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113-413: A common spectroscopic method for analysis is Fourier transform infrared spectroscopy (FTIR), where chemical bonds can be detected through their characteristic infrared absorption frequencies or wavelengths. These absorption characteristics make infrared analyzers an invaluable tool in geoscience, environmental science, and atmospheric science. For instance, atmospheric gas monitoring has been facilitated by

226-413: A Michelson interferometer adapted for FTIR, light from the polychromatic infrared source, approximately a black-body radiator, is collimated and directed to a beam splitter . Ideally 50% of the light is refracted towards the fixed mirror and 50% is transmitted towards the moving mirror. Light is reflected from the two mirrors back to the beam splitter and some fraction of the original light passes into

339-408: A functional group or substituent , and "radical" now implies "free". However, the old nomenclature may still appear in some books. The term radical was already in use when the now obsolete radical theory was developed. Louis-Bernard Guyton de Morveau introduced the phrase "radical" in 1785 and the phrase was employed by Antoine Lavoisier in 1789 in his Traité Élémentaire de Chimie . A radical

452-407: A cast thin film. The product was a road cone that had cracked in service, and many similar cones also failed because an anti-UV additive had not been used. Polymers are susceptible to attack by atmospheric oxygen , especially at elevated temperatures encountered during processing to shape. Many process methods such as extrusion and injection moulding involve pumping molten polymer into tools, and

565-552: A circuit and the product separates or fails. In the case of a seal or a tube, failure occurs when the wall of the device is penetrated. The carbonyl end groups that are formed are usually aldehydes or ketones , which can oxidise further to carboxylic acids . The net result is a high concentration of elemental oxygen on the crack surfaces, which can be detected using EDX in the ESEM. For example, two EDX spectra were obtained during an investigation into ozone cracking of diaphragm seals in

678-525: A commercial instrument could be built. Also an electronic computer was needed to perform the required Fourier transform, and this only became practicable with the advent of minicomputers , such as the PDP-8 , which became available in 1965. Digilab pioneered the world's first commercial FTIR spectrometer (Model FTS-14) in 1969. Digilab FTIRs are now a part of Agilent Technologies's molecular product line after Agilent acquired spectroscopy business from Varian . In

791-526: A constant grid, as pioneered by James W. Brault . This confers very high wavenumber accuracy on the resulting infrared spectrum and avoids wavenumber calibration errors. The near-infrared region spans the wavelength range between the rock-salt region and the start of the visible region at about 750 nm. Overtones of fundamental vibrations can be observed in this region. It is used mainly in industrial applications such as process control and chemical imaging . FTIR can be used in all applications where

904-454: A corner-cube mirror is parallel to the incoming ray, regardless of the orientation of the mirror about axes perpendicular to the axis of the light beam. A spectrometer with 0.001 cm resolution is now available commercially. The throughput advantage is important for high-resolution FTIR, as the monochromator in a dispersive instrument with the same resolution would have very narrow entrance and exit slits . In 1966 Janine Connes measured

1017-488: A dispersive spectrometer was used in the past (see external links). In addition, the improved sensitivity and speed have opened up new areas of application. Spectra can be measured in situations where very little energy reaches the detector. Fourier transform infrared spectroscopy is used in geology , chemistry, materials, botany and biology research fields. FTIR is also used to investigate various nanomaterials and proteins in hydrophobic membrane environments. Studies show

1130-479: A gas chromatograph. However this technique is little used compared to GC-MS (gas chromatography-mass spectrometry) which is more sensitive. The GC-IR method is particularly useful for identifying isomers, which by their nature have identical masses. Liquid chromatography fractions are more difficult because of the solvent present. One notable exception is to measure chain branching as a function of molecular size in polyethylene using gel permeation chromatography , which

1243-456: A group that is instead electron-withdrawing, the SOMO then interacts with the empty π* orbital. There are no electrons occupying the higher energy orbital formed, while a new SOMO forms that is lower in energy. This results in a lower energy and higher stability of the radical species. Both donating groups and withdrawing groups stabilize radicals. Another well-known albeit weaker form of delocalization

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1356-435: A higher temperature source, typically a tungsten-halogen lamp. The long wavelength output of these is limited to about 5 μm (2,000 cm ) by the absorption of the quartz envelope. For the far-IR, especially at wavelengths beyond 50 μm (200 cm ) a mercury discharge lamp gives higher output than a thermal source. Far-IR spectrometers commonly use pyroelectric detectors that respond to changes in temperature as

1469-581: A key role in the intermediary metabolism of various biological compounds. Such radicals can even be messengers in a process dubbed redox signaling . A radical may be trapped within a solvent cage or be otherwise bound. Radicals are either (1) formed from spin-paired molecules or (2) from other radicals. Radicals are formed from spin-paired molecules through homolysis of weak bonds or electron transfer, also known as reduction. Radicals are formed from other radicals through substitution, addition , and elimination reactions. Homolysis makes two new radicals from

1582-490: A key role in these defense mechanisms. These are often the three vitamins, vitamin A , vitamin C and vitamin E and polyphenol antioxidants . Furthermore, there is good evidence indicating that bilirubin and uric acid can act as antioxidants to help neutralize certain radicals. Bilirubin comes from the breakdown of red blood cells ' contents, while uric acid is a breakdown product of purines . Too much bilirubin, though, can lead to jaundice , which could eventually damage

