th  - 1988 - 51st ENC Conference

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172 JMAGIC ANGLE SPINNING SEPARATED LOCAL FIELD SPECTROSCOPY: SOME
EXPERIMENTAL OBSERVATIONS RELEVANT TO THE DETERMINATION OF C-H DISTANCES BY NMR:
Gretchen G. Webb* and Kurt W. Zilm, Department of Chemistry, Yale University 225
Prospect Street, New Maven, CT 06511
The calibration of the homonuclear scaling factor is very important in
obtaining accurate results for 13C-IH bond distances by separated local field
spectroscopy (SLF). It has generally been noted that the best fits of SLF data always
give an effective scaling factor that is significantly less than that measured on a
standard liquid sample if C-H distances from diffraction studies are used to calculate
the dipolar couplings. This discrepency has been attributed by other workers to the
effects of molecular motion or alternatively interpreted as indicating that C-H
distances are in fact longer than measured by either neutron or x-ray diffraction. In
this paper the problems with calibrating the homonuclear scaling factor in CPMAS
probes is discussed and it is suggested that scaling of 1H-13C J couplings for a
liquid sample may be the most accurate approach. Using this technique the scaling
factor for a semi-wlndowless MREV-8 sequence is found to be that predicted by theory.
When the scaling factor is determined from HAS SLF patterns the scaling factor is
found to always be reduced by the same amount if C-H bondlengths are assumed to be
1.09 A. This reduction in the scaling factor occurs for both CH and CH 2 groups and is
apparently independent of temperature down to 77K. The results indicate that molecular
and lattice libratlons are the principal sources of the reduction in observed dipolar
couplings.
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Determination of H-H Bond Distances in Transition Metal Dihydrogen Complexes
by Solid State NMR
M. Chinn, M. Cozine, M. Heinekey, G. Kubas, t J. Millar*and K. Zilm
Dept. of Chemistry, Yale University, New Haven, CT 06511
1"Los Alamos National Laboratory, Los Alamos, NM 87545
Molecular hydrogen (H 2) somewhat surprisingly acts as a ligand in a large number of transition metal
complexes LnM(Vl2-H2 ). These complexes are of great interest to the organometallic community since they
may model an intermediate in the oxidative addition of H 2 to form dihydrides. Not surprising is the fact that
the interaction of the H 2 ligand and the metal exhibits a wide degree of variation depending on the metal and
the basicity of the ligands, L. In the simplest model, stronger binding of the H 2 by the metal should result in
lengthening of the H-H bond. These distances have been studied by techniques such as x-ray and neutron
diffraction as well as by solution 1H T 1 measurements. We report measurements of H-H distances by a 1H
solid state selective pulse method which suppresses the homogeneous lineshape of the ligands, L, and
allows observation of the H 2 dipolar Pake pattern. These Pake patterns are complicated by torsional
oscillations of the H 2, but study of the lineshapes as a function of temperature in most cases leads to models
for the motion and allows determination of the H-H distances..In the Mo, Ru and W complexes studied to
date, observed powder pattems are -500 kHz in width, indicating bond distances ranging from 0.89 to 1.02
Angstroms.
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