Aromaticity

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Two different resonance forms of benzene (top) combine to produce an average structure (bottom) Benzene resonance structures.png
Two different resonance forms of benzene (top) combine to produce an average structure (bottom)

In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August Wilhelm Hofmann in 1855. [1] There is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds.

Contents

Aromaticity can also be considered a manifestation of cyclic delocalization and of resonance. [2] [3] [4] This is usually considered to be because electrons are free to cycle around circular arrangements of atoms that 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. This commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by Kekulé (see History section below). The model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to give rise to six one-and-a-half bonds. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.

Theory

Modern depiction of benzene Benzene-6H-delocalized.svg
Modern depiction of benzene

As is standard for resonance diagrams, a double-headed arrow is used to indicate that the two structures are not distinct entities, but merely hypothetical possibilities. Neither is an accurate representation of the actual compound, which is best represented by a hybrid (average) of these structures, which can be seen at right. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon-carbon bonds have the same length, intermediate between that of a single and that of a double bond.

A better representation is that of the circular π bond (Armstrong's inner cycle), in which the electron density is evenly distributed through a π-bond above and below the ring. This model more correctly represents the location of electron density within the aromatic ring.

The single bonds are formed with electrons in line between the carbon nuclei — these are called σ-bonds. Double bonds consist of a σ-bond and a π-bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. The following diagram shows the positions of these p-orbitals:

Benzene-orbitals.png

Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalized. This means that, instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry.

Benzene Orbitals.svg

History

The term "aromatic"

The first known use of the word "aromatic" as a chemical term — namely, to apply to compounds that contain the phenyl radical — occurs in an article by August Wilhelm Hofmann in 1855. [1] If this is indeed the earliest introduction of the term, it is curious that Hofmann says nothing about why he introduced an adjective indicating olfactory character to apply to a group of chemical substances only some of which have notable aromas. Also, many of the most odoriferous organic substances known are terpenes, which are not aromatic in the chemical sense. But terpenes and benzenoid substances do have a chemical characteristic in common, namely higher unsaturation indices than many aliphatic compounds, and Hofmann may not have been making a distinction between the two categories.

The structure of the benzene ring

Historic benzene formulae as proposed by Kekule. Historic Benzene Formulae Kekule (original).png
Historic benzene formulae as proposed by Kekulé.

In the 19th century chemists found it puzzling that benzene could be so unreactive toward addition reactions, given its presumed high degree of unsaturation. The cyclohexatriene structure for benzene was first proposed by August Kekulé in 1865. Over the next few decades, most chemists readily accepted this structure, since it accounted for most of the known isomeric relationships of aromatic chemistry.

Between 1897 and 1906, J. J. Thomson, the discoverer of the electron, proposed three equivalent electrons between each carbon atom in benzene.

An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson, who was apparently the first (in 1925) [6] to coin the term aromatic sextet as a group of six electrons that resists disruption.

In fact, this concept can be traced further back, via Ernest Crocker in 1922, [7] to Henry Edward Armstrong, who in 1890 wrote "the (six) centric affinities act within a cycle...benzene may be represented by a double ring (sic) ... and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition". [8] [ verification needed ]

Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as the electron, which was to be discovered only seven years later by J. J. Thomson. Second, he is describing electrophilic aromatic substitution, proceeding (third) through a Wheland intermediate, in which (fourth) the conjugation of the ring is broken. He introduced the symbol C centered on the ring as a shorthand for the inner cycle, thus anticipating Erich Clar's notation. It is argued that he also anticipated the nature of wave mechanics, since he recognized that his affinities had direction, not merely being point particles, and collectively having a distribution that could be altered by introducing substituents onto the benzene ring (much as the distribution of the electric charge in a body is altered by bringing it near to another body).

The quantum mechanical origins of this stability, or aromaticity, were first modelled by Hückel in 1931. He was the first to separate the bonding electrons into sigma and pi electrons.

Characteristics of aromatic (aryl) compounds

An aromatic (or aryl) compound contains a set of covalently bound atoms with specific characteristics:

  1. A delocalized conjugated π system, most commonly an arrangement of alternating single and double bonds
  2. Coplanar structure, with all the contributing atoms in the same plane
  3. Contributing atoms arranged in one or more rings
  4. A number of π delocalized electrons that is even, but not a multiple of 4. That is, 4n + 2 number of π electrons, where n=0, 1, 2, 3, and so on. This is known as Hückel's Rule.

