Hypercolor was a line of clothing T-shirts and shorts, that changed colour with heat. They were manufactured by Generra Sportswear Company of Seattle and marketed in the United States as Generra Hypercolor or Generra Hypergrafix and elsewhere as Global Hypercolor, they contained a thermochromic pigment made by Matsui Shikiso Chemical of Japan, that changed between two colours – one when cold, one when warm. The shirts were produced with several color change choices beginning in 1991; the effect could be permanently damaged when the clothing was washed in hotter than recommended water, bleached, or tumble-dried. Generra Sportswear Co. had been founded as a men's sportswear distributor and importer in Seattle in 1980. The company was sold to Texas-based Farah Manufacturing Co. in 1984 and bought back by its founders in 1989. In 1986, the company added womenswear items to their portfolio, they struggled to meet the overwhelming demand for Hypercolor products. Between February and May 1991 they sold $50 million in Hypercolor garments.
Generra went bankrupt due to mismanagement and fading demand in 1992. The Hypercolor business for the U. S. market was sold to The Seattle T-Shirt Company in 1993. The company emerged from bankruptcy in 1995 as a licensing business; the Generra name was acquired by Public Clothing Co. of New York in 2002. Today, Generra Co. is men's apparel brand headquartered in New York City. In the early 2000s, the technique was revived by a number of apparel brands. In 2010, the Hypercolor Trademark was acquired by Cincinnati Native Jay Banks, who recreated the color changing technology, they were wrongfully sued by a law firm to halt the launch of Hypercolor. Those Hypercolor Shirts were never released, are rumored to be in a storage container somewhere in the midwest. Substances that can change color due to a change in temperature are called thermochromes. There are two types of thermochromes: leuco dyes; the color change of Hypercolor shirts is based on combination of two colors: the color of the dyed fabric, which remained constant, the color of the thermochromic dye.
Droplets of the thermochromic dye mixture are enclosed in transparent microcapsules, a few micrometers in diameter, bound to the fibers of the fabric. The thermochromic droplets are a mixture of several chemicals — crystal violet lactone, a quaternary ammonium salt of a fatty acid dissolved in 1-dodecanol as solvent. Together, these lead to a reversible chemical reaction in response to temperature change that produces a change of color. At low temperatures, the mixture is a solid; the weak acid forms a colored complex with the leuco dye by causing the lactone ring in the center of the dye molecule to open. At high temperatures, above 24–27 °C, the solvent melts and the ammonium salt dissociates, allowing it to react with the weak acid; this reaction increases the pH, which leads to closing of the lactone ring of the dye to convert it to its colorless form. Therefore, at the low temperature the color of the shirt is the combination of the color of the encapsulated colored dye with the color of the dyed fabric, while at higher temperatures the capsules become colorless and the color of the fabric prevails.
"Thermochromic Fabric Displays". HowStuffWorks
In organic chemistry, the term aromaticity is used to describe a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. Aromatic molecules are stable, do not break apart to react with other substances. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have special stability. Since the most common aromatic compounds are derivatives of benzene, the word aromatic refers informally to benzene derivatives, so it was first defined. Many non-benzene aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the double-ringed bases in RNA and DNA. An aromatic functional group or other substituent is called an aryl group; the earliest use of the term aromatic was in an article by August Wilhelm Hofmann in 1855. Hofmann used the term for a class of benzene compounds, many of which have odors, unlike pure saturated hydrocarbons.
Aromaticity as a chemical property bears no general relationship with the olfactory properties of such compounds, although in 1855, before the structure of benzene or organic compounds was understood, chemists like Hofmann were beginning to understand that odiferous molecules from plants, such as terpenes, had chemical properties that we recognize today are similar to unsaturated petroleum hydrocarbons like benzene. In terms of the electronic nature of the molecule, aromaticity describes a conjugated system made of alternating single and double bonds in a ring; this configuration allows for the electrons in the molecule's pi system to be delocalized around the ring, increasing the molecule's stability. The molecule cannot be represented by one structure, but rather a resonance hybrid of different structures, such as with the two resonance structures of benzene; these molecules cannot be found in either one of these representations, with the longer single bonds in one location and the shorter double bond in another.
