Bruker BioSpin Solid State NMR, User Manual

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102 (327) BRUKER BIOSPIN User Manual Version 002
Practical CP/MAS Spectroscopy on Spin 1/2 Nuclei
2. Once these values are measured, any HH condition can be calculated. As-
sumed you want to cross polarize
119
Sn, the sample spins at 12 kHz. The con-
tact time is anticipated to be rather long, because
119
Sn atoms are large and
far away from protons. So the power level for the contact should not be too
high. Let us set the RF-field to 50 kHz for the contact. We decide to apply a
ramp shape on the proton contact pulse, covering the ±1 spinning sidebands.
This means that we need to apply a ramp from 38 to 62 kHz RF field, plus
some safety margin, about 35 to 65 kHz RF field on the proton ramp. For
119
Sn
we need to apply 50 kHz RF field. Since the RF field is proportional to the am-
plitude in a shape (RF-voltage output is proportional to shape amplitude val-
ue), the shape power must range from 65 kHz to 35 kHz, from 100 to about
50% amplitude. Use
calcpowlev
to calculate the changes in dB to achieve the
calculated RF fields (enter reference RF-field to calculate required RF field in-
stead of pulse lengths). In our case, the proton contact pulse power sp0 is cal-
culated at + 3.74 dB (65 kHz compared to 100 kHz), the power level for
119
Sn
is calculated at +1.94 dB (50 kHz compared to 62.5 kHz). Be sure to add the
calculated number for a desired RF-field lower than the reference field, sub-
tract the number if the desired RF-field is higher.
3. If such a table is not available, but an oscilloscope is, one can measure the
RF- voltage for the X contact pulse of the known (
13
C) HH condition, calculate
the pp-voltage for the unknown HH condition from the NMR-frequencies of the
two nuclei, and set this voltage for the unknown HH condition.
Hints, Tricks, Caveats for Multi-nuclear (CP-)MAS Spectroscopy 6.4
1. Since T
1
relaxation tends to be slow in solids, direct observation of hetero-nu-
clei is usually time consuming, so CP is widely used because the proton T
1
is
usually bearable. However, CP can only be used if the hetero-nucleus is cou-
pled to protons (or whatever nucleus the magnetization is drained from).
Whereas
13
C and
15
N usually bear directly bonded protons, this is not the case
for many other spin ½ hetero-nuclei. So the magnetization must come from
more remote substituents. More remote they may also be because atomic radii
increase as one goes to nuclei with higher atomic mass. In short: HH condi-
tions may be very sharp, T
I-S
may be long, but proton T
1 ρ
may still be short.
2. Chemical shift ranges and chemical shift anisotropies increase with nuclei of
higher order number and number of electrons in the outer shell. Therefore one
may be confronted with two problems:
- to find the signal somewhere within the possible chemical shift range
- to find the signal within a “forest” of spinning sidebands
3. Ease of setup therefore depends largely on the availability of a setup sample
with decent T
1
, efficient CP, and known chemical shift for referencing. Chapter
2 lists some useful setup samples together with known parameters.
Setup for Standard Heteronuclear Samples 15N, 29SI, 31P 6.5
1.
15
N on α -glycine: calculate HH condition as described above. Else:
Load α -glycine
13
C reference spectrum, set observe nucleus N15 in
edasp
- add 2 dB to
sp0
(
spnam0
=
ramp.100
)