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20
Double-CP 20
Double Cross Polarization (DCP) experiments use two consecutive cross polar-
ization steps. Usually, the first step transfers from protons to one type of X-nucle-
us (to achieve high sensitivity), the second step transfers to a different (Y) nucleus
in order to probe the dipolar coupling between X and Y. The sequence of transfers
is in principle arbitrary, but usually sensitivity is an issue, so transfer from protons
(to generate a large magnetization) and detection on the nucleus of higher sensi-
tivity (to gain signal intensity) is the standard procedure. Detection of the most
sensitive nucleus, protons, is also possible, but is difficult if the homonuclear pro-
ton-proton dipolar coupling is strong (see
"CRAMPS: General" on page 271
).
In this chapter, the most popular double CP experiment is described. Here, the
first CP step transfers magnetization from protons to
15
N. Then, in a second cross
polarization step, magnetization is transferred from
15
N to
13
C; the signal is finally
detected on
13
C under suitable proton decoupling. The purpose of this experiment
is to gain information about the C-N dipolar coupling which in turn provides special
information.
Naturally, the C-N-, or in general, the X-Y dipolar coupling is much smaller than
any dipolar coupling involving protons. For C-N, it is <2.5 kHz. This has some ex-
perimental consequences:
1. There is no need to decouple
15
N while observing
13
C, since the coupling is
spun out already at moderate spin rates.
2. The Hartmann-Hahn condition for this cross polarization is extremely sharp
and must be adjusted very carefully for every spin rate.
3. The magnetization transfer is substantially slower than from protons, meaning
that contact times are usually longer.
4. The transfer occurs (unlike CP from protons) not out of a bath of abundant
spins, but behaves (especially at high spin rates) more like a transfer between
spin pairs.
5. Labeled samples must be used so that an observable number of coupled spins
is present.
Advanced experimental schemes use tangential pulses to provide adiabatic con-
ditions during the cross polarization (S. Hediger et al.) or provide only selective
polarization transfer, Specific CP (Baldus et al.).
References:
1. J. Schaefer, T.A. Skokut, E.O. Stejskal R.A. McKay, and J.E. Varner, Proc. Nat. Acad. Sci. USA 78 ,
5978 (1981).
2. J. Schaefer, E.O. Stejskal, J.R. Garbow, and R.A. McKay, Quantitative Determination of the Concen-
trations of
13
C-
15
N Chemical Bonds by Double Cross-Polarization NMR, J. Magn. Reson.59,150-156
(1984).
3. M. Baldus, A.T. Petkova, J. Herzfeld, and R.G. Griffin, Cross Polarization in the tilted frame: assign-
ment and spectral simplification in heteronuclear spin systems. Mol. Physics 5, 1197-1207 (1998).