Whereas benzene is aromatic (6 electrons, from 3 double bonds), cyclobutadiene is not, since the number of π delocalized electrons is 4, which of course is a multiple of 4. The cyclobutadienide (2−) ion, however, is aromatic (6 electrons). An atom in an aromatic system can have other electrons that are not part of the system, and are therefore ignored for the 4n + 2 rule. In furan, the oxygen atom is sp² hybridized. One lone pair is in the π system and the other in the plane of the ring (analogous to C-H bond on the other positions). There are 6 π electrons, so furan is aromatic.

Aromatic molecules typically display enhanced chemical stability, compared to similar non-aromatic molecules. A molecule that can be aromatic will tend to alter its electronic or conformational structure to be in this situation. This extra stability changes the chemistry of the molecule. Aromatic compounds undergo electrophilic aromatic substitution and nucleophilic aromatic substitution reactions, but not electrophilic addition reactions as happens with carbon-carbon double bonds.

Many of the earliest-known examples of aromatic compounds, such as benzene and toluene, have distinctive pleasant smells. This property led to the term "aromatic" for this class of compounds, and hence the term "aromaticity" for the eventually discovered electronic property.

The circulating π electrons in an aromatic molecule produce ring currents that oppose the applied magnetic field in NMR. [9] The NMR signal of protons in the plane of an aromatic ring are shifted substantially further down-field than those on non-aromatic sp² carbons. This is an important way of detecting aromaticity. By the same mechanism, the signals of protons located near the ring axis are shifted up-field.

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.

Planar monocyclic molecules containing 4n π electrons are called antiaromatic and are, in general, destabilized. Molecules that could be antiaromatic will tend to alter their electronic or conformational structure to avoid this situation, thereby becoming non-aromatic. For example, cyclooctatetraene (COT) distorts itself out of planarity, breaking π overlap between adjacent double bonds. Relatively recently, cyclobutadiene was discovered to adopt an asymmetric, rectangular configuration in which single and double bonds indeed alternate; there is no resonance and the single bonds are markedly longer than the double bonds, reducing unfavorable p-orbital overlap. This reduction of symmetry lifts the degeneracy of the two formerly non-bonding molecular orbitals, which by Hund's rule forces the two unpaired electrons into a new, weakly bonding orbital (and also creates a weakly antibonding orbital). Hence, cyclobutadiene is non-aromatic; the strain of the asymmetric configuration outweighs the anti-aromatic destabilization that would afflict the symmetric, square configuration.

Importance of aromatic compounds

Aromatic compounds play key roles in the biochemistry of all living things. The four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine each serve as one of the 20 basic building-blocks of proteins. Further, all 5 nucleotides (adenine, thymine, cytosine, guanine, and uracil) that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines. The molecule heme contains an aromatic system with 22 π electrons. Chlorophyll also has a similar aromatic system.

Aromatic compounds are important in industry. Key aromatic hydrocarbons of commercial interest are benzene, toluene, ortho-xylene and para-xylene. About 35 million tonnes are produced worldwide every year. They are extracted from complex mixtures obtained by the refining of oil or by distillation of coal tar, and are used to produce a range of important chemicals and polymers, including styrene, phenol, aniline, polyester and nylon.

Types of aromatic compounds

The overwhelming majority of aromatic compounds are compounds of carbon, but they need not be hydrocarbons.

Neutral homocyclics

Benzene, as well as most other annulenes (cyclodecapentaene excepted) with the formula CnHn where n ≥ 4 and is an even number, such as cyclotetradecaheptaene.

Heterocyclics

In heterocyclic aromatics (heteroaromats), one or more of the atoms in the aromatic ring is of an element other than carbon. This can lessen the ring's aromaticity, and thus (as in the case of furan) increase its reactivity. Other examples include pyridine, pyrazine, imidazole, pyrazole, oxazole, thiophene, and their benzannulated analogs (benzimidazole, for example).

Polycyclics

Polycyclic aromatic hydrocarbons are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms (see also simple aromatic rings). Examples are naphthalene, anthracene, and phenanthrene.

Substituted aromatics

Many chemical compounds are aromatic rings with other functional groups attached. Examples include trinitrotoluene (TNT), acetylsalicylic acid (aspirin), paracetamol, and the nucleotides of DNA.

Atypical aromatic compounds

Aromaticity is found in ions as well: the cyclopropenyl cation (2e system), the cyclopentadienyl anion (6e system), the tropylium ion (6e), and the cyclooctatetraene dianion (10e). Aromatic properties have been attributed to non-benzenoid compounds such as tropone. Aromatic properties are tested to the limit in a class of compounds called cyclophanes.

