NationMaster - Encyclopedia: Infrared spectroscopy

Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify compounds or investigate sample composition. Infrared spectroscopy correlation tables are tabulated in the literature. Extremely high resolution spectrogram of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. ... For other uses, see Infrared (disambiguation). ... Legend Î³ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High... Absorption spectroscopy refers to a wide range of techniques where one measures how much light of a particular wavelength (color) is absorbed by a sample. ... Extremely high resolution spectrogram of the Sun showing thousands of elemental absorption lines (fraunhofer lines) Spectroscopy is the study of the interaction between radiation (electromagnetic radiation, or light, as well as particle radiation) and matter. ... The absorptions in this range do not apply only to bonds in organic molecules. ...

Theory

The infrared portion of the electromagnetic spectrum is divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The far-infrared, approximately 400-10 cm^-1 (1000–30 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The mid-infrared, approximately 4000-400 cm^-1 (30–1.4 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The higher energy near-IR, approximately 14000-4000 cm^-1 (1.4–0.8 μm) can excite overtone or harmonic vibrations. The names and classifications of these subregions are merely conventions. They are neither strict divisions nor based on exact molecular or electromagnetic properties. Wavenumber in most physical sciences is a wave property inversely related to wavelength, having SI units of reciprocal meters (mâˆ’1). ... This article is about the type of Electromagnetic radiation. ... Rotational spectroscopy or microwave spectroscopy studies the absorption and emission of electromagnetic radiation (typically in the microwave region of the electromagnetic spectrum) by molecules associated with a corresponding change in the rotational quantum number of the molecule. ... Rovibronic excitation stands for rotational vibrational electronic excitation of a molecule. ... In mathematics, a number of concepts employ the word harmonic. ...

Infrared spectroscopy exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels. These resonant frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, by the associated vibronic coupling. In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. In particular, in the Born-Oppenheimer and harmonic approximations, i.e. when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are determined by the normal modes corresponding to the molecular electronic ground state potential energy surface. Nevertheless, the resonant frequencies can be in a first approach related to the strength of the bond, and the mass of the atoms at either end of it. Thus, the frequency of the vibrations can be associated with a particular bond type. A quantum mechanical system can only be in certain states, so that only certain energy levels are possible. ... This article is about resonance in physics. ... A potential energy surface is generally used within the adiabatic or Bornâ€“Oppenheimer approximation in quantum mechanics and statistical mechanics to model chemical reactions and interactions in simple chemical and physical systems. ... In theoretical chemistry, the vibronic coupling terms (which are neglected within the Born-Oppenheimer approximation) are proportional to the interaction between electronic and nuclear motions of molecules. ... The Born-Oppenheimer approximation, also known as the adiabatic approximation, is a technique used in quantum chemistry and condensed matter physics in order to de-couple the motion of nuclei and electrons (i. ... The molecular Hamiltonian is an operator in quantum chemistry and atomic, molecular, and optical physics which describes the motions of electrons and nuclei in a polyatomic molecule. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... The quantum harmonic oscillator is the quantum mechanical analogue of the classical harmonic oscillator. ... Geometry of the water molecule Molecular geometry or molecular structure is the three dimensional arrangement of the atoms that constitute a molecule, inferred from the spectroscopic studies of the compound. ... Normal modes in an oscillating system are special solutions where all the parts of the system are oscillating with the same frequency (called normal frequencies or allowed frequencies). ... The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units. ...

Simple diatomic molecules have only one bond, which may stretch. More complex molecules have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups. For example, the atoms in a CH₂ group, commonly found in organic compounds can vibrate in six different ways, symmetrical and antisymmetrical stretching, scissoring, rocking, wagging and twisting; as shown below: Benzene is the simplest of the arenes, a family of organic compounds An organic compound is any member of a large class of chemical compounds whose molecules contain carbon and hydrogen; therefore, carbides, carbonates, carbon oxides and elementary carbon are not organic (see below for more on the definition controversy...

