H Spectroscopy Flashcards
What is 1H NMR spectroscopy?
Proton nuclear magnetic resonance is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules
What is the basic principle of NMR spectroscopy?
Every proton has nuclear spin and precess at Larmor frequency
NMR spectroscopy is based on the principle that every proton has a different chemical environment affecting the Larmor frequency of the proton (chemical shift) - this can be used to measure the free induction decay from the different protons
Fourier transform can be used to convert the different frequencies of FID to create an NMR spectrum
Explain shielded and deshielded environments
Protons with high electron density are considered shielded
Protons with low electron density are considered deshielded
The electrons surrounding the proton creates its own magnetic field which opposes the main magnetic field
Shielded protons experience a lower magnetic field and therefore a lower Larmor frequency whereas deshielded protons have a higher frequency
Where do shielded and deshielded protons appear on the NMR spectra?
Dieshielded on the left
Shielded on the right
What is an NMR spectrum?
A plot of the radio frequency applied against absorption
A signal in the spectrum is known as a resonance
The frequency of the signal is known as the chemical shift
If you are imaging a molecule with 5 different protons with different electronic environments, each proton will precess at a different frequency so following Fourier transform, there will be 5 peaks on the NMR spectra
What does each peak represent in the NMR spectrum?
Each peak represents a different proton or group of protons with the same chemical environment
Why do doublet peaks occurs in NMR spectra?
Individual magnetic spin states interact (spin-spin coupling) within a molecule
An isolated proton or a group of identical protons with the same chemical environment will give a single peak
Sometimes, a molecule will have protons close enough that their magnetic spin states interact with each other and affect each other’s precession frequencies - this causes a doublet to appear on the spectra
What is chemical shift in 1H NMR spectroscopy?
In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field
The electron cloud shields 1H nuclear spins from applied B0 field
Resonant frequency of 1H changes with molecule
This is expressed as a frequency ‘chemical’ shift
ppm with respect to fixed reference
What is 1H spectroscopic imaging used for?
This data is used to generate detailed information about the molecular structures, characteristics, components and behaviours of a medical sample
Used to look at different metabolites in the brain - can help to identify tumours
What does the resonant frequency of the nucleus depend on?
The gyromagnetic ratio
What is the gyromagnetic ratio?
The gyromagnetic ratio, often denoted by the symbol γ (gamma) is the ratio of the magnetic momentum in a particle to its angular momentum (around the axis)
At 3T what is the frequency of 1H?
128 MHz
Can you image different nuclei with NMR?
Other nuclei resonate at different frequencies so need separate RF coils
They have natural abundance and concentration so dealing with weaker signals
How do you boost the signal of low concentration atoms?
Hyperpolarisation
What is 2H used to image?
Deuterated infusions - Deuterium metabolic imaging (DMI)
Used to map metabolism - imaging of substrates and their metabolic products enriched with 2H isotope
Common 2H enriched substrates include glucose and acetate to study pathways involved in energy metabolism such as glycolysis or the TCA cycle
