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Topic: Aromaticity


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  C&EN: SCIENCE & TECHNOLOGY - A Deeper Discussion Of All-Metal Aromaticity-Antiaromaticity
aromaticity, meaning that overall, the compound is aromatic.
Therefore, it's the aromaticity from the lone pairs that overwhelms the paramagnetic contributions from the antiaromatic
MOs of aromatic molecules are lower than the p orbital energy of the free atom.
pubs.acs.org /cen/science/8150/print/8150sci1a.html   (1089 words)

  
  Aromaticity - Hückel's rule, Aromatic ions
Aromaticity has to do with the unusual stability of the compound benzene and its derivatives, as well as certain other unsaturated ring compounds.
The aromaticity of the benzene ring can be assessed by measuring its "resonance energy." One way to do this is by measuring its heat of hydrogenation.
Although aromatic rings are normally planar, with uninterrupted conjugation, that is not always the case.
www.chemistryexplained.com /A-Ar/Aromaticity.html   (1232 words)

  
 Aromaticity of Cyclopropa- and Dicyclopropapyridine and Phosphabenzene
Abstract: The fusion of a small ring to benzene has been advocated as a way to induce bond alternation and reduce aromaticity.
In this theoretical study (performed using the B3LYP/aug-cc-pVDZ method), we examine the effect of fusing 1 or 2 three-member rings to pyridine or phosphabenzene with comparison back to the parent system (benzene).
The effect of the fusion on the aromaticity of the 6 member ring is analyzed in terms of resonance stablization energy, magnetic properties (NMR shifts and NICS), and geometric parameters.
hackberry.chem.trinity.edu /eccc7   (84 words)

  
 Chemblog   (Site not responding. Last check: )
Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone.
Aromatic molecules are able to interact with each other in so-called π-π stacking: the π systems form two parallel rings overlap in a "face-to-face" orientation.
Aromatic molecules are also able to interact with each other in an "edge-to-face" orientation: the slight positive charge of the substituents on the ring atoms of one molecule are attracted to the slight negative charge of the aromatic system on another molecule.
chemblog.chemicalblogs.com /33_chemblog/archive/51_aromaticity.html   (1461 words)

  
 FSD: Chemical & Materials Sciences Division: Highlights: Observation of d-Orbital Aromaticity   (Site not responding. Last check: )
Aromaticity is an important concept in chemistry, usually referring to planar cyclic hydrocarbon molecules that exhibit delocalized π bonds, such as benzene.
However, d-orbital aromaticity requires significant d-d bonding interactions, and unlike valent s or p orbitals, d orbitals are spatially more contracted and their tendency to participate in chemical bonding strongly depends on the metal and its coordination environment.
clusters suggest that they represent a new class of d-orbital aromatic molecules, which may be good redox species and may exhibit novel chemical and catalytic properties.
www.pnl.gov /cmsd/highlights/20051103orbital.stm   (401 words)

  
 Aromaticity as a controlling influence on a ligand dissociation mechanism for Λ/Δ Interconversion in Metal ...
This concept of transition state aromaticity is a useful one which has been developed in organic chemistry to answer questions such as whether pericyclic reactions (which are thought to occur through conjugated cyclic transition states) will occur.
We emphasize again that aromatic rings tend to resist disruption (in this case equated with no dissociation), but the corollary is that anti-aromatic rings, as seen here for one ring, tend to strongly seek disruption.
Mnemonics for predicting such aromaticity in a single ring containing such π electrons are very well known, originating from the work of Huckel, and subsequently formulated as the 4n+2 rule.
www.ch.ic.ac.uk /rzepa/tris-chelates   (1338 words)

  
 What does "aromatic" really mean?
That is, one key feature of aromatic compounds is that there is a set of electrons in a loop orbital resulting from overlapping p orbitals around a ring.
Yes, it is. A manifestation of this is that the parent compound, cyclopentadiene, is a rather "strong" acid -- by the standards of H attached to hydrocarbons.
Aromatic compounds are more stable than we might expect when we see a structure showing single and double bonds.
www.geocities.com /Athens/Thebes/5118/obc/arom.htm   (1852 words)

  
 Aromaticity   (Site not responding. Last check: )
Aromaticity is a property associated with the extra stability of certain types of
The posts need to be regularly spaced, all around the field with all the posts standing upright in order to have an effective fence.
In order for a compound to be aromatic, all FOUR of these criteria must be met.
www.mhhe.com /physsci/chemistry/carey5e/Useful/aromaticity.html   (248 words)

