Paratropicity and antiaromaticity: role of the homo-lumo energy gap

Minsky, Abraham; Meyer, A. Y.; Rabinovitz, Mordecai

doi:10.1016/s0040-4020(01)96458-0

Cited by 104 publications

(69 citation statements)

References 42 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…CARBOCYCLIC DIANIONS. One of the characteristics of the X H NMR spectra of 4n ττ electron systems is the high-field (paratropic) shift of their bands (11,19). This paratropic shift is a result of the pronounced importance of the paramagnetic term of the proton-shielding constant in these 4n ΊΓ systems, which is nearly negligible in species that are not antiaromatic.…”

Section: Discussionmentioning

confidence: 99%

“…The concepts of aromaticity and antiaromaticity are difficult to understand because they are not physically observable (JO,11) and hence cannot be related directly to experimental data. The most com prehensive definitions of aromatic and antiaromatic molecules are based on energy content: An aromatic molecule is a species in which the ir-electron derealization reduces the energy content relative to a model compound lacking such cyclic conjugation; an antiaromatic molecule is a species in which the TT-electron derealization increases the energy content.…”

mentioning

confidence: 99%

“…Antiaromatic An ττ systems are expected to exhibit paratropic (high-field) shifts in their *H NMR spectra (J 1, 19) in contrast to the diatropic (low-field) shifts of (An + 2) ττ systems. Although the diatropic shifts revealed by (An + 2) ττ systems are wellestablished, the experimental support for the relationship between paratropicity and antiaromaticity is far less satisfactory (11).…”

mentioning

confidence: 99%

“…This correlation leads to a new theoretical index for antiaromaticity, namely, the magnitude of the energy gap, ΔΕ, between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (ΔΕ = ^HOMO ~ ELVMO)-An unequivocal correlation exists between the degree of paratropicity experienced by the An ττ dianions and the HOMO-LUMO energy gap as manifested by the line shapes (20)(21)(22) and chemical shifts (11) of their l H NMR bands. The paratropicities and delocalization patterns of dianions and their relationship to the topologies of systems will be dem onstrated in this chapter.…”

mentioning

confidence: 99%

See 3 more Smart Citations

NMR Studies of 4n π-Conjugated Polycyclic Anions

Rabinovitz

Cohen

1987

Advances in Chemistry

Self Cite

View full text Add to dashboard Cite

The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) de termines the line shape and chemical shift of 4n π polycyclic doubly charged systems. Narrower energy gaps correspond to more signif icant line broadening in the 1 H NMR bands. This line broadening is attributable to a singlet-triplet equilibrium that depends on the HOMO-LUMO gap. The topology of the system influences the par atropicity of these 4n π polycyclic dianions. Whereas the triphenylene dianion does not exhibit a 1 H NMR spectrum, a gradual change toward a highly resolved spectrum is obtained for the tri -benzo[a,c,i]phenazine dianion. A correlation between the HOMO -LUMO gap and the paratropic shift of these dianions is demonstrated. THE NOTI ON OF AROMATI CI TY is one of the most intriguing problems inorganic chemistry and has drawn the attention of chemists since Kekulé's ingenious proposal (1) in 1872 of the structure of benzene. The concept of antiaromaticity introduced by Breslow (2, 3) about a century later emphasized the perplexity of aromaticity. Nevertheless, numerous physical and chemical properties of many groups of compounds are attributed to their

show abstract

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

NMR Studies of 4n π-Conjugated Polycyclic Anions

Rabinovitz

Cohen

1987

Advances in Chemistry

Self Cite

View full text Add to dashboard Cite

show abstract

“…This small band gap of 3 contrasts starkly with the much larger S 0 -T 1 band gap of 1.86 eV calculated for 2 itself. Paratropicity in antiaromatic systems is usually attributed to a low-lying paramagnetic state, with band gaps ranging up to 2.1 eV [8,9]. For the (2/3) system the S 0 -T 1 band gap is substantially larger for the paratropic species than for the diatropic species.…”

Section: Methodsmentioning

confidence: 99%

Synthesis and Characterization of a New [4.0.4.0] Porphyrin-like Antiaromatic Macrocycle

Johnson

Slebodnick

Ibers

1997

J. Porphyrins Phthalocyanines

View full text Add to dashboard Cite

The antiaromatic macrocycle 2 has been prepared by the reaction of the bipyrrole dialdehyde 1 with hydrazine. In the crystal structure of 2· DMSO molecule 2 is nearly planar, the mean deviation of the 28 atoms of the core from the best least-squares plane being 0.227 Å. AM1 geometric optimization of the core of 2 leads to a fully planar macrocycle. Attempts to oxidize 2 to 3 were largely unsuccessful, yielding only limited evidence for the formation of 3. The antiaromatic precursor 2 turns out to be much more stable than the putative aromatic product 3. Molecular orbital calculations at the AM1/PM3 level indicate the S 0- T 1 gap for 3 is only about 0.2 eV and hence is readily accessible via intersystem crossing from any visible transition. This small band gap of 3 contrasts starkly with the much larger S 0- T 1 band gap of 1.86 eV calculated for 2 itself. Paratropicity in antiaromatic systems is usually attributed to a low-lying paramagnetic state, with band gaps ranging up to 2.1 eV. For the 2/3 system the S 0- T 1 band gap is substantially larger for the paratropic species than for the putative diatropic species.

show abstract