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 ↓


Nuclear magnetic resonance spectroscopy


Nuclear magnetic resonance spectroscopy, or NMR spectroscopy, is a powerful analytical technique used in organic chemistry to determine the structure of organic compounds. NMR involves the study of the interaction between magnetic fields and certain atomic nuclei. When a nucleus is placed in a magnetic field, it behaves similar to a tiny bar magnet. If the nucleus is compatible, it can absorb and reemit electromagnetic radiation, and this process is detected in NMR spectroscopy.

The most common type of NMR used in organic chemistry involves hydrogen nuclei, also known as proton NMR or 1H NMR. The basic principle relies on the magnetic properties of certain nuclei present in organic compounds. The nuclei of interest are exposed to a strong magnetic field and irradiated with radio frequency (RF) pulses.

Principle of NMR spectroscopy

NMR spectroscopy is based on the magnetic properties of certain atomic nuclei. When placed in a magnetic field, these nuclei absorb electromagnetic radiation at a frequency characteristic of the isotope. The main steps involved in this process are:

  • Placing the nuclei in a strong magnetic field.
  • Application of a radio frequency pulse to excite the nucleus.
  • Reception of the signal when the nucleus returns to its equilibrium state.

The data are often displayed in a spectrum that shows the frequency of the absorbed radio waves versus their intensity. Each peak in the NMR spectrum corresponds to a nucleus or group of nuclei in the compound.

Basic concepts

Magnetic fields and Larmor precession

When placed in a magnetic field, nuclei with spin will precess in the direction of the magnetic field at a frequency known as Larmor frequency. This frequency depends on the strength of the magnetic field and the type of nucleus.

Larmor frequency (ν) = γB₀/2π

Where:

  • ν is the Larmor frequency,
  • γ is the gyromagnetic ratio (a constant for each nucleus),
  • B₀ is the magnetic field strength.

Chemical shift

Chemical shift is a dimensionless number that indicates the position of the NMR signal relative to a standard reference compound. In proton NMR, the reference compound is typically tetramethylsilane (TMS). The chemical shift is measured in parts per million (ppm):

Chemical shift δ (ppm) = (ν_sample - ν_TMS) / ν_TMS × 10⁶

Different functional groups in organic molecules affect the chemical shift, helping chemists predict the structure of the compound.

Equipment

An NMR spectrometer consists of several major components:

  • A powerful magnet to produce a magnetic field.
  • A transmitter to generate and emit the RF pulse.
  • A receiver for detecting the NMR signal.
  • A computer to process and interpret the data.

Modern NMR spectrometers can be quite large because they use superconducting magnets cooled by liquid helium to maintain a strong magnetic field.

Analysis of NMR spectra

NMR spectra are analyzed to provide information about the number and type of hydrogen atoms in a molecule, their environment, and their connectivity:

  • The number of symbols corresponds to the number of specific hydrogen environments in the molecule.
  • Signal intensity (integration): Indicates the relative number of hydrogen atoms contributing to the signal.
  • Signal splitting (multiplicity): results from interactions between neighbouring hydrogen atoms (spin-spin coupling), which is described by n+1 rule.

Multiplicity of signs

The multiplicity of a signal indicates how many peaks are observed for a particular hydrogen atom and is due to the presence of neighbouring hydrogens. n+1 rule helps to estimate the multiplicity:

For a hydrogen atom with n adjacent hydrogens:

  • Singlet (n=0): One vertex, no neighbors.
  • Doublet (n=1): two peaks, one neighbouring.
  • Triplet (n=2): Three peaks, two neighbors.
  • Quartet (n=3): four peaks, three neighbors, etc.

Examples: ethanol

Consider the ethanol compound, CH₃CH₂OH. It has three different hydrogen environments: methyl group (CH₃), methylene group (CH₂), and hydroxyl hydrogen (OH). The NMR spectrum displays:

  • A triplet for the methyl group, which is split by two methylene hydrogens. 
    ---| | |---
  • A quartet for the methylene group, split by three methyl hydrogens. 
    ---| | | |---
  • A singlet for the hydroxyl group, which broadens due to exchange with water and other molecules. 
    ---|---

Conclusion

Nuclear Magnetic Resonance Spectroscopy is an essential tool in the structure determination of organic compounds. Its ability to provide detailed insight into the chemical environments of protons and their neighboring atoms makes it invaluable to chemists. Understanding the fundamental concepts of magnetic fields, chemical shifts, signal integration, and splitting patterns helps to understand complex molecular structures more easily. With continued advances in technology, NMR continues to provide even more detailed and accurate structural analysis.


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Nuclear magnetic resonance spectroscopy

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