Undergraduate
  1. General chemistry  1.1. Introduction to Chemistry  1.2. Atomic Structure  1.2.1. Subatomic particles  1.2.2. Atomic Model  1.2.3. Quantum numbers  1.2.4. Electron Configuration  1.2.5. Periodic trends  1.2.6. Isotopes and their applications  1.3. Chemical bond  1.3.1. Ionic bond  1.3.2. Covalent bonds  1.3.3. Metal Bond  1.3.4. Hybridization  1.3.5. Molecular geometry and VSEPR theory  1.3.6. Bond polarity and dipole moment  1.4. Stoichiometry  1.4.1. Mole concept  1.4.2. Empirical and molecular formula  1.4.3. Limiting reactant and excess reactant  1.4.4. Percent Yield and Purity  1.4.5. Dilution calculation  1.5. States of matter  1.5.1. Gas Laws  1.5.2. Matter and Intermolecular Forces  1.5.3. Solid and Crystal Structures  1.5.4. Phase diagram  1.5.5. Plasma and supercritical fluid  1.6. Chemical reactions  1.6.1. Types of Chemical Reactions  1.6.2. Balancing Chemical Equations  1.6.3. Thermochemical reactions  1.6.4. Reaction energy profiles  1.7. Solutions and Mixtures  1.7.1. Concentration units  1.7.2. Syndrome properties  1.7.3. Solubility and Solubility Rules  1.7.4. Henry's Law and Raoult's Law  1.7.5. Partition coefficient  1.8. Chemical equilibrium  1.8.1. Law of mass action  1.8.2. Le Chatelier's principle  1.8.3. Reaction quotient  1.8.4. To use this online calculator for Equilibrium Constant, enter Equilibrium Constant (Eq.) and hit the calculate button. Here is how the Equilibrium Constant calculation can be explained with given input values -> 0.05 = 0.05/0.  1.8.5. Acid-base balance  1.9. Acids and bases  1.9.1. Arrhenius, Brønsted–Lowry, and Lewis definitions  1.9.2. pH and pOH  1.9.3. Acid-base titration  1.9.4. Buffer Solution  1.9.5. Acid-base indicator  1.9.6. Polyprotic Acids and Bases  1.10. Electrochemistry  1.10.1. redox reactions  1.10.2. Galvanic and Electrolytic Cells  1.10.3. Nernst equation  1.10.4. Electrolysis  1.10.5. Faraday's laws of electrolysis  1.11. Thermodynamics  1.11.1. Laws of Thermodynamics  1.11.2. Enthalpy, entropy and Gibbs free energy  1.11.3. Spontaneity of reactions  1.11.4. Thermodynamic cycles (Hess's law, Carnot cycle)  1.12. Kinetics  1.12.1. Rate law and reaction order  1.12.2. Activation Energy and Catalyst  1.12.3. Collision theory  1.12.4. Reaction mechanism  1.12.5. Transition state theory
  2. Organic chemistry  2.1. Structure and relationships  2.1.1. Hybridisation in Carbon Compounds  2.1.2. Resonance and aromatization  2.1.3. Molecular orbitals  2.1.4. Inductive and mesomeric effects  2.2. Hydrocarbons  2.2.1. Hydrocarbons  2.2.2. Alkene  2.2.3. Alkynes  2.2.4. Aromatic compounds  2.2.5. Cycloalkanes and Conformation  2.3. Functional Group  2.3.1. Alcohols and phenols  2.3.2. Ethers and epoxides  2.3.3. Aldehyde and ketone  2.3.4. Carboxylic acids and derivatives  2.3.5. Amin  2.3.6. Esters and amides  2.3.7. Thiols and sulfides  2.4. Stereoscopic  2.4.1. Isomerism in Stereochemistry  2.4.2. Chirality and optical activity  2.4.3. Structural analysis  2.4.4. Diastereomers and Enantiomers  2.5.1. Substitution reactions  2.5.2. Elimination reactions  2.5.3. Addition reactions  2.5.4. Radical reactions  2.5.5. Rearrangement reactions  2.5.6. Pericyclic Reactions  2.6. Spectroscopy and structural analysis  2.6.1. Infrared Spectroscopy  2.6.2. Nuclear magnetic resonance spectroscopy  2.6.3. Mass Spectrometry  2.6.4. UV-visible spectroscopy  2.6.5. X-ray crystallography  2.7. Polymer chemistry  2.7.1. Addition and Condensation Polymers  2.7.2. Polymerization mechanism  2.7.3. Biodegradable Polymer  2.7.4. Conducting and smart polymers
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Ligands and coordination compounds  3.1.2. Crystal field theory  3.1.3. Molecular Orbital Theory in Coordination Compounds  3.1.4. Jahn–Taylor distortion  3.2. Main Group Chemistry  3.2.1. Alkali and alkaline earth metals  3.2.2. Halogens and Noble Gases  3.2.3. Transition metals and their complexes  3.2.4. Lanthanides and Actinides  3.3. Solid state chemistry  3.3.1. Crystal lattices and unit cells  3.3.2. Defects in solids  3.3.3. Electrical and magnetic properties  3.3.4. Superconductivity  3.4. Bio-inorganic chemistry  3.4.1. Metal ions in biology  3.4.2. Enzymatic reactions with metals  3.4.3. Metal poisoning and detoxification  3.4.4. Metalloproteins and Metalloenzymes
  4. Physical chemistry  4.1. Quantum Chemistry  4.1.1. Wave–particle duality  4.1.2. Schrödinger Equation  4.1.3. Quantum Numbers and Orbitals  4.1.4. Atomic and molecular spectroscopy  4.2. Statistical mechanics  4.2.1. Maxwell–Boltzmann distribution  4.2.2. Partitioning functions  4.2.3. Bose–Einstein and Fermi–Dirac statistics  4.3. Chemical thermodynamics  4.3.1. Gibbs Free Energy  4.3.2. Phase transition  4.3.3. Thermodynamic efficiency  4.4. Surface chemistry  4.4.1. Absorption and Catalysis  4.4.2. Colloids and Emulsions
  5. Analytical Chemistry  5.1. Classical methods  5.1.1. Gravimetric Analysis  5.1.2. titrimeter  5.2. Instrumental methods  5.2.1. Chromatography  5.2.2. Spectroscopy  5.2.3. Electroanalytical methods  5.2.4. X-ray diffraction
  6. Industrial Chemistry  6.1. Petrochemistry  6.2. Chemical engineering principles  6.3. Green Chemistry  6.4. Pharmaceuticals and Drug Synthesis
  7. Biochemistry  7.1. Carbohydrates  7.2. Proteins and enzymes  7.3. Lipids and membranes  7.4. Nucleic acids  7.5. Metabolism and Bioenergetics  7.6. Enzyme kinetics and mechanism

