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Aromaticity Effects on the Profiles of the Lowest Triplet-State Potential-Energy Surfaces for Rotation about the C[DOUBLE BOND]C Bonds of Olefins with Five-Membered Ring Substituents: An Example of the Impact of Baird’s Rule

Authors

  • Dr. Jun Zhu ,

    1. Department of Chemistry - BMC, Uppsala University, Box 576, 75123 Uppsala (Sweden), Fax: (+46) 18-4713818
    2. State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 (P. R. China)
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  • Dr. Heather A. Fogarty,

    1. Department of Chemistry - BMC, Uppsala University, Box 576, 75123 Uppsala (Sweden), Fax: (+46) 18-4713818
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  • Dr. Helene Möllerstedt,

    1. Department of Chemical and Biological Engineering/Organic Chemistry, Chalmers University of Technology, 412 96 Gothenburg (Sweden)
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  • Dr. Maria Brink,

    1. Department of Chemical and Biological Engineering/Organic Chemistry, Chalmers University of Technology, 412 96 Gothenburg (Sweden)
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  • Dr. Henrik Ottosson

    Corresponding author
    1. Department of Chemistry - BMC, Uppsala University, Box 576, 75123 Uppsala (Sweden), Fax: (+46) 18-4713818
    • Department of Chemistry - BMC, Uppsala University, Box 576, 75123 Uppsala (Sweden), Fax: (+46) 18-4713818
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Abstract

A density functional theory study on olefins with five-membered monocyclic 4n and 4n+2 π-electron substituents (C4H3X; X=CH+, SiH+, BH, AlH, CH2, SiH2, O, S, NH, and CH) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet-state (T1) potential-energy surface (PES) for twisting about olefinic C[DOUBLE BOND]C bonds. It exploited both Hückel’s rule on aromaticity in the closed-shell singlet ground state (S0) and Baird’s rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH2[DOUBLE BOND]CH(C4H3X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C4H3X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S0-antiaromatic/T1-aromatic (C5H4+) to strongly S0-aromatic/T1- antiaromatic (C5H4). Our hypothesis is that the shapes of the T1 PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T1E(T1)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between ΔE(T1) and the (anti)aromaticity changes of the C4H3X groups, as measured by the zz-tensor component of the nucleus-independent chemical shift ΔNICS(T1;1)zz, are found both for sets A and B separately (linear fits; r2=0.949 and 0.851, respectively) and for the two sets combined (linear fit; r2=0.851). For sets A and B combined, strong correlations are also found between ΔE(T1) and the degree of S0 (anti)aromaticity as determined by NICS(S0,1)zz (sigmoidal fit; r2=0.963), as well as between the T1 energies of the planar olefins and NICS(S0,1)zz (linear fit; r2=0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T1 PESs suitable for adiabatic Z/E photoisomerization.

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