th  - 1988 - 51st ENC Conference

25
-- I NUCLEAR MAGNETIC RESONANCE STUDIES OF GROUP VI METAL CARBONYLS ON
OXIDE SUPPORTS: William M. Shirley, Department of Chemistry, Wichita State
University, Wichita, Kansas 67208.
High-resolution solid-state NMR has become an important technique for
characterizing heterogeneous catalysts. In this study, 13C NMR was used to
characterize surface species prepared by allowing Cr(CO)6 , Mo(CO)6 , and W(CO)6 to
react with alumina and zeolite supports. Surface trlcarbonyl species, stable up to
200°C for all three metals, have been observed by NMR on either y-alumlna or a NaX
zeolite. The large downfleld shift of 247 ppm observed for Cr(CO)3/NaX indicates an
anionic species. The molybdenum and tungsten carbonyls on alumina show a strong
signal from an intermediate species with 4 or 5 CO llgands. Although diffuse
reflectance visible spectroscopy indicates an intermediate species for the chromium
carbonyl on the NaX zeolite, the NMR signal for this species is barely observable
using maglc-angle spinning (MAS). Interpretation of MAS spectra in the presence or
absence of cross polarization (CP) provides information on the mobility of surface
species on alumina. While the spinning sldebands of the 13C resonance from
Mo(CO)3/alumlna using CP/MAS indicate a very broad resonance (400 ppm), the
intermediate carbonyl has a smaller chemical shift anlsotropy and is not enhanced by
CP. The Mo(CO)3 species is apparently static while the intermediate species is
rather mobile. Mobility is also indicated for Cr(CO)3/NaX since the resonance is
only about 150 ppm wide. This resonance is narrow enough to be followed from a room
temperature powder pattern to a relatively narrow llne above 150°C using a
conventional liquids probe at 7 T.
_ _
26 I A NOVEL METHOD FOR DETERMING ACTIVATION ENERGIES AND
CORRELATION TIMES FROM NMR SPIN-LATTICE RELAXATION DATA
Morton A. Fineman *
Department of Physics
San Diego State University
San Diego,CA 92182
The task of deducing the activation energies and correlation times from spin-
lattice relaxation data consisting of relaxation time measurements at various
temperatures at a fixed Larmor frequency has, in the past, been accomplished
by employing complicated and tedious iterative programs. In this paper a novel
technique is described which permits one to find accurate values of the activation
energy and correlation times easily and quickly without the use of a computer.
The only requirement is that a minimum value for the spin-lattice relaxation
time is observed. A typical set of data from an NMR experiment on penta-
deuterated ethane will be treated to demonstrate this method. Results
obtained by this technique for several other compounds reported in the literature
will be compared to the recorded literature values.
The support of EE&G Incorporated , the US Federal Aviation Agency and
the US Navy is gratefully acknowledged.
110