14.7 Ultraviolet Spectroscopy
3 min read•may 7, 2024
is a powerful tool for detecting in molecules. It measures how compounds absorb UV light, revealing information about their electronic structure and helping identify functional groups.
The technique relies on the relationship between wavelength and energy absorption. By analyzing absorption patterns, chemists can determine a molecule's structure and even calculate its concentration using the .
Ultraviolet Spectroscopy
Detection of conjugated systems
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- Measures absorption of ultraviolet light by molecules detects presence of conjugated π electron systems
- Conjugated π electrons absorb UV light due to from ###Highest_occupied_molecular_orbital_()0### to ###Lowest_unoccupied_molecular_orbital()_0###
- Energy gap between HOMO and LUMO determines wavelength of light absorbed smaller gap absorbs longer wavelengths ()
- More extensive conjugation leads to smaller HOMO-LUMO gaps absorbs at longer wavelengths
- Functional groups with conjugated π systems (chromophores) contribute to UV absorption
- Alkenes, alkynes, aromatic rings (benzene), groups (aldehydes, ketones), (-N=N-)
- Conjugated systems are particularly important in UV spectroscopy due to their ability to absorb light in the UV-visible region of the
Wavelength vs energy absorption
- Energy of a photon is inversely proportional to its wavelength represented by equation
- = energy, = Planck's constant, = speed of light, = wavelength
- Shorter wavelengths correspond to higher energy transitions
- UV spectra plot absorbance () vs wavelength ()
- Absorbance peaks indicate wavelengths at which molecule absorbs UV light
- Wavelength of maximum absorption () depends on extent of conjugation
- More conjugation leads to longer values
- Conjugated dienes ( around 230 nm) vs conjugated trienes ( around 270 nm)
Concentration from Beer-Lambert law
- Beer-Lambert law relates absorbance to concentration and path length
- = absorbance, = , = path length, = concentration
- Molar absorptivity () measures how strongly a substance absorbs light at a given wavelength
- Unique for each substance and depends on the wavelength
- Units
- Higher values indicate stronger absorption
- Path length () is distance light travels through sample
- Usually 1 cm for standard cuvettes
- Concentration () can be calculated by rearranging Beer-Lambert law
- Requires known values for absorbance, molar absorptivity, and path length
- Example calculation:
- Measure absorbance () of sample at a specific wavelength
- Look up molar absorptivity () for the substance at that wavelength
- Measure or know the path length () of the cuvette
- Plug values into equation to calculate concentration
- This relationship forms the basis for in UV spectroscopy
Factors Affecting UV Spectra
- : The plot of absorbance vs. wavelength, showing characteristic peaks for specific compounds
- : The choice of solvent can influence the position and intensity of absorption peaks due to interactions with the analyte
- Electronic transitions: Different types of transitions (e.g., π→π*, n→π*) contribute to the observed UV spectrum, with each transition having characteristic energy requirements
Key Terms to Review (35)
200-400 nm: The 200-400 nm range refers to the ultraviolet (UV) portion of the electromagnetic spectrum, which lies between visible light and X-rays. This region is particularly important in the context of ultraviolet spectroscopy, a technique used to analyze the absorption and emission of UV radiation by molecules.
Absorbance (A): Absorbance measures the amount of light absorbed by a sample at a particular wavelength in ultraviolet spectroscopy. It quantifies how much light is absorbed as it passes through a substance, typically reported as a logarithmic function of the ratio of incident to transmitted light intensity.
Absorption Spectrum: The absorption spectrum is a graphical representation of the specific wavelengths of light that a substance absorbs when exposed to electromagnetic radiation. It is a fundamental concept in the field of ultraviolet spectroscopy, which is used to identify and quantify the presence of various compounds in a sample.
Azo Compounds: Azo compounds are a class of organic compounds containing the azo functional group (-N=N-), which features two nitrogen atoms double-bonded to each other. These compounds exhibit a wide range of applications, from dyes and pigments to pharmaceuticals and polymers.
Bathochromic Shift: A bathochromic shift, also known as a red shift, is a phenomenon in which the absorption or emission spectrum of a molecule is shifted to longer wavelengths (lower energy) compared to a reference compound. This shift is typically observed in ultraviolet and visible light spectroscopy and is closely related to the concepts of conjugation, aromaticity, and the chemistry of vision.
