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| {{For|a table of IR spectroscopy data|infrared spectroscopy correlation table}}
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| '''Infrared spectroscopy''' (IR spectroscopy) is the [[spectroscopy]] that deals with the [[infrared]] region of the [[electromagnetic spectrum]], that is light with a longer [[wavelength]] and lower [[frequency]] than [[visible light]]. It covers a range of techniques, mostly based on [[absorption spectroscopy]]. As with all spectroscopic techniques, it can be used to identify and study [[chemical]]s. A common laboratory instrument that uses this technique is a [[Fourier transform infrared spectroscopy|Fourier transform infrared]] (FTIR) [[spectrometer]]. | |
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| The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The higher-energy near-IR, approximately 14000–4000 cm<sup>−1</sup> (0.8–2.5 μm wavelength) can excite [[overtones|overtone]] or [[Harmonic (mathematics)|harmonic]] vibrations. The mid-infrared, approximately 4000–400 cm<sup>−1</sup> (2.5–25 μm) may be used to study the fundamental vibrations and associated [[Rotational-vibrational spectroscopy|rotational-vibrational]] structure. The far-infrared, approximately 400–10 [[wavenumber|cm<sup>−1</sup>]] (25–1000 μm), lying adjacent to the [[microwave]] region, has low energy and may be used for [[rotational spectroscopy]]. The names and classifications of these subregions are conventions, and are only loosely based on the relative molecular or electromagnetic properties.
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| ==Theory==
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| [[File:Bromomethane IR spectroscopy.svg|thumbnail|right|Sample of an IR spec. reading; this one is from [[bromomethane]], showing peaks around 3000, 1300 and 1000(on the x-axis).]]
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| Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are [[resonant frequency|resonant frequencies]], i.e. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates. The energies are determined by the shape of the molecular [[potential energy surface]]s, the masses of the atoms, and the associated [[vibronic coupling]].
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| In particular, in the [[Born–Oppenheimer approximation|Born–Oppenheimer]] and harmonic approximations, i.e. when the [[molecular Hamiltonian]] corresponding to the electronic [[ground state]] can be approximated by a [[quantum harmonic oscillator|harmonic oscillator]] in the neighborhood of the equilibrium [[molecular geometry]], the resonant frequencies are associated with the [[normal modes]] corresponding to the molecular electronic ground state potential energy surface. The resonant frequencies are also related to the strength of the bond and the [[atomic mass|mass of the atoms]] at either end of it. Thus, the frequency of the vibrations are associated with a particular normal mode of motion and a particular bond type.
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| ===Number of vibrational modes===
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| In order for a vibrational mode in a molecule to be "IR active," it must be associated with changes in the dipole. A permanent dipole is not necessary, as the rule requires only a change in dipole moment.<ref>{{cite book|last=Paula|first=Peter Atkins, Julio de|title=Elements of physical chemistry|year=2009|publisher=Oxford U.P|location=Oxford|isbn=978-0-19-922672-6|pages=459|edition=5th ed.}}</ref>
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| A molecule can vibrate in many ways, and each way is called a ''vibrational mode.'' For molecules with N atoms in them, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H<sub>2</sub>O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.
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| Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, e.g. N<sub>2</sub>, the band is not observed in the IR spectrum, but only in the Raman spectrum. Asymmetrical diatomic molecules, e.g. CO, absorb in the IR spectrum. More complex molecules have many bonds, and their vibrational spectra are correspondingly more complex, i.e. big molecules have many peaks in their IR spectra.
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| The atoms in a CH<sub>2</sub>X<sub>2</sub> group, commonly found in [[organic compound]]s and where X can represent any other atom, can vibrate in nine different ways. Six of these involve only the CH<sub>2</sub> portion: '''symmetric and antisymmetric stretching''', '''scissoring''', '''rocking''', '''wagging''' and '''twisting''', as shown below. (Note, that because CH<sub>2</sub> is attached to X<sub>2</sub> it has 6 modes, unlike H<sub>2</sub>O, which only has 3 modes. The rocking, wagging, and twisting modes do not exist for H<sub>2</sub>O, since they are rigid body translations and no relative displacements exist.)
