Name: ir absorption and bond dipole changes
Uploaded: 2024-12-05T18:32:09.000Z
Duration: 1 min 20 s
Channel: JoVE

Pour deux charges d’intensité égale et de signes opposés, comme dans l’acide chlorhydrique, le moment dipolaire est exprimé comme le produit de la charge sur l’un des atomes multiplié par la distance à l’autre atome.
Dans un champ électrique, en fonction de sa direction, la liaison est étirée ou comprimée alternativement avec un changement simultané du moment dipôle.
Lorsque le dipôle de liaison vibrante correspond à la fréquence de l’onde, la liaison absorbe de l’énergie. De telles vibrations sont appelées IR-actives.
Par exemple, dans les alcènes asymétriques, les vibrations d’étirement C=C sont actives IR et présentent de forts pics d’absorption. Alors que la vibration C=C dans les alcènes transsymétriques - sans moment dipolaire - conduit à une absence du pic d’étirement C=C.
De plus, plus la variation du moment dipolaire est élevée, plus la quantité de rayonnement absorbée est importante et plus l’intensité maximale de l’IR est élevée.
Par exemple, une liaison C=O, avec un moment dipolaire plus élevé qu’une liaison vinyle C=C, montre un changement plus important du moment dipolaire avec le temps, par rapport à une liaison C=C, et entraîne un pic plus intense.

région de fréquence ir: région d'empreinte digitale

IR Spectrum Peak Intensity: Dipole Moment

Les spectres IR sont divisés en deux régions principales: la région de diagnostic et la région d'empreinte digitale. La région de diagnostic du spectre se situe au-dessus de 1 500 cm⁻¹. Les absorptions résultant des vibrations des liaisons simples N–H, C–H et O–H s'étirent à des nombres d'ondes plus élevés et apparaissent sur le côté gauche du spectre. Les absorptions d'étirement des liaisons C≡C et C≡N se produisent entre 2 100 et 2 300 cm⁻¹. En revanche, celles résultant des absorptions d'étirement des liaisons C=O, C=N et C=C se produisent entre 1 600 et 1 850 cm⁻¹.

Les absorptions inférieures à 1 500 cm⁻¹ se situent dans la région d'empreinte digitale, résultant des vibrations des liaisons C–C, C–O et C–N. Cette région est également appelée région des spectres complexes et est plus difficile à analyser. Parfois, les spectres de molécules structurellement proches se chevauchent et ne sont pas distinguables dans la région de diagnostic. Cependant, ils peuvent être différenciés dans la région d'empreinte digitale car ils présentent un modèle de pics unique. Par exemple, les groupes O–H et C–H du butanol et du propanol présentent des absorptions par étirement similaires dans la région diagnostique, mais ils présentent un modèle de pics différent dans la région d'empreinte digitale, ce qui permet de les distinguer.

Région de fréquence IR: région d'empreinte digitale

Le moment dipolaire d’une liaison est le produit de la charge partielle sur l’un ou l’autre atome et de la distance entre eux. Les moments dipôles influencent l’efficacité ...

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Analytical Chemistry

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Unit 1

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Molecular Vibrational Spectroscopy

Spectroscopie vibrationnelle moléculaire

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

ir absorption and bond dipole changes

IR absorption depends on whether a bond has a changing dipole moment. A dipole moment comes from partial charge on each atom and the distance between them. It helps determine how strongly a bond absorbs infrared light and how intense the peak looks.

A bond in an electric field can be stretched or compressed depending on the field direction. Infrared radiation has an electric field that flips direction very quickly. So polar bonds are pulled back and forth as the field reverses. If this motion matches the bond’s natural vibration, the bond can absorb IR energy. These vibrations are called IR-active.

Symmetrical bonds with zero dipole moment behave differently. When they vibrate, there is no change in dipole moment, so no energy is absorbed. These vibrations are IR-inactive. This is seen in symmetrical alkenes, where the C=C stretch is absent, while asymmetrical alkenes show a strong C=C absorption band.

A larger change in dipole moment gives a stronger IR absorption band. That is why C=O bonds, which have a large dipole moment, usually show higher-intensity peaks than C=C bonds, which have a smaller dipole moment.

IR Absorption and Bond Dipole Changes

infrared (ir) spectroscopy: overview

IR Spectra and Functional Group ID

Infrared (IR) spectroscopy helps identify functional groups in molecules by measuring how they absorb infrared light. It is based on the idea that electromagnetic radiation can move atoms or molecules from a lower energy state to a higher one when the radiation matches the energy gap between those states. In IR spectroscopy, that absorption happens as molecular vibrations change.

ir spectroscopy: molecular vibration overview

ir-spectroscopy-molecular-vibration-overview

IR Spectroscopy: Vibration Types and Bands

IR spectroscopy explains how covalently bonded molecules absorb infrared radiation by moving to higher vibrational levels. The key motions are stretching and bending vibrations. These motions help describe how a bond responds when it absorbs IR energy. Stretching vibrations happen along the bond line and change the distance between two bonded atoms. They can be symmetric, where two bonds lengthen and shorten together, or asymmetric, where one bond moves one way while another moves the opposite...

