PHD
  1. Inorganic chemistry  1.1. Coordination chemistry  1.1.1. Ligand field theory  1.1.2. Crystal field theory  1.1.3. Molecular Orbital Theory for Coordination Compounds  1.1.4. Stability of Coordination Compounds  1.1.5. Electronic spectra of coordination compounds  1.1.6. Isomerism in Coordination Compounds  1.1.7. Kinetics and mechanism of coordination reactions  1.2. Organometallic Chemistry  1.2.1. Metal Carbonyls  1.2.2. Metal alkyl and aryl  1.2.3. Fischer and Schrock carbonaceans  1.2.4. Oxidative Addition and Reductive Elimination  1.2.5. Metal Hydrides  1.2.6. Catalysis by organometallic compounds  1.2.7. Metallocenes and Sandwich Complexes  1.2.8. CH activation  1.3. Solid state chemistry  1.3.1. Crystal structures and lattices  1.3.2. Band theory and electrical properties  1.3.3. Defects in the crystal  1.3.4. Synthesis of solid state materials  1.3.5. Magnetic and optical properties of solids  1.3.6. Superconductivity  1.3.7. Solid state ionics  1.4. Bio-inorganic chemistry  1.4.1. Metalloproteins and enzymes  1.4.2. Metal ion transport and storage  1.4.3. The role of metals in medicine  1.4.4. Bioinspired catalysis  1.4.5. Metal–DNA interactions  1.4.6. Metal Complexes in Medical Science  1.5. Lanthanides and Actinides  1.5.1. Electronic configuration and oxidation states in lanthanides and actinides  1.5.2. Spectral and Magnetic Properties in Lanthanides and Actinides  1.5.3. Separation and Extraction of Lanthanides and Actinides  1.5.4. Coordination chemistry of f-block elements  1.6. Main Group Chemistry  1.6.1. Chemistry of boron compounds  1.6.2. Chemistry of Silicon and Germanium Compounds  1.6.3. Chemistry of Phosphorus and Sulfur Compounds  1.6.4. Organosilicon chemistry  1.6.5. Noble Gas Chemistry
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition and substitution reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic Reactions  2.1.7. Photochemical reactions  2.2. Stereoscopic  2.2.1. Chirality and optical activity  2.2.2. Structural analysis  2.2.3. Stereoelectronic effects  2.2.4. Asymmetric synthesis  2.2.5. Dynamic Stereochemistry  2.3. Organometallic Chemistry in Organic Synthesis  2.3.1. Organolithium and organomagnesium reagents  2.3.2. Transition metal catalyzed coupling reactions  2.3.3. Palladium-catalyzed reactions  2.3.4. Olefin Metathesis  2.3.5. Gold and silver catalysis  2.4. Natural products chemistry  2.4.1. Alkaloids and terpenoids  2.4.2. Steroids and Flavonoids  2.4.3. Total synthesis of natural products  2.4.4. Biosynthesis of natural products  2.5. Supramolecular Chemistry  2.5.1. Host-Guest Chemistry  2.5.2. Self-assembly and molecular recognition  2.5.3. Supramolecular Catalysis  2.5.4. Molecular Machines
  3. Physical Chemistry  3.1. Thermodynamics  3.1.1. Laws of Thermodynamics  3.1.2. Chemical Potential and Equilibrium  3.1.3. Statistical thermodynamics  3.1.4. Non-equilibrium thermodynamics  3.2. Quantum Chemistry  3.2.1. Schrödinger equation and its applications  3.2.2. Molecular orbital theory  3.2.3. Density functional theory  3.2.4. Post-Hartree–Fock methods  3.3. Chemical kinetics  3.3.1. Reaction rate theory  3.3.2. Mechanism and rate laws  3.3.3. Catalysis and enzyme kinetics  3.3.4. Reaction dynamics  3.4. Spectroscopy and molecular structure  3.4.1. Infrared Spectroscopy  3.4.2. UV-visible spectroscopy  3.4.3. Nuclear Magnetic Resonance Spectroscopy  3.4.4. Mass Spectrometry  3.4.5. Raman Spectroscopy  3.5. Surface and Colloid Chemistry  3.5.1. Absorption and Catalysis  3.5.2. Surfactants and micelles  3.5.3. Colloidal Stability  3.5.4. Nanomaterials and Interfaces
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.1.4. Ion Chromatography  4.2. Electroanalytical techniques  4.2.1. Voltammetry and Polarography  4.2.2. Conductometry and potentiometry  4.2.3. Colometry  4.3. Spectroscopic Methods  4.3.1. Atomic absorption spectroscopy  4.3.2. Fluorescence Spectroscopy  4.3.3. X-ray diffraction  4.3.4. Electron paramagnetic resonance spectroscopy  4.4. Chemometrics  4.4.1. Data processing and statistical methods  4.4.2. Multidisciplinary analysis  4.4.3. Machine Learning in Analytical Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.2. Quantum chemistry methods  5.3. Computational drug design  5.4. Machine Learning in Chemistry  5.5. Ab initio molecular dynamics
  6. Biophysics and Medicinal Chemistry  6.1. Drug discovery and design  6.2. Enzyme kinetics and inhibition  6.3. Chemical biology approach  6.4. Protein-ligand interactions  6.5. Bioorthogonal chemistry
  7. Materials chemistry  7.1. Polymer chemistry  7.1.1. Polymerization mechanism  7.1.2. Functional Polymers  7.1.3. Biodegradable Polymer  7.1.4. Conducting and semiconducting polymers  7.2. Nano Chemistry  7.2.1. Nanoparticles and nanostructures  7.2.2. Carbon-based nanomaterials  7.2.3. Quantum dots  7.3. Energy and Environmental Chemistry  7.3.1. Battery and Fuel Cell Chemistry  7.3.2. Green chemistry and sustainable materials  7.3.3. Photocatalysis

PHD → Physical Chemistry → Spectroscopy and molecular structure ↓


UV-visible spectroscopy


UV-visible spectroscopy is a form of absorption spectroscopy that deals with the ultraviolet and visible region of the electromagnetic spectrum. This analytical technique is used to measure the absorbance or transmittance of a liquid or solid sample. It is an important tool in physical chemistry for the study and analysis of molecular structures.

