2. 2D NMR
• So far the NMR spectral methods we
have discussed have been one
dimensional (since they have a single
chemical shift x coordinate axis).
• With the development of more advanced
spectroscopic methods as computational
power has increased, it has become
possible to obtain two dimensional
spectra.
3. • In two dimensional
experiments, both the x and the y axes
have chemical shift scales and the 2D
spectra are plotted as a grid like a map.
• Information is obtained from the
spectra by looking at the peaks in the
grid and matching them to the x and y
axes.
4. • In a normal pulsed experiment the 90°
excitation pulse is followed immediately
by an acquisition phase in which the
FID is recorded and the data are
stored in the computer.
• In experiments that use normal pulse
sequences such as DEPT a preparation
phase is included before the data
acquisition.
5. WHY 2D NMR??
• 1D protein spectra are far too complex
for interpretation as most of the
signals overlap heavily.
• By the introduction of additional
spectral dimensions these spectra are
simplified and some extra information is
obtained.
7. COSY NMR
• Of the many types of two-dimensional
experiments, two find the most
frequent application.
• One of these is H-H Correlation
Spectroscopy, better known by its
acronym, COSY.
• In a COSY experiment, the chemical
shift range of the proton spectrum is
plotted on both axes.
8. • In COSY the application of two 90°
pulses to a spin system will give a signal
which varies with time t1 where t1 is
the time between the two pulses.
9. HOW TO INTERPRET A COSY
SPECTRUM??
• The first thing to note about the spectrum
is the proton NMR spectrum of the
compound being studied is plotted along
both the horizontal and vertical axes and
each axis is calibrated according to the
chemical shift values.
• The COSY spectrum shows distinct spot on
a corner extending from the upper right
corner of the spectrum down to the lower
left corner.
10. • By extending vertical and horizontal
lines from each spot on the diagonal
you can easily see that each spot on
the diagonal corresponds with the
same peak on each coordinate axis.
• The diagonal peaks serve only as
reference points. The important
peaks in the spectrum are the off
diagonal peaks.
