I: Low Frequency NMR and NQR Using a dc SQUID. II: Variable-temperature ¹³C CP/MAS of Organometallics

Thesis/Dissertation · Wed Nov 01 00:00:00 EST 1995

DOI:https://doi.org/10.2172/970015· OSTI ID:970015

Ziegeweid, Marcia A. ^[1]

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1. Univ. of California, Berkeley, CA (United States)

NMR and NQR at low frequencies are difficult prospects due to small nuclear spin polarization. Furthermore, the sensitivity of the inductive pickup circuitry of standard spectrometers is reduced as the frequency is lowered. I have used a cw-SQUID (Superconducting Quantum Interference Device) spectrometer, which has no such frequency dependence, to study the local atomic environment of ¹⁴N via the quadrupolar interaction. Because ¹⁴N has spin I = 1 and a 0-6 MHz frequency range, it is not possible to obtain well-resolved spectra in high magnetic fields. I have used a technique to observe ¹⁴N NQR resonances via their effect on neighboring protons mediated by the heteronuclear dipolar interaction to study peptides and narcotics. The sensitivity of the SQUID is not enough to measure low-frequency surface (or other low spin density) systems. The application of spin-polarized xenon has been previously used to enhance polarization in conventional NMR experiments. Because xenon only polarizes spins with which it is in contact, it is surface selective. While differences in chemical shifts between surface and bulk spins are not large, it is expected that the differences in quadrupole coupling constant should be very large due to the drastic change of the electric field gradient surrounding spins at the surface. With this in mind, I have taken preliminary steps to measure SQUID detected polarization transfer from Xe to another spin species at 4.2 K and in small magnetic fields (<50 G). In this regime, the spin-lattice relaxation of xenon is dependent on the applied magnetic field. The results of our efforts to characterize the relaxation of xenon are presented. The final section describes the solid-state variable-temperature (VT) one- and two-dimensional ¹³C cross polarization (CP)/magic angle spinning (MAS) NMR of Hf(η₅-C₅H₅)₂(η¹-C₅H₅)₂, Zr(η⁵-C₅H₅)₃(η¹-C₅H₅) and Sn(η¹-C₅H₅)₄. This work was undertaken in the hope of gaining insight into the intramolecuhrr dynamics, specifically which fluxional processes exist in the solid state, by what mechanism rearrangements are occurring, and the activation energies by which these processes are governed.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 970015
• Report Number(s): LBL-38324; TRN: US201002%%1055
• Resource Relation: Related Information: Designation of Academic Dissertation: doctoral thesis; Academic Degree: Ph.D.; Name of Academic Institution: University of California; Location of Academic Institution: Berkeley, CA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
CHEMICAL SHIFT
COUPLING CONSTANTS
ELECTRIC FIELDS
FREQUENCY DEPENDENCE
FREQUENCY RANGE
MAGNETIC FIELDS
NARCOTICS
PEPTIDES
POLARIZATION
PROTONS
QUADRUPOLES
RELAXATION
SENSITIVITY
SPECTRA
SPECTROMETERS
SPIN
SPIN-LATTICE RELAXATION
XENON