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

Denken Sie daran, dass die Schwingungsfrequenz einer Bindung proportional zur Quadratwurzel der Kraftkonstante ist, wobei nur die Auswirkungen der Bindungsfestigkeit und nicht die Faktoren der reduzierten Masse berücksichtigt werden.
Stärkere Bindungen mit höheren Kraftkonstanten schwingen also bei der IR-Absorption bei höheren Frequenzen. Schwächere Bindungen schwingen bei niedrigeren Frequenzen.
Zum Beispiel schwächt die in Alkoholen und Phenolen beobachtete intermolekulare Wasserstoffbrückenbindung die bestehende O-H-Bindung und beeinflusst dadurch die Position des charakteristischen O-H-Peaks.
In einer sehr verdünnten Lösung oder Gasphase führt das Fehlen intermolekularer H-Bindungen zu einem scharfen Peak bei ≈3600 cm−1.
Da die Moleküle in konzentrierten Lösungen unterschiedlich stark an der H-Bindung beteiligt sind, ergibt sich eine sehr breite Bande im Bereich von 3200–3550 cm−1.
In mäßig verdünnten Lösungen werden sowohl scharfe als auch breite Peaks beobachtet, die den freien bzw. H-gebundenen O-H-Schwingungen entsprechen.
Darüber hinaus verhindert das Vorhandensein sperriger Gruppen H-Bindungswechselwirkungen, die unabhängig vom Aggregatzustand des Alkohols oder Phenols nur einen scharfen Peak erzeugen.

verbreiterung der ir-spektrum-spitze: wasserstoffbrücken.

IR Spectrum Peak Broadening: Hydrogen Bonding

Die Schwingungsfrequenz einer Bindung ist direkt proportional zu ihrer Bindungsstärke. Daher schwingen stärkere Bindungen bei höheren Frequenzen, während schwächere Bindungen bei niedrigeren Frequenzen schwingen. Die Streckschwingung der starken O–H-Bindung in Alkoholen und Phenolen (sehr verdünnte Lösung oder Gasphase) erscheint als scharfer Peak bei 3600–3650 cm^-1.

Das Ausmaß der Wasserstoffbrücken beeinflusst jedoch die beobachtete Streckfrequenz und Bandverbreiterung. Intermolekulare oder intramolekulare Wasserstoffbrücken schwächen die O–H-Bindung, was zu einem Bereich von Bindungsstärken (3300–3400 cm^-1) führt. Daher erscheint die wasserstoffgebundene O–H-Streckschwingung in konzentrierten Lösungen als breites Signal in der IR-Spektroskopie. In mäßig verdünnten Lösungen erscheint das scharfe Signal der freien O–H-Streckung neben dem breiten Signal der wasserstoffgebundenen O–H.

Wenn die Anwesenheit sperriger Gruppen Wasserstoffbrücken verhindert, ist unabhängig vom physikalischen Zustand des Alkohols oder Phenols ein scharfes Band zu sehen. Ebenso zeigen Moleküle mit intramolekularen Wasserstoffbrücken ein breites Band, unabhängig von der Konzentration. Beispielsweise zeigen ortho-carbonylsubstituierte Phenole bei allen Konzentrationen ein breites Signal bei 3200 cm^-1.

Bei Carbonsäuren führt die intermolekulare Wasserstoffbrücke zwischen zwei Molekülen zur Bildung von Dimeren. Das O–H-Band erscheint als sehr breiter Peak (2400–3400 cm^-1) mit Mittelpunkt bei ≈3000 cm^-1.

Verbreiterung der IR-Spektrum-Spitze: Wasserstoffbrücken.

Die Schwingungsfrequenz einer Bindung ist direkt proportional zu ihrer Bindungsstärke. Dies hat zur Folge, dass stärkere Bindungen bei höheren Frequenzen schwingen, während schwächere Bindungen ...

ir-spectrum-peak-broadening-hydrogen-bonding

Education

Lehre

JoVE Core

Analytical Chemistry

Analytische Chemie

Unit 1

Einheit 1

Molecular Vibrational Spectroscopy

Molekülschwingungsspektroskopie

KEY TERMS AND CONCEPTS

ZENTRALE BEGRIFFE UND KONZEPTE

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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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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