Beer-Lambert Law: The Beer-Lambert law, also known as Beer's law, is a fundamental relationship in spectroscopy that describes the attenuation of light as it passes through a medium. It establishes a direct correlation between the concentration of an absorbing species in a solution and the amount of light absorbed by that solution.
Carbonyl: The carbonyl group is a functional group consisting of a carbon atom double-bonded to an oxygen atom. It is a key structural feature in many organic compounds, including aldehydes, ketones, carboxylic acids, and esters, and plays a crucial role in their chemical reactivity and properties.
Chromophore: A chromophore is a functional group or conjugated system within a molecule that is responsible for the molecule's color. It is the part of a molecule that absorbs specific wavelengths of light, leading to the observed color of the molecule.
Conjugated Systems: Conjugated systems refer to a series of alternating single and double bonds within a molecule, creating a continuous network of overlapping pi orbitals. This unique electronic structure has important implications for the absorption of ultraviolet light and the stability of the molecular orbitals in these types of compounds.
Electromagnetic spectrum: The electromagnetic spectrum encompasses all types of electromagnetic radiation, ranging from gamma rays with the shortest wavelengths to radio waves with the longest wavelengths. In organic chemistry, it plays a crucial role in structure determination by providing information about molecular vibrations and ion fragmentation patterns.
Electromagnetic Spectrum: The electromagnetic spectrum is the entire range of electromagnetic radiation, which includes various types of energy waves such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. This spectrum is organized based on the wavelength and frequency of the different forms of radiation, and it plays a crucial role in various areas of science, including organic chemistry.
Electronic Transitions: Electronic transitions refer to the process by which an electron in an atom or molecule moves from one energy level or orbital to another. This change in the electron's energy state is accompanied by the absorption or emission of a photon of specific wavelength or frequency, which is a fundamental concept in the fields of spectroscopy and quantum mechanics.
Ethanol: Ethanol, also known as ethyl alcohol, is a colorless, volatile, and flammable liquid that is the principal type of alcohol found in alcoholic beverages. It is an important organic compound with diverse applications in various fields, including as a fuel, solvent, and chemical feedstock.
Hexane: Hexane is a straight-chain alkane with the molecular formula C6H14. It is a colorless, volatile, and flammable liquid that is widely used as a solvent and in the production of various chemicals and fuels.
Highest occupied molecular orbital (HOMO): The Highest Occupied Molecular Orbital (HOMO) is the highest energy orbital containing electrons in a molecule under ground state conditions. It plays a crucial role in determining how a molecule interacts with light and other chemical entities.
HOMO: HOMO, or Highest Occupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the highest energy level occupied by electrons in a molecule. This term is crucial in understanding the stability, reactivity, and spectroscopic properties of organic compounds, particularly in the context of conjugated systems, pericyclic reactions, and the chemistry of vision.
Homotopic: In the context of 1H NMR spectroscopy and proton equivalence, homotopic protons are those that can be interchanged by a symmetry operation without changing the molecule's overall spatial arrangement. These protons have identical chemical environments and therefore exhibit identical chemical shifts in NMR spectroscopy.
Hyperchromic Effect: The hyperchromic effect refers to an increase in the intensity or absorbance of a chromophore's absorption spectrum, typically observed in ultraviolet (UV) and visible light spectroscopy. This phenomenon is closely linked to the study of aromatic compounds and their electronic transitions.
Lowest unoccupied molecular orbital (LUMO): The LUMO is the lowest energy molecular orbital that does not contain electrons but can accept them during chemical reactions or excitations. It plays a crucial role in determining the reactivity and properties of molecules, especially in conjugated systems analyzed by ultraviolet spectroscopy.
LUMO: LUMO, or Lowest Unoccupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the energy level of the highest-energy orbital that is not occupied by electrons in the ground state of a molecule. The LUMO is crucial in understanding the stability and reactivity of conjugated systems, as well as the behavior of molecules in various photochemical and pericyclic reactions.