  
 Aromaticity studies of benzofused thieno[3,2-b] furans
From the results it was emphasized that aromaticity of both isomeric heterocyclic is different and it is governed by the most stable heterocyclic moiety in the molecule.
In the π-electronic system of 1, aromaticity is presumably controlled by the benzo[b]thiophene unit and structure of 1 can be considered to be a substited benzo[b]thiophene with an attached enol-ether moiety.
On the other hand, aromaticity of 2 is governed by the stable thiophene ring and the molecule can be considered to be a phenyl (or phenoxy) substituted thiophene.
www.mdpi.org /ecsoc/ecsoc-5/Papers/a0013/a0013.htm   (1707 words)

  
 Aromaticity   (Site not responding. Last check: )
A ring is identified as aromatic if it is planar (all atoms are sp2 hybridized), doesn't form exocyclic double bonds and satisfies the Huckel's 4N+2 rule.
In any case, if the structure is not aromatic a nitrogen with valence greater than 3 and lower than 5 is considered as N+.
It is based on some rigorous rules which have been implemented to recognize most of the common aromatic structures.
www.disat.unimib.it /chm/Help/edragon/Aromaticity.html   (429 words)

  
 Aromaticity
lower than expected heats of combustion and hydrogenation for aromatic molecules.
Aromatic molecules undergo substitution reactions rather than addition reactions in order to
Special magnetic properties are observed when an aromatic species is placed in a magnetic field.
chem.stthomas.edu /pages/Ippoliti/aromaticity.html   (132 words)

  
 [No title]
The aromatic heterocycle pyridine is similar to benzene, and is often used as a weak base for scavanging protons.
The resulting planar ring meets the first requirement for aromaticity, and the π-system is occupied by 6 electrons, 4 from the two double bonds and 2 from the heteroatom, thus satisfying the Hückel Rule.
The non-bonding electron pair on the nitrogen is not part of the aromatic π-electron sextet, and may bond to a proton or other electrophile without disrupting the aromatic system.
www.cem.msu.edu /~reusch/VirtualText/react3.htm   (4132 words)

  
 Chapter 11 : Aromaticity
Aromaticity is a property associated with the extra stability of certain types of
The property of aromaticity is generally about a significant extra stability associated with a resonance delocalised structure.
The presence of aromatic systems can also influence the reactivity of functional groups directly attached to the aromatic system particularly in the benzylic position.
www.chem.ucalgary.ca /courses/351/Carey/Ch11/ch11-2.html   (119 words)

  
 [No title]
Aromatic compounds are highly unsaturated, cyclic molecules that show extra stability as compared to their open chain counterpart.
Huckel's rule for aromaticity states that a molecule must meet all of the following four conditions in order to be aromatic:
Examine their structures carefully to see if they meet all the four conditions for aromaticity.
www.uwosh.edu /faculty_staff/xie/tutorial/aromaticity.htm   (154 words)

  
 Aromaticity | English | Dictionary & Translation by Babylon
Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone.
It can also be considered a manifestation of cyclic delocalization and of resonance.This is usually considered to be because electrons are free to cycle around circular arrangements of atoms, which are alternately single- and double-bonded to one another.
These bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring identical to every other.
www.babylon.com /definition/Aromaticity   (174 words)

  
 OCHem
The connotation of the word is that the pi electrons of aromatic molecules possess unusual stability in comparison to pi electrons in simple alkenes.
It is possible to generalize this idea in terms of a simple formula that indicates the number of pi electrons required for a cyclic system to be considered aromatic.
Here n is an integer, 0,1,2,3,...., that represents the number of sets of degenerate bonding orbitals in the MO diagram of the molecule.
www.usm.maine.edu /~newton/Chy251_253/Lectures/Aromaticity/HuckelsRule.html   (1379 words)

  
 The aromaticity of Pericyclic reaction transition states
Of course, aromatic molecules can support chiral groups as substituents on the rings, but we exclude this class in our argument here, since we are concerned only with the nature of the aromatic structures themselves.
For relatively small sized rings (<14), aromaticity can be related to the degree of alternation in the bond lengths; no alternation indicates a high degree of aromaticity (and implied stability), whereas partitioning into short (double) and long (single) bond lengths indicates no aromaticity, or even anti-aromaticity.
The former is clearly non-aromatic whilst the latter is aromatic.
www.ch.ic.ac.uk /rzepa/pericyclic   (3688 words)

