A spectroscopic approach to identifying the out-of-plane conformations of Ni(II) meso-tetraphenylporphyrin in solution

Abstract

Ni(II) meso-tetraphenylporphyrin (NiTPP) is a model molecule whose substituent causes out-of-plane (oop) deformations of the porphyrin macrocycle. Although the conformers of NiTPP in the solution had been tentatively explored, the determination of its specific conformers was controversial. In this work, based on the conformational search through molecular dynamics (MD), three energy lowest-lying conformers of NiTPP were sorted out, namely, the S4 (staggered) conformer, the C2 (eclipsed) conformer, and the C1 (eclipsed-staggered) conformer. According to the high-precision free energy calculation based on density functional theory (DFT), the Raman and 1H-NMR spectra of the three conformers were calculated. The actual spectra of NiTPP in solution resulted from the sum spectra of the mixture conformers, so with the consideration of the corresponding conformation weights, the experimental spectra could be stimulated. Our results showed that only at the low-frequency ∼215 cm−1, did the Raman spectrum in solution differ significantly from that in solid powder. On the other hand, the 1H-NMR spectra of the NiTPP conformers were remarkably different, and none of the single conformer's 1H-NMR range could match the experimental 1H-NMR spectrum. However, with the consideration of the existence of three dominant conformations, the observed 1H-NMR spectrum could be well simulated, confirming the rational mixture of the three dominant conformers existing in the aqueous solution. Therefore, this work has not only resolved the different conformers of NiTPP in solution, but also provided an effective approach to combining DFT, Raman, and 1H-NMR analytical tools for identifying different oop conformations of metalloporphyrins in solution.