Molar Absorptivity: Molar absorptivity, also known as molar extinction coefficient, is a measure of the ability of a molecule to absorb light at a specific wavelength. It is a fundamental parameter in ultraviolet (UV) spectroscopy, which is used to quantify the concentration of a substance in a sample by measuring the amount of light it absorbs.
Molar absorptivity (ɛ): Molar absorptivity, denoted as ɛ, measures how well a chemical species absorbs light at a particular wavelength. It is a crucial parameter in ultraviolet spectroscopy for determining the concentration of substances in a solution.
Molar absorptivity (ε): Molar absorptivity, denoted as ε, is a measure of how well a chemical species absorbs light at a given wavelength, expressed in units of L mol^-1 cm^-1. It quantifies the capacity of molecules, particularly conjugated compounds, to absorb ultraviolet (UV) or visible light, which is fundamental in understanding their electronic structure and behavior.
Molar absorptivity (ϵ): Molar absorptivity, symbolized as ϵ, is a measure of how well a chemical species absorbs light at a given wavelength. It quantifies the absorption per mole of substance in a solution, facilitating the understanding of its concentration through ultraviolet spectroscopy.
Monochromator: A monochromator is an optical device that isolates a narrow band of wavelengths from a broader spectrum of light. It is a critical component in ultraviolet (UV) spectroscopy, allowing for the selection of a specific wavelength or range of wavelengths to be used in the analysis of chemical samples.
N → π*: The n → π* transition, also known as the nonbonding to antibonding pi transition, is an electronic transition that occurs in organic molecules when an electron in a nonbonding (n) orbital is excited and promoted to an antibonding (π*) orbital. This transition is particularly relevant in the context of ultraviolet (UV) spectroscopy, as it can provide valuable information about the structure and electronic properties of organic compounds.
Quantitative Analysis: Quantitative analysis is the process of determining the exact amounts or concentrations of chemical components in a sample. It involves the use of analytical techniques to measure the quantities of specific substances present in a material or solution.
Solvent Effects: Solvent effects refer to the influence that the surrounding solvent environment can have on the behavior and properties of chemical reactions, molecules, and spectroscopic measurements. The nature and polarity of the solvent can significantly impact the energetics, kinetics, and outcomes of various organic chemistry processes.
Spectrophotometer: A spectrophotometer is an analytical instrument used to measure the amount of light absorbed or transmitted by a sample at different wavelengths of the electromagnetic spectrum, particularly in the ultraviolet and visible light regions. It is a crucial tool for studying the chemical composition and properties of substances.
Ultraviolet (UV) spectroscopy: Ultraviolet spectroscopy is an analytical technique that uses ultraviolet light to analyze the chemical structure, particularly focusing on conjugated systems, of organic compounds. It measures the intensity of light absorbed by a sample at different UV wavelengths to determine various properties and structures.
Ultraviolet Spectroscopy: Ultraviolet spectroscopy is a technique used to analyze the absorption of ultraviolet (UV) light by molecules, providing information about their electronic structure and the presence of chromophores, which are functional groups that can absorb UV radiation. This analytical method is widely used in various fields, including organic chemistry, biochemistry, and materials science, to identify and characterize organic compounds.
UV-Vis Spectroscopy: UV-Vis spectroscopy is an analytical technique that measures the absorption or reflection of ultraviolet and visible light by a sample. It provides information about the electronic structure and conjugation of molecules, allowing researchers to identify and quantify organic compounds.
Woodward-Fieser Rules: The Woodward-Fieser rules are a set of empirical guidelines used to predict the ultraviolet (UV) absorption maxima of conjugated organic compounds. These rules provide a systematic approach to estimating the wavelength of maximum absorption based on the structure and substituents of the molecule.
λmax: λmax, or the wavelength of maximum absorption, is a key concept in ultraviolet (UV) spectroscopy that describes the specific wavelength at which a molecule absorbs light most strongly. This term is crucial in understanding the interpretation of UV spectra and the effects of conjugation on the absorption properties of organic compounds.
π → π*: The term 'π → π*' refers to an electronic transition that occurs in organic molecules during ultraviolet (UV) spectroscopy. It describes the excitation of an electron from a bonding π orbital to an antibonding π* orbital, which is a higher energy state within the molecule.