A special case of aromaticity is found in homoaromaticity where conjugation is interrupted by a single sp³ hybridized carbon atom.

When carbon in benzene is replaced by other elements in borabenzene, silabenzene, germanabenzene, stannabenzene, phosphorine or pyrylium salts the aromaticity is still retained. Aromaticity also occurs in compounds that are not carbon-based at all. Inorganic 6-membered-ring compounds analogous to benzene have been synthesized. Hexasilabenzene (Si6H6) and borazine (B3N3H6) are structurally analogous to benzene, with the carbon atoms replaced by another element or elements. In borazine, the boron and nitrogen atoms alternate around the ring. Quite recently, the aromaticity of planar Si56- rings occurring in the Zintl phase Li12Si7 was experimentally evidenced by Li solid state NMR. [10]

Metal aromaticity is believed to exist in certain metal clusters of aluminium.[ citation needed ]

Möbius aromaticity occurs when a cyclic system of molecular orbitals, formed from pπ atomic orbitals and populated in a closed shell by 4n (n is an integer) electrons, is given a single half-twist to correspond to a Möbius strip. A π system with 4n electrons in a flat (non-twisted) ring would be anti-aromatic, and therefore highly unstable, due to the symmetry of the combinations of p atomic orbitals. By twisting the ring, the symmetry of the system changes and becomes allowed (see also Möbius–Hückel concept for details). Because the twist can be left-handed or right-handed, the resulting Möbius aromatics are dissymmetric or chiral. As of 2012, there is no proof that a Möbius aromatic molecule was synthesized. [11] [12] Aromatics with two half-twists corresponding to the paradromic topologies were first suggested by Johann Listing. [13] In carbo-benzene the ring bonds are extended with alkyne and allene groups.

Y-aromaticity

Y-aromaticity is a concept which was developed to explain the extraordinary stability and high basicity of the guanidinium cation. Guanidinium does not have a ring structure but has six π-electrons which are delocalized over the molecule. However, this concept is controversial and some authors have stressed different effects. [14] [15] [16]

See also

Related Research Articles

<span class="mw-page-title-main">Aromatic compound</span> Compound containing rings with delocalized pi electrons

Aromatic compounds or arenes usually refers to organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying Hückel's Rule. Aromatic compounds have the following general properties:

<span class="mw-page-title-main">Covalent bond</span> Chemical bond by sharing of electron pairs

A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full valence shell, corresponding to a stable electronic configuration. In organic chemistry, covalent bonding is much more common than ionic bonding.

<span class="mw-page-title-main">Conjugated system</span> System of connected p-orbitals with delocalized electrons in a molecule

In theoretical chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability. It is conventionally represented as having alternating single and multiple bonds. Lone pairs, radicals or carbenium ions may be part of the system, which may be cyclic, acyclic, linear or mixed. The term "conjugated" was coined in 1899 by the German chemist Johannes Thiele.

In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures into a resonance hybrid in valence bond theory. It has particular value for analyzing delocalized electrons where the bonding cannot be expressed by one single Lewis structure. The resonance hybrid is the accurate structure for a molecule or ion; it is an average of the theoretical contributing structures.

In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century. The MOT explains the paramagnetic nature of O2, which VSEPR theory cannot explain.

<span class="mw-page-title-main">Delocalized electron</span> Electrons that are not associated with a single atom or covalent bond

In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.

<span class="mw-page-title-main">Annulene</span> Completely conjugated monocyclic hydrocarbons

Annulenes are monocyclic hydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds ('mancude'). They have the general formula CnHn (when n is an even number) or CnHn+1 (when n is an odd number). The IUPAC accepts the use of 'annulene nomenclature' in naming carbocyclic ring systems with 7 or more carbon atoms, using the name '[n]annulene' for the mancude hydrocarbon with n carbon atoms in its ring, though in certain contexts (e.g., discussions of aromaticity for different ring sizes), smaller rings (n = 3 to 6) can also be informally referred to as annulenes. Using this form of nomenclature 1,3,5,7-cyclooctatetraene is [8]annulene and benzene is [6]annulene (and occasionally referred to as just 'annulene').

<span class="mw-page-title-main">Hückel's rule</span> Method of determining aromaticity in organic molecules

In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π-electrons, where n is a non-negative integer. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. The succinct expression as the 4n + 2 rule has been attributed to W. v. E. Doering (1951), although several authors were using this form at around the same time.

Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy.

