Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Organic chemistry → Spectroscopy and structural determination ↓


NMR Spectroscopy


Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful and versatile analytical techniques used to determine the structure of molecules in the field of organic chemistry. This technique takes advantage of the magnetic properties of certain atomic nuclei. It provides detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.

Fundamentals of NMR spectroscopy

NMR spectroscopy is based on the interaction between nuclear spin and external magnetic fields. Atomic nuclei with an odd number of protons or neutrons have an intrinsic magnetic moment and spin, which makes them NMR-active. ¹H (protons) and ¹³C (carbon) are the most common nuclei analyzed using NMR. When these nuclei are subjected to a strong external magnetic field, they align themselves with or against the field, creating different energy levels.

Energy levels and the Zeeman effect

In the absence of an external magnetic field, the magnetic moments of the nucleus are randomly oriented. However, when a magnetic field is applied, the magnetic moments align with or opposite to the field. The energy difference between these states is called the Zeeman splitting. The frequency at which energy is absorbed and the nucleus transitions between these energy levels is called the Larmor frequency.

with a magnetic field energy ΔE

The Larmor frequency depends on the type of nucleus and the strength of the magnetic field, which is calculated from the following equation:

    ω₀ = γB₀

where ω₀ is the Larmor frequency, γ is the gyromagnetic ratio (a specific constant for each type of nucleus), and B₀ is the magnetic field strength.

Chemical shift

Chemical shift is an important parameter in NMR spectroscopy, providing insight into the chemical environment surrounding the nucleus. It is defined as the difference between the resonance frequency of the nucleus in the sample and that of the reference compound. Chemical shifts are measured in parts per million (ppm) and are largely affected by electron shielding and delocalization effects.

1 0 Chemical Shift (ppm)

Shielding occurs when the circulating electrons generate a local magnetic field that opposes the applied magnetic field, thereby decreasing the actual magnetic field experienced by the nucleus. In contrast, de-shielding occurs when the electrons increase the local magnetic field, thereby increasing the magnetic field at the nucleus.

Spin-spin coupling

Spin-spin coupling, or J-coupling, is an interaction between neighboring nuclei. It provides additional details about molecular structure through the splitting pattern of NMR signals. When nuclei are coupled, they split each other's NMR signals into multiples. The number of splits is related to the number of coupled neighboring nuclei.

For example, a proton will split into a doublet with a neighboring proton. A proton with two neighboring protons will appear as a triplet and so on. The coupling constant, J, is measured in hertz (Hz) and indicates the energy difference between the multiplet peaks.

    CH₃-CH₂-Br

In this example, the protons in the ethyl group show a typical splitting pattern:

  • CH₃ protons split into triplets (due to two CH₂ protons)
  • CH₂ protons split into quartets (due to three CH₃ protons)

NMR spectra interpretation

The NMR spectrum provides a lot of information that can be used to elucidate the structure of a compound. Key aspects include:

  • Number of signs: Indicates the number of different types of hydrogen or carbon environments in the molecule.
  • Chemical shift: Provides clues about the electronic environment and the type of functional groups present around the nucleus.
  • Integration: The area under each peak is proportional to the number of protons contributing to that signal.
  • Multiplicity: Splitting patterns reflect the number of neighbouring protons and their interactions.

Example analysis

Consider the NMR spectrum of ethanol (CH₃CH₂OH):

    1. Triplet (CH₃) at 1 ppm
    2. Quartet at 3.8 ppm (CH₂)
    3. Singlet (OH) at 5 ppm
CH₃ CH₂ OH

This spectrum indicates that ethanol has three different environments for the hydrogen atoms:

  • CH₃ group shows triplet because it is adjacent to CH₂ group.
  • Since CH₃ group has three protons, CH₂ group appears as a quartet.
  • OH group is represented as a singlet because it is not normally paired with other protons.

Types of NMR spectroscopy

There are several types of NMR spectroscopy that target different nuclei or specialize in particular analytical techniques:

  • ¹H NMR: Examines hydrogen atoms and is the most common type of NMR.
  • ¹³C NMR: Focuses on carbon atoms, providing information about the carbon skeleton of organic compounds. Since the natural abundance of ¹³C is about 1.1%, this requires a more sensitive approach.
  • ²D NMR: Uses two-dimensional techniques such as COSY, NOESY and HSQC to provide detailed interaction maps and connectivity information in molecules.

Applications of NMR spectroscopy

NMR spectroscopy has wide applications in research, pharmaceuticals, materials science, and other fields:

  • Structural elucidation: Determining the complete arrangement of atoms within complex organic molecules.
  • Quantitative analysis: Measuring the concentrations of components in mixtures.
  • Study of molecular dynamics: Investigating the conformational dynamics of molecules and enzyme catalysis.
  • Analysis of complex mixtures: Providing detailed data about metabolic pathways and natural product samples.

Limitations of NMR spectroscopy

Despite its versatility, NMR spectroscopy has some limitations:

  • Sensitivity: NMR is inherently less sensitive than other techniques, and requires larger sample sizes for analysis.
  • Cost: High-resolution NMR spectrometers and maintenance are expensive.
  • Time consumption: Detailed analysis and interpretation of complex spectra can be time consuming.

Conclusion

NMR spectroscopy remains an indispensable tool in organic chemistry, providing detailed insight into molecular structures and behaviors. With continued advances, its scope is expected to further broaden across a variety of scientific fields.


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NMR Spectroscopy

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