th  - 1987 - 51st ENC Conference

WF24
VT CP/MAS NMR OF ANTIFERROMAGNETIC METAL COMPLEXES
Gordon. C__~. Campbell and James F. Haw
Department of Chemistry, Texas A&M University
College Station, TX 77843
Drawing on our previous experience with variable-temperature (FT) CP/MAS
NMR of paramagnetic species 1,2, we are now applying the technique to the study of
antiferromagnetic materials. Antiferromagnetism is found in a wide variety of
transition metal dimers and complexes in which a low-lying triplet excited
electronic energy state lies above a singlet ground state. Spin pairing in the
singlet state at low temperatures leads to diamagnetism, while at higher
temperatures the triplet state is significantly populated, and these substances
exhibit the properties of paramagnetic materials.
It is thus difficult in many cases to observe CP/MAS NMR spectra of anti-
ferromagnetic solids at or above room temperature. Lowering the sample temp-
erature is found to significantly increase the amount of information available
from CP/MAS NMR data. Not only do resonances closer to metal centers become
observable, but the temperature dependence
of the chemical shifts may give quant-
itative information about the electronic
character of the species. A good example
of this is afforded by the 13C CP/MAS NMR
data for copper(ll) n-butyrate hydrate
dimer presented in the figure, where mag-
netically inequivalent sites (eg., CI and
CI') are also observable.
1. J. F. Haw and G. C. Campbell,
J__~. Ma~n. Reson., 66, 558 (1986).
VARIABLE TEPdPE]u.,tTURE C-13 CP/IdAS SPECTRA
OF COPPlg:I {11) N-BUTYRATE HYDRATE DIMER
¢~ ~K
2. G. C. Campbell, R. C. Crosby, and J. F. Haw,
J. Ma~n. Reson. 69 191 (1986). I 1 i