Chapter 3. Wideline Solids Module Operation
55
VNMR 6.1C User Guide: Solid-State NMR
01-999162-00 C0402
• Remember that for solid samples of quadrupolar nuclei, the 90
°
pulse usually cannot
be determined from a
pw
array.
3.6 Data Processing
Most data processing needs of wideline spectra are the same as that for other spectra. There
are, however, several specialized applications, for which software is provided. Since most
wideline spectra are collected in a spin-echo mode, it can be extremely important to start
acquisition, or at least Fourier transformation, on top of the echo. The FID display program
provides a point-by-point (if
dotflag='y'
), two-color display of the real and imaginary
channels of the FID in order to provide the best possible examination of the details of the
FID. One or two time cursors can also be displayed and are not constrained to fall on top
of individual data points but may be used to interpolate as well (for example, to estimate
the time of the echo). A left shift of the FID may be used to shift the FID until the echo
occurs at the first point of the FID.
Normally, the echo top is well enough defined so that left shifting removes all distortion.
However, this is not always the case, especially with very short T
2
echoes. A means of
fractionally left shifting has been provided as follows:
lsfid='n' phfid='n'
ft
ft('inverse',
n,expn)
jexp
n
scalesw=1.0/
n
df
wft
In this example,
n
in the
ft
command is a interpolation factor (power of 2);
exp
n
is an
experiment number for the interpolated FID;
df
interpolates the FID by a factor of
n
and
the echo top may be picked more accurately;
scalesw=1.0/
n
will correct
sw
.
It is also common to collect wideline spectra with the transmitter placed in the exact center
of the resonance. Software is provided to allow “phasing the FID” to place as much as
possible of the FID in the real channel. This operation means that the frequency-
independent phase shift of the spectrum is as close to zero as possible, which is beneficial
since frequency-domain phasing of wideline spectra can be difficult at best. In addition,
spectral symmetry can be forced by software that sets the imaginary channel of the FID to
zero.