1695-492: A lifetime that is less than a few nanoseconds. To avoid confusion, particularly for carbon-centered radicals, Griller and Ingold introduced the following definitions: "Stabilized should be used to describe a carbon-centered radical, R · , when the R−H bond strength is weaker than the appropriate C−H bond of alkane." "Persistent should be used to describe a radical that has a lifetime that is significantly greater than methyl [radical] under

1808-419: A maximum OPD of 10 m. The point in the interferogram corresponding to zero path difference has to be identified, commonly by assuming it is where the maximum signal occurs. This so-called centerburst is not always symmetrical in real world spectrometers so a phase correction may have to be calculated. The interferogram signal decays as the path difference increases, the rate of decay being inversely related to

1921-443: A microscope with linear or 2-D array detectors. The spatial resolution can approach 5 microns with tens of thousands of pixels . The images contain a spectrum for each pixel and can be viewed as maps showing the intensity at any wavelength or combination of wavelengths. This allows the distribution of different chemical species within the sample to be seen . This technique has been applied in various biological applications including

2034-413: A new lower-energy filled bonding-orbital and a singly-filled new SOMO, higher in energy than the original. While the energy of the unpaired electron has increased, the decrease in energy of the lone pair forming the new bonding orbital outweighs the increase in energy of the new SOMO, resulting in a net decrease of the energy of the molecule. Therefore, electron-donating groups help stabilize radicals. With

2147-464: A normal organic compound. Organic radicals are inherently electron deficient thus the greater the electronegativity of the atom on which the unpaired electron resides the less stable the radical. Between carbon, nitrogen, and oxygen, for example, carbon is the most stable and oxygen the least stable. Electronegativity also factors into the stability of carbon atoms of different hybridizations. Greater s-character correlates to higher electronegativity of

2260-463: A penetration depth of around 1 or 2 micrometers depending on sample conditions. The interferogram in practice consists of a set of intensities measured for discrete values of retardation. The difference between successive retardation values is constant. Thus, a discrete Fourier transform is needed. The fast Fourier transform (FFT) algorithm is used. The first FTIR spectrometers were developed for far-infrared range. The reason for this has to do with

2373-456: A photosensitizer. Typical chemical transformations with this singlet dioxygen species involve, among others, conversion of cellulosic biowaste into new poylmethine dyes. In chemical equations, radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows: Radical reaction mechanisms use single-headed arrows to depict the movement of single electrons: The homolytic cleavage of

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2486-435: A plane mirror that moves linearly to vary the path of one beam. In this arrangement the moving mirror must not tilt or wobble as this would affect how the beams overlap as they recombine. Some systems incorporate a compensating mechanism that automatically adjusts the orientation of one mirror to maintain the alignment. Arrangements that avoid this problem include using cube corner reflectors instead of plane mirrors as these have

2599-430: A prevalent radical, O 2 reacts with many organic compounds to generate radicals together with the hydroperoxide radical. Drying oils and alkyd paints harden due to radical crosslinking initiated by oxygen from the atmosphere. The most common radical in the lower atmosphere is molecular dioxygen. Photodissociation of source molecules produces other radicals. In the lower atmosphere, important radical are produced by

2712-487: A radical is the hydroxyl radical (HO · ), a molecule that has one unpaired electron on the oxygen atom. Two other examples are triplet oxygen and triplet carbene ( ꞉ CH 2 ) which have two unpaired electrons. Radicals may be generated in a number of ways, but typical methods involve redox reactions , ionizing radiation , heat, electrical discharges, and electrolysis are known to produce radicals. Radicals are intermediates in many chemical reactions, more so than

2825-521: A semiconductor fabrication factory. The EDX spectrum of the crack surface shows the high-oxygen peak compared with a constant sulfur peak. In contrast, the EDX spectrum of the unaffected elastomer surface spectrum shows a relatively low-oxygen peak compared with the sulfur peak. Fourier transform infrared spectroscopy Fourier-transform infrared spectroscopy ( FTIR ) is a technique used to obtain an infrared spectrum of absorption or emission of

2938-824: A series of 13-hydroxyoctadecadienoic acid and 9-hydroxyoctadecadienoic acid products that serve as signaling molecules that may trigger responses that counter the tissue injury which caused their formation. ROS attacks other polyunsaturated fatty acids, e.g. arachidonic acid and docosahexaenoic acid , to produce a similar series of signaling products. Reactive oxygen species are also used in controlled reactions involving singlet dioxygen 1 O 2 {\displaystyle {}^{1}\mathrm {O} _{2}} known as type II photooxygenation reactions after Dexter energy transfer ( triplet-triplet annihilation ) from natural triplet dioxygen 3 O 2 {\displaystyle {}^{3}\mathrm {O} _{2}} and triplet excited state of

3051-405: A solid, liquid, or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer , which measures intensity over a narrow range of wavelengths at a time. The term Fourier-transform infrared spectroscopy originates from the fact that a Fourier transform (a mathematical process)

3164-540: A spin-paired molecule by breaking a covalent bond, leaving each of the fragments with one of the electrons in the bond. Because breaking a chemical bond requires energy, homolysis occurs under the addition of heat or light. The bond dissociation energy associated with homolysis depends on the stability of a given compound, and some weak bonds are able to homolyze at relatively lower temperatures. Some homolysis reactions are particularly important because they serve as an initiator for other radical reactions. One such example