Undergraduate → Organic chemistry → Spectroscopy and structural analysis ↓


UV-visible spectroscopy


Ultraviolet-Visible (UV-Vis) spectroscopy is a widely used technique in the field of analytical chemistry and organic chemistry. It deals with the measurement of the absorption of UV or visible light by chemical substances. This technique helps to identify and analyze various organic compounds. In this comprehensive explanation, we will discuss in depth the principles, instrumentation, and applications of UV-Vis spectroscopy in organic chemistry. We will also include examples and diagrams to help you better understand this important analytical tool.

Principles of UV-visible spectroscopy

UV-Vis spectroscopy is based on the absorption of ultraviolet (UV) and visible light by molecules. The energy absorbed by the molecule causes electronic transitions, where electrons move from a lower energy level to a higher energy level. The wavelength of the absorbed light and the extent of absorption provide valuable information about the molecular structure of a substance.

Electronic transitions

Molecules are composed of different energy levels, and electrons can be excited from one energy level to another by absorbing energy. In UV-Vis spectroscopy, the most common transitions are:

  • σ → σ*: These transitions involve the excitation of electrons from the sigma bonding orbital to the antibonding sigma orbital. They require more energy and usually occur in the UV region.
  • n → σ*: Non-bonding to antibonding sigma transition. This usually occurs when lone pair electrons are excited.
  • π → π*: Exciting an electron from a pi bonding orbital to a pi antibonding orbital. This is common in molecules with double bonds and generally occurs in the visible range.
  • n → π*: Non-bonding to bonding pi transition. These energies typically lie in the visible spectrum for carbonyl groups and similar functional groups.

