| Abstract
| - The structures and relative energies of all expected isomers and intermediates of new carbaporphyrin systems(2-aza-21-carba-23-X(thia or oxa)-porphyrin with or without 5,10,15,20-tetraphenyl) have been calculatedusing density functional theory (DFT) methods. The two tautomers having inside N−H turned out to bealmost isoenergetic, with the relative energy difference less than 0.5−1.0 kcal/mol for both thiacarbaporphyrinand oxacarbaporphyrin. The tautomer having outside N−H is calculated to have only 1−3 kcal/mol higherenergy than the lowest-energy isomer for the thiacarbaporphyrin, whereas the difference increases up to about9−12 kcal/mol for the oxacarbaporphyrin system. The results explain why three isomers can coexist in thethiacarbaporphyrin, whereas only two isomers can be observed in the oxacarbaporphyrin. The calculatedstructures show that the relative stability of the isomers is not determined by the steric hindrance betweentwo hydrogen atoms inside the core ring but by the degree of the π-electron resonance of the core framework.The relative energy of the intermediate structure having two hydrogen atoms at the C(21) atom is calculatedto be about 3−6 and 11−15 kcal/mol for thiacarbaporphyrin and oxacarbaporphyrin, respectively. The overallenergy barriers of the tautomerism between the isomers with inside N−H are estimated to be 12−15 and20−22 kcal/mol for thiacarbaporphyrin and oxacarbaporphyrin, respectively. The shape of the core cavitymade by the four inside ligand atoms (C, N, X, and N) is almost regular square in the oxacarbaporphyrinsystems but rectangular or rhombic in the thiacarbaporphyrin systems.
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