Publication Date

2018-12-08

Availability

Embargoed

Embargo Period

2019-12-08

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Chemistry (Arts and Sciences)

Date of Defense

2018-06-27

First Committee Member

Jamie D. Walls

Second Committee Member

Rajeev Prabhakar

Third Committee Member

James N. Wilson

Fourth Committee Member

Edwin Rivera

Abstract

NMR spectroscopy is a versatile analytical platform with a well developed theoretical foundation that makes it a popular vehicle for biological, chemical, material, and medical research. Successfully deciphering an NMR spectrum in order to extract important structural and chemical information typically requires the separation of overlapping signals that can be limited by the available spectral resolution, which is itself influenced by numerous factors such as molecular motions, spin-spin couplings, and field inhomogeneities. In this dissertation, methods for improving spectral resolution in both liquid and solids are explored. To begin, a brief introduction to NMR theory is provided which includes an introduction to quantum mechanical treatments of spin systems, an overview of Average Hamiltonian Theory and description of Walls's method for designing pathway selective pulses, PSPs. In Chapter 2, the reverse INEPT pathway selective pulse, or RIPSP, is presented and shown to generate a pure rotation of an I-spin (e.g., proton) only for nonzero heteronuclear coupling to an S-spin (e.g., carbon-13) in I(n)S spin systems. When the RIPSP is applied to more complex systems, such as carbon-13 labeled sugars, it was shown that line narrowing occurred. The origin of this line narrowing is explored in Chapter three. A theory is proposed that the line sharpening is due to the contribution from multispin, single-quantum coherence coupled with correlated B(1) and B(0) field inhomogeneities. In Chapter four, focus is turned to the study of spin-1/2 dipolar solids where line narrowing under a CPMG or (pi) - pulse train is explored. A theory is developed that shows (pi) pulse imperfections in a CPMG pulse train generate an effective ``pulsed" spin-locking of single-quantum coherences with phase +/- (phi(X)phi(Y)) . Long-lived signals and line narrowing were observed in experiments on both magnetically dilute, random spin networks, such as those found in C(60) and C(70), and in adamantane and ferrocene samples.

Keywords

NMR; resolution improvement; pathway selective pulse; RIPSP; dipolar echoes

Available for download on Sunday, December 08, 2019

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