TBR Ultraviolet/Visible Spectroscopy

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CookieZine

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I have a few questions about UV-Vis. TBR states, "Because σ-bonds are so much stronger than π-bonds, the lowest energy absorbance for alkanes is significantly higher than the lowest energy absorbance for alkenes." Can someone explain this to me? I know that σ-bonds are much stronger than π-bonds but the second part of that sentence confuses me.

I also do not understand why the energy gap between π and π* decreases as the conjugation increases. The notion that λ max is directly proportional to the amount of conjugation is easy to remember but difficult for me to understand. When we talk about the wavelength of highest absorbance, λ max, what exactly are we talking about? In one sense I thought that if we bombard some compound photons in the UV-Vis range there would be one wavelength that the compound would absorb more so than the others. This doesn't make sense to me because I thought if λ max is 258 nm and if absorbance is a measure of how many photons are being absorbed then ran into a problem. For example, if λ max is 258 nm and absorbance is 100% then what would happen if you increased the wavelength to 259 nm? I know that absorbance cannot be more than 100% but would the increase in wavelength cause the absorbance to fall? If λ max is 258 nm then it must be the wavelength at which the compound experiences the greatest absorbance of photons. What happens for wavelengths that are greater than λ max? I think it may have to do with energy being quantized:



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Conjugated aldehydes and ketones have about the same π- π* absorbances as conjugated alkenes of the same number of π-bonds. However, conjugated aldehydes and ketones have other, more intense absorbances (ε > 10,000) that are of longer wavelength than their hydrocarbon counterparts. This is attributed to the n-to- π* transition associated with aldehydes and ketones, possible because of the lone pair of electrons on the carbonyl oxygen. What does the n-to- π* transition mean? Is this transition not found in other conjugated systems that have lone pairs of electrons, which would lead to more intense absorbances that are of longer wavelength than their hydrocarbon counterparts? For example, would conjugated ethers and esters exhibit this property?
 
I have a few questions about UV-Vis. TBR states, "Because σ-bonds are so much stronger than π-bonds, the lowest energy absorbance for alkanes is significantly higher than the lowest energy absorbance for alkenes." Can someone explain this to me? I know that σ-bonds are much stronger than π-bonds but the second part of that sentence confuses me.

Conjugated aldehydes and ketones have about the same π- π* absorbances as conjugated alkenes of the same number of π-bonds. However, conjugated aldehydes and ketones have other, more intense absorbances (ε > 10,000) that are of longer wavelength than their hydrocarbon counterparts. This is attributed to the n-to- π* transition associated with aldehydes and ketones, possible because of the lone pair of electrons on the carbonyl oxygen. What does the n-to- π* transition mean? Is this transition not found in other conjugated systems that have lone pairs of electrons, which would lead to more intense absorbances that are of longer wavelength than their hydrocarbon counterparts? For example, would conjugated ethers and esters exhibit this property?

In regards to this information - why would the more intense energy absorbances have a longer wavelength? Isn't energy inversely proportional to wavelength?
 
In regards to this information - why would the more intense energy absorbances have a longer wavelength? Isn't energy inversely proportional to wavelength?

The intensity of an absorbance has nothing to do with the energy of the excitation. The intensity of absorbance is governed by Beer's law, which states that absorbance is a function of extinction coefficient, path length, and concentration. If you've taken a UV spectrum before, you should remember that the more concentrated you make your sample, the more intense the absorption. OP seems to be convoluting these two separate ideas.
 
The intensity of an absorbance has nothing to do with the energy of the excitation. The intensity of absorbance is governed by Beer's law, which states that absorbance is a function of extinction coefficient, path length, and concentration. If you've taken a UV spectrum before, you should remember that the more concentrated you make your sample, the more intense the absorption. OP seems to be convoluting these two separate ideas.

Sweet! Thanks!
 
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