Practical matters L6 Flashcards
What does how much sample depends on
Natural abundance and magnetogyric ratio
Define magnetogyric ratio
describes the ratio of a particle’s magnetic moment to its angular momentum. the magnetogyric ratio describes how strongly a nucleus interacts with an external magnetic field and how it responds to the application of radiofrequency (RF) radiation.
What percentage of carbon in nmr active
For C only 1% is carbon 13 the remainder is carbon 12. Stronger samples are required for 13C than 1H
What does the larger magnetogyric ratio mean
The larger the magnetogyric ration the more sensitive the nucleus
What does the number of scans depend on in nmr
Signal-to-Noise Ratio (SNR): More scans improve SNR; signal increases linearly
Nuclei with lower natural abundance require more scans.
Less sensitive need more scans.
What is signal to noise ratio in NMR
SNR is the ratio of signal strength to background noise.
Higher SNR = clearer data.
Improves with numberofscans
What does the choice of solvent depend on in NMR?
No Proton Interference: Use deuterated solvents
Solubility of Sample: The solvent must dissolve the analyte.
Chemical Shift: Minimal overlap with sample peaks.
Purity: High purity to avoid contamination.
Why is deuterium invisible in a
H NMR spectrum?
Deuterium has a different nuclear spin (I = 1) than hydrogen causing it to not resonate at the same frequency.
Describe the solvent peak in 13C nmr
In 13C spectrum solvent is not invisible and the chance of finding 13C in solvent is the same a finding it in the sample. Much more solvent than sample solvent the solvent signal dominates spectrum
How do you record an NMR spectrum
1) sample placed in a magnet
2) sample spun
3) spectrometer uses 2H signal from deuterated solvent to lock magnetic field
4) sample shimmed
5) data accumulated to fid which then undergoes a Fourier transformation to produce spectrum
Why is the sample spun
Spinning creates a more homogeneous sample/field
What is the sample usually spun at
Sample usually spun at 10-15 HZ
Why does spectrometer uses 2H signal to lock magnetic field
Prevents field drift
What does sample shimmed mean/do
Very fine adjustment if magnetic field at sample. Poor shimming affects resolution of spectrum
