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For example, rings can include oxygen , nitrogen , or sulfur.
Molecules with one ring are called monocyclic as in benzene. Molecules with two rings are called bicyclic as in naphthalene. Molecules with more than two rings are called polycyclic as in anthracene. Simple monocyclic aromatic rings are usually five-membered rings like pyrrole or six-membered rings like pyridine. Fused aromatic rings consist of monocyclic rings that share their connecting bonds. The nitrogen N -containing aromatic rings can be separated into basic aromatic rings that are easily protonated , and form aromatic cations and salts e.
Hydrogen atoms connected to the carbon atoms are omitted for clarity. Geometry optimization procedure was performed for all compounds. Thus, the boron derivatives of the studied hydrocarbons are not planar. Due to the non-planar structures of the compounds, we decided to determine the aromaticities of boron derivatives with a plane of symmetry like in the parent hydrocarbons and without it like in relaxed structures of boron derivatives.
There are two exceptions from this typical behavior for boron-substituted hydrocarbons, in both cases for —BH—BH— substituted derivatives. The structure of compound 10b is flat; no loss of planarity is observed. On the other hand, in compound 5b , the boron-containing ring transforms, upon optimization, into a pyramidal structure with a five-membered ring four carbons and a BH group in the base of the pyramid and a second BH group at the top. For all other than 5b and 10b compounds, the planar structures are transition states between two equivalent non-planar minima. The energy barriers between planar and non-planar structures are different, from about 0.
Antiaromaticity in Monocyclic Conjugated Carbon Rings. Kenneth B. Wiberg. Department of Chemistry, Yale University, New Haven. Antiaromaticity in Monocyclic Conjugated Carbon Rings. Kenneth B. Wiberg. Department of Chemistry, Yale University, New Haven, Connecticut
Aromaticity data for whole structures of substituted hydrocarbons values for planar structures underlined. Variations of the HOMA total index for hydrocarbons of the a series are surprisingly small. Most HOMA total values for this group of compounds are between 0. Thus, all these compounds can be classified as moderate aromatic.
Only compound 1a the benzene derivative has higher HOMA total value 0. On the other side, HOMA total values for the compounds of the b series span over a much larger region. They are antiaromatic compounds 1b and 3b or non-aromatic 2b , 7b , 8b , 9b , 11b ; small aromatic properties are suggested by the HOMA total values for compounds 4b , 5b , 6b , 10b and especially 12b. The last compound can be considered as moderate aromatic. However, its moderate aromaticity is due to the fact that compound 12b , a derivative of coronene, is much larger than the other studied systems.
Thus, boron substitution strongly disturbs electronic structure probably only in a part of its carbon skeleton. The biggest difference in aromatic properties is observed between benzene derivatives of both series. Benzene is the smaller hydrocarbon considered in this work, so it is reasonable that in this case, introduction of the boron atoms can change completely its electronic structure. Differences between HOMA total values for planar and non-planar structures of a series are always small or very small. What is intriguing is that HOMA total values are usually a bit higher for relaxed non-planar structures.
This behavior is in opposite to the pure hydrocarbons where destroying the planar structure results in decreasing aromatic properties [ 45 ]. However, differences between magnetic susceptibilities of the same compound in its planar and non-planar structures are much greater than for the HOMA total results.
In addition, magnetic susceptibility calculations afforded sometimes quite unexpected values. Such an unexpected case is compound 2 , where significantly more negative values are predicted for the hydrocarbon with the —BH—BH— insertion, 2b. Another somewhat strange case is that of compound 4.
For this compound, a very huge, difficult to explain, change in magnetic properties occurred during transition from the flat to the relaxed structure without symmetry plane. Such a huge change is not observed for any other structure reported in this work. The origin of such unexpected artifact in the magnetic susceptibility data is worth of a future more detailed study.
Such a result can support the thesis about the multidimensional character of aromaticity, where structural and magnetic indices correspond to different manifestations of this property [ 32 ]. At the same time, there is some correlation between HOMA total data for two groups of boron-doped hydrocarbons, correlation coefficients being 0. Aromaticity data for individual rings of the —BH—BH— substituted hydrocarbons values for planar structures underlined, values for rings containing boron atoms in bold. It should be mentioned at this moment that there are some problems with using some standard aromaticity indices for the compounds containing boron atoms.
First of all, NICS index data look sometimes unreliable for these rings. This can be the result of the fact that electrons close to B atoms are freer to move than those of C atoms and create ring currents that produce these high NICS 0 values. The fact that electrons close to B atoms are more diffuse can influence also the PDI data, due to the higher values of para delocalization indices obtained in these rings. Thus, PDIs for rings containing B atoms are likely to be somewhat overestimated. Some changes appear for the rings that have a boundary with the ring containing boron atoms.