Beer–Lambert law

The Beer-Lambert law is a fundamental principle in UV-Vis spectroscopy, linking the absorption of light to the properties of the substance through which the light is traveling. This law is expressed mathematically as follows:

        A = εlc

Where:

  • A is the absorbance of the solution.
  • ε is the molar absorptivity or molar extinction coefficient, expressed in L/mol cm.
  • l is the path length of the sample cell (cuvette) in centimeters.
  • c is the concentration of the solution in mol/L.

The Beer-Lambert law shows that absorption is directly proportional to the concentration of the solution and the path length of the optical system. This relationship can be represented graphically to determine the unknown concentration.

Concentration (C) Absorption(A) A = εlc

Instrumentation of UV-visible spectroscopy

The basic components of a UV-Vis spectrophotometer include a light source, a monochromator or filter, a sample holder (usually a cuvette), a detector, and a display or data processor. Each of these plays an important role in accurately measuring absorbance.

Light source

A typical UV-Vis spectrophotometer uses either a deuterium lamp for the UV region or a tungsten lamp for the visible region. Some instruments combine the two to cover the entire UV-visible spectrum.

Monochromator

The monochromator separates light into its component wavelengths. It usually consists of a prism or diffraction grating, which disperses the light into its spectral components. By rotating the monochromator, a specific wavelength of light is separated and directed through the sample.

Sample holder

The sample is usually placed in a cuvette with a known path length. Cuvettes are often made of quartz or optical glass, since these materials do not absorb in the UV-Vis range.

Detectors

After passing through the sample, the light reaches the detector, which converts the transmitted light into an electrical signal. Common detectors include photomultiplier tubes and photodiodes.

Data processing and display

The signals from the detector are processed to calculate the absorbance or transmittance. The results are displayed digitally or graphically as an absorption spectrum, in which absorbance is plotted against wavelength.

Light source Sample cuvette Detectors

Applications of UV-visible spectroscopy

UV-Vis spectroscopy has many applications in organic chemistry and various other scientific disciplines:

Quantitative analysis

The most common use of UV-Vis spectroscopy is the quantitative determination of various analytes. Using the Beer-Lambert law, the concentration of a compound in solution can be accurately determined by measuring its absorbance at a specific wavelength.

For example, if you have a solution containing a colored compound, you can prepare a series of standard solutions of known concentrations, measure their absorbance, and create a calibration curve. By measuring the absorbance of an unknown sample and referring to the calibration curve, the concentration of the unknown solution can be calculated.

Concentration Absorption Calibration curve

Qualitative analysis and structural explanation

UV-Vis spectroscopy can also provide information about the electronic structure and conjugation of organic molecules. Below are some cases where UV-Vis spectroscopy is helpful in structural analysis:

  • Conjugated systems: Molecules with extensive conjugation absorb at longer wavelengths. For example, beta-carotene, with its long conjugated chain, absorbs in the visible region, giving carrots their orange color.
  • Aromatic compounds: Aromatic compounds such as benzene exhibit characteristic absorption bands, known as B-bands, which are due to π → π* transitions.
Wavelength (nm) Absorption Example spectrum

Limitations of UV-visible spectroscopy

Although UV-Vis spectroscopy is a versatile and powerful tool, it still has some limitations:

  • Non-specificity: Several compounds may have a similar absorption spectrum, making it sometimes difficult to distinguish between them based solely on UV-Vis analysis.
  • Sample preparation: Accurate results depend largely on accurate sample preparation and cleanliness of cuvettes.
  • Concentration range: Too high or too low concentrations can cause deviations from the Beer–Lambert law, affecting accuracy.

Conclusion

UV-visible spectroscopy is an important technique in the arsenal of tools available for analyzing and studying organic compounds. It provides valuable data about electronic transitions, helping to identify and quantify a wide range of substances. Understanding this technique enables chemists to interpret absorption data, elucidate molecular structures, and perform comprehensive chemical analysis. By understanding its principles, tools, applications, and limitations, one can take full advantage of this method in scientific investigations and practical applications.


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UV-visible spectroscopy

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