Rather, the molecule exhibits bond lengths in between those of double bonds. This seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds, was developed by August Kekulé; the model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to produce six one-and-a-half bonds. Benzene is a more stable molecule than would be expected without accounting for charge delocalization; as it is a standard for resonance diagrams, the use of a double-headed arrow indicates that two structures are not distinct entities but hypothetical possibilities. Neither is an accurate representation of the actual compound, best represented by a hybrid of these structures. A C=C bond is shorter than a C−C bond. Benzene is a regular hexagon—it is planar and all six carbon–carbon bonds have the same length, intermediate between that of a single and that of a double bond. In a cyclic molecule with three alternating double bonds, the bond length of the single bond would be 1.54 Å and that of the double bond would be 1.34 Å.
However, in a molecule of benzene, the length of each of the bonds is 1.40 Å, indicating it to be the average of single and double bond. A better representation is that of the circular π-bond, in which the electron density is evenly distributed through a π-bond above and below the ring; this model more represents the location of electron density within the aromatic ring. The single bonds are formed from overlap of hybridized atomic sp2-orbitals in line between the carbon nuclei—these are called σ-bonds. Double bonds consist of a π-bond; the π-bonds are formed from overlap of atomic p-orbitals below the plane of the ring. The following diagram shows the positions of these p-orbitals: Since they are out of the plane of the atoms, these orbitals can interact with each other 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 is considered to have π symmetry. The first known use of the word "aromatic" as a chemical term—namely, to apply to compounds that contain the phenyl group—occurs in an article by August Wilhelm Hofmann in 1855. 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. 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 than many aliphatic compounds, Hofmann may not have been making a distinction between the two categories. 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, hence the term "aromaticity" for the discovered electronic property.
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 pr
Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure, or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring. Lactones are formed by intramolecular esterification of the corresponding hydroxycarboxylic acids, which takes place spontaneously when the ring, formed is five- or six-membered. Lactones with three- or four-membered rings are reactive, making their isolation difficult. Special methods are required for the laboratory synthesis of small-ring lactones as well as those that contain rings larger than six-membered. Lactones are named according to the precursor acid molecule, with a -lactone suffix and a Greek letter prefix that specifies the number of carbon atoms in the heterocycle — that is, the distance between the relevant -OH and the -COOH groups along said backbone; the first carbon atom after the carbon in the -COOH group on the parent compound is labelled α, the second will be labeled β, so forth. Therefore, the prefixes indicate the size of the lactone ring: α-lactone = 3-membered ring, β-lactone = 4-membered, γ-lactone = 5-membered, etc.
The other suffix used to denote a lactone is -olide, used in substance class names like butenolide, cardenolide or bufadienolide. To obtain the preferred IUPAC names, lactones are named as heterocyclic pseudoketones by adding the suffix ‘one’, ‘dione’, ‘thione’, etc. and the appropriate multiplicative prefixes to the name of the heterocyclic parent hydride. The name lactone derives from the ring compound called lactide, formed from the dehydration of 2-hydroxypropanoic acid CH3-CH-COOH. Lactic acid, in turn, derives its name from its original isolation from soured milk. An internal dehydration within the same molecule of lactic acid would have produced alpha-propiolactone, a lactone with a 3-membered ring. Occurring lactones are saturated and unsaturated γ- and δ-lactones, to a lesser extent macrocyclic lactones; the γ- and δ-lactones are intramolecular esters of the corresponding hydroxy fatty acids. They contribute to the aroma of fruits, butter and other foods. Cyclopentadecanolide is responsible for the musklike odor of angelica root oil.
