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Proton affinities of dehydroporphyrin and subporphyrin in ground and excited states obtained by high level computations

High level computations have been employed for design of novel 'super-bases' containing dehydropyrrole moiety, which was incorporated in the large polycyclic structures such as porphyrin and subporphyrin. The basicity of porphyrin derivatives such as dedihydroporphyrin (P), dehydroporphyrin (HP) and dehydrosubporphyrin (P2) was studied in ground and excited states. Ground state total energies and proton affinities were obtained at HF/6-31G* and MP2 levels, while vertical excitation energies and excited state gas phase proton affinities were obtained by TDDFT/ B3LYP/6-31G*//HF/6-31G* method. Furthermore, aromaticity of these species was estimated by means of NICS values at GIAO/B3LYP/6-311+G**//6-31G* level. Basicities of porphyrin H2P and its subporphyrin analogue P2 indicate the importance of multiple nitrogen lone-pair coordination on acid/base properties of the molecules. There is a significant lp-lp interaction in P and stabilization of its protonated form. Planarization of the protonated structure leads to the complete reversal of the  -electron ring currents indicating aromaticity of the protonated form. P2 does not show significantly higher basicity than H2P what is due to smaller ring size that forces non-planar geometry.

DFT; NICS; TDDFT; proton affinity; basicity

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