Name: raman spectra and molecular fingerprints
Uploaded: 2024-12-05T18:32:10.000Z
Duration: 1 min 20 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Physical Science

Waves and Electromagnetic Radiation

3.2.9-12.X

Raman Spectroscopy: Overview

L’effet Raman se produit dans les molécules dont la polarisabilité des liaisons nettes change pendant les vibrations. Les pics dans le spectre représentent des modes vibratoires spécifiques des molécules, fournissant une empreinte digitale unique pour comprendre la composition moléculaire et les liaisons.
En spectroscopie Raman, un laser monochromatique de haute énergie interagit avec un échantillon. La majeure partie de la lumière est transmise à travers lui, mais une petite partie se disperse dans différentes directions, ce qui est enregistré en fonction de la longueur d’onde ou du nombre d’onde.
La fréquence du photon diffusé peut soit correspondre à la fréquence du photon incident, appelée diffusion de Rayleigh, soit présenter un décalage de fréquence, connu sous le nom de diffusion Raman.
La diffusion Raman se compose de la diffusion de Stokes et de la diffusion anti-Stokes, où les photons diffusés ont des fréquences plus basses et des fréquences plus élevées que les photons incidents, respectivement. 
L’intensité des pics Raman dépend de la polarisabilité des liaisons, de l’intensité du laser et de la concentration des groupes actifs Raman.
Cependant, les signaux de Stokes sont presque toujours plus forts que les signaux anti-Stokes en raison de la population plus élevée de molécules à l’état fondamental vibratoire dans la plupart des conditions.

spectroscopie raman: aperçu

Le principe de base de la spectroscopie Raman repose sur l'interaction entre la lumière et la matière, et plus précisément sur la diffusion inélastique des photons par les molécules. Lorsqu'un faisceau lumineux monochromatique, provenant généralement d'une source laser, interagit avec un échantillon, la majeure partie de la lumière diffusée a la même fréquence que la lumière incidente. C'est ce qu'on appelle la diffusion de Rayleigh.

Cependant, une petite fraction de la lumière diffusée présente un décalage de fréquence dû à l'échange d'énergie entre les photons incidents et les niveaux d'énergie vibrationnelle des molécules de l'échantillon. Ce phénomène est appelé diffusion Raman.

Pour obtenir des spectres Raman, l'échantillon est irradié par un faisceau monochromatique de lumière laser. Le rayonnement diffusé est collecté à un angle approprié (généralement 180°) à l'aide d'un spectromètre équipé d'un détecteur sensible. La mesure de la lumière diffusée décalée en fréquence en fonction de la longueur d'onde ou du nombre d'ondes fournit le spectre Raman de l'échantillon.

La diffusion Raman par résonance se produit lorsque la longueur d'onde d'excitation est proche d'une transition électronique de la molécule. Cela augmente l'intensité de la diffusion Raman, ce qui facilite la détection des signaux même faibles. Cependant, le choix d'une longueur d'onde d'excitation appropriée est crucial pour éviter les interférences de fluorescence, qui peuvent submerger le signal Raman. Dans de nombreux cas, des lasers proches de l'infrarouge ou visibles sont utilisés pour minimiser la fluorescence.

Lorsqu'une molécule absorbe un photon, elle est temporairement élevée à un niveau d'énergie virtuel. Cet état virtuel est de courte durée et la molécule revient rapidement à un niveau d'énergie vibratoire en émettant un photon. La différence d'énergie entre les photons incidents et émis correspond aux niveaux d'énergie vibratoire de la molécule.

Il existe deux types de diffusion Raman : Stokes et anti-Stokes. Dans la diffusion Stokes, le photon émis a une énergie plus faible (longueur d'onde plus longue) que le photon incident, tandis que dans la diffusion anti-Stokes, le photon émis a une énergie plus élevée (longueur d'onde plus courte).

Les décalages de fréquence du rayonnement diffusé de manière inélastique correspondent à la fréquence vibrationnelle de la molécule. En analysant ces décalages de fréquence, des informations précieuses sur la structure moléculaire et la composition chimique de l'échantillon peuvent être obtenues.

Les spectres Raman sont généralement présentés sous forme de tracés d'intensité en fonction du nombre d'ondes (inverse de la longueur d'onde). Les pics du spectre représentent des modes vibrationnels spécifiques des molécules de l'échantillon, fournissant une « empreinte digitale » unique pour l'identification.

Considérons un échantillon de tétrachlorure de carbone. La diffusion Stokes et anti-Stokes se produit lorsqu'il est irradié avec un laser monochromatique. L'intensité du signal Stokes sera plus forte que celle du signal anti-Stokes, car la population de molécules dans l'état vibrationnel fondamental est plus élevée que celle dans l'état excité à température ambiante.

Les modes vibrationnels peuvent être identifiés en analysant le spectre Raman du tétrachlorure de carbone. Les décalages de fréquence associés à ces modes vibrationnels fournissent des informations précieuses sur la composition chimique et la liaison de la molécule.

Spectroscopie Raman: aperçu

Le principe sous-jacent de la spectroscopie Raman est basé sur l'interaction entre la lumière et la matière, en particulier la diffusion inélastique des molécules ...

Education

Enseignement

JoVE Core

Analytical Chemistry

Chimie analytique

Unit 1

Unité 1

Molecular Vibrational Spectroscopy

Spectroscopie vibrationnelle moléculaire

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 illuminated with laser light and the scattered radiation is gathered at a suitable angle, often 180 degrees. A spectrometer with a sensitive detector measures the shifted light. The result is a Raman spectrum, which shows intensity against wavelength or wavenumber. The peaks in this plot come from specific vibrational modes in the sample.

Some Raman signals are stronger when the laser wavelength is close to an electronic transition. This is called resonance Raman scattering. It can make weak signals easier to detect. The choice of excitation wavelength also matters because fluorescence can interfere with the Raman signal. In many cases, visible or near-infrared lasers are used to reduce that problem.

Raman scattering involves a very short-lived virtual energy level. The molecule briefly moves through this state and then returns to a vibrational level by emitting a photon. The energy difference between the incoming and outgoing photons matches the molecule’s vibrational energy. Two common forms are Stokes scattering and anti-Stokes scattering. In Stokes scattering, the emitted photon has lower energy and a longer wavelength. In anti-Stokes scattering, the emitted photon has higher energy and a shorter wavelength.

The frequency shifts in a Raman spectrum give useful clues about molecular structure and chemical composition. Because each sample has its own set of vibrational peaks, the spectrum can act like a unique fingerprint for identification. Carbon tetrachloride is one example. When it is irradiated with monochromatic laser light, both Stokes and anti-Stokes scattering occur. The Stokes signal is stronger than the anti-Stokes signal because more molecules are in the ground vibrational state at room temperature. By analyzing these peaks, the vibrational modes of carbon tetrachloride can be identified, along with information about its bonding and composition.

Raman Spectra and Molecular Fingerprints

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 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...

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 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 Raman spectroscopy uses laser light, Stokes and anti-Stokes scattering, and spectra peaks to identify molecules like carbon tetrachloride.

Raman Spectra and Molecular Fingerprints | JoVE K12

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Raman Spectra and Molecular Fingerprints

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