ir spectroscopy: hooke's law approximation of molecular vibration

ir-spectroscopy-hooke-s-law-approximation-of-molecular-vibration

Molecular Bond Vibrations in IR Spectroscopy

Molecular bond vibrations can be modeled with IR spectroscopy using Hooke’s law. A covalently bonded heteronuclear diatomic molecule is treated like two masses connected by a spring. The atoms act like the masses, and the bond acts like the spring. Hooke’s law links the vibrational frequency to two main factors. The frequency increases with the force constant, K, which describes bond stiffness. It decreases with the reduced mass, μ, which combines the masses of both atoms into one value for...

ir spectrometers

IR Spectrometer Signal Paths

IR spectrometers use infrared light to compare a sample beam with a reference beam and record how each one behaves. The two main instruments are dispersive IR spectrometers and Fourier transform infrared, or FTIR, spectrometers. In a dispersive IR spectrometer, a hot wire produces the infrared beam. Mirrors split it into two equal beams. One beam passes through the sample, and the other serves as the reference. The beams then move into the monochromator, which separates the radiation into a...

ir spectrum

IR Spectrum Peak Regions and Patterns

Infrared (IR) spectroscopy shows how a molecule responds when IR radiation passes through it. The bonds in the molecule stretch or bend as they absorb that energy. This creates an absorption spectrum, which plots percent transmittance against wavenumber. Transmittance is the ratio of radiant power that passes through a sample to the radiant power from the source. When transmittance is multiplied by 100, it becomes percent transmittance, or %T. %T ranges from 100% for no absorption to 0% for...

ir absorption frequency: hybridization

C-H Stretching in Alkane, Alkene, and Alkyne IR

Hydrocarbons such as alkanes, alkenes, and alkynes show characteristic C–H stretching bands in infrared, or IR, spectra. These bands help identify how the carbon atoms are hybridized. The bond type affects the stretching frequency that appears in the spectrum. The difference comes from s character, which is the amount of s-orbital contribution in a hybrid orbital. Among sp, sp2, and sp3 hybridized orbitals, sp has the most s character at 50%. Because of that, the electrons are held closer to...

ir absorption frequency: delocalization

Conjugation Shifts IR Bond Stretching

Conjugation shifts IR bond stretching because electron delocalization changes bond strength. Electron delocalization means electrons spread across more than one atom instead of staying in one bond or one atom. It is common in conjugated systems, where alternating single and double bonds let pi-electrons move through the molecule. This electron movement helps stabilize the molecule. It also lowers the double bond character of the bonds in the conjugated network. When a bond has less double bond...

ir frequency region: x&ndash;h stretching

IR Peaks for C-H, O-H, and N-H Bonds

IR spectroscopy shows the stretching bands of C-H, O-H, and N-H bonds in the 2700-4000 cm-1 range. These X-H bonds are useful for identifying common functional groups in a spectrum. Their peaks appear in different parts of this region and often have different shapes and strengths. C-H stretching gives sharp bands in the 2850-3000 cm-1 range. O-H stretching appears in the 3650-3200 cm-1 range, and N-H stretching appears in the 3500-3100 cm-1 range. N-H stretching usually shows one or two sharp...

ir frequency region: alkyne and nitrile stretching

ir-frequency-region-alkyne-and-nitrile-stretching

Triple-Bond Signals in IR Spectra

Alkyne and nitrile groups both contain triple bonds, and they show clear stretching bands in the diagnostic IR region. These absorptions usually appear around 2100 to 2300 cm −1 . This makes them useful markers when reading an IR spectrum. Triple bonds absorb at a higher stretching frequency than double bonds and single bonds. The C≡C bond stretches at a higher frequency than a C=C or C–C bond. The C≡N bond also stretches at a higher absorption than C=N and C–N bonds. This higher stretching...

ir frequency region: alkene and carbonyl stretching

ir-frequency-region-alkene-and-carbonyl-stretching

IR Signals for C=C and C=O Bonds

IR spectroscopy helps identify alkene double bonds and carbonyl groups by their stretching signals. These bonds absorb in a diagnostic region of the IR spectrum. Alkenes also show vinylic C–H stretching and C–H out-of-plane bending bands. Those bands can help determine substitution patterns on the double bond. The exact stretching frequency can change with bond environment. Resonance, inductive effects, ring strain, dipole moment, and hydrogen bonding can all shift the absorption. That is why...

ir frequency region: fingerprint region

IR Spectra: Diagnostic vs Fingerprint Region

IR spectra are divided into a diagnostic region and a fingerprint region. The diagnostic region lies above 1500 cm −1 and appears on the left side of the spectrum. It shows key bond stretches that help identify functional groups. In this region, single-bond vibrations from N-H, C-H, and O-H stretches appear at higher wavenumbers. The C≡C and C≡N stretches occur between 2100 and 2300 cm −1. The C=O, C=N, and C=C stretches appear between 1600 and 1850 cm −1. The fingerprint region lies below...

