In the next lecture we will do an in-depth treatment of NMR
imaging, building on the foundation given in the last lecture. Please review
pages 95-97, and then read pages 98-110 before the next lecture.
Now some summary of the previous lecture:
1) How does one start out in drawing the lines connecting
peaks in a COSY map?
There is no one correct answer! Start with any peak you like
& you will still get the same result. The easiest way is to start with a
resolved peak or at one end of the molecule (if you know it) but that isn't
essential. I would recommend just getting a ruler and trying the 2-bromobutane
and sucrose COSY spectra in the class notes. Connections of cross peaks for
both of these COSY spectra are shown in the week 7 animations posted on the
website. Remember to draw lines parallel to the axes when connecting the cross
peaks and diagonal peaks. See me if you have difficulties. Remember that COSY
spectra are symmetric, i.e. they have identical information on either side of
the diagonal and so you can use either or both sides of the COSY map for
drawing your lines.
2) The advantages of 2D NMR relative to 1D NMR are greater
resolution of spectra and the greater amount of information obtained. Also, 2D
spectra use only non-selective pulses. For example, in a crowded, poorly
resolved spectrum it is much easier to do a COSY with non-selective pulses than
attempt to do a selective decoupling experiment to reveal scalar interactions.
3) The major disadvantage of 2D NMR relative to 1D NMR is
the vastly greater amount of spectrometer time required by the former. A
typical real 2D NMR experiment might have 200 (or more) t1 increments, i.e. 200
separate FIDs would have to be collected. This would
take 200 times as long as a comparable 1D experiment! Thus 2D experiments for
dilute samples take many hours.
4) Now a closing thought from a past student:
"What you get out of an NMR experiment depends upon the
pulse sequence!"
This student realized that what specific kind of NMR data we
obtain from the host of possibilities - chemical shifts, decouplings,
relaxation time measurements, NOEs, various kinds of
2D NMR, or NMR image data - depends on the specific type of pulse sequence used
to manipulate the nuclear spin system.