Name _________________________
Exam 3, 2008:
CHEM/BCMB 4190/6190/8189
3
b.
Explain your reasoning, in detail, for your answers to part ‘a’.
(
9 points
)
For the codeine molecule, of the assigned
13
C chemical shifts shown in the table, eight of
these are for methine (-CH) groups, three for methylene (-CH
2
) groups, and two for
methyl (-CH
3
) groups.
For the “DEPT(90)” experiment, as shown in the plot above,
when the pulse angle (
±
y) is 90 degrees, no signals are observed from -CH
2
and -CH
3
groups, and positive signals are observed from -CH groups.
Thus, eight signals are
observed in this experiment, corresponding to the eight -CH groups, at the chemical
shifts listed in the table.
For the “DEPT(45)-DEPT(135)” experiment, the signal
intensities from -CH and -CH
3
groups when
y is 45 degrees are the same as when
y is
135 degrees.
Subtracting the signals observed when
y is 135 degrees from the signals
observed when
y is 45 degrees leaves no signals for these groups.
However, for -CH
2
groups, when
y is 45 degrees the intensities are maximal with positive phase, and they
are also maximal, but with negative phase, when
y is 135 degrees.
Thus, subtracting
these gives very large signals for -CH
2
groups.
Therefore, for codeine, three large
signals will be observed in the “DEPT(45)-DEPT(135)” experiment from the three -CH
2
groups.