3277-444: Is hyperconjugation . In radical chemistry, radicals are stabilized by hyperconjugation with adjacent alkyl groups. The donation of sigma (σ) C−H bonds into the partially empty radical orbitals helps to differentiate the stabilities of radicals on tertiary, secondary, and primary carbons. Tertiary carbon radicals have three σ C-H bonds that donate, secondary radicals only two, and primary radicals only one. Therefore, tertiary radicals are

3390-409: Is α-tocopherol ( vitamin E ). The tocopherol radical itself is insufficiently stable for isolation, but the parent molecule is a highly effective hydrogen-atom donor. The C−H bond is weakened in triphenylmethyl (trityl) derivatives. A large variety of inorganic radicals are stable and in fact isolable. Examples include most first-row transition metal complexes. With regard to main group radicals,

3503-402: Is a spectrum of the signal at a series of discrete wavelengths. The range of wavelengths that can be used in the calculation is limited by the separation of the data points in the interferogram. The shortest wavelength that can be recognized is twice the separation between these data points. For example, with one point per wavelength of a HeNe reference laser at 0.633 μm ( 15 800  cm )

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3616-412: Is absorbed by the sample. Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process is rapidly repeated many times over a short time span. Afterwards, a computer takes all this data and works backward to infer what the absorption is at each wavelength. The beam described above is generated by starting with a broadband light source—one containing

3729-426: Is absorbed, and repeat for each different wavelength. (This is how some UV–vis spectrometers work, for example.) Fourier-transform spectroscopy is a less intuitive way to obtain the same information. Rather than shining a monochromatic beam of light (a beam composed of only a single wavelength) at the sample, this technique shines a beam containing many frequencies of light at once and measures how much of that beam

3842-456: Is an important example of a stable diradical. Singlet oxygen , the lowest-energy non-radical state of dioxygen, is less stable than the diradical due to Hund's rule of maximum multiplicity . The relative stability of the oxygen diradical is primarily due to the spin-forbidden nature of the triplet-singlet transition required for it to grab electrons, i.e., " oxidize ". The diradical state of oxygen also results in its paramagnetic character, which

3955-541: Is an important source of radicals (see eq. 1 below). These reactions give the chlorine radical, Cl , which catalyzes the conversion of ozone to O 2 , thus facilitating ozone depletion ( eq. 2.2 – eq. 2.4 below). Such reactions cause the depletion of the ozone layer , especially since the chlorine radical is free to engage in another reaction chain; consequently, the use of chlorofluorocarbons as refrigerants has been restricted. Radicals play important roles in biology. Many of these are necessary for life, such as

4068-465: Is apparent from the balanced equations. Radicals are important in combustion , atmospheric chemistry , polymerization , plasma chemistry, biochemistry , and many other chemical processes. A majority of natural products are generated by radical-generating enzymes. In living organisms, the radicals superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and thus blood pressure. They also play

4181-476: Is available. For combustion to occur, the energy barrier between these must be overcome. This barrier can be overcome by heat, requiring high temperatures. The triplet-singlet transition is also " forbidden ". This presents an additional barrier to the reaction. It also means molecular oxygen is relatively unreactive at room temperature except in the presence of a catalytic heavy atom such as iron or copper. Combustion consists of various radical chain reactions that

4294-507: Is conferred to the radical anion when the charge can be delocalized . Examples include alkali metal naphthenides , anthracenides , and ketyls . Hydrogen abstraction generates radicals. To achieve this reaction, the C-H bond of the H-atom donor must be weak, which is rarely the case in organic compounds. Allylic and especiall doubly allylic C-H bonds are prone to abstraction by O 2 . This reaction

4407-421: Is converted to a spectrum by Fourier transformation. This requires it to be stored in digital form as a series of values at equal intervals of the path difference between the two beams. To measure the path difference a laser beam is sent through the interferometer, generating a sinusoidal signal where the separation between successive maxima is equal to the wavelength of the laser (typically a 633 nm HeNe laser

4520-400: Is demonstrated by its attraction to an external magnet. Diradicals can also occur in metal-oxo complexes , lending themselves for studies of spin forbidden reactions in transition metal chemistry. Carbenes in their triplet state can be viewed as diradicals centred on the same atom, while these are usually highly reactive persistent carbenes are known, with N-heterocyclic carbenes being

4633-439: Is growing for more specialist applications. There are also derivative methods such as infrared microscopy , which allows very small areas to be analyzed in an optical microscope . One method of elemental analysis that is important in forensic analysis is energy-dispersive X-ray spectroscopy (EDX) performed in the environmental scanning electron microscope (ESEM). The method involves analysis of back-scattered X-rays from

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4746-444: Is no direct equivalent as the apparent wavelength is determined by the modulation frequency in the interferometer. The interferogram belongs in the length dimension. Fourier transform (FT) inverts the dimension, so the FT of the interferogram belongs in the reciprocal length dimension([L ]), that is the dimension of wavenumber . The spectral resolution in cm is equal to the reciprocal of

4859-523: Is possible using chlorinated solvents that have no absorption in the area in question. Measuring the gas evolved as a material is heated allows qualitative identification of the species to complement the purely quantitative information provided by measuring the weight loss. FTIR analysis is used to determine water content in fairly thin plastic and composite parts, more commonly in the laboratory setting. Such FTIR methods have long been used for plastics, and became extended for composite materials in 2018, when

