Triplet State Aromaticity: NICS Criterion, Hyperconjugation, and Charge Effects.
Aromaticity, one of the most important concepts in organic chemistry, has attracted considerable interest from both experimentalists and theoreticians. It remains unclear which NICS index is best to evaluate the triplet-state aromaticity. Here, we carry out thorough density functional theory (DFT) calculations to examine this issue. Our results indicate that among the various computationally available NICS indices, NICS(1)zz is the best for the triplet state. The correlations can be improved from 0.840 to 0.938 when only neutral species are considered, demonstrating the significant effect of the charge on the triplet-state aromaticity. In addition, calculations suggest that five-membered cyclic species with "hyperconjugative" aromaticity (and antiaromaticity) in the S0 state will become antiaromatic (and aromatic) in the T1 state, indicating an important role of hyperconjugation. Finally, a moderate correlation (r(2) =0.708) is identified between the NICS(1)zz values and spin distributions.
State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, Fujian, P. R. China.
Hongchao
H
Sun
State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, Fujian, P. R. China.
Ke
K
An
State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, Fujian, P. R. China. jun.zhu@xmu.edu.cn.
Jun
J
Zhu
ISE values; NICS; density functional calculations; five-membered rings; triplet state