Physics:Infrared spectroscopy

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Short description: Measurement of infrared radiation's interaction with matter
OVIRS instrument of the OSIRIS-REx probe is a visible and infrared spectrometer

Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. It can be used to characterize new materials or identify and verify known and unknown samples. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer (or spectrophotometer) which produces an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light absorbance (or transmittance) on the vertical axis vs. frequency, wavenumber or wavelength on the horizontal axis. Typical units of wavenumber used in IR spectra are reciprocal centimeters, with the symbol cm−1. Units of IR wavelength are commonly given in micrometers (formerly called "microns"), symbol μm, which are related to the wavenumber in a reciprocal way. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer. Two-dimensional IR is also possible as discussed below.

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 14,000–4,000 cm−1 (0.7–2.5 μm wavelength) can excite overtone or combination modes of molecular vibrations. The mid-infrared, approximately 4,000–400 cm−1 (2.5–25 μm) is generally used to study the fundamental vibrations and associated rotational–vibrational structure. The far-infrared, approximately 400–10 cm−1 (25–1,000 μm) has low energy and may be used for rotational spectroscopy and low frequency vibrations. The region from 2–130 cm−1, bordering the microwave region, is considered the terahertz region and may probe intermolecular vibrations.[1] The names and classifications of these subregions are conventions, and are only loosely based on the relative molecular or electromagnetic properties.

Uses and applications

File:Mary Moffit using Infrared spectrophotometer 2012 017 b1f6 79407z11s.tiff

US Food and Drug Administration scientist uses portable near infrared spectroscopy device to detect potentially illegal substances

It is also used in forensic analysis in both criminal and civil cases, for example in identifying polymer degradation. It can be used in determining the blood alcohol content of a suspected drunk driver.

IR spectroscopy has been used in identification of pigments in paintings[2] and other art objects[3] such as illuminated manuscripts.[4]


Infrared spectroscopy is utilized in the field of semiconductor microelectronics:[5] for example, infrared spectroscopy can be applied to semiconductors like silicon, gallium arsenide, gallium nitride, zinc selenide, amorphous silicon, silicon nitride, etc.

Another important application of infrared spectroscopy is in the food industry to measure the concentration of various compounds in different food products.[6][7]

Infrared spectroscopy is also used in gas leak detection devices such as the DP-IR and EyeCGAs.[8] These devices detect hydrocarbon gas leaks in the transportation of natural gas and crude oil.

Infrared spectroscopy is an important analysis method in the recycling process of household waste plastics, and a convenient stand-off method to sort plastic of different polymers (PET, HDPE, ...).[9]

Other developments include a miniature IR-spectrometer that's linked to a cloud based database and suitable for personal everyday use,[10] and NIR-spectroscopic chips[11] that can be embedded in smartphones and various gadgets.

In catalysis research it is a very useful tool to characterize the catalyst,[12][13][14] as well as to detect intermediates.[15]

Infrared spectroscopy coupled with machine learning and artificial intelligence also has potential for rapid, accurate and non-invasive sensing of bacteria.[16] The complex chemical composition of bacteria, including nucleic acids, proteins, carbohydrates and fatty acids, results in high-dimensional datasets where the essential features are effectively hidden under the total spectrum. Extraction of the essential features therefore requires advanced statistical methods such as machine learning and deep-neural networks. The potential of this technique for bacteria classification have been demonstrated for differentiation at the genus,[17] species[18] and serotype[19] taxonomic levels, and it has also been shown promising for antimicrobial susceptibility testing,[20][21][22] which is important for many clinical settings where faster susceptibility testing would decrease unnecessary blind-treatment with broad-spectrum antibiotics. The main limitation of this technique for clinical applications is the high sensitivity to technical equipment and sample preparation techniques, which makes it difficult to construct large-scale databases. Attempts in this direction have however been made by Bruker with the IR Biotyper for food microbiology.[23]

Theory

Sample of an IR spec. reading; this one is from bromomethane (CH3Br), showing peaks around 3000, 1300, and 1000 cm−1 (on the horizontal axis).

Infrared spectroscopy exploits the fact that molecules absorb frequencies that are characteristic of their structure. These absorptions occur at resonant frequencies, i.e. the frequency of the absorbed radiation matches the vibrational frequency. The energies are affected by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling.[24]

3D animation of the symmetric stretch-compress mode of the C–H bonds of bromomethane

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 neighbourhood of the equilibrium molecular geometry), the resonant frequencies are associated with the normal modes of vibration corresponding to the molecular electronic ground state potential energy surface. Thus, it depends on both the nature of the bonds and the mass of the atoms that are involved. Using the Schrödinger equation leads to the selection rule for the vibrational quantum number in the system undergoing vibrational changes:

△v=±1

The compression and extension of a bond may be likened to the behaviour of a spring, but real molecules are hardly perfectly elastic in nature. If a bond between atoms is stretched, for instance, there comes a point at which the bond breaks and the molecule dissociates into atoms. Thus real molecules deviate from perfect harmonic motion and their molecular vibrational motion is anharmonic. An empirical expression that fits the energy curve of a diatomic molecule undergoing anharmonic extension and compression to a good approximation was derived by P.M. Morse, and is called the Morse function. Using the Schrödinger equation leads to the selection rule for the system undergoing vibrational changes :

