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Effects on IR bands - IR Spectroscopy | Organic Chemistry PDF Download

1. Mesomeric and Inductive effects

Effects on IR bands - IR Spectroscopy | Organic Chemistry

2. Conjugation – By resonance, conjugation lowers the energy of a double or triple bond. The effect of this is readily observed in the IR spectrum.

Effects on IR bands - IR Spectroscopy | Organic Chemistry

Conjugation will lower the observed IR band for a carbonyl from 20-40 cm–1 provided conjugation gives a strong resonance contributor.

Effects on IR bands - IR Spectroscopy | Organic Chemistry

Effects on IR bands - IR Spectroscopy | Organic Chemistry


[Inductive effects are usually small, unless coupled with a resonance contributor, note –CHand –Cl above]


3. Steric effects – usually not important in IR spectroscopy, unless they reduce the strength of a bond (usually p) by interfering with proper orbital overlap. Here the methyl group in the structure at the right causes the carbonyl group to be slightly out of plane, interfering with resonance.

Effects on IR bands - IR Spectroscopy | Organic Chemistry


4. Strain effects: Changes in bond angle forced by the constraints of a ring will cause a slight change in hybridization, and therefore, bond strength. As bond angle decreases, carbon becomes more electronegative, as well as less sp2 hybridized (bond angle < 120°).

Effects on IR bands - IR Spectroscopy | Organic Chemistry


5. Hydrogen bonding 

Hydrogen bonding causes a broadening in the band due to the creation of a continuum of bond energies associated with it.

In the solution phase, these effects are readily apparent; in the gas phase where these effects disappear or in lieu of steric effects, the band appears as sharp as all other IR bands. H-bonding can interact with other functional groups to lower frequencies.

Effects on IR bands - IR Spectroscopy | Organic Chemistry

Illustration1: CH3CH2CN


Effects on IR bands - IR Spectroscopy | Organic Chemistry


Illustration2:

Effects on IR bands - IR Spectroscopy | Organic Chemistry


Effects on IR bands - IR Spectroscopy | Organic Chemistry


Answer:

 Effects on IR bands - IR Spectroscopy | Organic Chemistry


Illustraion3:


Effects on IR bands - IR Spectroscopy | Organic Chemistry


Effects on IR bands - IR Spectroscopy | Organic Chemistry


Answer:

Effects on IR bands - IR Spectroscopy | Organic Chemistry

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FAQs on Effects on IR bands - IR Spectroscopy - Organic Chemistry

1. What are the effects of IR bands in IR spectroscopy?
Ans. The effects of IR bands in IR spectroscopy refer to the changes or shifts observed in the absorption peaks of molecules when they interact with infrared radiation. These effects provide valuable information about the functional groups present in the molecule, allowing for identification and analysis.
2. How does the presence of different functional groups affect the IR bands?
Ans. Different functional groups in a molecule have characteristic vibrational frequencies, which result in specific IR bands. The presence of different functional groups will cause shifts or changes in the position, intensity, or shape of the IR bands. By analyzing these effects, chemists can determine the types of functional groups present in a sample.
3. Can the intensity of IR bands provide any information about the sample?
Ans. Yes, the intensity of IR bands can provide valuable information about the sample. The intensity of the absorption peaks is related to the concentration of the absorbing species, allowing for quantitative analysis. Moreover, the relative intensities of different bands can indicate the relative abundance of different functional groups in the sample.
4. How can the shape of IR bands be used for identification purposes?
Ans. The shape of the IR bands can be used for identification purposes by comparing them to reference spectra or databases. The shape of the bands is influenced by factors such as the strength of the bond, the presence of neighboring groups, and hydrogen bonding. By analyzing the shape of the bands, chemists can match them to known spectra and identify the compound.
5. Can the position of IR bands be used to differentiate between different compounds?
Ans. Yes, the position of IR bands can be used to differentiate between different compounds. Each functional group has characteristic vibrational frequencies, resulting in specific positions of absorption peaks. By comparing the positions of the IR bands in a sample to known spectra, chemists can determine the types of functional groups present and differentiate between different compounds.
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