UV-Vis Spectroscopy of Conjugated Alkenes - Online Tutor, Practice Problems & Exam Prep

Ultraviolet (UV) light can excite electrons in conjugated systems, promoting them from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). The energy gap between these orbitals influences the wavelength of maximum absorption (λ_max), which increases with more conjugated double bonds. The relationship between energy (E) and wavelength (λ) is given by the equation: E1=hc×λ. Thus, greater conjugation leads to lower energy and higher λ_max, potentially shifting absorption from UV to visible light.

UV-Vis Spectroscopy of Conjugated Alkenes Concept 1

Hey, everyone. So we can say here that energy from UV or light energy can promote an electron from a lower energy to higher energy molecular orbital, or MO. Here, if we take a look, we have 1,3-butadiene. Here, it possesses 4 pi-electrons. These pi-electrons, we could fit them within their atomic p orbitals.

Since there are 4 of them, we'd go up, up, up, up. And if we were to distribute it within this molecular orbital diagram here, we would be 1 up, 1 down, 1 up, 1 down. Here, HOMO represents the highest occupied molecular orbital, basically the orbital that's highest up with any electrons. LUMO would be our lowest unoccupied molecular orbital, the one that's right above it.

Now, here when we do UV radiation, we would excite one of these electrons, let's say this one, and it will be promoted up to a higher energy state, a higher molecular orbital. So we'd still have up, down, and then we'd have up, and then we'd have down here. As a consequence of this, this would no longer be our HOMO. This would no longer be our LUMO. This would be our new HOMO, the highest occupied molecular orbital, and then this would become our LUMO, the lowest unoccupied molecular orbital.

One thing we need to see from here is that this promotion would have an impact on our λ_max, our wavelength in terms of this. Remember, the higher up we go in terms of molecular orbital, the smaller the distance gets between our individual molecular orbitals. Alright. So just remember, applying some UV or visible light can excite one of these electrons, making it promote itself up to the next highest energy molecular orbital.

UV-Vis Spectroscopy of Conjugated Alkenes Concept 2

So we can say here that the transition from our pi molecular orbital to our pi star molecular orbital corresponds to the lambda max, which depends on the energy gap between our HOMO and LUMO molecular orbitals. We're going to say here that the energy gap decreases with an increase in our conjugated double bonds. And this is going to cause our lambda max to increase, giving us a higher lambda max. Now, remember from general chemistry, when we're talking about energy and wavelength, the equation that connects them together is that energy is equal to Planck's constant, which is h, times the speed of light, which is c, divided by the wavelength.

E = h ⋅ c / λ

Here you can see that energy acts as a numerator and wavelength acts as a denominator. This would mean that there's an inverse relationship between the two of them. As our energy goes down, our wavelength has to go up. So a decrease in energy would mean an increase in our lambda max. Now, if we take a look here, we have 1,3-butadiene which has 4 pi electrons.

We've applied ultraviolet radiation which caused one of our electrons to be promoted up to a higher level. In this form, the LUMO would now be technically here, and this would technically be the HOMO, the one with the highest occupied molecular orbital with an electron. We'd say here that if we calculated the lambda max of 1,3-butadiene, it'd be 217 nanometers. Over here, we have 1,3,5-hexatriene, so this has 6 pi electrons. So we would say here that initially it would be HOMO and this would be LUMO. But if we were to irradiate these electrons, this electron would be promoted up here. And, again, upon that promotion, this would technically become the HOMO, and this would become the LUMO here. Now we can say here that this is going to cause, this is going to give us a lambda max value now of 258 nanometers. So notice that ultraviolet radiation still caused the promotion of an electron from the HOMO up to the LUMO.

But notice that your lambda max value is higher. That's because there's more conjugation within 1,3,5-hexatriene than there is in 1,3-butadiene. Basically, the more conjugated we become as a compound, the higher our lambda max will be. And if we can get enough conjugation going on, we could transition from the ultraviolet spectrum to the visible light spectrum. We could actually interpret this increase in wavelength into colors.

And a vast majority of plants in nature are due because of highly conjugated compounds residing within them. Right? So just remember, as we're increasing our conjugation, we're going to decrease our energy, which is going to increase our lambda max. With enough of that, we can transition from UV to visible light.

