Bruker BioSpin Solid State NMR, User Manual

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204 (327) BRUKER BIOSPIN User Manual Version 002
Relaxation Measurements
Starting from the glycine spectrum, create a new data set, set parameters accord-
ing to
Table 16.1.
, and acquire a 1D spectrum. The relaxation delay after inver-
sion is controlled by a variable delay list – this can be created using
edlist
, and
the name of the list set as the parameter
vdlist.
This pulse program uses the method of Torchia, in which the phase of the contact
pulse, and the receiver, is inverted in alternate scans. In the first scan, the first 90°
pulse creates –z magnetization, and in the second scan it cz. The phase
cycling of the receiver means that the difference between the two scans is record-
ed. For short relaxation delays, neither relaxes significantly, and so the maximum
signal is recorded. At longer relaxation delays, both the +z magnetization (which
is larger than the equilibrium value as it is created by CP), and the –z magnetiza-
tion relax, and the recorded signal decays exponentially as a function of the relax-
ation delay. At long times both have relaxed back to equilibrium, and the two
scans yield a zero signal.
The resulting spectrum should be phased to give positive peaks – given the very
short recovery delay, no appreciable relaxation will have occurred. Now we can
set parameters for the 2D acquisition, as in
Figure 16.1.
. Since this is a pseudo-
2D experiment, the only relevant parameter in F1 is the number of points, which
should be the number of entries in the vd list. The most important setting is the
range of relaxation delays set in the vd list. Ideally, the list should run from times
short enough for no appreciable relaxation to occur, up to a few times the longest
T
1
value. Of course, the accurate relaxation time constants are not known in ad-
vance, but order of magnitude estimates can be obtained by running the 2D ex-
periment with a small number of relaxation delays, and a small number of scans
per slice. The relaxation delays should be approximately equally spaced in log(de-
lay), in order that decays with all time constants in the range are equally well char-
acterized. Data can always be improved either by increasing the number of
relaxation delays sampled, or by averaging more FIDs at each relaxation delay.
For the glycine sample, a suitable list of times would be:
100ms, 220ms, 450ms, 1s, 2.2s, 4.5s, 10s, 22s, 45s.
Table 16.1. Parameters for the 1D CP Inversion Recovery Experiment
Parameter Value Comments
Pulprog cpxt1
Vdlist See text Relaxation delays after inversion pulse. Short
value – to set spectrum phase correctly.
d1 3s Needs only to be 3x proton T1
pl1 X HH contact power standard cp setting
pl11 power for 90 degree pulses usually pl11<pl1 for short pulses
p1 Measured 90° X pulse length
at pl11 adequate for required excitation bandwidth
Ns 2 Should be enough to see a reasonable spectrum.