-C2 axis (pyrroles A and D, or pyrrole B and C).