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| {| class="wikitable"
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| |-
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| ! Symmetrical<br>stretching
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| ! Antisymmetrical<br>stretching
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| ! <br>Scissoring
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| |-
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| | [[Image:Symmetrical stretching.gif]]
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| | [[Image:Asymmetrical stretching.gif]]
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| | [[Image:Scissoring.gif]]
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| |-
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| ! Rocking
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| ! Wagging
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| ! Twisting
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| |-
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| | [[Image:Modo rotacao.gif]]
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| | [[Image:Wagging.gif]]
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| | [[Image:Twisting.gif]]
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| |}
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| These figures do not represent the "[[recoil]]" of the C atoms, which, though necessarily present to balance the overall movements of the molecule, are much smaller than the movements of the lighter H atoms.
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| ===Special effects===
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| The simplest and most important IR bands arise from the "normal modes," the simplest distortions of the molecule. In some cases, "overtone bands" are observed. These bands arise from the absorption of a photon that leads to a doubly excited vibrational state. Such bands appear at approximately twice the energy of the normal mode. Some vibrations, so-called 'combination modes," involve more than one normal mode. The phenomenon of [[Fermi resonance]] can arise when two modes are similar in energy; Fermi resonance results in an unexpected shift in energy and intensity of the bands etc.
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| ==Practical IR spectroscopy==
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| The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR is the same as the vibrational frequency of a bond, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This can be achieved by scanning the wavelength range using a [[monochromator]]. Alternatively, the whole wavelength range is measured at once using a [[Fourier transform]] instrument and then a [[transmittance]] or [[absorbance]] spectrum is generated using a dedicated procedure. Analysis of the position, shape and intensity of peaks in this spectrum reveals details about the molecular structure of the sample.
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| This technique works almost exclusively on samples with [[covalent bond]]s. Simple spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra. The technique has been used for the characterization of very complex mixtures. Spectra issues with infrared fluorescence are rare.
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| ===Sample preparation===
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| <!-- A link on Liquid paraffin targets this paragraph. If you change the sub-heading, please change the link as well. Thanks. -->
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| Gaseous samples require a sample cell with a long [[pathlength]] to compensate for the diluteness. The pathlength of the sample cell depends on the concentration of the compound of interest. A simple glass tube with length of 5 to 10 cm equipped with infrared windows at the both ends of the tube can be used for concentrations down to several hundred ppm. Sample gas concentrations well below ppm can be measured with a [[White cell (spectroscopy)|White's cell]] in which the infrared light is guided with mirrors to travel through the gas. White's cells are available with optical pathlength starting from 0.5 m up to hundred meters.
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| Liquid samples can be sandwiched between two plates of a salt (commonly [[sodium chloride]], or common salt, although a number of other salts such as [[potassium bromide]] or [[calcium fluoride]] are also used).<ref name=Har>{{cite book
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| |author = Laurence M. Harwood, Christopher J. Moody
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| |title = Experimental organic chemistry: Principles and Practice
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| |edition = Illustrated
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| |page = 292|isbn=0-632-02017-2 |year=1989
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| |publisher=Wiley-Blackwell}}</ref>
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| The plates are transparent to the infrared light and do not introduce any lines onto the spectra.
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| Solid samples can be prepared in a variety of ways. One common method is to crush the sample with an oily mulling agent (usually [[Nujol]]) in a [[marble]] or [[agate]] [[Mortar and pestle|mortar, with a pestle]]. A thin film of the mull is smeared onto salt plates and measured. The second method is to grind a quantity of the sample with a specially purified salt (usually [[potassium bromide]]) finely (to remove scattering effects from large crystals). This powder mixture is then pressed in a mechanical [[Machine press|press]] to form a translucent pellet through which the beam of the spectrometer can pass.<ref name=Har/> A third technique is the "cast film" technique, which is used mainly for polymeric materials. The sample is first dissolved in a suitable, non hygroscopic solvent. A drop of this solution is deposited on surface of KBr or NaCl cell. The solution is then evaporated to dryness and the film formed on the cell is analysed directly. Care is important to ensure that the film is not too thick otherwise light cannot pass through. This technique is suitable for qualitative analysis. The final method is to use [[microtomy]] to cut a thin (20–100 µm) film from a solid sample. This is one of the most important ways of analysing failed plastic products for example because the integrity of the solid is preserved.
