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

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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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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 T1 [ΔE(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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