4972-405: Is required to convert the raw data into the actual spectrum. The goal of absorption spectroscopy techniques (FTIR, ultraviolet-visible ("UV-vis") spectroscopy , etc.) is to measure how much light a sample absorbs at each wavelength. The most straightforward way to do this, the "dispersive spectroscopy" technique, is to shine a monochromatic light beam at a sample, measure how much of the light

5085-442: Is the basis of drying oils , such as linoleic acid derivatives. In free-radical additions , a radical adds to a spin-paired substrate. When applied to organic compounds, the reaction usually entails addition to an alkene. This addition generates a new radical, which can add to yet another alkene, etc. This behavior underpins radical polymerization , technology that produces many plastics . Radical elimination can be viewed as

5198-537: Is the basis of the radical chain reaction . The art of polymerization entails the method by which the initiating radical is introduced. For example, methyl methacrylate (MMA) can be polymerized to produce Poly(methyl methacrylate) (PMMA – Plexiglas or Perspex) via a repeating series of radical addition steps: Newer radical polymerization methods are known as living radical polymerization . Variants include reversible addition-fragmentation chain transfer ( RAFT ) and atom transfer radical polymerization ( ATRP ). Being

5311-429: Is the easiest and most straight forward sample type to test. Many polymer degradation mechanisms can be followed using IR spectroscopy, such as UV degradation and oxidation, among many other failure modes. Many polymers are attacked by UV radiation at vulnerable points in their chain structures. Thus, polypropylene suffers severe cracking in sunlight unless anti-oxidants are added. The point of attack occurs at

5424-433: Is the homolysis of halogens, which occurs under light and serves as the driving force for radical halogenation reactions. Another notable reaction is the homolysis of dibenzoyl peroxide, which results in the formation of two benzoyloxy radicals and acts as an initiator for many radical reactions. Classically, radicals form by one-electron reductions . Typically one-electron reduced organic compounds are unstable. Stability

5537-517: Is the inverse of the maximum OPD. For example, a maximum OPD of 2 cm results in a separation of 0.5 cm . This is the spectral resolution in the sense that the value at one point is independent of the values at adjacent points. Most instruments can be operated at different resolutions by choosing different OPD's. Instruments for routine analyses typically have a best resolution of around 0.5 cm , while spectrometers have been built with resolutions as high as 0.001 cm , corresponding to

5650-581: Is used). This can trigger an analog-to-digital converter to measure the IR signal each time the laser signal passes through zero. Alternatively, the laser and IR signals can be measured synchronously at smaller intervals with the IR signal at points corresponding to the laser signal zero crossing being determined by interpolation. This approach allows the use of analog-to-digital converters that are more accurate and precise than converters that can be triggered, resulting in lower noise. The result of Fourier transformation

5763-424: Is usually relatively easy to detect, owing to the strong absorption by the carbonyl group in the spectrum of polyolefins . Polypropylene has a relatively simple spectrum, with few peaks at the carbonyl position (like polyethylene). Oxidation tends to start at tertiary carbon atoms because free radicals here are more stable, so last longer and are attacked by oxygen. The carbonyl group can be further oxidised to break

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5876-427: The aging process itself. Similarly, the process of mito hormesis suggests that repeated exposure to radicals may extend life span. Because radicals are necessary for life, the body has a number of mechanisms to minimize radical-induced damage and to repair damage that occurs, such as the enzymes superoxide dismutase , catalase , glutathione peroxidase and glutathione reductase . In addition, antioxidants play

5989-556: The bolometer were required because of the low energy of the radiation. One such was the Golay detector . An additional issue is the need to exclude atmospheric water vapour because water vapour has an intense pure rotational spectrum in this region. Far-infrared spectrophotometers were cumbersome, slow and expensive. The advantages of the Michelson interferometer were well-known, but considerable technical difficulties had to be overcome before

6102-401: The lung . This process promotes the development of emphysema . Oxybenzone has been found to form radicals in sunlight, and therefore may be associated with cell damage as well. This only occurred when it was combined with other ingredients commonly found in sunscreens, like titanium oxide and octyl methoxycinnamate . ROS attack the polyunsaturated fatty acid , linoleic acid , to form

6215-421: The retardation . The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength. This results in a maximum at zero retardation, when there is constructive interference at all wavelengths, followed by series of "wiggles". The position of zero retardation is determined accurately by finding the point of maximum intensity in

6328-470: The tertiary carbon atom present in every repeat unit, causing oxidation and finally chain breakage. Polyethylene is also susceptible to UV degradation, especially those variants that are branched polymers such as low-density polyethylene . The branch points are tertiary carbon atoms, so polymer degradation starts there and results in chain cleavage, and embrittlement. In the example shown at left, carbonyl groups were readily detected by IR spectroscopy from

6441-399: The 20th century the word "radical" was used in chemistry to indicate any connected group of atoms, such as a methyl group or a carboxyl , whether it was part of a larger molecule or a molecule on its own. A radical is often known as an R group . The qualifier "free" was then needed to specify the unbound case. Following recent nomenclature revisions, a part of a larger molecule is now called

6554-500: The Fourier transform gives a similar result. There are three principal advantages for an FT spectrometer compared to a scanning (dispersive) spectrometer. Another minor advantage is less sensitivity to stray light, that is radiation of one wavelength appearing at another wavelength in the spectrum. In dispersive instruments, this is the result of imperfections in the diffraction gratings and accidental reflections. In FT instruments there

