Methyl quantum tunneling and nitrogen-14 NQR NMR studies using a SQUID magnetic resonance spectrometer

Thesis/Dissertation · Thu Jul 01 04:00:00 EDT 1993

DOI:https://doi.org/10.2172/10125943· OSTI ID:10125943

Black, Bruce Elmer ^[1]

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

Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) techniques have been very successful in obtaining molecular conformation and dynamics information. Unfortunately, standard NMR and NQR spectrometers are unable to adequately detect resonances below a few megahertz due to the frequency dependent sensitivity of their Faraday coil detectors. For this reason a new spectrometer with a dc SQUID (Superconducting Quantum Interference Device) detector, which has no such frequency dependence, has been developed. Previously, this spectrometer was used to observe ¹¹B and ²⁷Al NQR resonances. The scope of this study was increased to include ²³Na, ⁵¹V, and ⁵⁵Mn NQR transitions. Also, a technique was presented to observe ¹⁴N NQR resonances through cross relaxation of the nitrogen polarization to adjacent proton spins. When the proton Zeeman splitting matches one nitrogen quadrupoler transition the remaining two ¹⁴N transitions can be detected by sweeping a saturating rf field through resonance. Additionally, simultaneous excitation of two nitrogen resonances provides signal enhancement which helps to connect transitions from the same site. In this way, nitrogen-14 resonances were observed in several amino acids and polypeptides. This spectrometer has also been useful in the direct detection of methyl quantum tunneling splittings at 4.2 K. Tunneling, frequencies of a homologous series of carboxylic acids were measured and for solids with equivalent crystal structures, an exponential correlation between the tunneling frequency and the enthalpy of fusion is observed. This correlation provides information about the contribution of intermolecular interactions to the energy barrier for methyl rotation.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 10125943
• Report Number(s): LBL--34880; ON: DE94007038
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400102
665100
665412
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
CHEMICAL AND SPECTRAL PROCEDURES
DIRECT CURRENT
METHYL RADICALS
NITROGEN 14
NUCLEAR QUADRUPOLE RESONANCE
NUCLEAR TECHNIQUES IN CONDENSED MATTER PHYSICS
OPTIMIZATION
SPECTROMETERS
SPECTROSCOPY
SQUID DEVICES
STRUCTURAL CHEMICAL ANALYSIS
SUPERCONDUCTING DEVICES
TUNNEL EFFECT