Exam 4, 2008:
Tuesday, 16 December, 2008
Shown (below, right) are results from 1D
selective TOCSY NMR experiments (‘B’ and
‘C’) performed on the
cellobioside molecule, a disaccharide, shown
below. In ‘A’ is shown the simple
spectrum of this molecule.
The pulse sequence for the selective TOCSY
experiment is shown (above, right).
pulse is applied at the frequency
of a chosen signal, followed by the spin-lock
sequence and then detection.
In ‘B’ are
shown the results of applying the selective
pulse at the frequency of the signal for
, and in ‘C’ are shown the results
of applying the selective pulse at the
frequency of the signal for hydrogen 1.
both ‘B’ and ‘C’, the TOCSY mixing time
(spin-lock) was 200 ms.
Explain the results in
‘B’, and explain the results in ‘C’.
The signal for 1 is at the same frequency as water (HDO).
This is unimportant, so you can
Also, ignore the very large spikes (singlets) in ‘A’ at 3.4 and 3.65 ppm, which result
The selective TOCSY experiment gives correlations between the nucleus whose frequency
is selectively excited by the initial, selective 90
pulse and other nuclei in the spin system.
If the mixing time is long enough, very long range correlations can be observed.
the frequency of 1
was selectively excited, and correlations to all other hydrogens in the
sugar molecule on the left (2
, etc.) are observed.
No correlations to nuclei in the
other sugar (1, 2, 3, etc.) are observed, indicating that any couplings between 1
these hydrogens are extremely small.
Likewise, in ‘C’, correlations between 1 (whose
frequency was initially excited) and the other hydrogens in the sugar on the right (2, 3, 4,
etc.) are observed, but no correlations are observed to hydrogens in the sugar on the left.