6667-422: The ability of FTIR to directly determine the polarity at a given site along the backbone of a transmembrane protein. The bond features involved with various organic and inorganic nanomaterials and their quantitative analysis can be done with the help of FTIR. An infrared microscope allows samples to be observed and spectra measured from regions as small as 5 microns across. Images can be generated by combining

6780-413: The accuracy of the wavelength calibration. The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required. In practice the scan can be on either side of zero resulting in a double-sided interferogram. Mechanical design limitations may mean that for the highest resolution the scan runs to the maximum OPD on one side of zero only. The interferogram

6893-676: The analysis of tissue sections as an alternative to conventional histopathology , examining the homogeneity of pharmaceutical tablets , and for differentiating morphologically-similar pollen grains. The spatial resolution of FTIR can be further improved below the micrometer scale by integrating it into scanning near-field optical microscopy platform. The corresponding technique is called nano-FTIR and allows for performing broadband spectroscopy on materials in ultra-small quantities (single viruses and protein complexes) and with 10 to 20 nm spatial resolution. The speed of FTIR allows spectra to be obtained from compounds as they are separated by

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7006-432: The article: Infrared spectroscopy . FTIR spectrometers are mostly used for measurements in the mid and near IR regions. For the mid-IR region, 2−25 μm (5,000–400 cm ), the most common source is a silicon carbide (SiC) element heated to about 1,200 K (930 °C; 1,700 °F) ( Globar ). The output is similar to a blackbody. Shorter wavelengths of the near-IR, 1−2.5 μm (10,000–4,000 cm ), require

7119-485: The basic principles remain the same. Nowadays, the moving mirror of the interferometer moves at a constant velocity, and sampling of the interferogram is triggered by finding zero-crossings in the fringes of a secondary interferometer lit by a helium–neon laser . In modern FTIR systems the constant mirror velocity is not strictly required, as long as the laser fringes and the original interferogram are recorded simultaneously with higher sampling rate and then re-interpolated on

7232-458: The beam coming out of the interferometer has a different spectrum. As mentioned, computer processing is required to turn the raw data (light absorption for each mirror position) into the desired result (light absorption for each wavelength). The processing required turns out to be a common algorithm called the Fourier transform . The Fourier transform converts one domain (in this case displacement of

7345-512: The breaking bond is drawn with a "fish-hook" arrow to distinguish from the usual movement of two electrons depicted by a standard curly arrow. The second electron of the breaking bond also moves to pair up with the attacking radical electron. Radicals also take part in radical addition and radical substitution as reactive intermediates . Chain reactions involving radicals can usually be divided into three distinct processes. These are initiation , propagation , and termination . Until late in

7458-449: The carbon atom (due to the close proximity of s orbitals to the nucleus), and the greater the electronegativity the less stable a radical. sp-hybridized carbons (50% s-character) form the least stable radicals compared to sp -hybridized carbons (25% s-character) which form the most stable radicals. The delocalization of electrons across the structure of a radical, also known as its ability to form one or more resonance structures, allows for

7571-840: The central nervous system, while too much uric acid causes gout . Reactive oxygen species or ROS are species such as superoxide , hydrogen peroxide , and hydroxyl radical , commonly associated with cell damage. ROS form as a natural by-product of the normal metabolism of oxygen and have important roles in cell signaling. Two important oxygen-centered radicals are superoxide and hydroxyl radical . They derive from molecular oxygen under reducing conditions. However, because of their reactivity, these same radicals can participate in unwanted side reactions resulting in cell damage. Excessive amounts of these radicals can lead to cell injury and death , which may contribute to many diseases such as cancer , stroke , myocardial infarction , diabetes and major disorders. Many forms of cancer are thought to be

7684-400: The chain molecular weight or degree of polymerization : The higher the chain length the greater the mechanical strength (such as tensile strength ). By cleaving the chain, the molecular weight drops rapidly and there comes a point when it has little strength whatsoever, and a crack forms. Further attack occurs in the freshly exposed crack surfaces and the crack grows steadily until it completes

7797-488: The chain, so weakening the material by lowering the molecular weight , and cracks start to grow in the regions affected. The reaction occurring between double bonds and ozone is known as ozonolysis when one molecule of the gas reacts with the double bond: The immediate result is formation of an ozonide , which then decomposes rapidly so that the double bond is cleaved. This is the critical step in chain breakage when polymers are attacked. The strength of polymers depends on

7910-583: The development of commercially available gas analyzers which can distinguish between carbon dioxide, methane, carbon monoxide, oxygen, and nitric oxide. Ultraviolet (UV) spectroscopy is used where strong absorption of UV radiation occurs in a substance. Such groups are known as chromophores and include aromatic groups, conjugated system of bonds, carbonyl groups and so on. Nuclear magnetic resonance spectroscopy detects hydrogen atoms in specific environments, and complements both infrared (IR) spectroscopy and UV spectroscopy. The use of Raman spectroscopy

8023-425: The electron deficiency to be spread over several atoms, minimizing instability. Delocalization usually occurs in the presence of electron-donating groups, such as hydroxyl groups (−OH), ethers (−OR), adjacent alkenes, and amines (−NH 2 or −NR), or electron-withdrawing groups, such as C=O or C≡N. Delocalization effects can also be understood using molecular orbital theory as a lens, more specifically, by examining