△v=±1,±2,±3,⋅⋅⋅[25]

Number of vibrational modes

In order for a vibrational mode in a sample to be "IR active", it must be associated with changes in the molecular dipole moment. A permanent dipole is not necessary, as the rule requires only a change in dipole moment.[26]

A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N number of atoms, geometrically 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 examples linear carbon dioxide (CO2) has 3 × 3 – 5 = 4, while non-linear water (H2O), has only 3 × 3 – 6 = 3.[27]

Stretching and bending oscillations of the CO2 carbon dioxide molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.

Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum, but only in the Raman spectrum. Asymmetrical diatomic molecules, e.g. carbon monoxide (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.

The atoms in a CH2X2 group, commonly found in organic compounds and where X can represent any other atom, can vibrate in nine different ways. Six of these vibrations involve only the CH2 portion: two stretching modes (ν): symmetric (νs) and antisymmetric (νas); and four bending modes: scissoring (δ), rocking (ρ), wagging (ω) and twisting (τ), as shown below. Structures that do not have the two additional X groups attached have fewer modes because some modes are defined by specific relationships to those other attached groups. For example, in water, the rocking, wagging, and twisting modes do not exist because these types of motions of the H atoms represent simple rotation of the whole molecule rather than vibrations within it. In case of more complex molecules, out-of-plane (γ) vibrational modes can be also present.[28]

Symmetry

Direction
 Symmetric Antisymmetric
Radial Image:Symmetrical stretching.gif
Symmetric stretching (νs) 		Image:Asymmetrical stretching.gif
Antisymmetric stretching (νas)
Latitudinal Image:Scissoring.gif
Scissoring (δ) 		Image:Modo rotacao.gif
Rocking (ρ)
Longitudinal Image:Wagging.gif
Wagging (ω) 		Image:Twisting.gif
Twisting (τ)

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.


Practical IR spectroscopy

The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR matches the vibrational frequency of a bond or collection of bonds, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This measurement can be achieved by scanning the wavelength range using a monochromator. Alternatively, the entire wavelength range is measured using a Fourier transform instrument and then a transmittance or absorbance spectrum is extracted.

This technique is commonly used for analyzing samples with covalent bonds. The number of bands roughly correlates with symmetry and molecular complexity.

A variety of devices are used to hold the sample in the path of the IR beam These devices are selected on the basis of their transparency in the region of interest and their resilience toward the sample.

Materials for containing IR samples[29]
material transparency range (cm−1) comment
Sodium chloride 5000–650 attacked (dissolved) by water, small alcohols, some amines
Calcium fluoride 4200–1300 insoluble in most solvents
Silver chloride 5000–500 attacked (dissolved) by amines, organosulfur compounds
Typical IR solution cell. The windows are CaF2.

Sample preparation

Gas samples

Liquid samples

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).[30]

Solid samples

A useful way of analyzing 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.[31]

Comparing to a reference

Schematics of a two-beam absorption spectrometer. A beam of infrared light is produced, passed through an monochromator (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.


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.

Nevertheless, among different absorption-based techniques which are used for gaseous species detection, Cavity ring-down spectroscopy (CRDS) can be used as a calibration-free method. The fact that CRDS is based on the measurements of photon life-times (and not the laser intensity) makes it needless for any calibration and comparison with a reference [32]

Some instruments also automatically identify the substance being measured from a store of thousands of reference spectra held in storage.

FTIR

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 transformed to get the actual spectrum.

An alternate method for acquiring spectra is the "dispersive" or "scanning monochromator" method. In this approach, the sample is irradiated sequentially with various single wavelengths. The dispersive method is more common in 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.[33] There are other advantages, as well as some disadvantages,[33] but virtually all modern infrared spectrometers are FTIR instruments.

Infrared microscopy

Various forms of infrared microscopy exist. These include IR versions of sub-diffraction microscopy[34] such as IR NSOM,[35] photothermal microspectroscopy, Nano-FTIR and atomic force microscope based infrared spectroscopy (AFM-IR).

Other methods in molecular vibrational spectroscopy

Another method is electron energy loss spectroscopy (EELS), in which the energy absorbed is provided by an inelastically scattered electron rather than a photon. This method is useful for studying vibrations of molecules adsorbed on a solid surface.

high-resolution EELS (HREELS) is a technique for performing vibrational spectroscopy in a transmission electron microscope (TEM).[36] In combination with the high spatial resolution of the TEM, unprecedented experiments have been performed, such as nano-scale temperature measurements,[37][38] mapping of isotopically labeled molecules,[39] mapping of phonon modes in position- and momentum-space,[40][41] vibrational surface and bulk mode mapping on nanocubes,[42] and investigations of polariton modes in van der Waals crystals.[43] Analysis of vibrational modes that are IR-inactive but appear in inelastic neutron scattering is also possible at high spatial resolution using EELS.[44]

Computational infrared microscopy

By using computer simulations and normal mode analysis it is possible to calculate theoretical frequencies of molecules.[45]

Absorption bands

IR spectroscopy is often used to identify structures because functional groups give rise to characteristic bands both in terms of intensity and position (frequency). The positions of these bands are summarized in correlation tables as shown below.

