13.1
Spectroscopie infrarouge (IR): aperçu
Lorsque le rayonnement électromagnétique traverse un matériau, les atomes ou les molécules passent d'un état d'énergie inférieur à un état d'énergie s…
En faisant passer le rayonnement IR à travers les molécules, leurs atomes liés de manière covalente absorbent un rayonnement compris entre 2,5 et 25 micromètres de longueur d’onde lorsqu’ils passent de niveaux d’énergie vibratoire inférieurs à des niveaux d’énergie vibratoire supérieurs.
Les vibrations moléculaires qui produisent un changement substantiel du moment dipolaire de liaison sont appelées IR actives.
Le rayonnement IR est exprimé en centimètres inverses qui représentent le nombre d’onde spectroscopique calculé comme l’inverse de la longueur d’onde en centimètres.
En utilisant la formule de la vitesse de la lumière, la longueur d’onde, λ, est égale à la vitesse de la lumière, c divisée par sa fréquence, v.
La substitution de la longueur d’onde λ montre que le rayonnement IR est proportionnel à la fréquence divisée par la vitesse de la lumière dans ce milieu.
Les fréquences de 4000 à 400 cm−1 correspondent à la région vibratoire du spectre IR.
La spectroscopie IR est utilisée pour identifier les groupes fonctionnels présents dans une molécule.
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Q1: What wavelength range does IR spectroscopy cover?
Infrared spectroscopy operates in the 2.5 to 25 micrometer wavelength range, which corresponds to the vibrational region of the IR spectrum expressed as 4000 to 400 cm⁻¹ in wavenumber units. This range captures the frequencies at which molecular vibrations absorb IR radiation and transition between vibrational energy levels.
Q2: What does it mean for a molecule to be IR active?
A molecule is IR active when its vibrations produce a substantial change in bond dipole moment during absorption of IR radiation. For effective IR absorption, molecular vibrations must fluctuate the dipole moments, allowing the molecule to interact with the electromagnetic field of IR light and transition to higher vibrational energy levels.
Q3: How is IR radiation expressed in spectroscopy units?
IR radiation is expressed in inverse centimeters, representing the spectroscopic wavenumber calculated as the reciprocal of wavelength in centimeters. Using the relationship between wavelength, frequency, and the speed of light, IR radiation wavenumber is proportional to frequency divided by the speed of light in that medium.
Q4: Why is IR spectroscopy useful for identifying functional groups?
Different functional groups absorb IR radiation at varying frequencies, making the IR spectrum similar to a molecular fingerprint. Since each compound displays unique vibrational frequencies based on its functional groups, IR spectroscopy enables qualitative analysis by comparing an unknown compound's vibrational frequencies with known reference compounds.
Q5: How does IR spectroscopy determine molecular structure?
When electromagnetic radiation passes through a molecule, atoms absorb IR radiation corresponding to the energy difference between lower and higher vibrational energy states. By analyzing which frequencies are absorbed and their intensities, IR spectroscopy reveals the functional groups present and their arrangement, allowing determination of molecular structure.
Q6: Can IR spectroscopy measure the concentration of a substance?
Yes, IR spectroscopy can determine substance concentration through quantitative analysis using the Beer-Lambert law, which states that absorbance is proportional to concentration. This quantitative application complements IR's primary use in qualitative functional group identification for both organic and inorganic compounds.
Q7: What happens when IR radiation passes through a molecule?
When IR radiation passes through a molecule, covalently bonded atoms absorb radiation between 2.5 to 25 micrometers in wavelength, causing transitions from lower to higher vibrational energy levels. Only IR-active vibrations that change the bond dipole moment produce observable absorption peaks in the IR spectrum.