UV-Vis Spectroscopy of Conjugated Alkenes: Videos & Practice Problems

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

Energy from ultraviolet (UV) or visible light can promote an electron from a lower energy state to a higher energy molecular orbital (MO). For example, in 1,3-butadiene, which contains four π electrons, these electrons occupy atomic p orbitals. When represented in a molecular orbital diagram, the electrons are arranged as follows: one up, one down, one up, and one down. In this context, the highest occupied molecular orbital (HOMO) is the orbital with the highest energy that contains electrons, while the lowest unoccupied molecular orbital (LUMO) is the next higher energy orbital that is empty.

When UV radiation is applied, one of the π electrons can be excited to a higher energy state, resulting in a new arrangement of electrons: one up, one down, one up, and one down in the higher orbital. This transition alters the definitions of HOMO and LUMO; the original HOMO becomes the new LUMO, and the previously unoccupied orbital now becomes the new HOMO. This electron promotion affects the λ_max, or the maximum wavelength of absorption, as the energy gap between molecular orbitals decreases with increasing energy levels. Thus, applying UV or visible light can effectively excite electrons, promoting them to higher energy molecular orbitals and influencing the compound's optical properties.

UV-Vis Spectroscopy of Conjugated Alkenes Concept 2

The transition from the π molecular orbital to the π* molecular orbital corresponds to the wavelength at which maximum absorption occurs, known as λ_max. This λ_max is influenced by the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). As the number of conjugated double bonds in a molecule increases, the energy gap decreases, leading to an increase in λ_max. This relationship indicates that a higher degree of conjugation results in a longer wavelength of light being absorbed.

To understand the connection between energy and wavelength, we can refer to the equation:

E = \frac{hc}{\lambda}

In this equation, E represents energy, h is Planck's constant, c is the speed of light, and λ is the wavelength. The inverse relationship between energy and wavelength means that as energy decreases, wavelength increases. Therefore, a decrease in energy corresponds to an increase in λ_max.

For example, in 1,3-butadiene, which has four π electrons, the application of UV radiation promotes one electron from the HOMO to the LUMO, resulting in a λ_max of 217 nanometers. In contrast, 1,3,5-hexatriene, with six π electrons, exhibits a λ_max of 258 nanometers after similar irradiation. The increase in λ_max for hexatriene is attributed to its greater conjugation compared to butadiene.

As the degree of conjugation in a compound increases, the energy required for electronic transitions decreases, leading to higher λ_max values. With sufficient conjugation, compounds can absorb light in the UV range and transition into the visible light spectrum, allowing us to perceive colors. This phenomenon is prevalent in nature, as many plants contain highly conjugated compounds that contribute to their vibrant colors. In summary, increasing conjugation decreases energy and increases λ_max, potentially shifting absorption from UV to visible light.

UV-Vis Spectroscopy of Conjugated Alkenes Example 1

When analyzing the effects of UV light on a molecule with 8 π electrons, it is essential to understand the concepts of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). In this case, the molecule has 8 molecular orbitals, which are filled according to the Aufbau principle, where electrons occupy the lowest energy levels first.

Initially, the filling of the molecular orbitals would look like this: each of the 8 π electrons fills the orbitals in pairs, resulting in a configuration where the HOMO is the highest energy orbital that contains electrons, and the LUMO is the next available orbital that is unoccupied. Before irradiation, the HOMO is fully occupied, while the LUMO remains empty.

Upon irradiation with UV light, one of the electrons from the HOMO is promoted to the LUMO. This transition alters the electronic configuration of the molecule, resulting in a new HOMO and LUMO. The new HOMO will now be the orbital that previously was the LUMO, and the LUMO will be the next higher energy orbital that has just been filled with the promoted electron.

This process illustrates the fundamental principles of molecular orbital theory, where the promotion of electrons due to external energy input, such as UV light, can significantly affect the electronic properties of a molecule. Understanding these transitions is crucial for predicting the reactivity and behavior of molecules in photochemical processes.

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

Answer: (B)

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.