Of the occurring bicyclic lactones, phthalides are responsible for the odors of celery and lovage oils, coumarin for woodruff. Lactone rings occur as building blocks in nature, such as in ascorbic acid, nepetalactone, hormones, neurotransmitters, anticancer drugs, phytoestrogens. Many methods in ester synthesis can be applied to that of lactones. In one industrial synthesis of oxandrolone the key lactone-forming step is an organic reduction - esterification. In halolactonization, an alkene is attacked by a halogen via electrophilic addition with the cationic intermediate captured intramolecularly by an adjacent carboxylic acid. Specific methods include Yamaguchi esterification, Shiina macrolactonization, Baeyer–Villiger oxidation and nucleophilic abstraction; the γ-lactones γ-octalactone, γ-nonalactone, γ-decalactone, γ-undecalactone can be prepared in good yield in a one-step process by radical addition of primary fatty alcohols to acrylic acid, using di-tert-butyl peroxide as a catalyst. The most stable structure for lactones are the 5-membered γ-lactones and 6-membered δ-lactones because, as in all organic cycles, 5 and 6 membered rings minimize the strain of bond angles.
Γ-lactones are so stable that, in the presence of dilute acids at room temperature, 4-hydroxy acids undergo spontaneous esterification and cyclisation to the lactone. Β-lactones can only be made by special methods. Α-lactones can be detected as transient species in mass spectrometry experiments. The reactions of lactones are similar to those of esters, as exemplified by gamma-lactone in the following sections: Heating a lactone with a base will hydrolyse the lactone to its parent compound, the straight chained bifunctional compound. Like straight-chained esters, the hydrolysis-condensation reaction of lactones is a reversible reaction, with an equilibrium. However, the equilibrium constant of the hydrolysis reaction of the lactone is lower than that of the straight-chained ester i.e. the products are less favored in the case of the lactones. This is because although the enthalpies of the hydrolysis of esters and lactones are about the same, the entropy of the hydrolysis of lactones is less than the entropy of straight-chained esters.
Straight-chained esters give two products upon hydrolysis, making the entropy change more favorable than in the case of lactones which gives only a single product. Lactones can be reduced to diols using lithium aluminium hydride in dry ether; the reduction reaction will first break the ester bond of the lactone, reduce the aldehyde group to the alcohol group. For instance, gamma-lactones will be reduced to butane-1,4-diol, (CH2-2-CH2. Lactones react with ethanolic ammonia, which will first break the ester bond and react with the acidic -COOH group, because of the basic properties of ammonia, to form a difunctional group, i.e. alcohol and amide. Gamma-lactones will react to yield CH2-2-CO-NH2. Lactones form polyesters according to the formula: Sesquiterpene lactones, found in many plants, can react with other molecules via a Michael reaction. Lactones contribute sig
In geometry, a tetrahedron known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, four vertex corners. The tetrahedron is the simplest of all the ordinary convex polyhedra and the only one that has fewer than 5 faces; the tetrahedron is the three-dimensional case of the more general concept of a Euclidean simplex, may thus be called a 3-simplex. The tetrahedron is one kind of pyramid, a polyhedron with a flat polygon base and triangular faces connecting the base to a common point. In the case of a tetrahedron the base is a triangle, so a tetrahedron is known as a "triangular pyramid". Like all convex polyhedra, a tetrahedron can be folded from a single sheet of paper, it has two such nets. For any tetrahedron there exists a sphere on which all four vertices lie, another sphere tangent to the tetrahedron's faces. A regular tetrahedron is one, it is one of the five regular Platonic solids. In a regular tetrahedron, all faces are the same size and shape and all edges are the same length.
Regular tetrahedra alone do not tessellate, but if alternated with regular octahedra in the ratio of two tetrahedra to one octahedron, they form the alternated cubic honeycomb, a tessellation. The regular tetrahedron is self-dual; the compound figure comprising two such dual tetrahedra form a stellated octahedron or stella octangula. The following Cartesian coordinates define the four vertices of a tetrahedron with edge length 2, centered at the origin, two level edges: and Expressed symmetrically as 4 points on the unit sphere, centroid at the origin, with lower face level, the vertices are: v1 = v2 = v3 = v4 = with the edge length of sqrt. Still another set of coordinates are based on an alternated cube or demicube with edge length 2; this form has Coxeter diagram and Schläfli symbol h. The tetrahedron in this case has edge length 2√2. Inverting these coordinates generates the dual tetrahedron, the pair together form the stellated octahedron, whose vertices are those of the original cube. Tetrahedron:, Dual tetrahedron:, For a regular tetrahedron of edge length a: With respect to the base plane the slope of a face is twice that of an edge, corresponding to the fact that the horizontal distance covered from the base to the apex along an edge is twice that along the median of a face.
