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

Recall that the vibrational frequency of a bond is proportional to the square root of the force constant, considering only bond strength effects and not reduced mass factors.
Thus, stronger bonds with higher force constants vibrate at higher frequencies upon IR absorption. Weaker bonds vibrate at lower frequencies.
For instance, intermolecular hydrogen bonding observed in alcohols and phenols weakens the existing O&#8211;H bond, thereby influencing the position of the characteristic O&#8211;H peak.
In a very dilute solution or gas phase, the absence of intermolecular H-bonds results in a sharp peak at &#8776;3600 cm&#8722;1.
In concentrated solutions, as the molecules are involved in varying degrees of H-bonding , there results in a very broad band in the range of 3200&#8211;3550 cm&#8722;1.
In moderately dilute solutions, both sharp and broad peaks are observed corresponding to the free and H-bonded O&#8211;H vibrations, respectively.
Moreso, the presence of bulky groups prevents H-bonding interactions, generating only a sharp peak irrespective of the physical state of the alcohol or phenol.

ir spectrum peak broadening: hydrogen bonding

IR Spectrum Peak Broadening: Hydrogen Bonding

The vibrational frequency of a bond is directly proportional to its bond strength. As a result, stronger bonds vibrate at higher frequencies, while weaker bonds vibrate at lower frequencies. The stretching vibration of the strong O&#8211;H bond in alcohols and phenols (very dilute solution or gas phase) appears as a sharp peak at 3600&#8211;3650 cm&#8722;1.
However, the extent of hydrogen bonding influences the observed stretching frequency and band broadening. Intermolecular or intramolecular hydrogen bonding weakens the O&#8211;H bond, resulting in a range of bond strengths (3300&#8211;3400 cm&#8722;1). So, the hydrogen-bonded O&#8211;H stretching vibration in concentrated solutions appears as a broad signal in IR spectroscopy. In moderately dilute solutions, the sharp signal from the free O&#8211;H stretching appears beside the broad signal from the hydrogen-bonded O&#8211;H.
When the presence of bulky groups prevents hydrogen-bonding interactions, a sharp band is seen irrespective of the alcohol or phenol&#39;s physical state. Similarly, molecules exhibiting intramolecular hydrogen bonding show a broad band independent of concentration. For instance, ortho-carbonyl substituted phenols exhibit a broad signal at 3200 cm&#8722;1 at all concentrations.
In carboxylic acids, intermolecular hydrogen bonding between two molecules leads to the formation of dimers. The O&#8211;H band appears as a very broad peak (2400&#8211;3400 cm&#8722;1) centered at &#8776;3000 cm&#8722;1.

The vibrational frequency of a bond is directly proportional to its bond strength. As a result, stronger bonds vibrate at higher frequencies, while weaker ...

ir-spectrum-peak-broadening-hydrogen-bonding

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Analytical Chemistry

Unit 1

Molecular Vibrational Spectroscopy

KEY TERMS AND CONCEPTS

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

infrared (ir) spectroscopy: overview

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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.

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Triple-Bond Signals in IR Spectra

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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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IR Peak Splitting in Amines and Nitro Groups

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raman spectroscopy: overview

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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

analytical chemistry

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Hydrogen Bonding Effects in IR O-H Peaks

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