8136-451: The four methyl groups to impede the way of a reacting molecule so the structure is unstable. The stability of many (or most) organic radicals is not indicated by their isolability but is manifested in their ability to function as donors of H . This property reflects a weakened bond to hydrogen, usually O−H but sometimes N−H or C−H. This behavior is important because these H donors serve as antioxidants in biology and in commerce. Illustrative

8249-437: The full spectrum of wavelengths to be measured. The light shines into a Michelson interferometer —a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference . Different wavelengths are modulated at different rates, so that at each moment or mirror position

8362-661: The gasoline-air mixture, tetraethyl lead was once commonly added to gasoline. This prevents the combustion from initiating in an uncontrolled manner or in unburnt residues ( engine knocking ) or premature ignition ( preignition ). When a hydrocarbon is burned, a large number of different oxygen radicals are involved. Initially, hydroperoxyl radical (HOO ) are formed. These then react further to give organic hydroperoxides that break up into hydroxyl radicals (HO ). Many polymerization reactions are initiated by radicals. Polymerization involves an initial radical adding to non-radical (usually an alkene) to give new radicals. This process

8475-410: The high temperatures needed for melting may result in oxidation unless precautions are taken. For example, a forearm crutch suddenly snapped and the user was severely injured in the resulting fall. The crutch had fractured across a polypropylene insert within the aluminium tube of the device, and IR spectroscopy of the material showed that it had oxidised, possibly as a result of poor moulding. Oxidation

8588-514: The historical definition of radicals contends that the molecules have nonzero electron spin. However, in fields including spectroscopy and astrochemistry , the definition is slightly different. Gerhard Herzberg , who won the Nobel prize for his research into the electron structure and geometry of radicals, suggested a looser definition of free radicals: "any transient (chemically unstable) species (atom, molecule, or ion)". The main point of his suggestion

8701-399: The incident radiation. However, as any material has a limited range of optical transmittance, several beam-splitters may be used interchangeably to cover a wide spectral range. In a simple Michelson interferometer, one beam passes twice through the beamsplitter but the other passes through only once. To correct for this, an additional compensator plate of equal thickness is incorporated. For

8814-547: The intensity of IR radiation falling on them varies. The sensitive elements in these detectors are either deuterated triglycine sulfate (DTGS) or lithium tantalate (LiTaO 3 ). These detectors operate at ambient temperatures and provide adequate sensitivity for most routine applications. To achieve the best sensitivity the time for a scan is typically a few seconds. Cooled photoelectric detectors are employed for situations requiring higher sensitivity or faster response. Liquid nitrogen cooled mercury cadmium telluride (MCT) detectors are

8927-404: The interferogram. When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample. Commercial spectrometers use Michelson interferometers with a variety of scanning mechanisms to generate the path difference. Common to all these arrangements is the need to ensure that the two beams recombine exactly as the system scans. The simplest systems have

9040-419: The interferometer increases as the path in the other decreases. A quite different approach involves moving a wedge of an IR-transparent material such as KBr into one of the beams. Increasing the thickness of KBr in the beam increases the optical path because the refractive index is higher than that of air. One limitation of this approach is that the variation of refractive index over the wavelength range limits

9153-425: The intracellular killing of bacteria by phagocytic cells such as granulocytes and macrophages . Radicals are involved in cell signalling processes, known as redox signaling . For example, radical attack of linoleic acid produces a series of 13-hydroxyoctadecadienoic acids and 9-hydroxyoctadecadienoic acids , which may act to regulate localized tissue inflammatory and/or healing responses, pain perception, and

9266-500: The intramolecular interaction of the unpaired electron with a donating group's pair of electrons or the empty π* orbital of an electron-withdrawing group in the form of a molecular orbital diagram. The HOMO of a radical is singly-occupied hence the orbital is aptly referred to as the SOMO, or the Singly-Occupied Molecular Orbital. For an electron-donating group, the SOMO interacts with the lower energy lone pair to form

9379-404: The length of the interferogram from zero to the maximum OPD as this makes their contributions orthogonal. This results in a spectrum with points separated by equal frequency intervals. For a maximum path difference d adjacent wavelengths λ 1 and λ 2 will have n and (n+1) cycles, respectively, in the interferogram. The corresponding frequencies are ν 1 and ν 2 : The separation

9492-429: The maximal retardation in cm. Thus a 4 cm resolution will be obtained if the maximal retardation is 0.25 cm; this is typical of the cheaper FTIR instruments. Much higher resolution can be obtained by increasing the maximal retardation. This is not easy, as the moving mirror must travel in a near-perfect straight line. The use of corner-cube mirrors in place of the flat mirrors is helpful, as an outgoing ray from

9605-467: The mechanical tolerance needed for good optical performance, which is related to the wavelength of the light being used. For the relatively long wavelengths of the far infrared, ~10 μm tolerances are adequate, whereas for the rock-salt region tolerances have to be better than 1 μm. A typical instrument was the cube interferometer developed at the NPL and marketed by Grubb Parsons . It used a stepper motor to drive

9718-568: The method was introduced by Krauklis, Gagani and Echtermeyer. FTIR method uses the maxima of the absorbance band at about 5,200 cm−1 which correlates with the true water content in the material. Free radicals In chemistry , a radical , also known as a free radical , is an atom , molecule , or ion that has at least one unpaired valence electron . With some exceptions, these unpaired electrons make radicals highly chemically reactive . Many radicals spontaneously dimerize . Most organic radicals have short lifetimes. A notable example of

