Name: uv-vis spectroscopy and molecular absorption
Uploaded: 2024-12-05T18:32:04.000Z
Duration: 1 min 16 s
Channel: JoVE

Upon absorption of radiation, electrons in a molecule move from the highest occupied molecular orbital, HOMO, to the lowest unoccupied molecular orbital, LUMO, with higher potential energy.
In most molecules, the energy difference between the electronic levels of the HOMO and the LUMO varies from 125&#8211;650 kJ/mol.
Usually, the sigma orbitals are the lowest energy occupied molecular orbitals, followed by the pi and n or nonbonding orbitals. The unoccupied or antibonding orbitals contain the highest energy.
In alkanes, only sigma-to-sigma antibonding electronic transition is observed, while saturated molecules like alcohols, amines, and ethers show n-to-sigma antibonding transitions.
Alkenes and alkynes show pi to antibonding pi transitions, whereas carbonyl compounds show n to antibonding pi transitions.
As per the selection rule, transitions like the n to antibonding pi transition, which involves a change in an electron&#39;s spin quantum number, are forbidden.

uv&ndash;vis spectroscopy: molecular electronic transitions

UV&ndash;Vis Spectroscopy: Molecular Electronic Transitions

In Ultraviolet&#8211;Visible (UV&#8211;Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this process, the electrons in a molecule transit from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), moving to a higher energy state. This transition and the types of orbitals involved are central to understanding how molecules interact with UV&#8211;Vis light. The energy differences between the HOMO and the LUMO are referred to as the bandgap, which typically ranges from 125 to 650 kJ/mol.
A group of atoms in a molecule responsible for absorbing radiation is called a chromophore. When a chromophore undergoes structural changes, both the energy and intensity of the molecule&#39;s absorption change. Molecules with different chemical structures have band gaps and absorb radiation at different wavelengths, producing different absorption spectra.
In molecular orbital theory, the energy levels of orbitals influence the types of electronic transitions that can occur. Typically, the lowest-energy occupied molecular orbitals are the &#963; orbitals, corresponding to &#963; bonds. Next in energy are the &#960; orbitals, followed by nonbonding (n) orbitals that contain an unshared pair of electrons. The highest energy levels are found in unoccupied or antibonding orbitals (&#960;* and &#963;*). Certain restrictions, known as selection rules, must be considered during electron transitions. One important selection rule states that transitions involving a change in an electron&#39;s spin quantum number are not allowed. Such transitions are called &#34;forbidden&#34;&#160;transitions. A common example is a transition from the n to &#960;* orbitals, which are often seen in molecules containing lone pairs.

In Ultraviolet&#8211;Visible (UV&#8211;Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of ...

uvvis-spectroscopy-molecular-electronic-transitions

Education

JoVE Core

Analytical Chemistry

Unit 1

Introduction to Molecular Spectroscopy

KEY TERMS AND CONCEPTS

uv-vis spectroscopy and molecular absorption

UV-Vis spectroscopy shows how molecules absorb ultraviolet and visible light. It is used to study electronic structure by tracking how electrons move between molecular orbitals. The absorbed light must match the energy gap between the ground state and an excited state.

In this process, an electron moves from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. This jump raises the electron to a higher energy level. The energy difference between the HOMO and LUMO is called the bandgap, and it is often about 125 to 650 kJ/mol.

A chromophore is the part of a molecule that absorbs radiation. If a chromophore changes its structure, the energy and intensity of absorption can also change. Different molecular structures therefore absorb at different wavelengths and produce different absorption spectra.

Molecular orbital theory helps explain these transitions. The lowest-energy occupied orbitals are sigma (σ) orbitals, which correspond to σ bonds. Above them are pi (π) orbitals, then nonbonding (n) orbitals with an unshared pair of electrons. The highest energy levels are antibonding orbitals, including π* and σ*.

Selection rules also limit which electron transitions can happen. One important rule is that a change in spin quantum number is not allowed. These are called forbidden transitions. A common example is an n to π* transition, which is often seen in molecules with lone pairs.

UV-Vis Spectroscopy and Molecular Absorption

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dual nature of electromagnetic (em) radiation

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EM Spectrum, Waves, and Photon Energy

Electromagnetic radiation has both wave and particle features. It includes electric and magnetic field components that oscillate in planes perpendicular to each other. These fields are also perpendicular to the direction the radiation moves through space. As a wave, electromagnetic radiation is described by wavelength and frequency. Wavelength is the distance between two consecutive peaks or troughs. Frequency is the number of wave cycles that pass a point in a fixed time. The electromagnetic...

interaction of em radiation with matter: spectroscopy

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EM Radiation and Matter in Spectroscopy

Electromagnetic radiation and matter interact in ways that reveal details about atoms and molecules. In spectroscopy, scientists study how EM radiation is absorbed, emitted, transmitted, reflected, or scattered by a substance. This helps identify its chemical composition and properties. EM radiation behaves as both a wave and a particle. As a wave, it has intensity and amplitude. Intensity, also called irradiance, is the number of photons that strike a given area in a certain time. Amplitude...

molecular spectroscopy: absorption and emission

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Molecular Energy Levels in Spectroscopy

Molecular spectroscopy describes how molecules absorb and emit light by moving between energy levels. These levels are called quantum states. Unlike atoms, molecules also have rotational and vibrational energy levels, so their energy structure is more complex. Each level is separated by an energy gap. The gaps between electronic, vibrational, and rotational levels are very different in size. A diatomic molecule can be used to show these three kinds of energy levels. A molecule can absorb a...

