MOLECULAR CONFORMATION AND AROMATICITY OF N-SUBSTITUTED PORPHINE DERIVATIVES

UDC 535.37+541.65+543.4

  • Kruk Mikalai Mikalaevich – DSc (Physics and Mathematics), Professor, Head of the Department of Physics. Belarusian State Technological University (13a, Sverdlova str., 220006, Minsk, Republic of Belarus). E-mail: m.kruk@belstu.by

  • Gladkov Lev L’vovich – DSc (Physics and Mathematics), Associate Professor, Professor, the Department of Physical and Mathematical Basis of Informatics. Belarusian State Academy of Communications (8/2, F. Skorina str., 220114, Minsk, Republic of Belarus). E-mail: llglad@tut.by

  • Klenitsky Dmitry Vikentievich – PhD (Physics and Mathematics), Associate Professor, Assistant Professor, the Department of Physics. Belarusian State Technological University (13a, Sverdlova str., 220006, Minsk, Republic of Belarus). E-mail: klen@belstu.by

  • Krylov Andrei Borisovich – PhD (Biology), Associate Professor, Assistant Professor, the Department of Physics. Belarusian State Technological University (13a, Sverdlova str., 220006, Minsk, Republic of Belarus). E-mail: krylov_ab@belstu.by

DOI: https://doi.org/10.52065/2520-6141-2023-266-1-7

Keywords: porphine, N-substitution, hybridization, molecular orbitals, aromaticity.

For citation: Kruk M. M., Gladkov L. L., Klenitsky D. V., Krylov A. B. Molecular conformation and aromaticity of N-substituted porphine derivatives. Proceedings of BSTU, issue 3, Physics and Mathematics. Informatics, 2023, no. 1 (266), pp. 34–41. DOI: https://doi.org/10.52065/2520-6141-2023-266-1-7.

Abstract

Using the density functional theory method, the conformation of the porphine molecule and two its Nsubstituted derivatives were calculated. It has been found that substitution of a proton in the pyrrole fragment of the macrocycle with CH3 or CF3 group results in the formation of a nonplanar conformer. Herewith, a significant tilt of the substituted pyrrole ring with respect to the macrocycle mean plane is accompanied by a significant pyramidalization of the nitrogen atom: in porphine H2P the nitrogen atom has sp2 hybridization, in H(N-CH3)P porphyrin, the index of the degree of hybridization λ2 increases to 2,208, and in H(N-CF3)P porphyrin λ2 was found to be 2,667. The molecular orbitals of the macrocycle experience significant shifts, whose magnitude and direction are determined by the properties of the substituents. The orbitals energy increases upon addition of an electron-donating CH3 group and decreases upon addition of an electronwithdrawing CF3 group compared to that of the porphine. In the latter case, a significant shift in the electron density from the nitrogen to the CF3 group leads to a change in the order of the two lowest unoccupied molecular orbitals. Using the harmonic oscillator model for aromaticity (HOMA), the macrocycle aromaticity indices IHOMA were calculated for an annulene like 18-membered conjugation pathway and a 22-membered Schleyer conjugation pathway. It has been shown that N-substitution impairs the conjugation via inner Ca–N–Ca fragment of the pyrrole ring due to the pyramidalization of the nitrogen atom, with simultaneous increase in the HOMA aromaticity index of the 18-membered λ-conjugation contour including the outer Ca–Cb–Cb–Ca fragment of the pyrrole ring. The IHOMA value for each of the studied conjugation pathways depends linearly on the degree of hybridization of the pyrrole nitrogen atom. It is assumed that the control of the atom hybridization by substitution in core and on periphery of the tetrapyrrolic macrocycle can be the basis for the method of targeted change in its aromaticity.

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19.12.2022