th  - 1988 - 51st ENC Conference

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TRANSITIONS:
Laboratory,
INDIRECT DETECTION OF 14N ~M=2 (OVERTONE) NMR
A. N. Garroway* and J. B. Miller, Naval Research
Code 6122, Washington D. C. 20375-5000
For quadrupolar spin systems in high magnetic field,
the ~M=2 NMR transition is weakly allowed, due to the slight
distortion of the pure Zeeman states by the quadrupolar
perturbation. For such overtone transitions, the theory and
direct observation of the 14N resonance, occurring at about twice
the 14N Larmor frequency, have been already presented by other
workers. Here we extend the method by using the IH spin system
to detect indirectly the nitrogen overtone transition. First IH
dipolar order is created and then progressively destroyed by
repetitive contacts with the nitrogen rotating frame Zeeman
reservoir; we monitor the loss of the IH signal. The 14N
irradiation is stepped in frequency during the cross-relaxation
to compensate partly for the reduced effective 14N rf field
strength. The merit of the indirect over the direct detection
scheme is the corresponding increase in signal intensity. We
demonstrate 14N indirect detection methods on some crystalline
solids including hexamethylene tetramine.
14 3 I DIFFERENTIAL DEVELOPMENT OF MULTIPLE-QUANTUM COHERENCE IN A
LIQUID CRYSTAL: W. V. Gerasimowicz*l", A. N.Garroway, and J. B. Miller, Chemistry Division, Code
6122, Naval Research Laboratory, Washington, D. C. 20375-5000.
A 2:1 molar mixture of 4'-cyanophenyl-4-n-heptylbenzoate and 4'-cyano-phenyl-4-n-
butylbenzoate exhibits nematic liquid crystal behavior in the temperature range from 25 o
to 50o C. The proton NMR spectra suggest that two regimes of dipolar interaction are
present L e. those characteristic of partially-isolated proton spin pairs, presumably on the
phenyl rings, and weakly-coupled spins originating from the alkyl chain moieties
comprising the remainder of the system. A solid-echo pulse sequence permits the
observation and separation of these unique regions within the molecules on the basis of
their differing relaxation properties. We have combined this technique with multiple-
quantum NMR, so that the spins are first prepared by means of a variable delay solid-echo
sequence followed by MQ NMR experiments. Differential development of proton spin
coherence can then be distinguished (in this case) for different segments of the same
molecule. We find that over the course of the multiple-quantum preparation times, the
phenyl proton pairs do not interact appreciably with the remaining protons of the molecule.
1"Permanent Address: U. S. D. A., Eastern Regional Research Center, 600 E. Mermaid Lane,
Philadelphia, PA 19118
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