Name: hydrogen bonding effects in ir o-h peaks
Uploaded: 2024-12-05T18:32:10.000Z
Duration: 1 min 23 s
Channel: JoVE

Rappelons que la fréquence vibratoire d’une liaison est proportionnelle à la racine carrée de la constante de force, en ne considérant que les effets de force de liaison et non les facteurs de masse réduits.
Ainsi, des liaisons plus fortes avec des constantes de force plus élevées vibrent à des fréquences plus élevées lors de l’absorption IR. Les liaisons plus faibles vibrent à des fréquences plus basses.
Par exemple, la liaison hydrogène intermoléculaire observée dans les alcools et les phénols affaiblit la liaison O-H existante, influençant ainsi la position du pic O-H caractéristique.
Dans une solution très diluée ou une phase gazeuse, l’absence de liaisons H intermoléculaires se traduit par un pic brutal à ≈3600 cm−1.
Dans les solutions concentrées, comme les molécules sont impliquées dans divers degrés de liaison H, il en résulte une bande très large dans la gamme de 3200 à 3550 cm−1.
Dans les solutions modérément diluées, on observe des pics aigus et larges correspondant respectivement aux vibrations O-H libres et liées H.
De plus, la présence de groupes volumineux empêche les interactions de liaison H, ne générant qu’un pic net quel que soit l’état physique de l’alcool ou du phénol.

Élargissement du pic du spectre ir: liaison hydrogène.

IR Spectrum Peak Broadening: Hydrogen Bonding

La fréquence de vibration d'une liaison est directement proportionnelle à sa force de liaison. Par conséquent, les liaisons plus fortes vibrent à des fréquences plus élevées, tandis que les liaisons plus faibles vibrent à des fréquences plus basses. La vibration d'étirement de la liaison O–H forte dans les alcools et les phénols (solution très diluée ou phase gazeuse) apparaît comme un pic prononcé à 3 600–3 650 cm^-1.

Cependant, l'étendue de la liaison hydrogène influence la fréquence d'étirement observée et l'élargissement de la bande. La liaison hydrogène intermoléculaire ou intramoléculaire affaiblit la liaison O–H, ce qui entraîne une gamme de forces de liaison (3 300–3 400 cm^-1). Ainsi, la vibration d'étirement de la liaison hydrogène O–H dans les solutions concentrées apparaît comme un signal large en spectroscopie IR. Dans les solutions modérément diluées, le signal net de l'étirement de la liaison O–H libre apparaît à côté du signal large de la liaison hydrogène O–H.

Lorsque la présence de groupes volumineux empêche les interactions par liaison hydrogène, une bande nette est observée quel que soit l'état physique de l'alcool ou du phénol. De même, les molécules présentant une liaison hydrogène intramoléculaire présentent une large bande indépendante de la concentration. Par exemple, les phénols substitués par un groupe orthocarbonyle présentent un signal large à 3200 cm^-1 à toutes les concentrations.

Dans les acides carboxyliques, la liaison hydrogène intermoléculaire entre deux molécules conduit à la formation de dimères. La bande O–H apparaît comme un pic très large (2400–3400 cm^-1) centré à ≈3000 cm^-1.

Élargissement du pic du spectre IR: liaison hydrogène.

La fréquence vibratoire d’une liaison est directement proportionnelle à sa force d’adhérence. En conséquence, les liaisons les plus fortes vibrent à des fréquences plus élevées, tandis que les plus faibles ...

ir-spectrum-peak-broadening-hydrogen-bonding

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Molecular Vibrational Spectroscopy

Spectroscopie vibrationnelle moléculaire

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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 bond strengths produces a broad signal in concentrated solutions.

In moderately dilute solutions, both forms can appear. The free O-H stretch gives a sharp signal, and the hydrogen-bonded O-H stretch gives a broad one beside it. If bulky groups block hydrogen bonding, the band stays sharp no matter whether the alcohol or phenol is in a solid, liquid, or gas state.

Molecules with intramolecular hydrogen bonding also show a broad band at all concentrations. Ortho-carbonyl substituted phenols are one example, with a broad signal near 3200 cm−1. Carboxylic acids show an even broader O-H peak because two molecules form hydrogen-bonded dimers. Their band extends from 2400-3400 cm−1 and is centered at about 3000 cm−1.

Hydrogen Bonding Effects in IR O-H Peaks

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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...

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key terms and concepts

See how hydrogen bonding shifts and broadens O-H peaks in IR spectra, from sharp free O-H bands to broad signals in alcohols, phenols, and carboxylic acids.

Hydrogen Bonding Effects in IR O-H Peaks | JoVE K12

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Hydrogen Bonding Effects in IR O-H Peaks

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