lecture 9- nuclear magnetic resonance

applications to forensic science

• Drug Analysis: Identifies and quantifies drugs in biological samples.

• Post-Mortem Changes: Studies biochemical changes after death to determine cause and time of death.

• Body Fluid Identification: Identifies body fluids (blood, semen, etc.) for evidence.

• Explosive Detection: Identifies chemical components in explosives.

• Xenobiotic Examination: Detects foreign chemicals (drugs, toxins) in biological systems.

• Counterfeit Product Identification: Compares chemical compositions to detect counterfeit items.

NMR

• 1H NMR (Proton NMR): Identifies hydrogen atoms, revealing their environment and attachment to carbon atoms.

• 13C NMR (Carbon-13 NMR): Shows carbon atoms, providing insights into carbon-carbon connectivity and types of carbons (e.g., methyl, quaternary).

• NMR gives a detailed map of the carbon-hydrogen structure, revealing molecular identity, functional groups, and bonding patterns.

NMR key characteristics

Depends on nuclear magnetic moment: NMR measures the magnetic properties of nuclei.

• Active nuclei align in an applied field: Nuclei with magnetic moments align with an external magnetic field.

• Transitions detected: NMR detects transitions between energy levels when nuclei absorb energy.

• Small proportion of nuclei excited: Only a small fraction of nuclei are excited at any given time.

• Detects impurities: NMR can identify impurities in a sample.

• Requires at least 1 mg sample: A minimum sample size is needed for accurate analysis.

NMR components

• Magnet poles: Create a strong magnetic field for NMR analysis.

• Sweep coils: Generate varying magnetic fields for frequency scanning.

• Spinning sample tube: Rotates the sample for better resolution.

• Sweep generator: Controls the frequency range during analysis.

• Radio frequency transmitter: Sends radio waves to excite nuclei.

• Radio frequency receiver & amplifier: Detects the radio waves emitted by the sample.

• Control console and recorder: Manages the system and records data for analysis.

nuclear spin

Aligns with magnetic field: When a nucleus is placed in an external magnetic field, it behaves like a small magnet and aligns either parallel (in the same direction) or anti-parallel (in the opposite direction) to the field.

• Parallel spin: When the nuclear spin aligns with the magnetic field, it’s in a lower energy state.

• Anti-parallel spin: When the nuclear spin aligns opposite to the magnetic field, it’s in a higher energy state.

The transition between these energy levels (from parallel to anti-parallel) when a nucleus absorbs energy is what NMR detects. This is key for identifying molecules and their structures.

NMR active nuclei

• NMR active nuclei are those with a magnetic moment (spin) that can interact with the external magnetic field used in NMR.

• Nuclei with even mass and atomic numbers are usually not NMR active because they don't have a magnetic moment.

• Nuclei with odd mass numbers or even mass and odd atomic numbers are typically NMR active because they have nonzero spin and can be detected in an NMR experiment.

What does the chemical shift tell you in NMR

• Chemical shift (δ): Indicates the environment of the nucleus (protons or carbons).

• Units: Measured in ppm (parts per million).

Analysing NMR spectra

• Identify the peaks: Each peak corresponds to a specific proton or carbon environment.

• Determine the number of signals: The number of distinct signals tells you how many different environments of protons or carbons are present.

• Examine the chemical shifts: The position of each peak provides information about the type of environment (e.g., alkyl, aromatic, etc.).

• Analyze the splitting patterns (coupling): The number of splits in each peak (multiplet) gives clues about neighboring protons and their coupling.

• Integrate the peaks: The area under each peak shows the relative number of protons or carbons in each environment.

• Compare with known reference compounds: Compare your data with known spectra to help identify the compound.

Chemical shifts for carbon

Common ranges for 13C NMR shifts:

• 0–50 ppm: Alkyl carbons (C–C bonds in saturated chains).

• 50–100 ppm: Carbons attached to electronegative atoms (e.g., O, N).

• 100–150 ppm: Aromatic carbons (benzene rings and similar structures).

• 150–200 ppm: Carbonyl carbons (C=O in aldehydes, ketones, esters, etc.).

• >200 ppm: Aldehyde or carboxylic acid carbons (carbonyl groups)

Chemical shifts for protons

• For protons (1H NMR): Common shifts:

  • 0–3 ppm: Alkyl groups.

  • 3–5 ppm: Hydrogens attached to electronegative atoms (e.g., O, N).

  • 6–8 ppm: Aromatic protons.

  • 9–10 ppm: Aldehydes.

  • 10–12 ppm: Carboxylic acids.

Multiplets (splitting patterns)

• Multiplets: The splitting of a peak into multiple smaller peaks due to coupling between protons.

• Coupling constant (J): The distance between splits, usually measured in Hz, tells you the number of neighboring protons. N+1

• Common patterns:

  • Doublet (2 peaks): Coupled to 1 proton.

  • Triplet (3 peaks): Coupled to 2 protons.

  • Quartet (4 peaks): Coupled to 3 protons.

  • Multiplet: More than 4 peaks, often due to complex splitting.

Why Are There Differences in NMR for Different Nuclei?

• Gyromagnetic Ratio (γ): Different nuclei have unique gyromagnetic ratios, affecting how they interact with the magnetic field (e.g., 1H vs 13C).

• Magnetic Moment: Nuclei have different magnetic moments based on their spin, leading to different resonance frequencies.

• Electron Shielding: Nuclei experience varying levels of shielding from the electron cloud, affecting chemical shifts.

• Nuclear Spin: Nuclei with different spin numbers (e.g., 1/2 for 1H and 13C) interact differently with the magnetic field.

• Isotopic Effects: Isotopes like 12C vs 13C or 1H vs 2H have different NMR behaviors due to mass and nuclear properties.

• Resonance Frequency: Different nuclei resonate at different frequencies, causing distinct chemical shifts.

• Coupling Constants: Different nuclei have different coupling constants (J values) based on their interactions with nearby nuclei.

Integration

• Integration in NMR measures the area under each peak in the spectrum.

• The area corresponds to the relative number of protons (in 1H NMR) or carbons (in 13C NMR) in that specific environment.

• Higher integration value means more protons or carbons in that environment.

• Ratio of integrals between peaks gives the relative number of nuclei in different environments.

• Purpose: Helps determine the molecular composition and structure by showing how many protons or carbons are in each type of environment.