NMR Spectroscopy Study Notes

NMR Spectroscopy Study Notes

• NMR spectroscopy measures the difference in energy levels of nuclei within an analyte.

• Quantum numbers describe electron location and energy relative to the nucleus. The final quantum number describes the electron's ability to interact with an applied magnetic field.

• A nucleus's overall spin depends on the number of protons and neutrons.

  • Even numbers of protons and neutrons result in zero overall spin (e.g., 12C).
  • An odd number of protons plus neutrons results in a half-integer spin (e.g., 13C, 1H).
  • Odd numbers of protons and neutrons result in an integer spin (e.g., 2H).

• The total number of possible spin orientations is (2I + 1).

• Nuclei with two or more spin states are NMR active.

• An applied magnetic field separates spin states with different energies.

• The Larmor frequency is the frequency needed for a change in spin state. Calculation involves the magnetogyric ratio, Planck's constant, and the applied magnetic field strength.

• The relative populations of spin states in an applied magnetic field follow the Boltzmann distribution. The populations are nearly equal at a constant temperature.

• Continuous Wave NMR scans the applied magnetic field to identify the frequencies of absorption for a particular nucleus.

• Fourier Transform NMR applies a brief pulse of RF-electromagnetic radiation at a specific frequency to excite all nuclei at once.

Classical Description of NMR

• Nuclei with spin states align with an external magnetic field. Nucleus precession, at an angular velocity of γΒ₀, occurs around the z-axis.

• Radio frequency radiation can cause these aligned nuclei to transition into different states and align along the x-y plane. This is absorption, and when the angular velocity aligns with the RF frequency of the applied magnetic field. Absorption takes place.

• The nuclear's angular velocity of precession is a function of the nucleus's magnetogyric ratio (γ), the value of Bo, and the pulse duration.

• Relaxation of the nuclei returns the spins to their original alignment, with the relaxation time (T1) influencing signal speed

Relaxation Mechanisms

• Spin-lattice relaxation: A nucleus returns to its lower energy spin state by transferring energy to other species in the sample (lattice).
• Spin-spin relaxation: Two nuclei in different spin states can exchange energy if located in close proximity

FID

• Free induction decay (FID) measures the signal after the RF pulse, and decay is measured as the nuclei return to their original equilibrium state.
• The process is oscillatory and exponentially damped.
• A sample consisting of only one nucleus results in a simple FID that is transformed to a simple NMR spectrum with one peak.
• A sample containing multiple nuclei results in a more complex FID and spectrum.

Pulse Sequences

• Pulse sequences involve: Pulse width, acquisition time, and recycle delay.

Environmental Effects

• Chemical shifts reflect the slightly different local magnetic fields that nuclei experience in a molecule/compound causing different absorption frequencies

• Shielding occurs due to electron circulation around the nucleus. Shielding makes the observed magnetic field smaller.

• Chemical shifts are useful in determining structural information about molecules.

• Values of the coupling constant become smaller as the distance between nuclei increases.

NMR Spectrometers

• Components include, magnet (Bo), magnetic coils (B1), a sample changer, and a nucleus-dependent probe.

• The sample probe is oriented to allow a uniform field for the nuclei in the sample.

Quantitative Analysis

• Qualitative analysis uses the unique signatures of molecules in their spectra to identify the composition of samples.
• Quantitative NMR (qNMR) is useful to determine the relative concentrations of analyte (the substance of interest) and any internal standards.
• An internal standard is a pure form of a substance that is chemically unrelated to the analyte but has a simple NMR spectrum with peaks that do not overlap with the analyte or any other components in the sample.