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PHDPhysical ChemistrySpectroscopy 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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