9831-399: The mid-IR region, the beamsplitter is usually made of KBr with a germanium-based coating that makes it semi-reflective. KBr absorbs strongly at wavelengths beyond 25 μm (400 cm ), so CsI or KRS-5 are sometimes used to extend the range to about 50 μm (200 cm ). ZnSe is an alternative where moisture vapour can be a problem, but is limited to about 20 μm (500 cm ). CaF 2 is

9944-463: The mirror in cm) into its inverse domain (wavenumbers in cm ). The raw data is called an "interferogram". The first low-cost spectrophotometer capable of recording an infrared spectrum was the Perkin-Elmer Infracord produced in 1957. This instrument covered the wavelength range from 2.5 μm to 15 μm ( wavenumber range 4,000 cm to 660 cm ). The lower wavelength limit

10057-630: The modern chemical sense, as they are permanently bound to each other, and have no unpaired, reactive electrons; however, they can be observed as radicals in mass spectrometry when broken apart by irradiation with energetic electrons. In a modern context the first organic (carbon–containing) radical identified was the triphenylmethyl radical , (C 6 H 5 ) 3 C . This species was discovered by Moses Gomberg in 1900. In 1933 Morris S. Kharasch and Frank Mayo proposed that free radicals were responsible for anti-Markovnikov addition of hydrogen bromide to allyl bromide . In most fields of chemistry,

10170-539: The most abundant radical in the universe is also the most abundant chemical in the universe, H . Most main group radicals are not however isolable , despite their intrinsic stability. Hydrogen radicals for example combine eagerly to form H 2 . Nitric oxide (NO) is well known example of an isolable inorganic radical. Fremy's salt (Potassium nitrosodisulfonate, (KSO 3 ) 2 NO) is a related example. Many thiazyl radicals are known, despite limited extent of π resonance stabilization . Many radicals can be envisioned as

10283-499: The most common example. Triplet carbenes and nitrenes are diradicals. Their chemical properties are distinct from the properties of their singlet analogues. A familiar radical reaction is combustion . The oxygen molecule is a stable diradical , best represented by O–O . Because spins of the electrons are parallel, this molecule is stable. While the ground state of oxygen is this unreactive spin-unpaired ( triplet ) diradical, an extremely reactive spin-paired ( singlet ) state

10396-537: The most stable and primary radicals the least stable. Most simply, the greater the steric hindrance the more difficult it is for reactions to take place, and the radical form is favored by default. For example, compare the hydrogen-abstracted form of N -hydroxypiperidine to the molecule TEMPO . TEMPO, or (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, is too sterically hindered by the additional methyl groups to react making it stable enough to be sold commercially in its radical form. N -Hydroxypiperidine, however, does not have

10509-418: The most widely used in the mid-IR. With these detectors an interferogram can be measured in as little as 10 milliseconds. Uncooled indium gallium arsenide photodiodes or DTGS are the usual choices in near-IR systems. Very sensitive liquid-helium-cooled silicon or germanium bolometers are used in the far-IR where both sources and beamsplitters are inefficient. An ideal beam-splitter transmits and reflects 50% of

10622-599: The moving mirror, recording the detector response after each step was completed. With the advent of cheap microcomputers it became possible to have a computer dedicated to controlling the spectrometer, collecting the data, doing the Fourier transform and presenting the spectrum. This provided the impetus for the development of FTIR spectrometers for the rock-salt region. The problems of manufacturing ultra-high precision optical and mechanical components had to be solved. A wide range of instruments are now available commercially. Although instrument design has become more sophisticated,

10735-428: The noise level at the expense of some reduction in resolution. For rapid calculation the number of points in the interferogram has to equal a power of two. A string of zeroes may be added to the measured interferogram to achieve this. More zeroes may be added in a process called zero filling to improve the appearance of the final spectrum although there is no improvement in resolution. Alternatively, interpolation after

10848-408: The persistency. This distinction is necessary because these two types of stability do not always correlate with each other. For example, benzylic radicals, which are known for their weak benzylic C−H bond strength, are thermodynamically stabilized due to resonance delocalization. However, these radicals are kinetically transient because they can undergo rapid, diffusion-limited dimerization, resulting in

10961-468: The photodissociation of nitrogen dioxide to an oxygen atom and nitric oxide (see eq. 1.1 below), which plays a key role in smog formation—and the photodissociation of ozone to give the excited oxygen atom O(1D) (see eq. 1.2 below). The net and return reactions are also shown ( eq. 1.3 and eq. 1.4 , respectively). In the upper atmosphere, the photodissociation of normally unreactive chlorofluorocarbons (CFCs) by solar ultraviolet radiation

11074-549: The products of breaking of covalent bonds by homolysis . The homolytic bond dissociation energies , usually abbreviated as "Δ H  °" are a measure of bond strength. Splitting H 2 into 2 H , for example, requires a Δ H  ° of +435 kJ/mol , while splitting Cl 2 into two Cl requires a Δ H  ° of +243 kJ/mol. For weak bonds, homolysis can be induced thermally. Strong bonds require high energy photons or even flames to induce homolysis. Diradicals are molecules containing two radical centers. Dioxygen (O 2 )

