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Chapter 6. Solid-State NMR Experiments

111

VNMR 6.1C User Guide: Solid-State NMR

01-999162-00 C0402

Parameters

sedra2

uses

UNITY

INOVA and UNITYplus parameters

tpwr

tpwrm

dpwr

cppwr

dipolr

crossp

dblvl2

, and

cntct

. See

page 85

for a description of these

parameters.

pwx

is the observe 90

pulse, in

s. It determines the length of

pulses of SEDRA.

cycles

is the number of 8-pulse SEDRA cycles before acquisition. Each SEDRA cycle

has a length of 8 rotor periods and a

pulse is applied at the middle of each rotor period.

Usually

cycles

is an integer from 0 to the maximum number of cycles. Typical values are

0 to 8.

sedraflag

set to

'n'

causes a 90

pulse to be applied at the middle of each 8 pulse

SEDRA cycle. This pulse partially refocuses the effects of SEDRA and provides a control.
If

sedraflag

is set to

'y'

, a 90

pulse is not applied, allowing full SEDRA evolution.

Usually,

sedraflag

is set to

'n','y'

. For a simultaneous array of

cycles

and

sedraflag

, set

array

'cycles,sedraflag'

, and not the reverse.

phaseflag

is set to

'n'

for normal operation. If

phaseflag

is set to

'y'

and

sedraflag

is set to

'n'

, 90

pulses are applied with 180

phase alternation for

successive SEDRA cycles. Better refocusing of the control pulse sequence should lead to a
larger SEDRA effect.

(Caution: setting

phaseflag

'y'

is not fully tested.)

Reference

Guillion, T.; Vega, J. Chem. Phys. Lett. 1993, 194, 423.

6.20 REDOR1—Rotational Echo Double Resonance

NMR measurement of internuclear distance, by using multidimensional methods, has been
of great importance for the determination of biomolecular structures in solution. It is
desirable to obtain similar data from the solid-state for study of membrane protein
structures that have been largely inaccessible with solution state methods. REDOR,
Rotational Echo Double Resonance, is a useful approach to this goal.

Figure 49

is a diagram

of the pulse sequence.

The REDOR experiment provides internuclear distance data between a pair of heteronuclei
(often

C and

N) by means of their mutual dipolar interaction. REDOR is a high-

resolution solids experiment and is therefore performed with magic angle spinning, MAS,
and usually proton cross-polarization, CP. The heteronuclear dipolar interaction is partially
averaged by MAS. Multiple

C and

N pulses, synchronized with the MAS rotor speed,

reintroduce dipolar information into the spectrum as a variation of spectral intensity with
time. Because REDOR is often performed with cross-polarization and proton decoupling,
it is a triple-resonance experiment requiring a three channel spectrometer and a triple-
resonance probe.

After initial cross-polarization, a variable number of

pulses are applied to the

channel, each synchronized with the end of successive rotor periods, with acquisition
beginning at the following rotor period. At the middle of each rotor period,

pulses are

applied to the

N channel. These

N pulses cause signal loss, due to the dipolar

interaction. A spectrum without

N pulses serves as a control experiment. The typical plot

is (So-S)/S, where S is the spectral intensity the and

N pulses and So is the intensity

without. The carbon-nitrogen bond distance is determined by simulation of the curve.