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Substituent Effect in the Cation Radicals of Monosubstituted Benzenes.

ABSTRACT: In 30 monosubstituted benzene cation radicals, studied at the ωB97XD/aug-cc-pVTZ level, the phenyl rings usually adopt a compressed form, but a differently compressed form-equivalent to an elongated one-may coexist. The computational and literature ionization potentials are well correlated. The geometrical and magnetic aromaticity, estimated using HOMA and NICS indices, show the systems to be structurally aromatic but magnetically antiaromatic or only weakly aromatic. The partial charge is split between the substituent and ring and varies the most at C(ipso). In the ring, the spin is 70%, concentrated equally at the C(ipso) and C(p) atoms. The sEDA(D) and pEDA(D) descriptors of the substituent effect in cation radicals, respectively, were determined. In cation radicals, the substituent effect on the σ-electron system is like that in the ground state. The effect on the π-electron systems is long-range, and its propagation in the radical quinone-like ring is unlike that in the neutral molecules. The pEDA(D) descriptor correlates well with the partial spin at C(ipso) and C(p) and weakly with the HOMA(D) index. The correlation of the spin at the ring π-electron system and the pEDA(D) descriptor shows that the electron charge supplied to the ring π-electron system and the spin flow oppositely.

SUBMITTER: Dobrowolski JC

PROVIDER: S-EPMC8269098 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The structure of 30 monosubstituted benzenes in the first excited triplet T1 state was optimized with both unrestricted (U) and restricted open shell (RO) approximations combined with the ?B97XD/aug-cc-pVTZ basis method. The substituents exhibited diverse ?- and ?-electron-donating and/or -withdrawing groups. Two different positions of the substituents are observed in the studied compounds in the T1 state: one distorted from the plane and the other coplanar with a quinoidal ring. The majority of the substituents are ?-electron donating in the first group while ?-electron withdrawing in the second one. Basically, U- and RO-?B97XD approximations yield concordant results except for the B-substituents and a few of the planar groups. In the T1 state, the studied molecules are not aromatic, yet aromaticity estimated using the HOMA (harmonic oscillator model of aromaticity) index increases from ca. -0.2 to ca. 0.4 with substituent distortion, while in the S1 state, they are only slightly less aromatic than in the ground state (HOMA ?0.8 vs ?1.0, respectively). Unexpectedly, the sEDA(T1) and pEDA(T1) substituent effect descriptors do not correlate with analogous parameters for the ground and first excited singlet states. This is because in the T1 state, the geometry of the ring changes dramatically and the sEDA(T1) and pEDA(T1) descriptors do not characterize only the functional group but the entire molecule. Thus, they cannot provide useful scales for the substituents in the T1 states. We found that the spin density in the T1 states is accumulated at the Cipso and Cp atoms, and with the substituent deformation angle, it nonlinearly increases at the former while decreases at the latter. It appeared that the gap between singly unoccupied molecular orbital and singly occupied molecular orbital (SUMO-SOMO) is determined by the change of the SOMO energy because the former is essentially constant. For the nonplanar structures, SOMO correlates with the torsion angle of the substituent and the ground-state pEDA(S0) descriptor of the ?-electron-donating substituents ranging from 0.02 to 0.2 e. Finally, shapes of the SOMO-1 instead of SOMO frontier orbitals in the T1 state somehow resemble the highest occupied molecular orbital ones of the S0 and S1 states. For several planar systems, the shape of the U- and RO-density functional theory-calculated SOMO-1 orbitals differs substantially.

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Substituent Effect in the Cation Radicals of Monosubstituted Benzenes.

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