Rules of the Day 1-25-08

1. Adjacent nuclei have magnetic fields associated with their spins. The spins of equivalent adjacent nuclei can be either +1/2 or -1/2, and at room temperature they are found in about a 50:50 mixture at any given nucleus (very slight excess of lower energy +1/2). These can add to give n+1 different spin combinations in the proportions predicted by Pascal's triangle. Each different spin combination produces a different magetic field, which leads to n+1 splittings in the peaks of the NMR spectra of the adjacent (no more than three bonds away) nuclei.

2. The distance between peaks split in this way is called the coupling constant ("J").

3. THEORY: When there are two sets of adjacent H atoms, the number of peaks multiply. For example, a CH2 group with a CH2 group and a CH3 group on either side should show 3 x 4 = 12 splittings! You can say this group is a "triplet of quartets" (or a "quartet of triplets").

4. 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.

5. 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.

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

Finish making a Roadmap for 310M reactions. Reread: Section 13.9-13.13, Problems: 13.19-13.25 in book. THEN read the third "NMR handout" called How Medical MRI Works