UV-Vis Spectroscopy of Conjugated Alkenes Example 1

Hey, everyone. So in this question, it says, the following molecule is irradiated with UV light. Which pi orbitals correspond to HOMO and LUMO? If we take a look here, we have 8 pi electrons within this molecule. And because of that, we're going to have 8 molecular orbitals.

We fill this in up, down, up, down, up, down, up, down. Before we've done any radiation, this would be our HOMO (highest occupied molecular orbital), and this would be our LUMO. Now here, we would irradiate. And technically here, there should be a little bit more distance up here.

Alright. So we're going to say here we're irradiating this with UV light. So this would be up, down, up, down, up, down, up. But then this one here gets promoted up here, making this our new HOMO and our new LUMO. This is how we would depict the transition from non-irradiated to irradiated.

Again, we have 8 pi electrons, so we're going to have 8 molecular orbitals. We would fill them up, filling from lowest to highest up, following the Aufbau principle. When we irradiate it, one of the electrons in the HOMO will be promoted up to the LUMO, giving us this new interpretation of the irradiated molecule. Right? So this is the approach you would take for a question like this.

Rank the following molecules in increasing order of wavelength of max absorption.

• A. ii < iv < iii < i
• B. ii < iv < i < iii
• C. iv < ii < i < iii
• D. iii < i < iv < ii

Here’s what students ask on this topic:

What is the relationship between conjugation and λmax in UV-Vis spectroscopy?

In UV-Vis spectroscopy, the relationship between conjugation and λmax (wavelength of maximum absorption) is direct. As the number of conjugated double bonds in a molecule increases, the energy gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) decreases. This lower energy gap results in a higher λmax. The equation that describes the relationship between energy (E) and wavelength (λ) is:

$E=\frac{hc}{\lambda }$

where h is Planck's constant and c is the speed of light. As energy decreases, wavelength increases, leading to a higher λmax. This shift can even move absorption from the UV to the visible light spectrum.

How does UV light excite electrons in conjugated systems?

UV light excites electrons in conjugated systems by promoting them from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). When UV radiation is absorbed, an electron in the HOMO gains enough energy to jump to the LUMO. This process is crucial for understanding the absorption characteristics of conjugated molecules. The energy required for this transition corresponds to the energy gap between the HOMO and LUMO, which influences the wavelength of maximum absorption (λmax). The more conjugated the system, the smaller the energy gap, resulting in a higher λmax.

Why does increased conjugation lead to a higher λmax?

Increased conjugation leads to a higher λmax because it reduces the energy gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). As conjugation increases, the π-electrons are more delocalized, lowering the energy required for an electron to transition from the HOMO to the LUMO. This lower energy corresponds to a longer wavelength of maximum absorption (λmax). The relationship between energy (E) and wavelength (λ) is given by:

$E=\frac{hc}{\lambda }$

Thus, as energy decreases, λmax increases.

What is the significance of the HOMO-LUMO gap in UV-Vis spectroscopy?

The HOMO-LUMO gap is significant in UV-Vis spectroscopy because it determines the energy required for electronic transitions. This energy gap directly influences the wavelength of maximum absorption (λmax). A smaller HOMO-LUMO gap means that less energy is needed to excite an electron from the HOMO to the LUMO, resulting in a higher λmax. The relationship between energy (E) and wavelength (λ) is described by:

$E=\frac{hc}{\lambda }$

Therefore, understanding the HOMO-LUMO gap helps predict the absorption properties of conjugated molecules.

How does the λmax of 1,3-butadiene compare to that of 1,3,5-hexatriene?

The λmax of 1,3-butadiene is lower than that of 1,3,5-hexatriene. 1,3-butadiene has 4 π-electrons and a λmax of 217 nm, while 1,3,5-hexatriene has 6 π-electrons and a λmax of 258 nm. The increased conjugation in 1,3,5-hexatriene reduces the energy gap between the HOMO and LUMO, resulting in a higher λmax. This demonstrates the principle that more conjugated systems absorb light at longer wavelengths due to the smaller energy gap between their molecular orbitals.