Ultraviolet Region

Topic: Molecular Structure And Absorption Spectra

Ultraviolet/Visible (UV/Vis) is a type of absorption spectroscopy used in organic chemistry that focuses on the slightly shorter, more energetic wavelengths of radiation in the ultraviolet and visible rea of the spectrum

The ultraviolet region of the electromagnetic spectrum refers to light with shorter wavelengths (higher energy) than the violet end of the visible range, although the two regions are related in their interactions with electrons in organic compounds. The wavelengths in the UV and visible ranges of the electromagnetic spectrum are strong enough to induce electronic excitation, promoting ground state valence electrons into excited states. In general, UV/Vis spectroscopy is used two kinds of molecules. It is useful in monitoring complexes of transition metals. The easy promotion of electrons from ground to excited states in the closely-spaced d-orbitals of many transition metals give them their bright color (by absorbing wavelengths in the visible region).

When a compound absorbs UV light, if that wavelength corresponds to the energy gap, E, between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), it can promote an electron from the HOMO to the LUMO. The wavelength of the absorbed light and the energy are related by the following equation: E=hc/λ, where E = energy (in Joules, J), h = Planck’s constant (6.626×10-34 J·s), c = speed of light (~3×108 m/s), and λ = wavelength (m).

If a compound absorbs only UV light and not visible light, it will appear colorless (transparent in solution, or white in solid form), as is the case for many organic compounds. However, as shown in the example below, changes in conjugation can have an effect on the HOMO-LUMO gap and therefore the wavelength of light that will be absorbed by a molecule. Increased conjugation from left to right means longer (lower energy) wavelengths are enough to promote an electron. As with visible spectra, UV spectra are plotted as absorbance vs. wavelength. This example can be extended to titration indicators which undergo a structural change to form or break extended conjugation in the molecule so the indicator compound changes from colorless to colored or vice versa (absorbing only UV light or absorbing visible light).

UV/Vis spectroscopy is used to study highly conjugated organic systems. Molecular orbital theory tells us that when molecules have conjugated π-systems, orbitals form many bonding, non-bonding, and anti-bonding orbitals. These orbitals can be reasonably close together in energy. In fact, the orbitals are close enough to allow promotion of electrons between electronic states through absorption of ultraviolet or even visible photons. The wavelength of the maximum absorption for any compound is directly related to the extent of conjugation in the molecule. The more extensive the conjugated system is, the longer the wavelength of maximum absorption will be.


Key Points

• The ultraviolet region of the electromagnetic spectrum refers to light with shorter wavelengths (higher energy) than the violet end of the visible range, although the two regions are related in their interactions with electrons in organic compounds.

• The wavelengths in the UV and visible ranges of the electromagnetic spectrum are strong enough to induce electronic excitation, promoting ground state valence electrons into excited states

• Compounds which absorb only UV light and not visible light will appear colorless.

• Increased conjugation decreases the HOMO-LUMO gap and therefore increases the wavelength of light absorbed by a compound.

• Molecular orbital theory tells us that when molecules have conjugated π-systems, orbitals form many bonding, non-bonding, and anti-bonding orbitals.

• The wavelength of the maximum absorption for any compound is directly related to the extent of conjugation in the molecule: the more extensive the conjugated system is, the longer the wavelength of maximum absorption will be.


Key Terms

Ultraviolet region: The region of the electromagnetic spectrum between visible light and X-rays, with a wavelength falling between 400 and 10 nanometers.

Wavelength: The distance between successive crests of a wave, especially points in a sound wave or electromagnetic wave.

 π-systems: Used to describe the situation that occurs when π systems (e.g. double bonds) are “linked together.”

Conjugated system: A system of connected p orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability.

HOMO: “Highest Occupied Molecular Orbital” is the molecular orbital that is highest in energy and also contains at least one electron.

LUMO: “Lowest Unoccupied Molecular Orbital” is the molecular orbital which is the lowest in energy of those which contain no electrons.



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