ir spectrum peak intensity: amount of ir-active bonds

ir-spectrum-peak-intensity-amount-of-ir-active-bonds

IR Peak Intensity and Bond Dipole Change

IR peak intensity depends on how much a bond’s dipole moment changes during vibration. When infrared light passes through a molecule, absorption happens only when a stretching or bending motion causes a substantial change in that bond dipole moment. That is what makes a molecule infrared active. Vibrational transitions usually fall in the infrared region from 4000 to 400 cm-1. Molecular bonds can stretch and bend in different ways, so an IR spectrum shows peaks with different intensities. The...

ir spectrum peak intensity: dipole moment

IR absorption depends on whether a bond has a changing dipole moment. A dipole moment comes from partial charge on each atom and the distance between them. It helps determine how strongly a bond absorbs infrared light and how intense the peak looks. A bond in an electric field can be stretched or compressed depending on the field direction. Infrared radiation has an electric field that flips direction very quickly. So polar bonds are pulled back and forth as the field reverses. If this motion...

ir spectrum peak broadening: hydrogen bonding

ir-spectrum-peak-broadening-hydrogen-bonding

Hydrogen Bonding Effects in IR O-H Peaks

Hydrogen bonding changes the O-H stretching peak in IR spectroscopy. The bond’s vibrational frequency depends on bond strength. Stronger bonds vibrate at higher frequencies, while weaker bonds vibrate at lower frequencies. In very dilute alcohol or phenol samples, or in the gas phase, the strong O-H bond gives a sharp peak at 3600-3650 cm−1. When intermolecular hydrogen bonding or intramolecular hydrogen bonding weakens the O-H bond, the stretch shifts to about 3300-3400 cm−1. This range of...

ir spectrum peak splitting: symmetric vs asymmetric vibrations

ir-spectrum-peak-splitting-symmetric-vs-asymmetric-vibrations

IR Peak Splitting in Amines and Nitro Groups

IR peak splitting can reveal how identical bonds in a polyatomic group are moving. These bonds may stretch in phase, which is symmetric stretching, or out of phase, which is asymmetric stretching. The two motions often create different IR peak shapes and positions. As a general rule, asymmetric stretching appears at a higher frequency than symmetric stretching. This pattern is useful for spotting functional groups in an IR spectrum. It also helps explain why some groups show one peak while...

applications of ir spectroscopy: overview

IR Spectroscopy Uses in Science and Forensics

IR spectroscopy is a useful tool for finding out what materials are made of. It is non-destructive, so the sample can often be kept intact. It gives chemical information about functional groups and the bonds between atoms in a molecule. This makes IR spectroscopy helpful in organic synthesis, pharmaceutical research, and materials science. Scientists use it to identify and characterize organic and inorganic compounds. It also helps with structural elucidation, which means working out a...

attenuated total reflectance (atr) infrared spectroscopy: overview

attenuated-total-reflectance-atr-infrared-spectroscopy-overview

ATR Infrared Spectroscopy for Material ID

ATR infrared spectroscopy is a method for identifying the composition of materials. It is used in chemistry, materials science, forensic science, and other areas where sample characterization is needed. The technique is useful because it usually needs little to no sample preparation and can be applied to many kinds of samples. The process starts when infrared, or IR, radiation is directed onto a diamond or germanium crystal. Because this crystal has a high refractive index, the IR beam...

raman spectroscopy: overview

Raman Spectra and Molecular Fingerprints

Raman spectroscopy uses laser light to study how molecules scatter light and reveal their vibrational energy levels. A monochromatic beam, or single-color light, hits the sample first. Most of the scattered light keeps the same frequency as the incoming light. This is called Rayleigh scattering. A small part of the light shifts in frequency because the photons exchange energy with molecular vibrations. That shifted light is Raman scattering. To collect a Raman spectrum, the sample is...

raman spectroscopy instrumentation: overview

raman-spectroscopy-instrumentation-overview

Raman Spectroscopy Setup and Detectors

Raman spectroscopy uses a laser, a sample holder, a wavelength selector, and a detector to measure scattered light from a sample. The laser is monochromatic, meaning it has one main wavelength. It usually uses visible or near-infrared light to create a tightly focused beam. That beam hits the sample and some of the light scatters. Liquid and gas samples are often placed in ordinary glass capillaries. Solid samples can be tested as powders packed into capillaries or as potassium bromide pellets.

key terms and concepts

Learn how bond dipole moments affect IR absorption, peak intensity, IR-active vibrations, and why C=O bands are stronger than C=C bands.

IR Absorption and Bond Dipole Changes | JoVE K12

analytical chemistry

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IR Absorption and Bond Dipole Changes

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