The UV-visible spectrum typically covers the range from 200 nm to 800 nm. Absorption of light in these regions by molecules causes electronic transitions, typically from the ground state to an excited state. Understanding these transitions helps chemists gain valuable information about the electronic structure of molecules.

Basic principles

In UV-visible spectroscopy, an incident light beam is passed through a sample. Some portion of the light is absorbed, and the remainder is transmitted through the sample. A spectrometer measures the intensity of the light passing through the sample and compares it to the intensity of the light before it entered the sample. This comparison provides absorption or transmittance data, which can give information about the substance.

The central principle underlying the Beer-Lambert law is often used to relate the absorption of light to the properties of the substance through which the light is traveling. This law is given as:

A = εlc

Where:

  • A is the measured absorbance (no unit).
  • ε is the molar absorptivity (L mol-1 cm-1).
  • l is the path length of the cuvette containing the sample (cm).
  • c is the concentration of the absorbing species (mol L-1).

Components of UV-visible spectrophotometer

A UV-visible spectrophotometer mainly consists of the following components:

  • Light source: Typically, a deuterium lamp (for UV) and a tungsten lamp (for visible) are used to generate a broad spectrum of light.
  • Monochromator: It separates light of a single wavelength from a broad spectrum.
  • Cuvette: A small container in which the sample solution is placed. Cuvettes are made from materials such as quartz (for UV) and glass (for visible) to avoid absorption.
  • Detector: Converts the transmitted light into an electrical signal. Photodiode or photomultiplier tube is commonly used.
  • Display: Displays absorption or transmittance values and can produce a spectrum showing how these values change with wavelength.

Electronic transitions

Electronic transitions are the basis of absorption in UV-visible spectroscopy. Molecules absorb light energy that excites electrons from lower energy orbitals (e.g., non-bonding or π orbitals) to higher energy (anti-bonding or σ orbitals).

energy ground state excited state

The most common types of electronic transitions observed in UV-visible spectroscopy include:

  • σ → σ* transitions: require high energy and do not usually occur in the UV-visible range except at very short wavelengths.
  • n → σ* transition: This involves lone pairs and is moderately rapid.
  • π → π* transitions: These transitions, observed in unsaturated systems such as alkenes and aromatics, are extremely intense and fall within the typical range of UV-visible spectroscopy.
  • n → π* transition: is less sharp and occurs in lower energy range because the non-bonding electron moves into the π* orbital.

Applications of UV-visible spectroscopy

UV–visible spectroscopy has a wide range of applications, including:

  • Qualitative analysis: Helps in identification of compounds by matching their spectrum with known spectrum.
  • Quantitative analysis: Used to determine the concentration of a sample using the Beer-Lambert law.
  • Monitoring reactions: By observing the changes in the spectrum during the reaction, the reaction kinetics can be studied.
  • Purity testing: Verify the purity of organic and inorganic compounds by analyzing spectral lines.

In addition, UV-visible spectroscopy is widely used in biochemistry to study proteins and nucleic acids. Aromatic amino acids and nucleotide bases strongly absorb UV light, which allows macromolecule concentrations and structure to be determined.

Wavelength (nm) Absorption

This spectrum is an example of a UV-visible spectrum that shows different peaks corresponding to different wavelengths of absorbed light. This reveals information about the type of transitions occurring in the molecule.

Factors affecting UV-visible absorption

Several factors affect the absorption spectra in UV–visible spectroscopy:

  • Concentration: According to the Beer-Lambert law, absorbance is directly proportional to concentration.
  • Path length: Increasing the path length in the cuvette increases the absorption capacity.
  • Solvent effect: Different solvents can shift the absorption maximum, because solvent polarity affects the energy levels.
  • Temperature: Increased temperature can cause broadening of spectral lines due to increased molecular motion.

The polarity of the solvent can cause a shift known as a bathochromic (red shift) or hypochromic (blue shift) shift. Similarly, hyperchromic and hypochromic effects refer to an increase or decrease in absorption capacity.

Advantages and limitations of UV-visible spectroscopy

Advantages:

  • Non-destructive technology.
  • Quick and easy execution.
  • High precision and reproducibility.
  • Wide applicability to both organic and inorganic compounds.

Limitations:

  • Compounds with similar spectra cannot be distinguished.
  • Quantitative analysis requires calibration.
  • Not suitable for measuring turbid, highly scattering samples.

Conclusion

UV-visible spectroscopy is a powerful and versatile tool in physical chemistry. From analyzing the electronic nature of molecules to determining concentrations in solution, this technique provides a wealth of information, making it an integral part of chemical, biological, and materials science research.

Overall, understanding the intricacies of UV-visible spectroscopy, from its principles to applications, enhances a researcher's ability to harness the enormous potential this technique offers in the exploration of molecular structures.


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UV-visible spectroscopy

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