The infrared spectra of a sample is collected by passing a beam of infrared light through the sample. Examination of the transmitted light reveals how much energy was absorbed at each wavelength. This can be done with a monochromatic beam, which changes in wavelength over time, or by using a Fourier transform instrument to measure all wavelengths at once. From this, a transmittance or absorbance spectrum can be produced, showing at which IR wavelengths the sample absorbs. Analysis of these absorption characteristics reveals details about the molecular structure of the sample. Image File history File links Symmetrical_stretching. ... Image File history File links Asymmetrical_stretching. ... Image File history File links Scissoring. ... Image File history File links Rocking. ... Image File history File links Wagging. ... Image File history File links Twisting. ... Something which is monochromatic has a single color. ... In mathematics, the Fourier transform is a certain linear operator that maps functions to other functions. ... In the fields of optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. ... In spectroscopy, the absorbance A is defined as , where I is the intensity of light at a specified wavelength Î» that has passed through a sample (transmitted light intensity) and is the intensity of the light before it enters the sample (or incident light intensity). ...

This technique works almost exclusively on samples with covalent bonds. 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 however. â€œCovalentâ€ redirects here. ...

Sample preparation

Gaseous samples require little preparation beyond purification, but a sample cell with a long pathlength (typically 5-10 cm) is normally needed, as gases show relatively weak absorbances. In chemistry, the path length is defined as the distance that light (UV/VIS) travels through a sample in an analytical cell. ...

Liquid samples can be sandwiched between two plates of a high purity salt (commonly sodium chloride, or common salt, although a number of other salts such as potassium bromide or calcium fluoride are also used). The plates are transparent to the infrared light and will not introduce any lines onto the spectra. Some salt plates are highly soluble in water, and so the sample, washing reagents and the like must be anhydrous (without water). R-phrases 36 S-phrases none Flash point Non-flammable Related Compounds Other anions NaF, NaBr, NaI Other cations LiCl, KCl, RbCl, CsCl, MgCl2, CaCl2 Related salts Sodium acetate Supplementary data page Structure and properties n, Îµr, etc. ... Potassium bromide (KBr) is a salt, used as an anticonvulsant and a sedative in the 1800s. ... Calcium fluoride (CaF2) is an insoluble ionic compound of calcium and fluorine. ... As a general term, a substance is said to be anhydrous if it contains no water. ...

Solid samples can be prepared in two major ways. The first is to crush the sample with a mulling agent (usually nujol) in a marble or agate mortar, with a pestle. A thin film of the mull is applied onto salt plates and measured. Nujol is an important material used in infrared spectroscopy. ... For other uses, see Marble (disambiguation). ... For other uses, see Agate (disambiguation). ...

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 crushed in a mechanical die press to form a translucent pellet through which the beam of the spectrometer can pass. Potassium bromide (KBr) is a salt, used as an anticonvulsant and a sedative in the 1800s. ...

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.

Typical method

A beam of infrared light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. 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. Image File history File links IR_spectroscopy_apparatus. ... Image File history File links IR_spectroscopy_apparatus. ...

Summary of absorptions of bonds in organic molecules

Wavenumbers listed in cm^-1. The absorptions in this range do not apply only to bonds in organic molecules. ... Image File history File links Download high resolution version (829x176, 11 KB)This version should display better. ... Wavenumber in most physical sciences is a wave property inversely related to wavelength, having SI units of reciprocal meters (mâˆ’1). ...

Uses and applications

Infrared spectroscopy is widely used in both research and industry as a simple and reliable technique for measurement, quality control and dynamic measurement. The instruments are now small, and can be transported, even for use in field trials. 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). Some machines will also automatically tell you what substance is being measured from a store of thousands of reference spectra held in storage.

By measuring at a specific frequency over time, changes in the character or quantity of a particular bond can be measured. This is especially useful in measuring the degree of polymerization in polymer manufacture. Modern research machines can take infrared measurements across the whole 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. A polymer (from Greek: Ï€Î¿Î»Ï…, polu, many; and Î¼ÎÏÎ¿Ï‚, meros, part) is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. ...

