Lecture 11&12: NMR Flashcards
- Nuclear
atomic nuclei (positively charged central core of atom with protons and neutrons – contains most of its mass)
- Magnetic
magnetism – force generated in matter by the motion of charges (electron or proton) within its atoms
- Resonance
amplification phenomenon generated when the applied oscillating force is equal (or close) to the natural frequency of the system (think wave-particle duality)
applications of NMR
- Power analysis method for chemical samples, biological compounds, and medicines
- Structural info for molecules containing hydrogen, carbon, phosphorus…
- Used for dissolved materials (solid state (ssNMR) is more complex and expensive)
- Info regarding motion in molecules, structural flexibility and how they interact in chemical reactions
- Determine how elements arrange in space (from linear molecules to tertiary proteins)
- Evaluate proportions of solid and liquid components in fatty foodstuffs like margarines and low-fat spreads
- Pharmaceutical studies and medicine;
o Dynamic studies, diagnoses of tissue abnormalities, pH control in diabetes, body scanning (MRI)
nuclear spin
- NMR occurs as nuclei of certain atoms possess spin
- Spin characterised by nuclear spin quantum number, I, which may take integer and half integer values
- Zero spin are not amenable to NMR observation
- Schematic of 1H and 13C atoms both have spin as nuclei have odd number of protons or neutrons so can be detected by NMR.
nuclear magnetic resonance
- Nuclei possess magnetic moment which is acted upon by external field
- Nuclei can take up 2I+1 possible orientations in this field
o If I=1/2 the nuclei may orientate in 2 quantized ways
Parallel (a) to the field or antiparallel (B) to it. a is slightly lower energy. - The lower energy a energy state may be excited to the higher B level by EM radiation oscillating at appropriate Larmor frequency and therefore induce NMR.
- Beta state may lose excess energy by relaxation and fall to alpha state.
fourier transform
- Mathematical manipulation of the interference pattern of various superimposed sine and cosine frequencies displayed in the time domain by the Fourier transform extracts the components and gives a frequency domain spectrum of the relative intensity of each signal plotted against its frequency.
o Done by NMR software
why are Deuterated solvents used for NMR
o Field-frequency lock system corrects any drift in magnetic field using deuterium resonance
o As more solvent than sample is present in NMR tube, a pronated 1H solvent would overshadow peaks of interest
Features of the 1H spectrum:
- Proton resonance distributed along frequency axis
o Each proton sits in distinct chemical environment characterised by its chemical shift with units of ppm. - Different peaks with different intensitites relating to number of protons giving rise to the signal – integration
units of chemical shift
ppm
chemical shift in external magnetic field
nuclei precesses proportional to:
o Magnetogyric ratio of nucleus
o Strength of magnetic field
shielding and deshielding
- Induced magnetic field opposes applied magnetic field (B0)
- Effect of B0 on nucleus us reduced (shielding)
o Higher electron density, greater the shielding. - Reducing electron density, nucleus feels more of B0 (deshielded)
factors affecting chemical shift
intramolecular effects
inductive effects
anisotropy
mesomeric effects
hydrogen bonding
solvents
intramolecular effects
result in changes in chemical shift
increased branching leads to increased shift
inductive effects
electronegativity
shielding affected by partial changes
decreases electron density, reduces shielding and increases shift
anisotropy
uneven distribution of e- in polar covalent bonds create partial changes
aromatic rings - less shielded protons so lower shift
- Intensity (peak area) of resonance is proportional to…
number of nuclei giving rise to it
