what are all 2D NMR techniques?

2D NMR spectroscopy involves several techniques, each designed to provide specific information about molecular structure and interactions. Here’s an overview of the most commonly used 2D NMR techniques:

1. COSY (Correlation Spectroscopy)

• Purpose: Detects through-bond scalar couplings between nuclei, typically protons. It provides information on which protons are coupled to each other.
• Application: Useful for determining the connectivity of atoms in small to medium-sized organic molecules.

2. NOESY (Nuclear Overhauser Effect Spectroscopy)

• Purpose: Detects through-space interactions between nuclei (usually protons) that are close in space but not necessarily bonded. It provides information on spatial proximity.
• Application: Widely used in studying the 3D structure of larger molecules like proteins and nucleic acids, as well as conformational analysis.

3. TOCSY (Total Correlation Spectroscopy)

• Purpose: Provides information on the entire spin system of coupled nuclei, allowing the identification of all protons within a spin system.
• Application: Particularly useful in analyzing complex mixtures or overlapping signals in organic compounds.

4. HSQC (Heteronuclear Single Quantum Coherence)

• Purpose: Correlates protons with directly attached heteronuclei (e.g., $^{13}C$, $^{15}N$). It gives a one-bond correlation between the proton and the heteronucleus.
• Application: Often used for studying large biomolecules, providing valuable information about carbon or nitrogen environments in the molecule.

5. HMQC (Heteronuclear Multiple Quantum Coherence)

• Purpose: Similar to HSQC but generally more sensitive. It also correlates protons with directly attached heteronuclei.
• Application: Used in situations where higher sensitivity is required for detecting heteronuclei like $^{13}C$ and $^{15}N$.

6. HMBC (Heteronuclear Multiple Bond Correlation)

• Purpose: Detects long-range correlations between protons and heteronuclei (e.g., $^{13}C$ or $^{15}N$) over two or more bonds.
• Application: Useful for identifying quaternary carbons or other heteroatoms that are not directly bonded to protons.

7. ROESY (Rotating-Frame Overhauser Effect Spectroscopy)

• Purpose: Similar to NOESY, but operates under conditions where the rotating-frame Overhauser effect is dominant. It is used when NOESY is less effective due to molecular dynamics.
• Application: Useful for molecules with intermediate molecular weights where NOESY might not provide clear data.

8. INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment)

• Purpose: Provides direct $^{13}C-^{13}C$ connectivity information. This technique is challenging because it requires a high concentration of the sample due to the low natural abundance of $^{13}C$.
• Application: Helps in elucidating carbon skeletons of complex organic molecules.

9. DQF-COSY (Double Quantum Filtered COSY)

• Purpose: Enhances the resolution of COSY spectra by filtering out signals that do not arise from double-quantum coherence, thus improving the clarity of correlation peaks.
• Application: Used when standard COSY spectra have overlapping peaks or complex patterns that need resolution.

10. J-Resolved Spectroscopy

• Purpose: Separates chemical shift and coupling constant information into two dimensions, allowing the determination of coupling constants independently of chemical shifts.
• Application: Provides clear insights into the spin-spin coupling constants, useful for complex splitting patterns in NMR spectra.

11. HETCOR (Heteronuclear Correlation)

• Purpose: Correlates chemical shifts between protons and heteronuclei, offering insights into which protons are bonded to which heteroatoms.
• Application: Useful for studying the connectivity between protons and heteronuclei like $^{13}C$, $^{15}N$, and $^{31}P$.

12. DEPT (Distortionless Enhancement by Polarization Transfer)

• Purpose: Provides information on the number of attached protons to carbon atoms. This technique is often combined with other 2D techniques to simplify spectra.
• Application: Widely used in structural elucidation, especially for distinguishing between CH, CH2, and CH3 groups in a molecule.

These 2D NMR techniques are indispensable tools for understanding molecular structures and interactions in complex chemical and biological systems. Each technique has its specific applications, and often, multiple techniques are used in conjunction to provide a complete picture of the molecular architecture.