<span class="mw-page-title-main">Silabenzene</span> Chemical compound

A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene, trisilabenzene, etc.

In chemistry, bond order is a formal measure of the multiplicity of a covalent bond between two atoms. As introduced by Linus Pauling, bond order is defined as the difference between the numbers of electron pairs in bonding and antibonding molecular orbitals.

Simple aromatic rings, also known as simple arenes or simple aromatics, are aromatic organic compounds that consist only of a conjugated planar ring system. Many simple aromatic rings have trivial names. They are usually found as substructures of more complex molecules. Typical simple aromatic compounds are benzene, indole, and pyridine.

<span class="mw-page-title-main">Borazine</span> Boron compound

Borazine, also known as borazole, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as “inorganic benzene”. Like benzene, borazine is a colourless liquid with an aromatic odor.

<span class="mw-page-title-main">Cyclooctadecanonaene</span> Chemical compound

Cyclooctadecanonaene or [18]annulene is an organic compound with chemical formula C
18H
18. It belongs to the class of highly conjugated compounds known as annulenes and is aromatic. The usual isomer that [18]annulene refers to is the most stable one, containing six interior hydrogens and twelve exterior ones, with the nine formal double bonds in the cis,trans,trans,cis,trans,trans,cis,trans,trans configuration. It is reported to be a red-brown crystalline solid.

The Hückel method or Hückel molecular orbital theory, proposed by Erich Hückel in 1930, is a simple method for calculating molecular orbitals as linear combinations of atomic orbitals. The theory predicts the molecular orbitals for π-electrons in π-delocalized molecules, such as ethylene, benzene, butadiene, and pyridine. It provides the theoretical basis for Hückel's rule that cyclic, planar molecules or ions with π-electrons are aromatic. It was later extended to conjugated molecules such as pyridine, pyrrole and furan that contain atoms other than carbon and hydrogen (heteroatoms). A more dramatic extension of the method to include σ-electrons, known as the extended Hückel method (EHM), was developed by Roald Hoffmann. The extended Hückel method gives some degree of quantitative accuracy for organic molecules in general and was used to provide computational justification for the Woodward–Hoffmann rules. To distinguish the original approach from Hoffmann's extension, the Hückel method is also known as the simple Hückel method (SHM).

<span class="mw-page-title-main">Cyclic compound</span> Molecule with a ring of bonded atoms

A cyclic compound is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon, none of the atoms are carbon, or where both carbon and non-carbon atoms are present. Depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be aromatic or non-aromatic; in the latter case, they may vary from being fully saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size numbers in the many billions.

<span class="mw-page-title-main">Aromatic ring current</span> Electric current observed in aromatic compounds

An aromatic ring current is an effect observed in aromatic molecules such as benzene and naphthalene. If a magnetic field is directed perpendicular to the plane of the aromatic system, a ring current is induced in the delocalized π electrons of the aromatic ring. This is a direct consequence of Ampère's law; since the electrons involved are free to circulate, rather than being localized in bonds as they would be in most non-aromatic molecules, they respond much more strongly to the magnetic field.

<span class="mw-page-title-main">Homoaromaticity</span> Organic molecular structure

Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.

<span class="mw-page-title-main">Möbius aromaticity</span>

In organic chemistry, Möbius aromaticity is a special type of aromaticity believed to exist in a number of organic molecules. In terms of molecular orbital theory these compounds have in common a monocyclic array of molecular orbitals in which there is an odd number of out-of-phase overlaps, the opposite pattern compared to the aromatic character to Hückel systems. The nodal plane of the orbitals, viewed as a ribbon, is a Möbius strip, rather than a cylinder, hence the name. The pattern of orbital energies is given by a rotated Frost circle (with the edge of the polygon on the bottom instead of a vertex), so systems with 4n electrons are aromatic, while those with 4n + 2 electrons are anti-aromatic/non-aromatic. Due to incrementally twisted nature of the orbitals of a Möbius aromatic system, stable Möbius aromatic molecules need to contain at least 8 electrons, although 4 electron Möbius aromatic transition states are well known in the context of the Dewar-Zimmerman framework for pericyclic reactions. Möbius molecular systems were considered in 1964 by Edgar Heilbronner by application of the Hückel method, but the first such isolable compound was not synthesized until 2003 by the group of Rainer Herges. However, the fleeting trans-C9H9+ cation, one conformation of which is shown on the right, was proposed to be a Möbius aromatic reactive intermediate in 1998 based on computational and experimental evidence.

