th  - 1988 - 51st ENC Conference

CALCULATION OF 2951MAS NMR CHEMICAL SHIFT FROM SILICATE
MINERAL STRUCTURE: Sherriff, Barbara L. " and G:rundy H Douglas
Department of Geology, McMasterUniversity, Hamilton, Ontario, L8S4MI...
There have been many attempts to correlate 295i MAS nmr chemical shift
with various parameters of silicate mineral structure. In our studies
of mineral systems such as scapolites and Feldspars we Found these
correlations to be inadequate For the interpretation of the complex nmr
spectra.
Silicate crystal structures were retrieved from a database and
manipulated with a computer graphics modelling program• Further
calculations revealed a simple correlation between 29Si MAS nmr
chemical shift and molecular geometry that Is applicable to all
silicate minerals. It is based on the magnetic anisotropy and valence
of the bond between the terminal oxygen atoms of the silicate
tetrahedron and the second nearest neighbour cation to the silicon.
The correlation, which is based on 76 data points and has a
correlationn coefficient of 0.911 with a standard deviation of 0.7ppm,
can be used to calculate the chemical shift and hence to assess the
validity of different structural models.
X-ray diffraction methods can only determine the average of the
AI-O and Si-O lengths for each tetrahedral (T) site in the case of
minerals with AI-Si disorder. Comparison of measured chemical shifts
with those calculated For structures with different T-O distances can
give an estimate of AI content.
8 NONLINEAR INCOHERENT SPECTROSCOPY
J. Paff and B. BiOmich*
Max-Planck-Institut fur Polymerforschung, 6500 Mainz, F.R. Germany
In incoherent spectroscopy the Fourier transforms of the nonlinear
cross-correlation functions of excitation and response are multidimen-
sional spectra which correspond to the nonlinear susceptibilities. In
stochastic NMR spectroscopy the Fourier transform of the cross-correla-
tion algorithm has been applied in the past for the computation of 2D
spectra in terms of 2D cross-sections through the 3D spectra of the
third order nuclear magnetic susceptibility. 1
We have tested the explicit time domain third order cross-correla-
tion for the derivation of 2D cross-sections through the 3D time corre-
lation function. After 2D FT one obtains z-COSY or exchange and MQ type
2D spectra. This approach is of interest, since the evaluation can be
executed in an analog fashion in parallel for each data point of the 2D
time domain matrix. In this way the multiplex advantage may be introdu-
ced to the additional dimension in 2D spectroscopy with the ultimate
goal to measure a complete 2D spectrum within a few TI. The procedure is
presently being implemented to obtain dead time free 2D ESR spectra,
taking advantage of the low power of continuous stochastic excitation.
The state of the art is described, and examples from NMR spectroscopy
are given.
i) B. BiOmich. Progr. NMR Spectrosc. 19, 331 (1987).
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