Excited-State Aromaticity Reversals in Naphthalene and Anthracene
Peter B Karadakov1, Muntadar A H Al-Yassiri1
1Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K.
The Journal of Physical Chemistry. A
|April 3, 2023
Summary
Aromaticity in naphthalene and anthracene shifts between electronic states. Singlet states can be more antiaromatic than triplet states, challenging assumptions for polycyclic aromatic hydrocarbons.
Area of Science:
- Computational chemistry
- Quantum chemistry
- Theoretical chemistry
Background:
- Aromaticity is a key concept in understanding the stability and reactivity of cyclic molecules.
- Previous studies on smaller systems like benzene suggest similarities in aromaticity between singlet (S1) and triplet (T1) states.
- Polycyclic aromatic hydrocarbons (PAHs) exhibit complex electronic structures that may alter these relationships.
Purpose of the Study:
- To investigate aromaticity reversals in the ground (S0) and excited electronic states (singlet S1, S2 and triplet T1-T3) of naphthalene and anthracene.
- To analyze the magnetic shielding distributions associated with these electronic states.
- To compare the aromaticity of singlet and triplet states in PAHs and challenge existing assumptions.
Main Methods:
- Calculation of off-nucleus isotropic magnetic shielding distributions.
- Utilizing complete-active-space self-consistent field (CASSCF) wavefunctions.
- Employing gauge-including atomic orbitals (GIAOs) for accurate magnetic property calculations.
Main Results:
- Naphthalene's S0, S1 (1Lb), and S2 (1La) states show shielding patterns analogous to fused benzene rings.
- In anthracene, the S1 state is aromatic and S2 is antiaromatic, with shielding patterns resembling extended naphthalene states.
- The lowest antiaromatic singlet state in both molecules is significantly more antiaromatic than the respective T1 state.
Conclusions:
- Aromaticity reversals occur between ground and excited electronic states in naphthalene and anthracene.
- The electronic state ordering in anthracene leads to an aromatic S1 and antiaromatic S2 state.
- The significant antiaromaticity of singlet states compared to triplet states in these PAHs indicates that simple analogies from smaller molecules do not hold.
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