Main article: Chemistry:Infrared spectroscopy correlation table
List of main IR spectroscopy bands. For example, the carboxyl group will contain a C = O band at 1700 cm−1 and an OH band at 3500 cm−1 (total group -COOH). Wavenumbers listed in cm−1.

Regions

A spectrograph is often interpreted as having two regions.[46]

  • functional group region ≥1,500 cm−1

In the functional region there are one to a few troughs per functional group.[46]

  • fingerprint region <1,500 cm−1

In the fingerprint region there are many troughs which form an intricate pattern which can be used like a fingerprint to determine the compound.[46]

Badger's rule

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. Originally published by Richard McLean Badger in 1934,[47] this rule states that the strength of a bond (in terms of force constant) correlates with the bond length. That is, increase in bond strength leads to corresponding bond shortening and vice versa.

Isotope effects

The different isotopes in a particular species may exhibit different fine details in infrared spectroscopy. For example, the O–O stretching frequency (in reciprocal centimeters) of oxyhemocyanin is experimentally determined to be 832 and 788 cm−1 for ν(16O–16O) and ν(18O–18O), respectively.

By considering the O–O bond as a spring, the frequency of absorbance can be calculated as a wavenumber [= frequency/(speed of light)]

ν~=12πckμ

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:

μ=mAmBmA+mB

(mi is the mass of atom i).

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

ν~(16O)ν~(18O)=98≈832788.

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 29Si, the lifetime increases to 19 ps. In similar manner, when the silicon atom is changed to 30Si, the lifetime becomes 27 ps.[48]

Two-dimensional IR

Main article: Physics:Two-dimensional infrared spectroscopy
Pulse Sequence used to obtain a two-dimensional Fourier transform infrared spectrum. The time period τ1 is usually referred to as the coherence time and the second time period τ2 is known as the waiting time. The excitation frequency is obtained by Fourier transforming along the τ1 axis.

Nonlinear two-dimensional infrared spectroscopy[49][50] 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 ω1 that was excited by the initial pump pulses and the frequency ω3 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.

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, COSY and 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.[51]

See also

References

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  43. ↑ "Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope". Nature Communications 8 (1). July 2017. doi:10.1038/s41467-017-00056-y. PMID 28733660. Bibcode: 2017NatCo...8...95G. 
  44. ↑ "Vibrational spectroscopy at atomic resolution with electron impact scattering". Nature Physics 15 (12): 1237–1241. 2019. doi:10.1038/s41567-019-0675-5. Bibcode: 2019NatPh..15.1237V. 
  45. ↑ "Theoretical Infrared Spectra: Quantitative Similarity Measures and Force Fields". Journal of Chemical Theory and Computation 16 (5): 3307–3315. May 2020. doi:10.1021/acs.jctc.0c00126. PMID 32271575. 
  46. ↑ 46.0 46.1 46.2 Smith, Janice Gorzynski (2011). "Chapter 13 Mass Spectrometry and Infrared Spectroscopy". in Hodge, Tami; Nemmers, Donna; Klein, Jayne. Organic chemistry (3rd ed.). New York, NY: McGraw-Hill. pp. 463–488. ISBN 978-0-07-337562-5. http://highered.mheducation.com/sites/007340277x/student_view0/index.html. Retrieved 2018-06-30. 
  47. ↑ "A Relation Between Internuclear Distances and Bond Force Constants". The Journal of Chemical Physics 2 (3): 128. 1934. doi:10.1063/1.1749433. Bibcode: 1934JChPh...2..128B. https://authors.library.caltech.edu/10440/1/BADjcp34.pdf. 
  48. ↑ "Isotope dependence of the lifetime of the vibration of oxygen in silicon". Physical Review Letters 96 (22). June 2006. doi:10.1103/PhysRevLett.96.225503. PMID 16803320. Bibcode: 2006PhRvL..96v5503K. 
  49. ↑ "Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy". J. Phys. Chem. B 102 (31): 6123–6138. 1998. doi:10.1021/jp9813286. 
  50. ↑ "Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations". Annual Review of Physical Chemistry 51 (1): 691–729. 2000. doi:10.1146/annurev.physchem.51.1.691. PMID 11031297. Bibcode: 2000ARPC...51..691M. 
  51. ↑ "Two-dimensional infrared spectroscopy of antiparallel beta-sheet secondary structure". Journal of the American Chemical Society 126 (25): 7981–90. June 2004. doi:10.1021/ja049811j. PMID 15212548. Bibcode: 2004JAChS.126.7981D. https://pure.rug.nl/ws/files/6680272/2004JAmChemSocDemirdoven.pdf. 



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