In other words, if C is the centroid of the base, the distance from C to a vertex of the base is twice that from C to the midpoint of an edge of the base. This follows from the fact that the medians of a triangle intersect at its centroid, this point divides each of them in two segments, one of, twice as long as the other. For a regular tetrahedron with side length a, radius R of its circumscribing sphere, distances di from an arbitrary point in 3-space to its four vertices, we have d 1 4 + d 2 4 + d 3 4 + d 4 4 4 + 16 R 4 9 = 2.
The visible spectrum is the portion of the electromagnetic spectrum, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or light. A typical human eye will respond to wavelengths from about 380 to 740 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 430–770 THz; the spectrum does not contain all the colors. Unsaturated colors such as pink, or purple variations like magenta, for example, are absent because they can only be made from a mix of multiple wavelengths. Colors containing only one wavelength are called pure colors or spectral colors. Visible wavelengths pass unattenuated through the Earth's atmosphere via the "optical window" region of the electromagnetic spectrum. An example of this phenomenon is when clean air scatters blue light more than red light, so the midday sky appears blue; the optical window is referred to as the "visible window" because it overlaps the human visible response spectrum. The near infrared window lies just out of the human vision, as well as the medium wavelength infrared window, the long wavelength or far infrared window, although other animals may experience them.
In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, described the phenomenon in his book Opticks, he was the first to use the word spectrum in this sense in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different-colored bands. Newton hypothesized light to be made up of "corpuscles" of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more than violet in glass; the result is that red light is bent less than violet as it passes through the prism, creating a spectrum of colors. Newton divided the spectrum into six named colors: red, yellow, green and violet.
He added indigo as the seventh color since he believed that seven was a perfect number as derived from the ancient Greek sophists, of there being a connection between the colors, the musical notes, the known objects in the solar system, the days of the week. The human eye is insensitive to indigo's frequencies, some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right but as a shade of blue or violet. Evidence indicates that what Newton meant by "indigo" and "blue" does not correspond to the modern meanings of those color words. Comparing Newton's observation of prismatic colors to a color image of the visible light spectrum shows that "indigo" corresponds to what is today called blue, whereas "blue" corresponds to cyan. In the 18th century, Johann Wolfgang von Goethe wrote about optical spectra in his Theory of Colours. Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors.
Goethe argued. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges with white between them; the spectrum appears only. In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by William Herschel and Johann Wilhelm Ritter, Thomas Young, Thomas Johann Seebeck, others. Young was the first to measure the wavelengths of different colors of light, in 1802; the connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century. Their theory of color vision proposed that the eye uses three distinct receptors to perceive color. Many species can see light within frequencies outside the human "visible spectrum". Bees and many other insects can detect ultraviolet light. Plant species that depend on insect pollination may owe reproductive success to their appearance in ultraviolet light rather than how colorful they appear to humans.
Birds, can see into the ultraviolet, some have sex-dependent markings on their plumage that are visible only in the ultraviolet range. Many animals that can see into the ultraviolet range cannot see red light or any other reddish wavelengths. Bees' visible spectrum ends at about 590 nm. Birds can see some red wavelengths; the popular belief that the common goldfish is the only animal that can see both infrared and ultraviolet light is incorrect, because goldfish cannot see infrared light. Dogs are thought to be color blind but they have been shown to be sensitive to colors, though not as many as humans; some snakes can "see" radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes, other snakes with the organ may detect warm bodies from a meter away. It may be used in thermoregulation and predator detection. (See
Dichloromethane is a geminal organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is used as a solvent. Although it is not miscible with water, it is polar, miscible with many organic solvents. Natural sources of dichloromethane include oceanic sources, macroalgae and volcanoes. However, the majority of dichloromethane in the environment is the result of industrial emissions. DCM is produced by treating either chloromethane or methane with chlorine gas at 400–500 °C. At these temperatures, both methane and chloromethane undergo a series of reactions producing progressively more chlorinated products. In this way, an estimated 400,000 tons were produced in the US, Japan in 1993. CH4 + Cl2 → CH3Cl + HCl CH3Cl + Cl2 → CH2Cl2 + HCl CH2Cl2 + Cl2 → CHCl3 + HCl CHCl3 + Cl2 → CCl4 + HClThe output of these processes is a mixture of chloromethane, dichloromethane and carbon tetrachloride; these compounds are separated by distillation. DCM was first prepared in 1839 by the French chemist Henri Victor Regnault, who isolated it from a mixture of chloromethane and chlorine, exposed to sunlight.