For rings with boron atoms, transforming the molecular structure from planar to non-planar results sometimes in a substantial change. It can be noticed that in non-planar structures, some rings containing boron atoms, compounds 3a and 6a as well as 1b , 2b and 3b , switch their antiaromatic properties into non-aromatic ones. Almost all rings increase their HOMA values in non-planar structures, and this is the rule for all the rings with boron atoms. In the molecular orbital picture, the six p atomic orbitals of benzene combine to give six molecular orbitals.
Three of these orbitals, which lie at lower energies than the isolated p orbital and are therefore net bonding in character one molecular orbital is strongly bonding, while the other two are equal in energy but bonding to a lesser extent are occupied by six electrons, while three destabilized orbitals of overall antibonding character remain unoccupied. The result is strong thermodynamic and kinetic aromatic stabilization.
Not all compounds with alternating double and single bonds are aromatic. Cyclooctatetraene , for example, possesses alternating single and double bonds. The molecule typically adopts a "tub" conformation.
This effect is due to the placement of two electrons into two degenerate nonbonding or nearly nonbonding orbitals of the molecule, which, in addition to drastically reducing the thermodynamic stabilization of delocalization, would either force the molecule to take on triplet diradical character, or cause it to undergo Jahn-Teller distortion to relieve the degeneracy. This has the effect of greatly increasing the kinetic reactivity of the molecule.
Because the effect is so unfavorable, cyclooctatetraene takes on a nonplanar conformation and is nonaromatic in character, behaving as a typical alkene. Because antiaromaticity is a property that molecules try to avoid whenever possible, only a few experimentally observed species are believed to be antiaromatic. Cyclobutadiene and cyclopentadienyl cation are commonly cited as examples of antiaromatic systems. In a conjugated pi-system, electrons are able to capture certain photons as the electrons resonate along a certain distance of p-orbitals - similar to how a radio antenna detects photons along its length.
Typically, the more conjugated longer the pi-system is, the longer the wavelength of photon can be captured. In other words, with every added adjacent double bond we see in a molecule diagram, we can predict the system will be less likely to absorb yellow light appear more red to our eyes and more likely to absorb red light appear more yellow to our eyes.
Many dyes make use of conjugated electron systems to absorb visible light , giving rise to strong colors. For example, the long conjugated hydrocarbon chain in beta-carotene leads to its strong orange color. When an electron in the system absorbs a photon of light of the right wavelength , it can be promoted to a higher energy level. In this model the lowest possible absorption energy corresponds to the energy difference between the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO.
Conjugated systems of fewer than eight conjugated double bonds absorb only in the ultraviolet region and are colorless to the human eye.
History In , Kenichi Fukui published a paper in the Journal of Chemical Physics titled "A molecular theory of reactivity in aromatic hydrocarbons. It is a colorless or white solid with an acrid odor. Cyclooctatetraene in a tub conformation. As a result, the net magnetic field outside the ring is greate Folders related to Aromatic ring current: Physical organic chemistry Revolvy Brain revolvybrain Electric current Revolvy Brain revolvybrain Nuclear magnetic resonance Revolvy Brain revolvybrain. You have access to this article. Carcerand topic Crystal structure of a nitrobenzene bound within a hemicarcerand reported by Cram and coworkers in Chem. Theory Fukui real.
With every double bond added, the system absorbs photons of longer wavelength and lower energy , and the compound ranges from yellow to red in color. Compounds that are blue or green typically do not rely on conjugated double bonds alone. This absorption of light in the ultraviolet to visible spectrum can be quantified using ultraviolet—visible spectroscopy , and forms the basis for the entire field of photochemistry. Conjugated systems that are widely used for synthetic pigments and dyes are diazo and azo compounds and phthalocyanine compounds.
Conjugated systems not only have low energy excitations in the visible spectral region but they also accept or donate electrons easily. Phthalocyanines , which, like Phthalocyanine Blue BN and Phthalocyanine Green G , often contain a transition metal ion, exchange an electron with the complexed transition metal ion that easily changes its oxidation state.
Pigments and dyes like these are charge-transfer complexes. Porphyrins have conjugated molecular ring systems macrocycles that appear in many enzymes of biological systems. As a ligand , porphyrin forms numerous complexes with metallic ions like iron in hemoglobin that colors blood red. Hemoglobin transports oxygen to the cells of our bodies. Porphyrin—metal complexes often have strong colors. A similar molecular structural ring unit called chlorin is similarly complexed with magnesium instead of iron when forming part of the most common forms of chlorophyll molecules, giving them a green color.
Another similar macrocycle unit is corrin , which complexes with cobalt when forming part of cobalamin molecules, constituting Vitamin B12 , which is intensely red. The corrin unit has six conjugated double bonds but is not conjugated all the way around its macrocycle ring. Conjugated systems form the basis of chromophores , which are light-absorbing parts of a molecule that can cause a compound to be colored.