Rules of the Day 1-31-07

1. The systematic way to match a molecule to an NMR spectrum is to A) Determine the number of equivalent sets of H atoms in the molecule (i.e. number of signals) from its structure, B) Determine relative integrations of those signals, [A) and B) alone often go a long way toward determining if a molecule matches the spectrum] C) Predict splitting patterns you expect, D) Predict chemical shift based on comparison to table. E) Look for items like "messed up" signals indicating a chiral center or broad singlet indicating an OH group etc., F) Go celebrate in your own responsible way because you are done. Click here to download a pdf of the spectra I showed in class today.

2. There are three different ways to take an NMR spectrum: Constant magnetic field and scan radio frequencies, constant frequencies and scan magnetic field, and flip all the spins instantaneously with a multi-frequency pulse, then monitor the rate at which the spins "relax" back to the +1/2 spin state. This latter approach uses a mathematical algorithm called Fourier Transform (FT) to reconstruct individual resonance frequencies for the different equivalent sets of protons so the spectra can be plotted.

3. All modern NMR instruments use the FT method because it allows many spectra to be taken quickly of a sample, then averaged to give a high signal-to-noise ratio.

4. NMR is incredibly powerful in chemistry in biochemistry today because scientists can use it to determine what atoms are close to each other in space, even if they are not bonded to each other. Thus you can not only determine covalent structure of a molecule, but also how it folds or associates in solution. Click here for a pdf of a recent paper describing our use of NMR to determine the structure of a new molecule bound to DNA.

5. The popular medical diagnostic technique of magnetic resonance imaging (MRI) is based on the same principles as NMR, namely the flipping (i.e. resonance) of nuclear spins of protons by radio frequency irradiation when a patient is placed in a strong magnetic field. Click here for a handout that describes MRI in greater detail.

6. Magnetic field gradients are used to gain imaging information, and rotation of the gradient around the center of the object gives imaging in an entire plane (i.e. slice inside patient).

7. In an MRI image, you are looking at individual slices that when stacked make up the three-dimensional image of relative amounts of protons, especially the protons from water and fat, in the different tissues.

Homework: Finish working on the second homework problem set, due Fri., 2-2-07, BEFORE CLASS. Click here to download the pdf.

We assume you are done making a Roadmap for 310M reactions. Read: Sections 15.1 and 15.2 , Problems: 15.1-15.4, 15.7 in the book. You will not turn these in but they are very helpful to PREPARE for the next lecture.