Techniques have been developed to assess the quality of tea-leaves using infrared spectroscopy. This will mean that highly trained experts (also called 'noses') can be used more sparingly, at a significant cost saving.^[1]

Infrared spectroscopy has been highly successful for applications in both organic and inorganic chemistry. Infrared spectroscopy has also been successfully utilized in the field of semiconductor microelectronics^[2]: for example, infrared spectroscopy can be applied to semiconductors like silicon, gallium arsenide, gallium nitride, zinc selenide, amorphous silicon, silicon nitride, etc. Not to be confused with Silicone. ... This article is about the chemical compound. ... Gallium nitride (GaN) is a wide bandgap semiconductor material used in optoelectronic, high-power and high-frequency devices. ... Zinc selenide (ZnSe), is a light yellow binary solid compound. ... Silicon nitride (Si3N4) is hard, solid substance, that can be obtained by direct reaction between silicon and nitrogen in high temperatures. ...

Isotope effects

The different isotopes in a particular species may give fine detail in infrared spectroscopy. For example, the O-O stretching frequency of oxyhemocyanin is experimentally determined to be 832 and 788 cm^-1 for ν(¹⁶O-¹⁶O) and ν(¹⁸O-¹⁸O) respectively. Single Oxygenated Hemocyanin protein from Octopus Hemocyanins (also spelled haemocyanins) are respiratory proteins containing two copper atoms that reversibly bind a single oxygen molecule (O2). ...

By considering the O-O as a spring, the wavelength of absorbance, ν can be calculated:

where k is the spring constant for the bond, and μ is the reduced mass of the A-B system: Reduced mass is an algebraic term of the form that simplifies an equation of the form The reduced mass is typically used as a relationship between two system elements in parallel, such as resistors; whether these be in the electrical, thermal, hydraulic, or mechanical domains. ...

The reduced masses for ¹⁶O-¹⁶O and ¹⁸O-¹⁸O can be approximated as 8 and 9 respectively. Thus

Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is a measurement technique for collecting infrared spectra. Instead of recording the amount of energy absorbed when the frequency of the infra-red light is varied (monochromator), the IR light is guided through an interferometer. After passing the sample the measured signal is the interferogram. Performing a mathematical Fourier transform on this signal results in a spectrum identical to that from conventional (dispersive) infrared spectroscopy. FTIR of a thin film of ethanol in the liquid phase. ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ... In mathematics, the Fourier transform is a certain linear operator that maps functions to other functions. ...

FTIR spectrometers are cheaper than conventional spectrometers because building of interferometers is easier than the fabrication of a monochromator. In addition, measurement of a single spectrum is faster for the FTIR technique because the information at all frequencies is collected simultaneously. This allows multiple samples to be collected and averaged together resulting in an improvement in sensitivity. Because of its various advantages, virtually all modern infrared spectrometers are FTIR instruments.

Two-dimensional infrared spectroscopy

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.

Nonlinear two-dimensional infrared spectroscopy^[3]^[4] 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 are applied to the sample. This is followed by a waiting time, where the system is allowed to relax. The waiting time typically 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 1

that was excited by the initial pump pulses and the frequency

ω 3

excited by the probe pulse after the waiting time. This allows the observation of coupling between different vibrational modes. Because of its extremely high time resolution it can be used to follow changes in molecular configurations taking place on a picosecond timescale. It is still a largely unexplored technique and is becoming increasingly popular for fundamental research. Correlation spectroscopy is one of several types of two-dimensional nuclear magnetic resonance (NMR) spectroscopy. ...

Like in two-dimensional nuclear magnetic resonance (2DNMR) spectroscopy this technique spreads the spectrum in two dimensions and allow 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 involve the excitation to overtones. These excitations result in excited state absorption peaks located below the diagonal and cross peaks. In 2DNMR two distinct techniques, COSY and NOESY, are frequently used. The cross peaks in the first are related to the scalar coupling, while in the later 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 proteins.^[5] Correlation spectroscopy is one of several types of two-dimensional nuclear magnetic resonance (NMR) spectroscopy. ... Correlation spectroscopy is one of several types of two-dimensional nuclear magnetic resonance (NMR) spectroscopy. ...

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