spectrophotometry: introduction

Spectrophotometry for Absorption Spectra

Spectrophotometry measures how a material absorbs, reflects, diffracts, or transmits electromagnetic radiation. It compares the intensity of radiation before and after it interacts with the material. The result shows how the material responds to different wavelengths. A spectrophotometer is the device used for this measurement. It includes a source of electromagnetic radiation, a slot for the sample, and a detector. The detector measures the radiation that leaves the material, including...

ultraviolet and visible (uv&ndash;vis) spectroscopy: overview

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UV-Vis Spectroscopy for Light Absorption

UV-Vis spectroscopy measures how materials absorb ultraviolet and visible light. It is a useful analytical method because it is versatile, simple to use, and gives results quickly. Scientists use it for both qualitative and quantitative analysis. When UV-Vis radiation passes through a sample, molecules absorb specific wavelengths based on the energy needed for electronic transitions. This absorption moves atoms or molecules from a ground state to an excited state. If a molecule absorbs UV...

UV-Vis spectroscopy shows how molecules absorb ultraviolet and visible light. It is used to study electronic structure by tracking how electrons move between molecular orbitals. The absorbed light must match the energy gap between the ground state and an excited state. In this process, an electron moves from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. This jump raises the electron to a higher energy level. The energy difference between...

uv&ndash;vis spectrometers

UV-Vis Spectrophotometer Parts and Samples

A UV-visible spectrophotometer measures how much UV and visible light a sample absorbs. It uses a light source, a wavelength selector, a sample holder, and a detector to build the spectrum. This makes it useful for checking liquid samples in the lab. The light source depends on the region being measured. A deuterium lamp provides UV radiation, and a tungsten lamp provides visible radiation. A monochromator or prism acts as a diffraction grating and splits the incoming light into different...

uv&ndash;vis spectrum

How Molecular Structure Shapes UV-Vis Bands

UV-Vis spectroscopy shows how a molecule absorbs light in the ultraviolet and visible ranges. When light passes through a substance, some of it is absorbed and the rest is reflected or transmitted. Molecules that absorb between 180 and 400 nm give a UV spectrum, while absorption in the 400 to 780 nm range gives a visible spectrum. A UV-Vis spectrum is a plot of absorbance versus wavelength. The y-axis shows molar absorptivity, ε, or log ε, and the x-axis shows wavelength. Absorbance describes...

uv&ndash;vis spectroscopy: beer&ndash;lambert law

Using Absorbance to Find Concentration

The Beer-Lambert law links absorbance, concentration, and path length in a solution. It is based on ideas from Johann Heinrich Lambert and August Beer. Their work helps explain how light changes as it passes through a medium. Lambert's law says that when light passes through a medium, the loss in intensity is directly proportional to the original intensity and the path length. Beer’s law adds that the transmittance of a solution stays constant if the product of concentration and path length...

photoluminescence: fluorescence and phosphorescence

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Light Emission from Excited Molecules

Photoluminescence is light emission from molecules after they absorb light energy. A substance takes in photons, which raises electrons to excited energy levels. The electrons then relax back to the ground state and release light. This process appears in many materials, including semiconductors, organic compounds, and inorganic compounds. The main role of electron spin helps separate photoluminescence into fluorescence and phosphorescence. In a singlet spin state, two electrons share the same...

variables affecting phosphorescence and fluorescence

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Factors That Change Light Emission in Molecules

Fluorescence and phosphorescence depend on several factors that change how molecules emit light. These effects matter in analytical chemistry, biological imaging, and materials science because they help scientists detect molecular properties and see cellular structures. Molecular structure is one of the main influences. Aromatic compounds with low-energy pi-bonding to pi-non-bonding transitions often give strong fluorescence. Aliphatic and alicyclic carbonyl compounds, as well as highly...

deactivation processes: jablonski diagram

Jablonski Diagram: Light Loss Pathways

Jablonski diagrams show how excited molecules lose energy after absorbing light. They are used to track both light-emitting and non-light-emitting steps in luminescence. The diagram gives a simple picture of the energy states a molecule can move through. The lowest line in a Jablonski diagram is the ground-state energy of the molecule. This state is usually a singlet state, and it represents the condition of most molecules in solution at room temperature. The lines above it show vibrational...

photoluminescence: applications

Fluorescent and Phosphorescent Analysis

Photoluminescence is useful in chemical analysis because it is sensitive and selective. It can support both direct and indirect measurements of an analyte, which is the substance being tested. When an analyte has a strong quantum yield, it can be measured directly by fluorescence or phosphorescence. Indirect analysis is used when the analyte is not naturally fluorescent or phosphorescent, or when its quantum yield is poor. One approach is to react the analyte with a reagent to make a...

fluorescence and phosphorescence: instrumentation

fluorescence-and-phosphorescence-instrumentation

Measuring Fluorescence and Phosphorescence

Fluorometers and spectrofluorometers are instruments used to measure molecular fluorescence. They do this by choosing specific excitation and emission wavelengths. The two instruments use different light sources and wavelength-selection methods. A fluorometer uses absorption interference filters to select the excitation and emission wavelengths. Its excitation source is usually a low-pressure mercury vapor lamp. This lamp gives intense lines across the ultraviolet and visible regions.

key terms and concepts

Learn how UV-Vis spectroscopy measures molecular absorption, HOMO-LUMO bandgaps, chromophores, orbital types, and allowed versus forbidden electronic transitions.

UV-Vis Spectroscopy and Molecular Absorption | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

UV-Vis Spectroscopy and Molecular Absorption

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