<span class="mw-page-title-main">Kekulene</span> Chemical compound

Kekulene is a polycyclic aromatic hydrocarbon which consists of 12 fused benzene rings arranged in a circle. It is therefore classified as a [12]-circulene with the chemical formula C48H24. It was first synthesized in 1978, and was named in honor of August Kekulé, the discoverer of the structure of the benzene molecule.

References

  1. 1 2 A. W. Hofmann (1855). "On Insolinic Acid". Proceedings of the Royal Society. 8: 1–3. doi:10.1098/rspl.1856.0002.
  2. ↑ Schleyer, Paul von Ragué (2001). "Introduction: Aromaticity". Chemical Reviews. 101 (5): 1115–8. doi:10.1021/cr0103221. PMID   11749368.
  3. ↑ A. T. Balaban, P. v. R. Schleyer and H. S. Rzepa (2005). "Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons". Chemical Reviews . 105 (10): 3436–3447. doi:10.1021/cr0300946.
  4. ↑ Schleyer, Paul von Ragué (2005). "Introduction: Delocalization Pi and Sigma". Chemical Reviews. 105 (10): 3433. doi: 10.1021/cr030095y .
  5. ↑ August Kekulé (1872). "Ueber einige Condensationsproducte des Aldehyds". Liebigs Ann. Chem. 162 (1): 77–124. doi:10.1002/jlac.18721620110.
  6. ↑ Armit, James Wilson; Robinson, Robert (1925). "CCXI.?Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases". Journal of the Chemical Society, Transactions. 127: 1604. doi:10.1039/CT9252701604.
  7. ↑ Ernest C. Crocker (1922). "Application Of The Octet Theory To Single-Ring Aromatic Compounds". J. Am. Chem. Soc. 44 (8): 1618–1630. doi:10.1021/ja01429a002.
  8. ↑ Henry Edward Armstrong (1890). "The structure of cycloid hydrocarbon". Proceedings of the Chemical Society (London). 6 (85): 95–106. doi:10.1039/PL8900600095.
  9. ↑ Merino, Gabriel; Heine, Thomas; Seifert, Gotthard (2004). "The Induced Magnetic Field in Cyclic Molecules". Chemistry – A European Journal. 10 (17): 4367. doi:10.1002/chem.200400457.
  10. ↑ Alexander Kuhn, Puravankara Sreeraj, Rainer Pöttgen, Hans-Dieter Wiemhöfer, Martin Wilkening,Paul Heitjans (2011). "Li NMR Spectroscopy on Crystalline Li12Si7: Experimental Evidence for the Aromaticity of the Planar Cyclopentadienyl-Analogous Si56− Rings". Angew. Chem. Int. Ed. 50 (50): 12099. doi:10.1002/anie.201105081.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ↑ D. Ajami, O. Oeckler, A. Simon, R. Herges (2003). "Synthesis of a Möbius aromatic hydrocarbon". Nature. 426 (6968): 819–21. doi:10.1038/nature02224. PMID   14685233.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ↑ Claire Castro, Zhongfang Chen, Chaitanya S. Wannere, Haijun Jiao, William L. Karney, Michael Mauksch, Ralph Puchta, Nico J. R. van Eikema Hommes, Paul von R. Schleyer (2005). "Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius [4 n]Annulene Aromaticity". J. Am. Chem. Soc. 127 (8): 2425–2432. doi:10.1021/ja0458165.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ↑ Rzepa, Henry S. (2005). "A Double-Twist Möbius-Aromatic Conformation of [14]Annulene". Organic Letters. 7 (21): 4637–9. doi:10.1021/ol0518333. PMID   16209498.
  14. ↑ Alberto Gobbi, Gemot Frenking (1993). "Y-Conjugated compounds: the equilibrium geometries and electronic structures of guanidine, guanidinium cation, urea, and 1,1-diaminoethylene". Journal of the American Chemical Society. 115: 2362–2372. doi:10.1021/ja00059a035.
  15. ↑ Kenneth B. Wiberg (1990). "Resonance interactions in acyclic systems. 2. Y-Conjugated anions and cations". Journal of the American Chemical Society. 112: 4177–4182. doi:10.1021/ja00167a011.
  16. ↑ R. Caminiti, A. Pieretti, L. Bencivenni, F. Ramondo, N. Sanna (1996). "Amidine N−C(N)−N Skeleton:  Its Structure in Isolated and Hydrogen-Bonded Guanidines from ab Initio Calculations". The Journal of Physical Chemistry. 100: 10928–10935. doi:10.1021/jp960311p.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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