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

A density functional theory study on olefins with five‐membered monocyclic 4 n and 4 n +2 π‐electron substituents (C 4 H 3 X; X=CH + , SiH + , BH, AlH, CH 2 , SiH 2 , 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 (T 1 ) potential‐energy surface (PES) for twisting about olefinic CC bonds. It exploited both Hückel’s rule on aromaticity in the closed‐shell singlet ground state (S 0 ) and Baird’s rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH 2 CH(C 4 H 3 X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C 4 H 3 X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S 0 ‐antiaromatic/T 1 ‐aromatic (C 5 H 4 + ) to strongly S 0 ‐aromatic/T 1 ‐ antiaromatic (C 5 H 4 − ). Our hypothesis is that the shapes of the T 1 PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T 1 [Δ E (T 1 )], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between Δ E (T 1 ) and the (anti)aromaticity changes of the C 4 H 3 X groups, as measured by the zz ‐tensor component of the nucleus‐independent chemical shift ΔNICS(T 1 ;1) zz , are found both for sets A and B separately (linear fits; r 2 =0.949 and 0.851, respectively) and for the two sets combined (linear fit; r 2 =0.851). For sets A and B combined, strong correlations are also found between Δ E (T 1 ) and the degree of S 0 (anti)aromaticity as determined by NICS(S 0 ,1) zz (sigmoidal fit; r 2 =0.963), as well as between the T 1 energies of the planar olefins and NICS(S 0 ,1) zz (linear fit; r 2 =0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T 1 PESs suitable for adiabatic Z / E photoisomerization.