th  - 1988 - 51st ENC Conference

I
12 o I SPECIATION OF WATER IN GLASSES BY HIGH-SPEED 1H MAS-NMR
Hellmut Eckert *+, James P. Yesinowski*§, Lynn A. Silver Y , and Edward M.
Stolper ¥, + Department of Chemistry, UC Santa Barbara, Goleta CA 93106,
§Division of Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena, CA 91125, YDivision of Geological and Planetary
Sciences, California Institute of Technology, Pasadena, CA 91125.
The state of water in silicate glasses has received considerable attention both
in geology and materials science and much effort has been devoted to
identifying and quantitating the H20 and OH species present. This work
describes the first application of high resolution 1H nuclear magnetic resonance
methods to this problem. 1H MAS-NMR results are reported on a series of
synthetic and naturally-occurring silicate glasses containing 0.04 to 9.4 wt.%
H20. Spinning at 8 kHz results in substantial line-narrowing; in addition,
extensive spinning sideband patterns are observed that reflect the
inhomogeneous character of the 1H-1H dipolar couplings within anisotropically
constrained water molecules. The 1H MAS-NMR spectra can be simulated as the
sum of the individual spectra of the model compounds tremolite (OH) and
analcite (H20), and the species concentrations thus obtained are in good
agreement with IR results. The agreement of the experimental 1H MAS-NMR
spectra with simulations based on compounds, in which the hydrogen-bearing
species are structurally isolated, indicates that no preferential clustering
occurs.
121 J BROADBAND SPIN DECOUPLING IN THE PRESENCE OF SCALAR INTERACTIONS
A. J. Shaka ~, C. J. Lee and A. Pines, University of California, Berkeley,
Berkeley, California 94720
Successful broadband spin decoupling sequences like WALTZ-16 are based on
an underlying model of isolated IS spin pairs. Almost all organic molecules
of interest, however, contain networks of scalar-coupled protons. We have
analyzed the effect of proton scalar coupling on broadband decoupling and
devised a new series of broadband decoupling sequences which we call the
DIPSI sequences. The DIPSI sequences of'fer the same high standard of decoupling
as WALTZ-16 in the isolated IS case, but over somewhat smaller bandwidths.
in the presence of proton-proton coupling the DIPSI sequences are superior
to WALTZ-16, offering an improvement in resolution and sensitivity for
carbon-13 spectroscopy.
The principles used to construct the DIPSI ~eq,,~nrP~ can be applied to
a number of related experiments in two-dimensional spectroscopy.
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