Triplet State Aromaticity: NICS Criterion, Hyperconjugation, and Charge Effects

Abstract

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.

Adaptive aromaticity in S0 and T1 states of pentalene incorporating 16 valence electron osmium

“…To further evaluate the (anti)aromaticity of these osmacycles, we performed the nucleus-independent chemical shift (NICS) calculations in both their S 0 and the T 1 states 36,37 , two-dimensional NICS grids are capable to display the magnetic shielding in aromatic rings and de-shielding in antiaromatic rings 38,39 . Compared with other NICS indices, the NICS(1) zz is chosen here as it can be easily computed and also highly effective in evaluating π aromaticity in both the S 0 and the T 1 states 40,41 . All these complexes in the S 0 state (1-S 0 , 2-S 0 , and 3-S 0 ) and complex 2 in the T 1 state (2-T 1 ) are aromatic, according to the significantly negative (shielded) NICS(1) zz values in these osmacycles (Fig.…”

“…For instance, the lowest triplet state of acenaphthylene has a larger contribution from excitation localized at an ethylenic bond than rather that delocalized across the entire π network 13 . In a recent study, we found that the localization of the spin density is found for antiaromatic species in the T 1 state, whereas triplet-state aromatic compounds have a tendency to delocalize their spin densities 41 . How about the population of the spin density in complexes 1-T 1 , 2-T 1 , 3-T 1 , and 2′?…”

Rational Design of a Carbon‐Boron Frustrated Lewis Pair for Metal‐free Dinitrogen Activation

“…To evaluate aromaticity of these rings, we performed the nucleus‐independent chemical shift (NICS) calculations. Compared with other NICS indices, the NICS(1) zz value is chosen here as it can be easily computed and also highly effective in π aromaticity in both the S 0 and T 1 states . The computed NICS(1) zz values in these three rings of 8 are −29.7, −26.4, and −25.2 ppm, respectively, indicating aromaticity of product 8 (Figure b).…”

“…Compared with otherN ICS indices, the NICS(1) zz value is chosen here as it can be easily computed and also highly effective in p aromaticity in both the S 0 and T 1 states. [67] The computed NICS(1) zz values in these three rings of 8 are À29.7, À26.4, and À25.2 ppm, respectively,i ndicating aromaticity of product 8 (Figure 4b). Note that the NICS(1) zz values in these two five-membered rings of 7 are À24.4 and À7.2 ppm, respectively,w hereas in TS7B they became more negative (À28.9 and À16.6 ppm, respectively).…”

Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes

“…[7] Later on, Ottosson proposed the cross-hyperconjugated orbital topology to justify hyperconjugative (anti)aromaticity in the ground state or the lowest excited states, [8] and some of us extended this concept to triplet state systems. [9] Houk and coworkers explored the difference in Diels-Alder reactivity of substituted cyclopentadienes and cyclopropenes also using hyperconjugative aromaticity. [10] More recently, Zhu, Zhao and co-workers analyzed the role of some ligands in the electron delocalization of cyclopentadiene and found that the AuPPh 3 moiety induces a stronger delocalization than main group substituents such as SnH 3 .…”