Infrared spectroscopy

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

Simple

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 (ν_s) and antisymmetric (ν_as); 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.^[27]

Symmetry Direction	Symmetric	Antisymmetric
Radial	Symmetric stretching (νs)	Antisymmetric stretching (νas)
Latitudinal	Scissoring (δ)	Rocking (ρ)
Longitudinal	Wagging (ω)	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.

The simplest and most important or fundamental IR bands arise from the excitations of normal modes, the simplest distortions of the molecule, from the ground state with vibrational quantum number v = 0 to the first excited state with vibrational quantum number v = 1. In some cases, overtone bands are observed. An overtone band arises from the absorption of a photon leading to a direct transition from the ground state to the second excited vibrational state (v = 2). Such a band appears at approximately twice the energy of the fundamental band for the same normal mode. Some excitations, so-called combination modes, involve simultaneous excitation of 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.^{[citation needed]}

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 is the same as 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 generated using a dedicated procedure.^{[citation needed]}

This technique is commonly used for analyzing 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.^{[citation needed]}

Sample preparation

Gas samples

Gaseous samples require a sample cell with a long

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).^[28] The plates are transparent to the infrared light and do not introduce any lines onto the spectra. 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).^{[citation needed]}

Solid samples

Solid samples can be prepared in a variety of ways. One common method is to crush the sample with an oily

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.^{[citation needed]}

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.^{[citation needed]}

Comparing to a reference

It is typical to record spectrum of both the sample and a "reference". This step controls for a number of variables, e.g. infrared detector, which may affect the spectrum. The reference measurement makes it possible to eliminate the instrument influence.^{[citation needed]}

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).^{[citation needed]}

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 ^[29]

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

FTIR

]

An alternate method for acquiring spectra is the "dispersive" or "scanning

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.^[30] There are other advantages, as well as some disadvantages,^[30]

but virtually all modern infrared spectrometers are FTIR instruments.

Infrared microscopy

Various forms of

atomic force microscope based infrared spectroscopy

(AFM-IR).

Other methods in molecular vibrational spectroscopy

Infrared spectroscopy is not the only method of studying molecular vibrational spectra. Raman spectroscopy involves an inelastic scattering process in which only part of the energy of an incident photon is absorbed by the molecule, and the remaining part is scattered and detected. The energy difference corresponds to absorbed vibrational energy.^{[citation needed]}

The selection rules for infrared and for Raman spectroscopy are different at least for some molecular symmetries, so that the two methods are complementary in that they observe vibrations of different symmetries.^{[citation needed]}

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.

Recently, high-resolution EELS (HREELS) has emerged as a technique for performing vibrational spectroscopy in a transmission electron microscope (TEM).^[33] In combination with the high spatial resolution of the TEM, unprecedented experiments have been performed, such as nano-scale temperature measurements,^[34][35] mapping of isotopically labeled molecules,^[36] mapping of phonon modes in position- and momentum-space,^[37][38] vibrational surface and bulk mode mapping on nanocubes,^[39] and investigations of polariton modes in van der Waals crystals.^[40] Analysis of vibrational modes that are IR-inactive but appear in

inelastic neutron scattering is also possible at high spatial resolution using EELS.^[41] Although the spatial resolution of HREELs is very high, the bands are extremely broad compared to other techniques.^[33]

Computational infrared microscopy

By using computer simulations and normal mode analysis it is possible to calculate theoretical frequencies of molecules.^[42]

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: Infrared spectroscopy correlation table

Regions

A spectrograph is often interpreted as having two regions.^[43]

• functional group region ${\displaystyle \geq 1,500{\text{ cm}}^{-1}}$

In the functional region there are one to a few troughs per functional group.^[43]

• fingerprint region ${\displaystyle <1,500{\text{ cm}}^{-1}}$

In the fingerprint region there are many troughs which form an intricate pattern which can be used like a fingerprint to determine the compound.^[43]

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,^[44] 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)]

${\displaystyle {\tilde {\nu }}={\frac {1}{2\pi c}}{\sqrt {\frac {k}{\mu }}}}$

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:

${\displaystyle \mu ={\frac {m_{\mathrm {A} }m_{\mathrm {B} }}{m_{\mathrm {A} }+m_{\mathrm {B} }}}}$

(${\displaystyle m_{i}}$ is the mass of atom ${\displaystyle i}$).

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

${\displaystyle {\frac {{\tilde {\nu }}(^{16}\mathrm {O} )}{{\tilde {\nu }}(^{18}\mathrm {O} )}}={\sqrt {\frac {9}{8}}}\approx {\frac {832}{788}}.}$

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.^[45]

Two-dimensional IR

Two-dimensional infrared correlation spectroscopy analysis combines multiple samples of infrared spectra to reveal more complex properties. 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.^{[citation needed]}

Main article: Two-dimensional infrared spectroscopy

Nonlinear two-dimensional infrared spectroscopy

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 (

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.^[48]

See also

• Applied spectroscopy
• Astrochemistry
• Atomic and molecular astrophysics
• Atomic force microscopy based infrared spectroscopy (AFM-IR)
• Cosmochemistry
• Far-infrared astronomy
• Forensic chemistry
• Forensic engineering
• Forensic polymer engineering
• Infrared astronomy
• Infrared microscopy
• Infrared multiphoton dissociation
• Infrared photodissociation spectroscopy
• Infrared spectroscopy correlation table
• Infrared spectroscopy of metal carbonyls
• Near-infrared spectroscopy
• Nuclear resonance vibrational spectroscopy
• Photothermal microspectroscopy
• Raman spectroscopy
• Rotational-vibrational spectroscopy
• Time-resolved spectroscopy
• Vibrational spectroscopy of linear molecules

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External links

Wikimedia Commons has media related to Infrared spectroscopy.

• Infrared spectroscopy for organic chemists
• Organic compounds spectrum database Archived 2013-01-14 at the Wayback Machine