DCM's volatility and ability to dissolve a wide range of organic compounds makes it a useful solvent for many chemical processes. Carbon diselenide is produced by reacting selenium powder with dichloromethane vapor near 550°C. 2 Se + CH2Cl2 → CSe2 + 2 HClIt is used as a paint stripper and a degreaser. In the food industry, it has been used to decaffeinate coffee and tea as well as to prepare extracts of hops and other flavorings, its volatility has led to its use as an aerosol spray propellant and as a blowing agent for polyurethane foams. The chemical compound's low boiling point allows the chemical to function in a heat engine that can extract mechanical energy from small temperature differences. An example of a DCM heat engine is the drinking bird; the toy works at room temperature. DCM chemically welds certain plastics. For example, it is used to seal the casing of electric meters. Sold as a main component of plastic welding adhesives, it is used extensively by model building hobbyists for joining plastic components together.
It is referred to as "Di-clo." It is used in the garment printing industry for removal of heat-sealed garment transfers, its volatility is exploited in novelty items: bubble lights and jukebox displays. DCM is used in the material testing field of civil engineering. DCM is the least toxic of the simple chlorohydrocarbons, but it is not without health risks, as its high volatility makes it an acute inhalation hazard, it can be absorbed through the skin. Symptoms of acute overexposure to dichloromethane via inhalation include difficulty concentrating, fatigue, headaches, numbness and irritation of the upper respiratory tract and eyes. More severe consequences can include suffocation, loss of consciousness and death. DCM is metabolized by the body to carbon monoxide leading to carbon monoxide poisoning. Acute exposure by inhalation has resulted in optic hepatitis. Prolonged skin contact can result in DCM dissolving some of the fatty tissues in skin, resulting in skin irritation or chemical burns, it may be carcinogenic, as it has been linked to cancer of the lungs and pancreas in laboratory animals.
Other animal studies showed salivary gland cancer. Research is not yet clear as to. DCM crosses the placenta. Fetal toxicity in women who are exposed to it during pregnancy, has not been proven. In animal experiments, it was fetotoxic at doses that were maternally toxic but no teratogenic effects were seen. In people with pre-existing heart problems, exposure to DCM can cause abnormal heart rhythms and/or heart attacks, sometimes without any other symptoms of overexposure. People with existing liver, nervous system, or skin problems may worsen after exposure to methylene chloride. In many countries, products containing DCM must carry labels warning of its health risks. In February 2013, the U. S. Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health warned that at least 14 bathtub refinishers have died since 2000 from DCM exposure; these workers had been working alone, in poorly ventilated bathrooms, with inadequate or no respiratory protection, no training about the hazards of DCM.
OSHA has since issued a DCM standard. In the European Union, the European Parliament voted in 2009 to ban the use of DCM in paint-strippers for consumers and many professionals; the ban took effect in December 2010. In Europe, the Scientific Committee on Occupational Exposure Limit Values recommends for DCM an occupational exposure limit of 100 ppm and a short-term exposure limit of 200 ppm. Concerns about its health effects have led to a search for alternatives in many of these applications. On March 15, 2019, the U. S. Environmental Protection Agency issued a final rule to prohibit the manufacture and distribution of methylene chloride in all paint removers for consumer use, effective in 180 days. Dichloromethane is not classified as an ozone-depleting substance by the Montreal Protocol; the U. S. Clean Air Act does not regulate dichloromethane as an ozone depleter. According to the EPA, the atmospheric lifetime of dichloromethane is short, such that the substance decomposes before reaching the ozone layer.
