Rules of the Day 1-28-08

1. PRACTICE: For alkyl groups complex splittings simplify because coupling constants ("J") are all about the same. In practice, if there are n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is an approximation, but almost always true on spectra taken with all but the most sophisticated NMR spectrometers.

2. For alkenes or ring structures such as cyclopropanes the splitting does not simplify (no bond rotation) and you see full multiplicative splitting ("doublet of doublets", etc.) Click here to go to Pictures of the Day for today in which the NMR spectra for an alkene and a cyclic structure are explained.

3. Bottom Line, chemical shift tells you what functional groups are present, integration tells you how many equivalent H atoms there are and splitting patterns tell you how the atoms are connected to each other.

4. Deuterium atoms do not show up in 1H-NMR spectra, so deuerated solvents are used to dissolve NMR samples.

5. The H atoms of relatively acidic functional groups (alcohols, carboxylic acids, amines) exchange rapidly, so they often do not split adjacent protons, and they can be replaced (signal disappears) with deuterium by adding a drop of D2O to the NMR sample.

6. H-bonding changes the location of a signal for H-bonding groups in a concentration dependent manner explaining why -OH and -NH2 group signals can vary so much in location.

7. Non-equivalent H atoms on the same C atom can split each other (called geminal coupling), for example on alkenes or small rings. This coupling usually has very small coupling constants, so is difficult to see on some spectra. See Friday's POTD for today for examples.

8. The splitting of a -CH2- group adjacent to a chiral center will be "messed up", that is split into many peaks. This is useful for identifying chiral centers in molecules.

9. 13C NMR tells you how many different types of carbon atoms are in a molecule. Because 13C atoms are so rare, you don't find two in the same molecule and there is no splitting (12C nuclei do NOT have spin quantum numbers of 1/2 so they do not split a 13C signal). Integration is NOT accurate with 13C NMR. We will not use 13C NMR in this class, but you will see it in lab.

Homework: Start working on the second homework problem set, due Fri. , 2-1-08, BEFORE CLASS. Click here to download the pdf.

We assume you are now finished making a Roadmap for 310M reactions. Read: Nothing new , Problems: Nothing new