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Recommendation
Aromaticity
Michael Faraday discovered benzene in 1825

Chemists in the early 19th century first just used the term “aromatic”
Aromatic compounds are more stables often far
currents or “aromaticity”, whereas positive values (i.e., deshielded) at each
point denote paratropic ring currents and “antiaromaticity”.
Negative
Interior positions
P. v. R. Schleyer, and F. Puhlhofer, Org. Lett. 4, 2873 (2002)
Magnetic criteria of aromaticity
Ring currents
Nucleus-Independent Chemical Shifts（NICS）
Ring Currents
NICS
P. v. R. Schleyer et al., J. Am. Chem. Soc. 118, 6317 (1996). 200 180 160 140 120 100 80 60 40 20 0
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The ability to sustain a diatropic ring current is the defining characteristic of aromatic species. Cyclic electron delocalization results in enhanced stability, bond length equalization,and special magnetic as well as chemical and physical properties.
LMO−NICS Method
IGLO formalism: the σ and π subspaces are separated the aromatic ring current can be related to the shielding
contributions arising from C-C π electrons. The canonical MOs are transformed into localized MOs
NICS Technique
Benzene
Cyclobutadiene
Compounds which exhibit significant exalted diamagnetic susceptibility are aromatic. Those compounds with exalted paramagnetic susceptibility may be antiaromatic. P. von RagueSchleyer et al., Org. Lett. 3, 2465 (2001)
Depending strongly on the equations used (various
isodesmic, homodesmotic, and hyperhomodesmotic reactions) and on the choice of reference molecules
Isomerization stabilization energy (ISE)
CMO-NICS Method
Individual
canonical molecular orbital (CMO) contributions to the magnetic shielding of atoms, as well as to the NICS of aromatic compounds, can be computed by the widely used GIAO method.
No precise quantitative definition and is not directly measurable experimentally
•A strong usually pleasant odor
•Extra stabilization
•Bond length equalization •A tendency to undergo electrophilic substitution rather than addition reactions. •Magnetic susceptibility exaltation
Aromaticity
Beauty
Recommendation
„We have not solved the problem of what aromaticity is…, but we all agree that aromaticity can be defined only artificially, by convention, if we do not want to go to the extreme of abandoning the notion altogether. However, classification and theory are not ends in themselves. If they generate new experimental work, new compounds, new processes, new methods—they are good; if they are sterile—they are bad‟.
Recommended that NICS(1) values, i.e., 1 Å above the ring centers, where the local contributions are diminished relative to the ring current effects, be employed as an aromaticity index, rather than NICS(0) (computed in the ring centers) A. Stanger, Journal of Organic Chemistry 71, 883 (2006)
bond length equalization, planarity and symmetry.
On the basis of geometrical considerations, molecules should
show a decrease in aromatic character when they possess a high degree of bond alternation and deviate significantly from planarity
Chemical reactivity criteria
Based on the chemical behavior : selectrophilic
aromatic substitution, rather than additions
Dominated by the transition state rather than the initial state energy Has many exceptions
The GIAO technique provides the most convenient
as well as popular way to arrive at the orbital contributions to the shielding tensor.
Energetic criteria of aromaticity
The energy is more stable Method
Compares the excess of stability of the structure due
to cyclic electron delocalization relative to a wellchosen reference system
Seminar I wuxia
AROMATICITY AND THE CRITERIA
Wuxia 1102
Professor：Zichao Tang Xueming Yang
Contents
1
2 3
Aromaticity concept
Criteria of aromaticity
Relationship among Criteria
Vague 暧昧的 Nebulous 朦胧的
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Aromaticity: “the most vexing and fascinating key chemical concept”
more stables and their geometries tend to be more regular than they “ought to be”.
Aromaticity: a time-dependent phenomenon
Aromaticity: “the most vexing and fascinating key chemical concept”
NICS
Technique
Significantly negative (i.e., magnetically shielded) NICS values in interior positions of rings or cages indicate the presence of induced diatropic ring