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| In [[photoacoustic spectroscopy]] the need for sample treatment is minimal. The sample, liquid or solid, is placed into the sample cup which is inserted into the photoacoustic cell which is then sealed for the measurement. The sample may be one solid piece, powder or basically in any form for the measurement. For example, a piece of rock can be inserted into the sample cup and the spectrum measured from it.
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| It is important to note that spectra obtained from different sample preparation methods will look slightly different from each other due to differences in the samples' physical states.
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| ===Comparing to a reference===
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| [[Image:IR spectroscopy apparatus.svg|thumbnail|right|400px|Schematics of a two-beam absorption spectrometer. A beam of infrared light is produced, passed through an [[interferometer]] (not shown), and then split into two separate beams. One is passed through the sample, the other passed through a reference. The beams are both reflected back towards a detector, however first they pass through a splitter, which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained. This "two-beam" setup gives accurate spectra even if the intensity of the light source drifts over time.]]
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| To take the infrared spectrum of a sample, it is necessary to measure both the sample and a "reference" (or "control"). This is because each measurement is affected by not only the light-absorption properties of the sample, but also the properties of the instrument (for example, what light source is used, what [[infrared detector]] is used, etc.). The reference measurement makes it possible to eliminate the instrument influence. Mathematically, the sample transmission spectrum is divided by the reference transmission spectrum.
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| The appropriate "reference" depends on the measurement and its goal. The simplest reference measurement is to simply remove the sample (replacing it by air). However, sometimes a different reference is more useful. For example, if the sample is a dilute solute dissolved in water in a beaker, then a good reference measurement might be to measure pure water in the same beaker. Then the reference measurement would cancel out not only all the instrumental properties (like what light source is used), but also the light-absorbing and light-reflecting properties of the water and beaker, and the final result would just show the properties of the solute (at least approximately).
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| A common way to compare to a reference is sequentially: first measure the reference, then replace the reference by the sample and measure the sample. This technique is not perfectly reliable; if the infrared lamp is a bit brighter during the reference measurement, then a bit dimmer during the sample measurement, the measurement will be distorted. More elaborate methods, such as a "two-beam" setup (see figure), can correct for these types of effects to give very accurate results. The [[Standard addition]] method can be used to statistically cancel these errors.
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| ===FTIR===
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| {{Main|Fourier transform infrared spectroscopy}}
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| [[File:FTIR-interferogram.svg|thumb|An interferogram from an [[FTIR]] measurement. The horizontal axis is the position of the mirror, and the vertical axis is the amount of light detected. This is the "raw data" which can be [[Fourier transform]]ed to get the actual spectrum.]]
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| '''[[Fourier transform]] infrared (FTIR) spectroscopy''' is a measurement technique that allows one to record infrared spectra. Infrared light is guided through an [[interferometer]] and then through the sample (or vice versa). A moving mirror inside the apparatus alters the distribution of infrared light that passes through the interferometer. The signal directly recorded, called an "interferogram", represents light output as a function of mirror position. A data-processing technique called [[Fourier transform]] turns this raw data into the desired result (the sample's spectrum): Light output as a function of infrared [[wavelength]] (or equivalently, [[wavenumber]]). As described above, the sample's spectrum is always compared to a reference.
