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Organic chemistry
Course: Organic chemistry > Unit 14
Lesson 2: UV/Vis SpectroscopyAbsorption in the visible region
Physical basis of our perception of color. Example of beta-carotene, the molecule that makes carrots orange. Created by Jay.
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What happens to the energy which is absorbed by the carrot? Why wont the pi* electrons undergo relaxation and emit blue light?(8 votes)- Because flourescence (emission of blue light) competes with other processes, check the Jablonski diagram. Radationless relaxation should dominate as a process.(5 votes)
- Minimum how many conjugated double bond should be present in a molecule to exhibit colour?(2 votes)
- It also depends on the functional groups on the molecule. If you really want to calculate the colors, you should check out Hückel theory, Hückel matrix, etc.(2 votes)
He said that beta-carotene absorbs mostly blue light waves and reflects orange, but what about green, yellow and red light waves? Are they absorbed too?(2 votes)
When he mentioned blue light, Jay was referring to the violet, blue, and green colors; these are considered to be part of the "blue" wavelengths of light. Likewise, yellow, orange, and red are considered to be part of the "orange" wavelengths. So beta-carotene absorbs a mixture of wavelengths in the "blue" region and reflects a mixture of wavelengths in the "orange" region, the latter comprising the particular color that we perceive. :)(1 vote)
I understand that a lower deltaE corresponds to a longer wavelength, but wasn't this due to a n-->pi* transition? How does a conjugated system/ pi-->pi* transition give a smaller delta E and longer wavelength? Thank you in advance!(2 votes)
In a conjugated system there are multiple bonding and antibonding molecular orbitals. As a result of how the individual atomic orbitals interact to create them, those molecular orbitals are spread out over a range of energies. Compared to a single (non-conjugated) π bond, this leads to the highest energy bonding molecular orbitals being closer in energy to the lowest energy antibonding molecular orbitals.(1 vote)
If electrons absorbs energy delta E, to be excited, then they would emit same amount of energy E as a form of photon(light) when go back to ground state. => conservation of energy. That means we see the color which are absorbed by electrons, right? I don't understand why we don't see color that subsance absorbed. Please help me..(1 vote)
Once an electron has been excited to a higher energy level, it can drop back to any lower energy level, not just the one from which it came.
That’s why you can see many different colours.(2 votes)
what is a action spectrum?(1 vote)- In this example, which part of the visible spectrum is the high %T? Is it <425nm and >525nm?(1 vote)
i get that beta-carotene absorbs the electromagnetic waves of blue end of the spectrum of light, with the orange/red end being reflected, but seeing as beta-carotene is not the only thing to make up carrots and in fact is only a small part of the vegetable (could be any orange example!), why does this colour dominate? why don't all the other components affect the colour (or do they)? what about the cellulose molecules, lignin, etc?(1 vote)- The concentration plays an important role, many biomolecules (probably cellulose, lignin) absorb light in the UV range, so our perception of color remains unaffected as we cannot detect changes in the UV range.(1 vote)
- I need to know more about chromophores and how and when transition could be occurred between sigma pi and n in molecular orbitals(1 vote)
- why absorption in uv visible spectra appear as bands,not as sharp peaks?(1 vote)
- Molecules are always vibrating. This causes the energy levels of the molecular orbitals to undergo small changes.
He explains this in the previous video.(1 vote)
Video transcript
On the right we have the dot structure for beta carotene which
is an orange molecule that is responsible for
the color of carrots. On the left is the absorption
spectrum for beta carotene. And the reason why beta
carotene has a color is because it absorbs
light in the visible region of the electromagnetic spectrum. The visible region starts at
approximately 400 nanometers, so if I draw a line right here, to the left of that line
would be the Ultra-Violet, the UV region of the
electromagnetic spectrum, and on the right would
be the visible region. So we see that beta carotene absorbs light with wavelengths of approximately
450 to 500 nanometers for a range and it absorbs strongly in the visible region. To explain why beta carotene is orange, we need to look at a
little bit more detail at the visible region of the
electromagnetic spectrum, and so here we have the different colors. The colors in the visible region, so essentially the colors in the rainbow. Approximately 400 nanometers, we're talking about violet light, alright, so we have
violet light right here, if we go beyond violet light, you're in the ultraviolet
region or the UV region. The visible region goes to
somewhere around 700 nanometers, or little bit beyond that, so
when you're in 700 nanometers, you're talking about red light. And if you go just past red light you are in the infrared region of the electromagnetic spectrum. And here I have six colors, I have red, orange, yellow, green, blue, and violet, and when Isaac Newton did his
famous experiment with a prism and he wrote down seven
colors, he included indigo, because he wanted to have seven colors in the visible regions, so
you usually memorize ROYGBIV for the colors of the rainbow. But the reason I've left out indigo here is because this allows us
to better see a color wheel. Isaac Newton was the first
to represent a color wheel, and you get a color wheel
by taking the violet and moving it over here and taking the red and moving it over here,
and so you put the violet right next to the red and
so you get a color wheel. It's useful to look at a color wheel, because it allows you
to see the relationship between complementary colors, for example, if I wanted to know the
complementary color for red, all I have to do is look
across on my color wheel and I can see that the
complementary color is green. The complementary color for violet, if I look directly across,
that would be yellow, and then finally, the
complementary color for blue would be orange, and this is useful because it allows you to think about why things appear to be a certain color. For example, if I look at this
orange sheet of paper here, and we try to understand why
this sheet of paper is orange, we know that white light consists of all these different wavelengths, we know white light consists
of all the different colors of the rainbow here. We could simplify that even further and we could think about white light being two complementary colors,
so we can say oh, okay, so this part consists of
blue wavelengths of light and then on the right here,
this part of the color wheel consists of orange wavelengths of light and we can think about white light consisting of blue wavelengths
and orange wavelengths so if we have white light coming in, here are the blue wavelengths of light, and then we have the orange
wavelengths of light, so this is an oversimplified
way to think about white light striking our orange object here. So if the object absorbs the
blue wavelengths of light, so we are absorbing the
blue wavelengths of light therefore we are reflecting
the orange wavelengths. So if we reflect the
orange wavelengths of light and our eye happens to be right here, we see the object as being orange. Our brains perceive the
object as being orange because we are seeing the
reflected orange light. And so that's how to think about why something appears
to be a certain color. If I go back up here
to beta carotene again, so I look and see where
beta carotene is absorbing. Beta carotene is absorbing
somewhere in the range of 450 to 500 nanometers and
those are blue wavelengths of light, right, if I look at down here so 450 to 500 nanometers, we're absorbing the blue
wavelengths of light. Therefore, we are reflecting
the orange wavelengths. And so we perceive beta
carotene to be orange. And so that's a little bit of the theory behind why we perceive something as having a certain color. In the next video we're
going to talk about how this dot structure of beta carotene allows the molecule to be colored.