Give Basic Theory of UV Spectroscopy. #Spectroscopy #Organic Chemistry

U.V. spectroscopy is based on the electronic excitation of molecules. The absorptions from the ultraviolet regions supply energy which is sufficient to raise an electron form one energy level to a higher one.

According to the molecular orbital approach the absorbed energy of light is used to transfer an electron from a bonding or non-bonding molecular orbital to an anti-bonding molecular orbital.
For example in acetone all single bonds are sigma bond, while double bond is pi bond and lone pairs of electrons with oxygen which don’t take part in bonding are non bonding electrons.
Now, the electron thus transferred may be a sigma (𝛔) bond electron or a pi (𝛑) bond electron or it may be an electron of a lone-pair (n).i.e. a non-bonding pair.

The possible electronic transitions (→) that are involved in ultraviolet and visible region are:
𝛔 → 𝛔*
𝛔 → 𝛑*
𝛑 → 𝛔*
𝛑 → 𝛑*
n → 𝛔*
n → 𝛑*
A 𝛑 → 𝛑* transition occurs with a compound containing double bond and an n → 𝛑* transition occurs with a compound containing heteroatom, e.g. Thus increasing amounts of energy are required to raise n, 𝛑 and 𝛔 electrons from their bonding to the corresponding anti-bonding molecular orbitals. In fact, 𝛔 electrons are held very tightly and hence the energy required for 𝛔 → 𝛔* transition is so much that it requires absorption of wavelengths from the far ultraviolet region.

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[saimahafeez]

I was looking for this concept from a couple of days, coincidentally, I came across this video, you solved my biggest problem, THANKS ALOT

[kagomealmariha]

Thanks.. very clear and easy to understand

[mariaengland]

Great job explaining this concept! Thanks so much!

[ytb]

great video. i think your pi antibonding symbol is missing the star symbol.

[saimahafeez]

I've one request, if possible plz make aa video on WHY PHENOPHTHALEIN DOES NOT CHANGE COLOR IN ACIDIC MEDIUM? I mean what's the basic principle or reason that electrons are not excited in acidic medium

[shaheersyed]

this was very informative thank you very much

[jboww]

So it’s difficult to achieve a transition for an alkane since it is so far into UV, what types of compounds can go into VIS wavelengths? I’m assuming compounds with a higher number of pi bonds/higher degree of conjugation? For a compound with a dozen conjugated double bonds, is the wavelength of the energy needed to perform a pi to pi* transition the same for each of those double bonds? Like can you consider all the pi bonds in a molecule as the same pi bond?