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| There is an alternate method for taking spectra (the "dispersive" or "scanning [[monochromator]]" method), where one wavelength at a time passes through the sample. The dispersive method is more common in [[Ultraviolet-visible spectroscopy|UV-Vis spectroscopy]], but is less practical in the infrared than the FTIR method. One reason that FTIR is favored is called "[[Fellgett's advantage]]" or the "multiplex advantage": The information at all frequencies is collected simultaneously, improving both speed and [[signal-to-noise ratio]]. Another is called "Jacquinot's Throughput Advantage": A dispersive measurement requires detecting much lower light levels than an FTIR measurement.<ref name=white>[http://books.google.com/books?id=t2VSNnFoO3wC&pg=PA7 ''Chromatography/Fourier transform infrared spectroscopy and its applications'', by Robert White, p7]</ref> There are other advantages, as well as some disadvantages,<ref name=white/> but virtually all modern infrared spectrometers are FTIR instruments.
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| ==Absorption bands==
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| IR spectroscopy is often used to identify structures because [[functional group]]s give rise to characteristic bands both in terms of intensity and position (frequency). The positions of these bands is summarized in correlation tables as shown below.
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| {{Main|Infrared spectroscopy correlation table}}
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| [[Image:IR summary version 2.gif|centre|frame]]
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| Wavenumbers listed in [[wavenumber|cm<sup>−1</sup>]].
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| ===Badger's rule===
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| For many kinds of samples, the assignments are known, i.e. which bond deformation(s) are associated with which frequency. In such cases further information can be gleaned about the strength on a bond, relying on the empirical guideline called Badger's rule. This rule states that the strength of a bond correlates with the frequency of its vibrational mode. That is, increase in bond strength leads to corresponding frequency increase and vice versa.
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| ==Uses and applications==
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| Infrared spectroscopy is a simple and reliable technique widely used in both organic and inorganic chemistry, in research and industry. It is used in quality control, dynamic measurement, and monitoring applications such as the long-term unattended measurement of CO<sub>2</sub> concentrations in greenhouses and growth chambers by infrared gas analyzers.
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| It is also used in [[forensic analysis]] in both criminal and civil cases, for example in identifying [[polymer degradation]]. It can be used in detecting how much alcohol is in the blood of a suspected drunk driver measured as 1/10,000 g/mL = 100 μg/mL.{{Citation needed|date=June 2011}}
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| A useful way of analysing solid samples without the need for cutting samples uses ATR or [[attenuated total reflectance]] spectroscopy. Using this approach, samples are pressed against the face of a single crystal. The infrared radiation passes through the crystal and only interacts with the sample at the interface between the two materials.
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| With increasing technology in computer filtering and manipulation of the results, samples in solution can now be measured accurately (water produces a broad absorbance across the range of interest, and thus renders the spectra unreadable without this computer treatment).
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| Some instruments will also automatically tell you what substance is being measured from a store of thousands of reference spectra held in storage.
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| Infrared spectroscopy is also useful in measuring the degree of polymerization in [[polymer]] manufacture. Changes in the character or quantity of a particular bond are assessed by measuring at a specific frequency over time. Modern research instruments can take infrared measurements across the range of interest as frequently as 32 times a second. This can be done whilst simultaneous measurements are made using other techniques. This makes the observations of chemical reactions and processes quicker and more accurate.
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| Infrared spectroscopy has also been successfully utilized in the field of semiconductor microelectronics:<ref>{{cite book |last= Lau|first= W.S. |coauthors= |title= Infrared characterization for microelectronics|url=http://books.google.com/?id=rotNlJDFJWsC&printsec=frontcover |publisher= World Scientific |year= 1999 |isbn=981-02-2352-8}}</ref> for example, infrared spectroscopy can be applied to semiconductors like [[silicon]], [[gallium arsenide]], [[gallium nitride]], [[zinc selenide]], amorphous silicon, [[silicon nitride]], etc.
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| The instruments are now small, and can be transported, even for use in field trials.
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| ==Isotope effects==
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| The different isotopes in a particular species may give fine detail in infrared spectroscopy. For example, the O–O stretching frequency (in reciprocal centimeters) of oxy[[hemocyanin]] is experimentally determined to be 832 and 788 cm<sup>−1</sup> for ν(<sup>16</sup>O–<sup>16</sup>O) and ν(<sup>18</sup>O–<sup>18</sup>O), respectively.