Ozone concentrations measured at the midlatitudes from the g
In chemistry, resonance is a way of describing bonding in certain molecules or ions by the combination of several contributing structures into a resonance hybrid in valence bond theory. It has particular value for describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. Under the framework of valence bond theory, resonance is an extension of the idea that the bonding in a chemical species can be described by a Lewis structure. For many chemical species, a single Lewis structure, consisting of atoms obeying the octet rule bearing formal charges, connected by bonds of positive integer order, is sufficient for describing the chemical bonding and rationalizing experimentally determined molecular properties like bond lengths and dipole moment. However, in some cases, more than one Lewis structure could be drawn, experimental properties are inconsistent with any one structure. In order to address this type of situation, several contributing structures are considered together as an average, the molecule is said to be represented by a resonance hybrid in which several Lewis structures are used collectively to describe its true structure.
For instance, in NO2–, nitrite anion, the two N–O bond lengths are equal though no single Lewis structure has two N–O bonds with the same formal bond order. However, its measured structure is consistent with a description as a resonance hybrid of the two major contributing structures shown above: it has two equal N–O bonds of 125 pm, intermediate in length between a typical N–O single bond and N–O double bond. According to the contributing structures, each N–O bond is an average of a formal single and formal double bond, leading to a true bond order of 1.5. By virtue of this averaging, the Lewis description of the bonding in NO2– is reconciled with the experimental fact that the anion has equivalent N–O bonds; the resonance hybrid represents the actual molecule as the "average" of the contributing structures, with bond lengths and partial charges taking on intermediate values compared to those expected for the individual Lewis structures of the contributors, were they to exist as "real" chemical entities.
The contributing structures differ only in the formal apportionment of electrons to the atoms, not in the actual physically and chemically significant electron or spin density. While contributing structures may differ in formal bond orders and in formal charge assignments, all contributing structures must have the same number of valence electrons and the same spin multiplicity; because electron delocalization lowers the potential energy of a system, any species represented by a resonance hybrid is more stable than any of the contributing structures. The difference in potential energy between the actual species and the energy of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy; the magnitude of the resonance energy depends on assumptions made about the hypothetical "non-stabilized" species and the computational methods used and does not represent a measurable physical quantity, although comparisons of resonance energies computed under similar assumptions and conditions may be chemically meaningful.
Molecules with an extended π system such as linear polyenes and polyaromatic compounds are well described by resonance hybrids as well as by delocalised orbitals in molecular orbital theory. Resonance is to be distinguished from isomerism. Isomers are molecules with the same chemical formula but are distinct chemical species with different arrangements of atomic nuclei in space. Resonance contributors of a molecule, on the other hand, can only differ in the way electrons are formally assigned to atoms in the Lewis structure depictions of the molecule; when a molecular structure is said to be represented by a resonance hybrid, it does not mean that electrons of the molecule are "resonating" or shifting back and forth between several sets of positions, each one represented by a Lewis structure. Rather, it means that the set of contributing structures represents an intermediate structure, with a single, well-defined geometry and distribution of electrons, it is incorrect to regard resonance hybrids as interconverting isomers though the term "resonance" might evoke such an image.
Symbolically, the double headed arrow A ⟷ B is used to indicate that A and B are contributing forms of a single chemical species. A non-chemical analogy is illustrative: one can describe the characteristics of a real animal, the narwhal, in terms of the characteristics of two mythical creatures: the unicorn, a creature with a single horn on its head, the leviathan, a large, whale-like creature; the narwhal is not a creature that goes back and forth between being a unicorn and being a leviathan, nor do the unicorn and leviathan have any physical existence outside the collective human imagination. Describing the narwhal in terms of these imaginary creatures provides a reasonably good description of its physical characteristics. Due to confusion