  
 Aromaticity studies of benzofused thieno[3,2-b] furans
From the results it was emphasized that aromaticity of both isomeric heterocyclic is different and it is governed by the most stable heterocyclic moiety in the molecule.
In the &#960;-electronic system of 1, aromaticity is presumably controlled by the benzo[b]thiophene unit and structure of 1 can be considered to be a substited benzo[b]thiophene with an attached enol-ether moiety.
On the other hand, aromaticity of 2 is governed by the stable thiophene ring and the molecule can be considered to be a phenyl (or phenoxy) substituted thiophene.
www.mdpi.net /ecsoc-5/a0013/a0013.htm   (1723 words)

  
 [No title]   (Site not responding. Last check: )
Aromatics by Gary Trammell and Srinivas Vuppuluri at the University of Illinois at Springfield
The aromaticity of the tropylium ion means that it forms easily for a carbocation.
Answer: It is a (relatively) strong acid due to the aromaticity of its conjugate base.
www.mpcfaculty.net /ron_rinehart/12A/chp11pag.htm   (1440 words)

  
 THE AROMATICITY OF C706- VERSUS C606-:
The fullerenes’ closed and hollow structure makes it possible to "sense” the aromaticity from the inside, outside and the surface of the cage, and to examine the global aromaticity of the entire fullerene as well as its local properties.
upon reduction, from modestly aromatic in the neutral state, to a high level of delocalization and aromaticity in the hexaanion.
Distinction between rings of different size (hexagons and pentagons) and in different areas on the fullerene skeleton was enabled by adding a methylene bridge to the fullerene.
www.weizmann.ac.il /ICS/chemistry66/057Sternfeld.html   (286 words)

  
 Aromaticity
this compound cannot be aromatic because it is not cyclic
this compound is not aromatic because one of the carbons
this compound is not aromatic because it has 4 pi electrons.
www2.chem.ucla.edu /~mouser/General/labzone/130BL/problems/hint/aromaticity.html   (90 words)

  
 [No title]   (Site not responding. Last check: )
For a compound to be aromatic it must meet all of these criteria:
Benzene with its 6 pi electrons meets all three criteria and is aromatic.
Naphthalene and azulene both have 10 continuous pi electrons in a planar molecule and are aromatic.
people.uis.edu /gtram1/organic/aromatics/aromaticity.htm   (218 words)

  
 chem-bla-ics
Now, aromaticity is a fuzzy concept, and there is no general agreement on what it is. Some say it is smelly compounds, others say ring systems which apply to the Hückel rule.
Aromaticity beyond that is even less well defined than aromaticity in general.
I asked Peter about this, and whether OpenNMR predicted shifts could indeed confirm aromaticity of compounds, and he replied and showed that the predicted spectra could be used to distinguish between C-C and C=C bonds.
chem-bla-ics.blogspot.com   (1397 words)

  
 Aromaticity
However, for polycyclic aromatic compounds such as naphthalene, the 4n+2 rule still applies for determining aromaticity, but in computing the energies via Huckel, there are no degeneracies.
Also, the buckyball is considered to be aromatic, but it is not planar.
The resonance energy is obtained by comparing the heats of hydrogenation of alkene fragments to the heat of hydrogenation of the aromatic molecule.
www.newton.dep.anl.gov /newton/askasci/1993/chem/CHEM008.HTM   (616 words)

  
 Julia Neus: Aromatizität - Geschichte und mathematische Analyse eines fundamentalen chemischen Begriffs   (Site not responding. Last check: )
Aromaticity is one of the most important concepts not only in organic chemistry.
The central issue is whether all these parameters essentially refer to the same basic property of aromaticity, or whether there are several independent factors behind the received notion that allow to distinguish between different types of aromaticity.
Others have supported the idea that aromaticity is a single, one-dimensional concept, because of the high correlation coefficients they obtained.
www.hyle.org /publications/books/neus/index.html   (462 words)

  
 CMS—Aromaticity Provides Bucky Stability in Aza-Fullerines   (Site not responding. Last check: )
Using quantum mechanical, density-functional calculations, Manaa and coworkers showed that the extended aromaticity of eight all-carbon hexagonal rings plays the stabilizing role for this novel molecule.
In this new structure, the top and bottom of the azafullerene each consists of a triphenylene-type unit connected to three nitrogen atoms.
Manaa, D. Sprehn, and H. Ichord,"Prediction of Extended Aromaticity for a Novel C48N12 Aza-Fullerene Structure" (pdf), Journal of the American Chemical Society, 124, 13990 (2002).
www-cms.llnl.gov /s-t/aromaticity.html   (233 words)

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