NMR Spectroscopy

NMR Spectroscopy

Introduction to NMR Spectroscopy

  • NMR (Nuclear Magnetic Resonance) Spectroscopy: A powerful analytical technique used to determine the structure of organic compounds based on the magnetic properties of atomic nuclei.

  • 2D NMR (Two-Dimensional NMR): A specialized form of NMR that provides additional information compared to 1D NMR, allowing for better resolution and analysis of complex spectra.

Learning Outcomes

  • Differences between 1D and 2D NMR:

    • 1D NMR: Provides a spectrum representing one dimension of information (e.g., chemical shifts of protons).

    • 2D NMR: Offers insights into the interactions between different nuclei, thereby displaying correlations between signals.

  • Useful 2D NMR Experiments:

    • COSY (Correlation Spectroscopy)

    • HSQC (Heteronuclear Single Quantum Coherence)

  • Structural Identification: How 2D NMR spectra assist in identifying structures of molecules.

Suggested Reading

  • Williams, D. & Fleming, I. (2008): Spectroscopic Methods in Organic Chemistry, Chapters: 3.17, 3.18, 3.19, 3.22.

Proton and Carbon NMR of Ethanol

  • Ethanol Structure:

    • Functional groups present:

    • Hydroxyl group (OH)

    • Methylene group (CH2)

    • Methyl group (CH3)

  • Assignment of NMR Spectra: Linking specific signals to their corresponding protons or carbons in the molecular structure.

Purpose of Additional Dimensions in NMR

  • Complex Spectra Challenge: Example of 1H NMR of codeine reveals multiple protons (22, 19, 3, & 4). The aim is to label each peak to its respective proton and carbon.

  • Assignment Process: Refers to the systematic approach of correlating NMR signals to specific molecular constituents.

Linking Signals to Functional Groups

  • Factors to Consider:

    • Chemical Shifts: The resonance frequency of a nucleus in the magnetic field, indicative of its chemical environment.

    • Shapes of Signals: The form of the peaks can indicate different types of hydrogen environments.

    • Integrals: Represents the number of protons contributing to each signal—important for quantifying occurrences.

    • Coupling Constants: Measure of the interaction between coupled protons, allowing identification of interconnected hydrogen signals.

  • Challenges: Busy spectra with overlapping signals complicate the assignment process, necessitating advanced techniques.

COSY - Correlation Spectroscopy

  • Overview:

    • Proton Spectrum Arrangement: Proton spectrum displayed on both axes of the diagram (x and y).

    • Diagonal ‘Blobs’: Represents peaks in the proton spectrum. Notably, cross peaks (non-diagonal blobs) illustrate correlations between protons that are coupled.

    • Elimination of J Value Calculation: This process simplifies the analysis by indicating coupling without needing to determine coupling constants.

COSY Example with Ethanol

  • **Proton Ranges in Ethanol: **

    • The peaks represent different proton environments (e.g., CH3, CH2, & OH).

Codeine COSY NMR

  • Complexity:

    • Multiple peaks (reference numberings and positions shown) indicating the presence and interaction of various protons within the codeine structure.

HSQC - Heteronuclear Single Quantum Coherence Spectroscopy

  • Conceptual Framework:

    • X-Axis: Proton spectrum.

    • Y-Axis: Carbon spectrum.

    • Cross Peaks: Displayed when a proton (chemical shift X) is directly associated with a carbon (chemical shift Y).

  • Insights: No cross peaks indicate no proton-carbon bond, such as with quaternary carbons or when protons are not bound to carbons.

HSQC - DEPT Features

  • Cross Peak Characteristics:

    • CH and CH3: Positive peaks.

    • CH2: Positive peaks, but shown with distinctive coloring for differentiation.

  • Application to Ethanol and Codeine:

    • Highlights how to interpret which protons correspond to specific carbons based on peak correlations.

Identification Implications of 2D NMR Techniques

  • Connecting Protons and Carbons: The ability to trace which protons correspond to certain carbons assists in structural identification.

  • Cross-peak Analysis: Absence of cross peaks may correlate to certain functional groups like OH or NH (for protons) and quaternary carbons (for carbons).

Variants of 2D NMR Techniques

  • Types of 2D NMR:

    • Homonuclear: Both axes represent protons (Example: COSY).

    • Heteronuclear: One axis denotes proton NMR, another denotes carbon NMR (Example: HSQC).

  • Numerous Combinations Possible:

    • Proton/proton (homonuclear)

    • Carbon/carbon (homonuclear)

    • Proton/carbon (heteronuclear)

    • Proton/nitrogen (heteronuclear)

    • Proton/fluorine (heteronuclear)

    • Any combination as long as nuclei are NMR active.

Additional Variants of 2D NMR

  • NOESY (Nuclear Overhauser Effect Spectroscopy):

    • Identifies spatial proximity between protons; cross peak intensity reflects distance.

  • Importance: Diverse types of 2D NMR provide comprehensive insights into molecular structures.

Insights into 3D NMR Spectroscopy

  • Structure:

    • One axis for proton, one for carbon, and the final for another nucleus, such as nitrogen, forming a three-dimensional representation of the spectrum.

  • Cross Peaks in 3D NMR: Located within the cube structure, showcasing their x, y, z coordinates, invaluable for detailed molecular analysis in complex molecules like proteins and DNA.

Suggested Video Resources

  • Links to instructional videos on NMR techniques:

    • https://www.youtube.com/watch?v=37g2t2XJhxu

    • https://www.youtube.com/watch?v=66lvCelMABY

References

  • Williams, D. & Fleming, I. (2008): Spectroscopic Methods in Organic Chemistry, McGraw-Hill.