Looking into some NMR usecases for my project, I realize my knowlegde from the Organic Chemistry course some four years ago has quite much faded away. Luckily I found this excellent NMR intro lecture on YouTube:
Below are some quick notes from the lecture, for my reference:
You know the atoms, but need to discern the structure
- One usually knows the number of atoms, since we have very good techniques for determining the chemical formula for compounds, so the NMR is just for determining the structure.
One peak for each equivalent set of hydrogens/carbons
- Use symmetry rules to slice out the largest possible "sections" of a molecule that does not have any intermal symmetry, and count the number of carbon/hydrogens in it!
- So... when reasoning from the spectrum towards a molecular structure, one can start with determining the amount of symmetry of the molecule! (which makes this indeed an interesting search problem(!))
Basically four types of information given
- Number of peaks
- Chemical shift
- Height of peaks
- Splitting ((n+1) rule + Pascal's triangle)
General facts/strategies to note
- Electrons provide shielding for magnetic fields
- Upfield = Shielded
- Downfield = De-shielded
- Electronegative atoms de-shield nearby protons (at least)
- Normal alkanes normally absorb between 0 and 5
- Rule of thumb: If the proton is on a carbon with no electronegative atoms connected, the shift will be less than 2.5, etc.
- The longer an alkane (less H per CHx unit), the further to the left (approaching shift 1.25 or so)
- Alkenes (with one double bond) are typically found between 4.5 and 10 on the chemical shift scale.
- Try to identify the fragments corresponding to the peaks, and then put them together in a reasonable way (AND always start with the degree of unsaturation, which you can get from the chemical formula).