C-NMR Flashcards
What is the natural abundance and magnetogyric ratio of Carbon-13 compared to H-1?
Carbon-13 has a very low natural abundance (~1.1%) and its magnetogyric ration is about 1/4 of the strength of Hydrogens.
Essentially, there is less C-13 in a sample so the instruments used need to be more sensitive to detect it.
How do Carbons couple with other atoms?
Homonuclear coupling (C-C) is possible but rarely observed.
Heteronuclear coupling (C-H) is far more common.
How can you tell the multiplicity of a proton-coupled Carbon?
Similar to proton-NMR, you can use the (N+1) rule for proton-coupled carbons. It is simply the number of Hydrogens attached (N) +1 to get the multiplicity of a carbon peak.
So, a methyl (CH3) carbon would be a quartet (4 peaks) because (n+1 = 3+1 = 4)
What problems arise with proton-coupled NMR spectra?
When carbons are coupled with protons, some peaks may overlap which can make it difficult to discern which peaks belong to which atom
What is Broad-Band Decoupling?
This is when all protons in a molecule are simultaneously irradiated with a broad spectrum of frequencies which cause all coupled protons to decouple.
What do broad-band decoupled peaks look like?
All broad-band decoupled Carbon peaks appear as a singlet as there is no coupling occurring (N+1 = 0+1 = 1)
What is the Nuclear Overhauser Effect (NOE)?
Essentially, the NOE Effect occurs between two different nuclei close in space. If one is irradiated (so it no longer appears in NMR spectra), the intensity of the other nuclei peaks change.
The change in intensity can be positive or negative, it depends on the atom type
Carbon-13 intensity usually increases if it is a small molecule
How is the NOE effect affected in C-NMR by nearby protons (H)?
The NOE effect increases when there are more protons nearby. Generally, the enhancement of a signal occurs when there are more protons attached.
How does distance affect the NOE Effect?
The NOE effect decreases with distance. This is NOT bond distance, it refers to physical distance in space. The max is roughly 4 Angstroms.
How do peak intensities in C-NMR differ from H-NMR?
In C-NMR peaks do NOT integrate to give the correct number of Carbons
What affects peak intensity in C-NMR?
NOE effect and relaxation affects Carbon peak intensity
What is relaxation in C-NMR?
Relaxation is essentially the time it takes for excited nuclei to return to their ground state.
NMR is done in pulses, so if a pulse is sent out before a nuclei relaxes to its ground state, it may not be entirely accurate. in this case, increase the time between peaks to get more accurate spectra results.
How can the NOE effect help determine stereochemistry?
The NOE effect is particularily useful as it tells how close in space certain protons are to the irradiated nuclei. This can help determine if molecules are cis or trans based on proton proximity in space.
What is magnetic Dipole-Dipole interaction?
Essentially, the magnetic dipoles of nearby protons interact with each other and affect peak intensity. If the dipoles are aligned, the peak intensity will be stronger, if not then it will be weak.
These dipole interactions are what cause the different relaxation times in different nuclei
Entirely dependant on proximity in SPACE, not through bond distance.
What is chemical and magnetic equivalence in C-NMR
In decoupled C-NMR (which is most), magnetic equivalence is irrelevant.
Chemical equivalence is based on surrounding environment. If two carbons have the same environment, they will have the same chemical shifts
