Dear BCM 230,

1) In the next lecture we will begin our coverage of TWO-DIMENSIONAL NMR, which will last two weeks and then lead into our discussion of NMR imaging (i.e. MRI). Please read pp. 68-80 of the Class Notes in preparation for the next lecture. In the next lab the 500/600 lab groups will do C-13 and P-31 spectroscopy and acquire data with heteronuclear decoupling and heteronuclear NOE. The 7T lab group will continue imaging.

2) A reminder my office hours are 3-4 PM Th and by appointment. Please do not hesitate to contact me if you wish to meet at another time. Other reminders: all the emails I send you are archived on the course web site. All the animations presented in class are also available on the web site.

Now some key points from the last lecture:

1) Comparison of scalar (J) coupling and the NOE. Match the letters from each section below to see the contrast between the two effects.

Scalar coupling:

a. Takes place through chemical bonds (scalar).

b. Is inherently present due to molecular structure.

c. Effects precessional frequencies.

d. Is removed by decoupling DURING the acquisition of a FID.

The NOE:

a. Takes place through space (dipolar).

b. Is only produced by the spectroscopist.

c. Effects spin state populations.

d. Is produced by saturation of a peak (or peaks) BEFORE the acquisition of a FID.

2) Below I am going to re-emphasize the practical uses of spin coupling and the NOE, as well as contrast HOMOnuclear (meaning proton-proton) with HETEROnuclear (usually meaning proton-some other kind of nucleus) coupling and NOE.

First, both homonuclear (proton-proton) spin coupling/decoupling and the homonuclear (proton-proton) NOE are important techniques to IDENTIFY or assign the peaks in proton NMR spectra.

Homonuclear spin coupling/decoupling enables the spectroscopist to identify nuclear signals connected by a small number (usually 3) of chemical bonds.

The homonuclear NOE effect allows one to identify protons close together in space (i.e. a few angstroms apart).

Both the proton-proton NOE and spin coupling/decoupling provide the spectroscopist ways to systematically identify or assign the peaks in an NMR spectrum and say which peak arises from which specific proton in a molecule.

The practical utility of heteronuclear (proton to some other nucleus) spin decoupling and the heteronuclear NOE comes from their use in acquiring spectra of low gamma nuclei with directly attached protons. Examples are spectra of C-13 and P-31. All are normally acquired with broadband decoupling of all protons. C-13 spectra are generally also acquired with irradiation of protons to give a proton to C-13 NOE effect. The use of heteronuclear decoupling and heteronuclear NOE gives SIMPLIFIED spectra with INCREASED signal-to-noise, for a summary see p.64. The simplification of the spectra is another example of SPECTRAL EDITING as was discussed in lecture #4.

3) Query: How does one determine the magnitude of the B2 field necessary for either decoupling (during acquisition of the FID to collapse multiplets) or NOE (prior to the B1 pulse to alter populations)?

The magnitude of the B2 field to accomplish either of these goals is determined empirically. In lab last Friday on the 500/600 spectrometer we varied the B2 field in the homonuclear decoupling experiment during data acquisition of the FID to collapse the multiplets of adjacent spin-coupled nuclei. Some groups also varied the B2 power to effectively saturate a proton in order to observe NOEs at nearby protons. In either experiment the user simply varies the B2 power to achieve the desired result.

4) In last Fridays lecture and lab we saw several new (to the class) pulse sequences. For the first three weeks of class we considered only the basic 1-pulse experiment. Then in week 4 we learned the T1 inversion-recovery experiment and the spin-echo experiment. Then in week 5 we saw pulse sequences for homo- and heteronuclear decoupling, homo- and heteronuclear NOE, and solvent saturation.

The point is that through the DESIGN of the pulse sequence we can produce specific types of effects and extract SPECIFIC KINDS of information from the nuclear spin system!