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High Resolution NMR Studies of Phase Transitions in Model H-Bonded Solids

Title: High Resolution NMR Studies of Phase Transitions in Model H-Bonded Solids.
Name(s): Günaydın Şen, Özge, author
Dalal, Naresh, professor directing dissertation
Brooks, James, outside committee member
Alabugin, Igor, committee member
Steinbock, Oliver, committee member
Fu, Riqiang, committee member
Department of Chemistry and Biochemistry, degree granting department
Florida State University, degree granting institution
Type of Resource: text
Genre: Text
Issuance: monographic
Date Issued: 2007
Publisher: Florida State University
Place of Publication: Tallahassee, Florida
Physical Form: computer
online resource
Extent: 1 online resource
Language(s): English
Abstract/Description: This dissertation presents comprehensive NMR studies of three related hydrogen-bonded compounds ammonia borane (NH 3BH3), ammonium dihydrogen arsenate (NH4H2AsO4) and ammonium dihydrogen phosphate (NH4H2PO4). All these compounds were known to exhibit phase transitions but the transition mechanisms were not well understood. Here we utilized as a probe the isotropic chemical shift, δ iso, as well as spin-lattice relaxation time, T1, using the magic angle spinning (MAS) technique at fields as high as 21.1 Tesla (900 MHz for 1H), to obtain a deeper understanding of their transition mechanism. NH3BH3, known as a potential source for hydrogen storage, was investigated via 15N MAS around its phase transition temperature TP ~225 K. Both δiso and T1 exhibited strong anomalies around the TP. At T Similar 15N NMR studies were carried out on the antiferroelectrics NH 4H2AsO4 (ADA) and NH4H2PO4 (ADP). Both δiso, and T1 exhibited anomalies around 216 K for ADA. It was found that τ c increased from about 100 ps to 400 ps, and the Arrhenius activation energy remains almost constant but τ0 increases by a factor of 2.5 just below the T N (216 K). 15N MAS-NMR was also measured at 21.1 Tesla, which helped us to investigate the mechanism to a depth not possible with lower fields. The results imply that the motion of the NH 4+ site plays an important role in the transition mechanism. On ADP, the data above 148 K were similar to those for ADA. However, despite numerous attempts, we were unable to detect 15N signals below its T N, but it was possible to follow the transition via 31P NMR. The 31P signals exhibited anomalous increase in the linewidth, and a change in δ iso around 148 K. Parallel measurements were conducted on the 75As NMR on ADA. Taken together these studies establish, in contrast to earlier models, that the antiferroelectric transitions in ADA and ADP also involve both order-disorder and displacive components.
Identifier: FSU_migr_etd-3917 (IID)
Submitted Note: A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Degree Awarded: Summer Semester, 2007.
Date of Defense: June 20, 2007.
Keywords: Antiferroelectric, Hydrogen Storage, Order-Disorder, H-Bonding, Phase Transition, P31, As75. Magic Angle Spinning Spin-Lattice Relaxation Time, N15, Nuclear Magnetic Resonance, Displacive
Bibliography Note: Includes bibliographical references.
Advisory Committee: Naresh Dalal, Professor Directing Dissertation; James Brooks, Outside Committee Member; Igor Alabugin, Committee Member; Oliver Steinbock, Committee Member; Riqiang Fu, Committee Member.
Subject(s): Chemistry
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Owner Institution: FSU

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Günaydın Şen, O. (2007). High Resolution NMR Studies of Phase Transitions in Model H-Bonded Solids. Retrieved from