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| By considering the O–O bond as a spring, the wavenumber of absorbance, ν can be calculated:
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| :<math>\nu = \frac{1}{2 \pi c} \sqrt{\frac{k}{\mu}}</math>
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| where ''k'' is the spring constant for the bond, ''c'' is the speed of light, and ''μ'' is the [[reduced mass]] of the A–B system:
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| :<math>\mu = \frac{m_A m_B}{m_A + m_B}</math>
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| (<math>m_i</math> is the mass of atom <math>i</math>).
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| The reduced masses for <sup>16</sup>O–<sup>16</sup>O and <sup>18</sup>O–<sup>18</sup>O can be approximated as 8 and 9 respectively. Thus
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| :<math>\frac{\nu(^{16}O)}{\nu(^{18}O)} = \sqrt{\frac{9}{8}} \approx \frac{832}{788}.</math>
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| Where <math>\nu</math> is the wavenumber; [wavenumber = frequency/(speed of light)]
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| The effect of isotopes, both on the vibration and the decay dynamics, has been found to be stronger than previously thought. In some systems, such as silicon and germanium, the decay of the anti-symmetric stretch mode of interstitial oxygen involves the symmetric stretch mode with a strong isotope dependence. For example, it was shown that for a natural silicon sample, the lifetime of the anti-symmetric vibration is 11.4 ps. When the isotope of one of the silicon atoms is increased to <sup>29</sup>Si, the lifetime increases to 19 ps. In similar manner, when the silicon atom is changed to <sup>30</sup>Si, the lifetime becomes 27 ps.<ref>{{cite journal|doi=10.1103/PhysRevLett.96.225503|title=Isotope Dependence of the Lifetime of the 1136-cm-1 Vibration of Oxygen in Silicon|year=2006|last1=Kohli|first1=K.|last2=Davies|first2=Gordon|last3=Vinh|first3=N.|last4=West|first4=D.|last5=Estreicher|first5=S.|last6=Gregorkiewicz|first6=T.|last7=Izeddin|first7=I.|last8=Itoh|first8=K.|journal=Physical Review Letters|volume=96|pages=225503|pmid=16803320|issue=22|bibcode=2006PhRvL..96v5503K}}</ref>
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| ==Two-dimensional IR==
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| '''Two-dimensional infrared correlation spectroscopy analysis''' is the application of 2D correlation analysis on infrared spectra. By extending the spectral information of a perturbed sample, spectral analysis is simplified and resolution is enhanced. The 2D synchronous and 2D asynchronous spectra represent a graphical overview of the spectral changes due to a perturbation (such as a changing concentration or changing temperature) as well as the relationship between the spectral changes at two different wavenumbers.
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| {{Main|Two-dimensional infrared spectroscopy}}
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| [[Image:2dir pulse sequence newversion.png|thumb|300px|right|Pulse Sequence used to obtain a two-dimensional Fourier transform infrared spectrum. The time period <math>\tau_1</math> is usually referred to as the coherence time and the second time period <math>\tau_2</math> is known as the waiting time. The excitation frequency is obtained by Fourier transforming along the <math>\tau_1</math> axis.]]
