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2.2 IR Spectra 132.3 Wavenumber
All signals point in the same direction.
The units of wavenumber are inverse centimeters, and the values range from 400 cm-1 to 4,000 cm-1.
The terms wavenumber and wavelength should not be confused.
A larger wavenumber means higher energy.
The signals on the right side of the spectrum correspond with lower energy radiation than the signals on the left side of the spectrum.
We will start with wavenumber.
Stronger bonds will have a higher wavenumber of absorption.
You can compare the bonds below.
Smaller atoms give bonds that undergo higher frequencies of absorption.
You can compare the bonds below.
The smallest atom (H) is involved in the C--H bond.
Because single bonds are the weakest bonds, they appear on the right side of the spectrum.
Double bonds appear at higher wavenumber because they are stronger than single bonds, while triple bonds appear at higher wavenumber because they are stronger than double bonds.
The left side of the spectrum contains signals that stretch at a high wavenumber because hydrogen has the smallest mass.
The most useful information is provided by the diagnostic region.
There are signals that arise from the stretching of double bonds, triple bonds, and bonds to H. Most single bonds have stretching and bending vibrations in the fingerprint region.
It is more difficult to analyze the signals in this region.
A bond that is bending might be what appears like a C--C stretch.
Each compound has a unique pattern of signals in this region, which is why it's called the fingerprint region.
The IR spectrum of propanol and ethanol will look the same in their diagnostic regions, but their fingerprints will be different.
We will only focus on the signals that appear in the diagnostic region for the rest of the chapter.
If you see any characteristic signals in your textbook or lecture notes, you should check them out.
The highlighted bonds should be ranked in order of decreasing wavenumber.
The wavenumber of absorption is affected by factors that affect the strength of a bond.
There are bonds to hydrogen on the left side of the IR spectrum.
We will compare different kinds of bonds.
The wavenumber of absorption depends on the state of the carbon atom.
The C --H bond will be shorter.
The weakest bond is the C --H bond.
We draw a line at 3000 cm-1.
The signals to the right of the line come from C --H bonds.
The signal to the left of 3000 cm-1 does not come from an alkane.
An alkene can produce a signal at 3100 cm-1, while an alkyne can produce a signal at 3300 cm-1.
Double bonds and internal triple bonds do not have any C --H bonds.
Let's look at the effects of resonance on bond strength.
There is a second compound.
In your textbook, you will learn more about the p systems.
We will analyze the effect of conjugate on the IR absorption of the carbonyl group.
The saturated ketone group produces a signal at a lower wavenumber than the carbonyl group.
We must draw resonance structures for each compound in order to understand why.
Let's start with the substance.
There are two resonance structures for Ketones.
The carbonyl group is drawn as a double bond in the first resonance structure and a single bond in the second resonance structure.
The contribution from each resonance structure must be considered in order to determine the nature of this bond.
The second resonance structure has charge separation and a carbon atom that has less than an octet of electrons.
The second resonance structure contributes to the overall character of the carbonyl group.
The carbonyl group of a ketone has a double-bond character.
Take a look at the resonance structures for a conjugate, unsaturated ketone.
Unconjugated, unsaturated ketones have three resonance structures instead of two.
The carbonyl group is drawn as a single bond in the third resonance structure.
This resonance structure shows charge separation as well as a carbon atom with less than an octet of electrons.
The resonance structure contributes to the overall character of the compound.
The carbonyl group of a saturated ketone has more single-bond character than the carbonyl group of a third resonance structure.
The signal at a lower wavenumber is produced by a slightly weaker bond with more single-bond character.
There is a similar trend.
A signal at 1740 cm-1 is the average, but a signal at 1710 cm-1 is the average.
The carbonyl group is a weaker bond due to resonance.
The compound has three carbonyl groups.
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