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 ↓


Nuclear Magnetic Resonance Spectroscopy


Nuclear magnetic resonance spectroscopy, often abbreviated as NMR spectroscopy, is a powerful analytical tool used in chemistry to determine the structure of molecules, understand molecular dynamics, and obtain quantitative and qualitative information on samples. It has unique sensitivity to the local environment of specific nuclei, making it exceptionally valuable in elucidating molecular structure.

Fundamentals of NMR

NMR spectroscopy is based on the magnetic properties of certain atomic nuclei. Not all atomic nuclei are suitable for NMR. The main criterion is that the nucleus must have an intrinsic magnetic moment, which generally occurs when the nucleus has spin not equal to zero. An important example is the hydrogen nucleus (^1H), which is abundantly present in organic compounds and has a spin of 1/2.

When a nucleus with spin is placed in an external magnetic field, it experiences a force that tends to align it with the field. However, due to quantum mechanics, the nucleus can only take certain orientations with respect to the magnetic field. The more energetically favorable orientation is parallel to the magnetic field.

1/2 spin nuclei: H (^1H), C (^13C), N (^15N), P (^31P)

How NMR works

When placed in a magnetic field, nuclei can absorb energy and transition between different spin states. The frequency of energy required for this transition falls in the radiofrequency region of the electromagnetic spectrum.

This resonance frequency, designated as ω, is proportional to the strength of the applied magnetic field and the magnetic properties of the nucleus. Mathematically, it can be expressed as:

ω=γB₀

Where ω is the angular frequency, γ is the gyromagnetic ratio, and B₀ is the magnetic field strength.

Tools and techniques

An NMR spectrometer consists of several major components: a strong magnet to apply the magnetic field, a radiofrequency transmitter to excite the sample, a receiver to detect the emitted radiofrequency signals, and a computer to process the data.

Sample preparation

The sample is usually dissolved in a solvent that does not interfere with the NMR spectrum. Deuterated solvents, such as deuterated chloroform (CDCl₃) or deuterated water (D₂O), are commonly used because they do not contain protons that would otherwise give rise to strong signals.

NMR experiments

The sample is placed in a magnetic field. Radiofrequency pulses are then applied. These pulses excite the nuclei, which transition to higher energy states. As the excited nuclei return to their lower energy states, they emit radiofrequency radiation. The detector captures this emitted signal.

Data collection

The emitted signals are called free induction decays (FID), which are collected over time. The FID is a complex decaying sine wave, and its analysis forms the basis of the NMR spectrum.

NMR spectra interpretation

Data obtained from an NMR experiment are often represented as a plot of frequency against intensity. Feature-rich spectra allow for in-depth analysis of molecular structures.

Chemical shift

The position of the signal in the ^1H or ^13C NMR spectrum is known as the chemical shift. It provides information about the electronic environment around the nucleus. Chemical shifts are expressed in parts per million (ppm) and are calculated relative to a standard reference compound, usually tetramethylsilane (TMS).

Spin-spin coupling

Nuclei in molecules affect each other's magnetic environments, leading to interactions called spin-spin coupling. These interactions result in the NMR signal being split into multiple peaks, called multiplets. The distance between these split peaks is known as the coupling constant, expressed in hertz (Hz).

Integration

The area under an NMR signal is proportional to the number of nuclei contributing to that signal. This information is important for determining the relative number of hydrogen atoms in different chemical environments within the molecule.

Example - Ethanol

Let us consider the NMR spectrum of ethanol (CH₃CH₂OH). The spectrum consists of three primary signals:

  • A triplet corresponding to the methyl group (CH₃) due to coupling with the adjacent methylene proton.
  • A quartet for the methylene group (CH₂), due to coupling of the methyl group with three protons.
  • A single peak for the hydroxyl group (OH), which is often broadened due to rapid proton exchange.

Advanced technologies and applications

NMR spectroscopy has expanded beyond traditional ^1H and ^13C NMR to include two-dimensional (2D) NMR, multinuclear NMR, and solid-state NMR. These techniques provide even deeper insight into molecular structure and dynamics.

2D NMR

Two-dimensional NMR techniques, such as COSY (COrrelation SpectroscopY) and HSQC (Heteronuclear Single Quantum Coherence), are used to understand complex molecular connectivity and spatial relationships within molecules. In 2D NMR, data are plotted on two axes, usually frequency versus frequency, which provides information about proton-proton, carbon-proton, and other interactions.

Polyatomic NMR

NMR spectroscopy is not limited to hydrogen and carbon. It can also study other nuclei such as ^15N, ^19F, ^29Si, and ^31P, each of which provides unique perspectives on the environment and structure of the molecule.

Let's look at ^31P NMR as an example:

  • Useful in the study of organophosphate and phosphine ligands.
  • Can provide information about chemical changes of phosphorus, and phosphorus-hydrogen or phosphorus-carbon coupling.

Solid state NMR

In solid-state NMR, samples are in solid form rather than in solution. This technique is particularly beneficial for studying materials such as polymers, proteins, and biosolids that are difficult to observe in solution.

Features of solid-state NMR include:

  • Magic-angle spinning (MAS) for averaging anisotropic interactions.
  • Cross-polarization (CP) techniques for signal amplification.

Applications of NMR Spectroscopy

NMR spectroscopy is widely used in various fields of research and industry.

Organic and inorganic chemistry

NMR is a primary tool for elucidating molecular structures, confirming synthetic compounds, and discovering reaction mechanisms. It also provides important insights into stereochemistry and isomers.

Biochemistry and molecular biology

NMR spectroscopy is used to study the shape and interaction dynamics of proteins, nucleic acids, and complex biomolecules in solution. It is invaluable in drug discovery and design.

Medical imaging - MRI

A type of NMR used in medical imaging is known as magnetic resonance imaging (MRI). In MRI, radiofrequencies and strong magnets create detailed images of organs and tissues, which are useful for diagnosis and medical research.

Challenges and considerations in NMR

NMR spectroscopy, despite its strong capabilities, presents challenges:

  • Sensitivity: Some nuclei may provide weak signals that require higher concentrations or more extended storage times.
  • Equipment cost and size: High-field NMR spectrometers are expensive and take up a lot of space in the laboratory.
  • Data interpretation: Drawing accurate conclusions from complex spectra requires a skilled analyst.

Despite these challenges, NMR spectroscopy remains an important tool in the arsenal of chemists. Its ability to provide detailed information on the structure and dynamics of molecular systems makes it indispensable in research and industry.


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Nuclear Magnetic Resonance Spectroscopy

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