Rules of the Day 1-30-08

1. We have told you the maximum number of peaks/signals you might see in a 1H NMR spectrum, but due to coincidental overlap, there may appear to be fewer and you might have difficulty understanding the peak splitting patterns.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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. 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). 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. [Memorize the preceding passage, as it will be worth 12 points on each exam this semester. I am not kidding] Click here for a handout on MRI.

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: 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.