th  - 1987 - 51st ENC Conference

MF29
MAGIC ANGLE SPINNING OF MATRIX ISOLATED REACTIVE INTERMEDIATES
Kurt W. Zilm, Ronald A. Merrill , Marc M. Greenburg, and Jerome A. Berson
Department of Chemistry, Yale University, P.O. Box 66~6
New Haven, Corm 06511
Isolation in inert gas matrices or rigid glasses is widely
"recognized as an invaluable tool for direct spectroscopic investigation
of reactive intermediates. CP/MAS NMR has proven to be a most powerful
technique for studying the structure of amorphous and polycrysta~llne
solids. However, until now combining MAS technology with matrix
isolation techniques for the study of ground state slnglet reactive
intermediates by NMR has proven to be problematic.
We have developed a CP/MAS probe deslgn which allows rapid transfer
and subsequent magic angle spinning of sealed 5 mm NMR tubes while
maintaining the samples temperature at 77 K. The MAS turbine design is
similar to that reported by Gay in 1984, except that the sample tube
extends 3 cm below the turbine. The maximum spinning rate obtained with
our current implementation of this design is 3 kHz. The spinning is very
stable and remarkably forgiving of imbalances in the tube, both at the
sample end and at the seal. The unique aspect of this probe design is
the use of separate gas supplies for cooling and spinning the sample.
Unlike previous low temperature MAS designs, this MAS probe allows rapid
transfer of samples prepared outside the probe into the precooled probe
without warming the sample or exposing it to the atmosphere. With a
probe of this design one can employ typical photochemical or pyrolytic
gas phase deposition techniques for preparing intermediates. Virtually
any ground state slnglet species which can be prepared and is long lived
at 77 K, can now be investigated by CP/MAS NMR.
Using this probe design we have observed the first C-13 CP/HAS NHR
spectrum of a matrix isolated reactive intermediate. The species is the
singlet biradical 3,4-dlmethylenefuran which gives a single resonance at
100ppm relative to TMS for the methylene carbons. The biradical, which
is deep purple, was prepared by photolysis of the corresponding C-13
labeled(98% at both methylene carbons) azo precursor at 77 K in a 2-MTHF
glass. After photolysis the decrease in the intensity of the precursor
peak at 58ppm agrees well with intensity of the new resonance at 100ppm.
Conclusive assignment of this llne to the biradical can be made on the
basis of this observation, photobleachlng, and chemical trapping
experiments. The presence of the NMIR signal demonstrates directly and
unambiguously the singlet nature of the biradical. Applications to other
reactive species will also be presented.