11187-689: The proliferation of malignant cells. Radical attacks on arachidonic acid and docosahexaenoic acid produce a similar but broader array of signaling products. Radicals may also be involved in Parkinson's disease , senile and drug-induced deafness , schizophrenia , and Alzheimer's . The classic free-radical syndrome, the iron-storage disease hemochromatosis , is typically associated with a constellation of free-radical-related symptoms including movement disorder, psychosis, skin pigmentary melanin abnormalities, deafness, arthritis, and diabetes mellitus. The free-radical theory of aging proposes that radicals underlie

11300-406: The property of returning any incident beam in a parallel direction regardless of orientation. Systems where the path difference is generated by a rotary movement have proved very successful. One common system incorporates a pair of parallel mirrors in one beam that can be rotated to vary the path without displacing the returning beam. Another is the double pendulum design where the path in one arm of

11413-447: The range to 25 μm (400 cm ) and caesium iodide 50 μm (200 cm ). The region beyond 50 μm (200 cm ) became known as the far-infrared region; at very long wavelengths it merges into the microwave region. Measurements in the far infrared needed the development of accurately ruled diffraction gratings to replace the prisms as dispersing elements, since salt crystals are opaque in this region. More sensitive detectors than

11526-516: The result of reactions between radicals and DNA , potentially resulting in mutations that can adversely affect the cell cycle and potentially lead to malignancy. Some of the symptoms of aging such as atherosclerosis are also attributed to radical induced oxidation of cholesterol to 7-ketocholesterol. In addition radicals contribute to alcohol -induced liver damage, perhaps more than alcohol itself. Radicals produced by cigarette smoke are implicated in inactivation of alpha 1-antitrypsin in

11639-448: The reverse of radical addition. In radical elimination, an unstable radical compound breaks down into a spin-paired molecule and a new radical compound. Shown below is an example of a radical elimination reaction, where a benzoyloxy radical breaks down into a phenyl radical and a carbon dioxide molecule. The generation and reactivity of organic radicals are dependent on both their thermodynamic stability and kinetic stability, also known as

11752-456: The same condition." While relationships between thermodynamic stability and kinetic persistency is highly case-dependent, organic radicals can be generally stabilized by any or all of these factors: the presence of electronegativity, delocalization, and steric hindrance. The compound 2,2,6,6-tetramethylpiperidinyloxyl illustrates the combination of all three factors. It is a commercially available solid that, aside from being magnetic, behaves like

11865-503: The sample as a result of interaction with the electron beam. Automated EDX is further used in a range of automated mineralogy techniques for identification and textural mapping. In all three spectroscopic methods, the sample usually needs to be present in solution, which may present problems during forensic examination because it necessarily involves sampling solid from the object to be examined. In FTIR, three types of samples can be analyzed: solution ( KBr ), powder, or film. A solid film

11978-411: The sample compartment. There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the retardation or optical path difference (OPD). An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of

12091-473: The shortest wavelength would be 1.266 μm ( 7900 cm ). Because of aliasing , any energy at shorter wavelengths would be interpreted as coming from longer wavelengths and so has to be minimized optically or electronically. The spectral resolution, i.e. the separation between wavelengths that can be distinguished, is determined by the maximum OPD. The wavelengths used in calculating the Fourier transform are such that an exact number of wavelengths fit into

12204-490: The singlet radical can initiate. The flammability of a given material strongly depends on the concentration of radicals that must be obtained before initiation and propagation reactions dominate leading to combustion of the material. Once the combustible material has been consumed, termination reactions again dominate and the flame dies out. As indicated, promotion of propagation or termination reactions alters flammability. For example, because lead itself deactivates radicals in

12317-530: The temperature of the atmosphere of Venus by recording the vibration-rotation spectrum of Venusian CO 2 at 0.1 cm resolution. Michelson himself attempted to resolve the hydrogen H α emission band in the spectrum of a hydrogen atom into its two components by using his interferometer. FTIR is a method of measuring infrared absorption and emission spectra. For a discussion of why people measure infrared absorption and emission spectra, i.e. why and how substances absorb and emit infrared light, see

12430-442: The usual material for the near-IR, being both harder and less sensitive to moisture than KBr, but cannot be used beyond about 8 μm (1,200 cm ). Far-IR beamsplitters are mostly based on polymer films, and cover a limited wavelength range. Attenuated total reflectance (ATR) is one accessory of FTIR spectrophotometer to measure surface properties of solid or thin film samples rather than their bulk properties. Generally, ATR has

12543-483: The width of features in the spectrum. If the OPD is not large enough to allow the interferogram signal to decay to a negligible level there will be unwanted oscillations or sidelobes associated with the features in the resulting spectrum. To reduce these sidelobes the interferogram is usually multiplied by a function that approaches zero at the maximum OPD. This so-called apodization reduces the amplitude of any sidelobes and also

12656-435: Was chosen to encompass the highest known vibration frequency due to a fundamental molecular vibration . The upper limit was imposed by the fact that the dispersing element was a prism made from a single crystal of rock-salt ( sodium chloride ), which becomes opaque at wavelengths longer than about 15 μm; this spectral region became known as the rock-salt region. Later instruments used potassium bromide prisms to extend

12769-466: Was then identified as the root base of certain acids (the Latin word "radix" meaning "root"). Historically, the term radical in radical theory was also used for bound parts of the molecule, especially when they remain unchanged in reactions. These are now called functional groups . For example, methyl alcohol was described as consisting of a methyl "radical" and a hydroxyl "radical". Neither are radicals in

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