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| '''Nonlinear two-dimensional infrared spectroscopy'''<ref>{{cite journal|author = P. Hamm, M. H. Lim, R. M. Hochstrasser|title = Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy| journal = J. Phys. Chem. B|volume = 102|issue = 31|pages = 6123|year = 1998|doi = 10.1021/jp9813286}}</ref><ref>{{cite journal|author = S. Mukamel|title = Multidimensional Fentosecond Correlation Spectroscopies of Electronic and Vibrational Excitations|journal = Annual Review of Physics and Chemistry|volume =51|issue = 1|pages = 691|year = 2000|doi = 10.1146/annurev.physchem.51.1.691|pmid = 11031297|bibcode = 2000ARPC...51..691M }}</ref> is the infrared version of [[correlation spectroscopy]]. Nonlinear two-dimensional infrared spectroscopy is a technique that has become available with the development of [[femtosecond]] infrared laser pulses. In this experiment, first a set of pump pulses is applied to the sample. This is followed by a waiting time during which the system is allowed to relax. The typical waiting time lasts from zero to several picoseconds, and the duration can be controlled with a resolution of tens of femtoseconds. A probe pulse is then applied, resulting in the emission of a signal from the sample. The nonlinear two-dimensional infrared spectrum is a two-dimensional correlation plot of the frequency ω<sub>1</sub> that was excited by the initial pump pulses and the frequency ω<sub>3</sub> excited by the probe pulse after the waiting time. This allows the observation of coupling between different vibrational modes; because of its extremely fine time resolution, it can be used to monitor molecular dynamics on a picosecond timescale. It is still a largely unexplored technique and is becoming increasingly popular for fundamental research.
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| As with two-dimensional nuclear magnetic resonance (2DNMR) spectroscopy, this technique spreads the spectrum in two dimensions and allows for the observation of cross peaks that contain information on the coupling between different modes. In contrast to 2DNMR, nonlinear two-dimensional infrared spectroscopy also involves the excitation to overtones. These excitations result in excited state absorption peaks located below the diagonal and cross peaks. In 2DNMR, two distinct techniques, [[Correlation spectroscopy#COSY|COSY]] and [[Correlation spectroscopy#NOESY|NOESY]], are frequently used. The cross peaks in the first are related to the scalar coupling, while in the latter they are related to the spin transfer between different nuclei. In nonlinear two-dimensional infrared spectroscopy, analogs have been drawn to these 2DNMR techniques. Nonlinear two-dimensional infrared spectroscopy with zero waiting time corresponds to COSY, and nonlinear two-dimensional infrared spectroscopy with finite waiting time allowing vibrational population transfer corresponds to NOESY. The COSY variant of nonlinear two-dimensional infrared spectroscopy has been used for determination of the secondary structure content of proteins.<ref>{{cite journal|author = N. Demirdöven, C. M. Cheatum, H. S. Chung, M. Khalil, J. Knoester, A. Tokmakoff|title= Two-dimensional infrared spectroscopy of antiparallel beta-sheet secondary structure|journal = [[Journal of the American Chemical Society]]|volume = 126|pages = 7981|year = 2004|doi = 10.1021/ja049811j|pmid = 15212548|issue = 25}}</ref>
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| ==See also==
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| {{col-begin}}
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| {{col-3}}
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| *[[Applied spectroscopy]]
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| *[[Far-infrared astronomy]]
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| *[[Forensic chemistry]]
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| *[[Forensic engineering]]
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| *[[Forensic polymer engineering]]
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| *[[Forensic science]]
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| {{col-3}}
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| *[[Infrared astronomy]]
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| *[[Microscopy#Infrared microscopy|Infrared microscopy]]
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| *[[Infrared spectroscopy correlation table]]
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| *[[Near-infrared spectroscopy]]
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| *[[Photothermal microspectroscopy]]
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| *[[Infrared spectroscopy of metal carbonyls]]
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| {{col-3}}
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| *[[Raman spectroscopy]]
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| *[[Rotational spectroscopy]]
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| *[[Rotational-vibrational spectroscopy]]
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| *[[Time-resolved spectroscopy]]
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| {{col-end}}
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| ==References==
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| {{reflist|2}}
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| ==External links==
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| {{commons category}}
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| *A useful gif animation of different vibrational modes: [http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/irspec1.htm SHU.ac.uk]
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| *[http://www.jkwchui.com/2010/09/pictorial-guide-to-interpreting-infrared-spectra/ Illustrated guide to basic IR spectra interpretation]
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| *[http://www2.dq.fct.unl.pt/qoa/jas/ir.html Infrared spectroscopy for organic chemists]
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| *[http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng Organic compounds spectrum database]
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| {{BranchesofSpectroscopy}}
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| [[Category:Infrared spectroscopy| ]]
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